Prepared in cooperation with Clean Water Services

Plankton Communities and Summertime Declines in 

Algal Abundance Associated with Low Dissolved Oxygen 

in the Tualatin River, Oregon 

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5037
Front Cover: Tualatin River downstream of Rood Bridge. (Photograph by Kurt Carpenter, July 19, 2005.)

Inset 1: Stephanodiscus binderanus, a filamentous centric diatom. (Photograph courtesy of Rex Lowe, Bowling Green State University.)

Inset 2: Anabaena flos-aquae, a colonial blue-green algae. (Photograph by Kurt Carpenter, July 15, 2008.)

Inset 3: Cryptomonas erosa, a Cryptophyte unicellular flagellate. (Photograph by Kurt Carpenter, June 30, 2008.)

Inset 4: Bosmina longirostris. (Photograph by Kurt Carpenter, June 26, 2006.)

Back Cover: Plankton tow sample vials showing longitudinal changes in plankton abundance and composition. (Photograph by Kurt Carpenter, June 6, 2006.)
Plankton Communities and Summertime Declines in 

Algal Abundance Associated with Low Dissolved 

Oxygen in the Tualatin River, Oregon 

By Kurt D. Carpenter and Stewart A. Rounds

Prepared in cooperation with Clean Water Services

Scientific Investigations Report 2013–5037

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director 

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Carpenter, K.D., and Rounds, S.A., 2013, Plankton communities and summertime declines in algal abundance 

associated with low dissolved oxygen in the Tualatin River, Oregon: U.S. Geological Survey Scientific Investigations 

Report 2013–5037, 78 p.
iii

Contents

Executive Summary.......................................................................................................................................1

Introduction.....................................................................................................................................................3

Patterns in Longitudinal and Seasonal Phytoplankton Abundance.............................................9

Primary Factors Controlling Phytoplankton Growth .....................................................................10

Study Background, Objectives, Scope, and Approach ................................................................11

Hydrology and Study Area Description...........................................................................................12

Sample Collection and Data Analysis Methods .....................................................................................15

Water-Sample Collection and Processing......................................................................................15

Field Parameters and Streamflow ...................................................................................................15

Plankton-Sample Collection, Processing, and Identifications....................................................15

Plankton Bioassay Experiments.......................................................................................................16

Quality Assurance Data.....................................................................................................................16

Data Analysis Methods......................................................................................................................17

Climate, Streamflow, and Water-Quality Conditions..............................................................................18

Streamflow and Sources of Flow.....................................................................................................18

Bioavailable Nutrients........................................................................................................................23

Patterns in Plankton Populations..............................................................................................................27

Phytoplankton......................................................................................................................................27

Wapato Lake Algal Bloom.................................................................................................................31

Zooplankton .........................................................................................................................................31

Multivariate Analyses of Phytoplankton Assemblages and Environmental Data.............................37

Nonmetric Multidimensional Scaling (NMDS) Ordinations.........................................................37

Spearman Rank Correlations, PCA, and BEST Analyses .............................................................37

Bioassay Experiment Results.....................................................................................................................44

Declines in Phytoplankton Populations ...................................................................................................44

Changes in River Hydrology and Wastewater Management, 1991–2008...........................................48

Case Studies of Low Dissolved Oxygen Events and Bloom Crashes in 2003–08 ..............................52

Low Dissolved Oxygen Events..........................................................................................................52

Specific Bloom Crashes ..........................................................................................................54

2003 ................................................................................................................................54

2004 ................................................................................................................................54

2005 ................................................................................................................................54

2006 ................................................................................................................................54

2007 ................................................................................................................................58

2008 ................................................................................................................................58
iv

Evaluation of Hypotheses to Explain Phytoplankton Declines.............................................................61

Light Limitation ....................................................................................................................................61

Reduced Algal “Seed Source” to Inoculate the River .................................................................62

Higher Summer Streamflows............................................................................................................62

Changes in the Dominant Sources of Flow ....................................................................................64

Water Storage Reservoirs and Flow Augmentation ............................................................65

WWTF Discharges.....................................................................................................................65

Zooplankton Grazing...........................................................................................................................69

Phosphorus Limitation........................................................................................................................69

Conclusions and Implications for River Management ..........................................................................71

Possible Future Studies, Monitoring, and Research .............................................................................72

Monitoring Needs...............................................................................................................................72

Bioassay Experiments........................................................................................................................72

Studies of Zooplankton—Phytoplankton Interactions and Fish Predation...............................73

Artificial Neural Network Models....................................................................................................73

Periodic Revisitation of the Multivariate Analyses.......................................................................73

Acknowledgments.......................................................................................................................................73

References Cited..........................................................................................................................................74

Appendixes ...................................................................................................................................................77

Appendix A. Quality-Assurance Data for Water Quality, Chlorophyll-a, Phytoplankton, and 

Zooplankton from the Tualatin River, Oregon, 2006–08 ...................................................77

Appendix B. Algal Taxa Biovolume for Samples Collected from the Tualatin River, Oregon, 

2006–08....................................................................................................................................77

Appendix C. Zooplankton Taxa Counts for Samples Collected from the Tualatin River, 

Oregon, 2006–08.....................................................................................................................77

Contents— Continued
v

Figures

1. Map of the Tualatin River basin, Oregon, showing the location of selected

sampling sites, streamflow gages, and continuous water-quality monitors......................4

2. Map of the Tualatin River basin, Oregon, showing the location of

water-quality and plankton sampling sites...............................................................................5

3. Graph showing average flow in the Tualatin River at West Linn , Oregon, July

and August 1929–2010..................................................................................................................7

4. Time series of phytoplankton chlorophyll-a and dissolved oxygen

concentrations in the lower Tualatin River, Oregon, July and August 2001–08 .................8

5. Biplot of chlorophyll-a and dissolved oxygen concentrations in the Tualatin

River at Stafford Road, July and August, 1991–2009...............................................................9

6. Graph showing longitudinal pattern in average chlorophyll-a concentrations

in the Tualatin River, Oregon, May–October 1991–2009.........................................................9

7. Graph showing changes in the average monthly phytoplankton chlorophyll-a

in the Tualatin River, Oregon, highlighting the 1991–2000 and 2002–2009 periods ..........10

8. Graph showing trends in the average August chlorophyll-a concentration and

the percentage of time in August that the minimum dissolved oxygen (DO)

standard was violated in the lower Tualatin River, Oregon, 1991–2009.............................11

9. Graphs showing annual patterns in snowpack, solar radiation, streamflow,

water-quality, and algal conditions in the Tualatin River, Oregon, 1991−2009..................19

10. Graphs showing natural flow, wastewater treatment facility discharges, and

flow augmentation in the Tualatin River at Farmington (river mile [RM] 33.3)

and West Linn (RM 1.8), Oregon, 2006–08...............................................................................20

11. Graphs showing streamflow and flow augmentation in the lower Tualatin

River, Oregon, showing the timing of longitudinal samplings, 2006–08 .............................21

12. Graphs showing percentage of natural flow, flow augmentation, and

wastewater treatment facility effluent in the Tualatin River at Farmington and

West Linn, Oregon, 2006−08......................................................................................................22

13. Graphs showing daily treated effluent discharges from the Rock Creek and

Durham wastewater treatment facilities, Oregon, May–October 1991–2008...................23

14. Time series of dissolved nitrite-plus-nitrate, ammonia, and soluble reactive

phosphorus concentrations at select sites in the Tualatin River, Oregon, 2006–08 ........25

15. Biplot of chlorophyll-a and ammonia and soluble reactive phosphorus in the

lower Tualatin River, Oregon, 2007...........................................................................................26

16. Time series of total algal biovolume and percentage of major phytoplankton

groups (divisions) at selected sites in the Tualatin River, Oregon, 2006−08 .....................32

17. Graphs showing longitudinal pattern in total zooplankton density in the

Tualatin River, Oregon, 2006−08................................................................................................34

18. Graphs showing seasonal pattern in zooplankton abundance and percentage

of major groups at select sites in the Tualatin River, Oregon, 2006−08 .............................35

19. Time series of algal biovolume and zooplankton density at select sites in the

Tualatin River, Oregon, 2006−08................................................................................................36
vi

20. Ordination plots of phytoplankton samples from the Tualatin River, Oregon,

2006–08, highlighting the two upstream sites, the group of samples collected

during and after the Anabaena flos-aquae bloom associated with the draining of

Wapato Lake, and downstream sites with Wapato-affected samples removed,

showing separation of samples collected in May–July versus August–September......38

21. Graphs showing changes in chlorophyll-a and dissolved oxygen concentrations

during bioassay experiments on May 19, June 11, and June 30, 2008, Tualatin

River, Oregon ...............................................................................................................................45

22. Time series of water releases from Barney Reservoir and Hagg Lake, Oregon, by

Clean Water Services for flow augmentation, July and August, 1991–2009 ....................46

23. Time series of chlorophyll-a concentration and turbidity in the Tualatin River at

Rood Bridge, Oregon, July–August 1992–2009 ......................................................................46

24. Time series of turbidity in the Tualatin River at Cherry Grove, Oregon,

June–August 1991–2009 ............................................................................................................47

25. Time series of chlorophyll-a concentrations and turbidity in the Tualatin River at

Elsner Road, Oregon, July and August 1991–2009 ................................................................47

26. Graph showing trend in the chlorophyll-a growth ratio in the reach between

Rood Bridge and Elsner Road, Tualatin River, Oregon, during August, 1991–2008..........47

27. Graph showing relation between turbidity and chlorophyll-a concentrations in

the Tualatin River at Elsner Road, Oregon, May–August 1992–2009..................................48

28. Time series of streamflow in the Tualatin River at Cherry Grove, Oregon, August

1991–2009 .....................................................................................................................................49

29. Time series of streamflow in the Tualatin River at West Linn, Oregon, 1991–2011..........49

30. Time series of streamflow in the Tualatin River at West Linn, Oregon, July and

August, 1991–2011 ......................................................................................................................50

31. Graphs showing relations between the percentage of natural flow, flow

augmentation, and wastewater treatment facility (WWTF) effluent

andchlorophyll-a anddiatom biovolume in the Tualatin River, Oregon, at river

miles 38.4 and 24.5 ......................................................................................................................51

32. Graph showing dissolved oxygen (DO) concentrations and water temperature in

the Tualatin River at Oswego Dam, Oregon, 2004–08, highlighting algal blooms

and low-DO events .....................................................................................................................53

33. Time series of chlorophyll-a, turbidity, and streamflow in the Tualatin River and

daily rainfall and solar maximum at the Durham wastewater treatment facility,

Oregon, June–October 2003 .....................................................................................................57

Figures— Continued
vii

Figures— Continued

34. Time series of chlorophyll-a, streamflow, and water-quality conditions in the

Tualatin River, Oregon, June–August 2006.............................................................................58

35. Time series of chlorophyll-a and turbidity in the Tualatin River at the Oswego

Dam, and percent biovolume of diatoms and cryptophyte algae at Stafford Road,

and rainfall and solar radiation at the Durham wastewater treatment facility

(WWTF), Oregon, June–August 2007 ......................................................................................59

36. Time series of chlorophyll-a and turbidity in the Tualatin River at Oswego Dam,

streamflow at West Linn, and percentage of wastewater treatment facility

(WWTF) effluent; rain and solar radiation at the Durham WWTF; and biovolume

of diatoms, cryptophyte algae, and blue-green algae at RM 24.5 near Scholls,

Oregon, June–August 2008 ......................................................................................................60

37. Biplot of streamflow and chlorophyll-a in the lower Tualatin River at the Oswego

Dam , Oregon, 2001–08...............................................................................................................63

38. Graph showing effect of the September 1993 Hagg Lake flow augmentation

experiment on chlorophyll-a concentrations in the Tualatin River at Elsner Road,

Oregon ..........................................................................................................................................63

39. Graph showing percentage of natural flow in the Tualatin River at Farmington,

Oregon, June–August 1991–2008.............................................................................................64

40. Graphs showing relative biovolume of select algal taxa showing potential

responses to natural flow and flow augmentation in the Tualatin River, Oregon,

2006–08 .........................................................................................................................................66

41. Graphs showing comparison of total algal biovolume in the Tualatin River

upstream and downstream from the Durham WWTF and Fanno Creek, Oregon,

2006–08 .........................................................................................................................................67

42. Graphs showing average percent relative biovolume of select taxa showing

potential positive responses and negative responses to percentage of

wastewater treatment facility (WWTF) effluent in the Tualatin River, Oregon,

2006–08 .........................................................................................................................................68

43. Graph showing pattern in chlorophyll-a and soluble reactive phosphorus

concentrations in the Tualatin River at Boones Ferry, Oregon, June–August 2004 ........70
viii

Tables

1. History of water-quality conditions and management actions in the Tualatin

River basin, Oregon ......................................................................................................................6

2. Sampling sites and data collection activities in the Tualatin River basin,

Oregon, 2006–08..........................................................................................................................13

3. Estimated average water velocities used to estimate travel times in four

reaches of the Tualatin River, Oregon, over a range of summer streamflows.................18

4. Quality of treated effluent from the Rock Creek and Durham wastewater

treatment facilities, Oregon, May–October 2006–08 ............................................................24

5. Taxa richness and abundance (biovolume) of major algal groups (divisions)

in the main-stem Tualatin River, Oregon, 2006–08.................................................................27

6. Dominant algal taxa in the Tualatin River, Oregon, 2006–08................................................30

7. Dominant zooplankton taxa in the Tualatin River, Oregon, 2006–08...................................33

8. Average phytoplankton percent biovolume for non-Wapato affected lower

river samples for May–July and August–September 2006–08............................................39

9. Summary statistics for environmental variables included in the multivariate

analysis of phytoplankton assemblages in the Tualatin River, Oregon .............................40

10. Spearman rank correlation matrix for environmental variables and select

groups of algae and zooplankton for the lower Tualatin River with

Wapato-affected samples removed........................................................................................41

11. Summary of BEST analyses listing the top environmental variables

explaining patterns in the phytoplankton species composition in the Tualatin

River, Oregon, 2006–08...............................................................................................................42

12. Average percent biovolumes of select algal taxa grouped according to

chlorophyll-a and seasonal growth phase in the Tualatin River, Oregon .........................43

13. Pairwise analysis of similarity (ANOSIM) for phytoplankton samples

collected each year by sequential sampling date, lower Tualatin River (river

mile 11.5–3.4), Oregon ................................................................................................................43

14. Low-dissolved oxygen events in the lower Tualatin River at the Oswego

Dam, Oregon, 2004–08, and potential contributing factors..................................................52

15. Algal bloom–crash sequences in the Tualatin River, Oregon, during 2003–08,

and possible contributing factors ............................................................................................55
ix

Conversion Factors, Datums, and Abbreviations and 

Acronyms

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.001562 square mile (mi2)

square mile (mi2) 2.59 square kilometer (km2)

acre-foot (acre-ft) 43,560 cubic feet (ft3)

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

mile per hour (mi/h) 1.609 kilometer per hour (km/h)

SI to Inch/Pound

Multiply By To obtain

Volume

cubic meter (m3) 35.3147 cubic foot (ft3)

liter (L) 0.2642 gallon (gal)

Mass

microgram (μg) 0.000001 gram (g)

gram (g) 0.035273 ounce, avoirdupois (oz)

kilogram (kg) 2.2046 pound (lb)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F=(1.8×°C)+32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: 

°C=(°F-32)/1.8.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Algal biovolumes for phytoplankton are given in cubic micrometers, or microns (μm), per 

milliliter of sample volume (μm3/mL). 

Algal cell densities for phytoplankton are given in cells per milliliter of sample (cells/mL), and 

zooplankton densities are given in number of organisms per cubic meter (#/m3).
x

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Abbreviations and Acronyms

Abbreviation or Acronym Definition

ANN Artificial Neural Network model

ANOSIM analysis of similarity

BEST Bio-Env Stepwise multivariate analysis

BOD biochemical oxygen demand 

Chl-a phytoplankton chlorophyll-a

CWS Clean Water Services

DO dissolved oxygen 

FNU formazin nephelometric unit

JWC Joint Water Commission 

NTU nephelometric turbidity unit

ODEQ Oregon Department of Environmental Quality

ORWSC USGS Oregon Water Science Center

PAR photosynthetically active radiation

RM river mile

SOD sediment oxygen demand 

SRP soluble reactive phosphorus

SIMPER similarity percentage analysis

TMDL Total Maximum Daily Load

TVID Tualatin Valley Irrigation District 

USGS U.S. Geological Survey

WWTF wastewater treatment facility 

99W Highway 99 west

Conversion Factors, Datums, and Abbreviations and 

Acronyms— Continued
Plankton Communities and Summertime Declines in 

Algal Abundance Associated with Low Dissolved 

Oxygen in the Tualatin River, Oregon

By Kurt D. Carpenter and Stewart A. Rounds

Executive Summary

Phytoplankton populations in the Tualatin River in 

northwestern Oregon are an important component of the 

dissolved oxygen (DO) budget of the river and are critical 

for maintaining DO levels in summer. During the low-flow 

summer period, sufficient nutrients and a long residence 

time typically combine with ample sunshine and warm water 

to fuel blooms of cryptophyte algae, diatoms, green and 

blue-green algae in the low-gradient, slow-moving reservoir 

reach of the lower river. Algae in the Tualatin River generally 

drift with the water rather than attach to the river bottom as 

a result of moderate water depths, slightly elevated turbidity 

caused by suspended colloidal material, and dominance of 

silty substrates. Growth of algae occurs as if on a “conveyor 

belt” of streamflow, a dynamic system that is continually 

refreshed with inflowing water. Transit through the system can 

take as long as 2 weeks during the summer low-flow period. 

Photosynthetic production of DO during algal blooms is 

important in offsetting oxygen consumption at the sedimentwater interface caused by the decomposition of organic 

matter from primarily terrestrial sources, and the absence of 

photosynthesis can lead to low DO concentrations that can 

harm aquatic life.

The periods with the lowest DO concentrations in 

recent years (since 2003) typically occur in August following 

a decline in algal abundance and activity, when DO 

concentrations often decrease to less than State standards 

for extended periods (nearly 80 days). Since 2003, algal 

populations have tended to be smaller and algal blooms 

have terminated earlier compared to conditions in the 1990s, 

leading to more frequent declines in DO to levels that do 

not meet State standards. This study was developed to 

document the current abundance and species composition 

of phytoplankton in the Tualatin River, identify the possible 

causes of the general decline in algae, and evaluate hypotheses 

to explain why algal blooms diminish in midsummer.

Plankton and water-quality sample data from 2006 to 

2008 were combined with parts of a larger discrete-sample 

and continuous water-quality monitoring dataset and 

examined to identify patterns in water-quality and algal 

conditions since 1991, with a particular emphasis on 2003–08. 

Longitudinal plankton surveys were conducted in 2006–08 

at six sites between river miles (RM) 24.5 and 3.4 at 2- to 

3-week intervals, or 5–6 per season, and in-situ bioassay 

experiments were conducted in 2008 to examine the potential 

effects of wastewater treatment facility (WWTF) effluent and 

phosphorus additions on phytoplankton biomass and algal 

photosynthesis. Phytoplankton and zooplankton community 

composition, streamflow, and water-quality data were analyzed 

using multivariate statistical techniques to gain insights into 

plankton dynamics to determine what factors might be most 

tied to the abundance and characteristics of the phytoplankton 

assemblages, and identify possible causes of their declines.

The connection between low-DO events and algal 

declines was clearly evident, as bloom crashes were nearly 

always followed by periods of low DO. Algal blooms 

occurred each year during 2006–08, producing maximum 

chlorophyll-a (Chl-a) values in June or July generally in the 

range of 50–80 micrograms per liter (μg/L). Bloom crashes 

and absence of sufficient algal photosynthesis in mid- to 

late-summer contributed to minimum DO concentrations that 

were less than the State standard of 6.5 milligrams per liter 

(mg/L) based on the 30-day mean daily concentration, for 

62–74 days each year. At times, the absolute minimum State 

standard (4 mg/L DO) also was not met. To learn more about 

why low-DO events occurred, specific algal declines during 

2003–08 were scrutinized to determine their likely causal 

factors. From this information, a series of hypotheses were 

formulated and evaluated in terms of their ability to explain 

recent declines in algal populations in the river in late summer. 

Meteorological, streamflow, turbidity, water temperature, 

and conductance conditions in the Tualatin River during the 

2006–08 summer seasons were not atypical. Natural flow 
2 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

comprised the majority (70–80 percent) of flow each year 

during spring, but then reduced to 38–40 percent during 

midsummer when WWTF effluent—which contributed 

as much as 36 percent—and flow augmentation releases 

comprised a greater fraction of the flow. Summer 2008 

was unusual, however, in the prolonged influence from the 

Wapato Lake agricultural area near Gaston in the upper part 

of the basin. The previous winter flooding and levee breach 

at Wapato Lake caused a much greater area of inundation. As 

a result, drainage from this area continued into July, much 

later than normal. A subsequent algal bloom in Wapato Lake 

then seeded the upper Tualatin River, and this drainage had 

a profound effect on the downstream plankton community. A 

large blue-green algae bloom developed—the largest in recent 

memory—prompting a public health advisory for recreational 

contact for about two weeks.

Algal growths and surface blooms are a common 

feature of the Tualatin River. Most of the dominant algae 

have growth forms and morphologies that are well suited 

for planktonic life, employing spines and gas vacuoles to 

resist settling, forming colonies, and producing mucilage (or 

toxins) to resist zooplankton grazing. In 2006–08, 143 algal 

taxa were identified in 117 main-stem samples; diatoms and 

green algae were more diverse than blue-green, golden, and 

cryptophyte algae, although these later groups sometimes 

dominated the overall volumetric abundance (biovolume). The 

most frequently occurring taxa, occurring in 97–99 percent 

of samples, were flagellated cryptophytes Cryptomonas 

erosa and Rhodomonas minuta. Other important algal taxa 

included centric diatoms Stephanodiscus, Cyclotella, and 

Melosira species and colonial green algae Scenedesmus

and Actinastrum. These taxa comprised the majority of 

the algal biovolume during much of the growing season. A 

general seasonal trend in the phytoplankton assemblages was 

observed, with dominance by filamentous centric diatoms 

Stephanodiscus and Melosira in spring and early summer, 

and flagellated cryptophytes and green algae, particularly 

Chlamydomonas sp., in late-summer; or, in 2008, dominance 

by blue-green algae Anabaena flos-aquae and Aphanizomenon 

flos-aquae during the Wapato Lake bloom event.

There were 99 zooplankton taxa identified from 

the Tualatin River in 2006–08, composed primarily 

of cladocerans, copepods, and rotifers. A seasonal 

increase in zooplankton abundance was observed in 

early summer just as or shortly after the phytoplankton 

population began to increase, with populations growing to 

15,000−120,000 organisms per cubic meter in the lower river. 

Zooplankton abundance showed a predictable and distinct 

longitudinal downstream increase, particularly downstream 

of Highway 99W (RM 11.6). Although grazing rates were 

not measured, the data suggest that, at times, zooplankton 

grazing may affect algal abundance and species composition 

in the Tualatin River, with diatoms becoming relatively 

less abundant and flagellated cryptophytes and green algae 

relatively more abundant during periods when zooplankton 

densities were highest.

Multivariate statistical analyses identified soluble reactive 

phosphorus (SRP), natural flow, flow augmentation, and 

WWTF effluent as important factors influencing Tualatin 

River phytoplankton populations, with zooplankton density 

(particularly rotifers and copepods), specific conductance, 

chloride, and water temperature also having an important 

influence. Although SRP was highly correlated with the 

plankton communities, that correlation was likely the result 

of high or low algal activity (uptake) as SRP concentrations 

were often reduced to low levels during blooms. While 

previous studies have already established that phosphorus, 

among other factors such as flow, places a theoretical cap 

on the size of the phytoplankton population in the river, 

sometimes algal declines occur when SRP concentrations are 

apparently sufficient. To identify alternative causal factors, 

additional analyses were performed without SRP to focus 

on other water-quality parameters, zooplankton density, and 

flow factors. Considering data for all 3 years and including 

just those samples from the lower Tualatin River not affected 

by the 2008 Wapato Lake drainage event, three factors 

(percentage of reservoir flow augmentation, total natural 

flow, and rotifer density) best explained variations in the 

phytoplankton assemblages.

Analyses focusing on the possible causes of algal 

declines included the above multivariate analyses, scrutiny 

of 10 specific instances of declines in algal populations 

during 2003–08 including several bloom–crash sequences, 

and analyses of historic routine watershed monitoring data 

from Clean Water Services. Six factors were hypothesized to 

be important in causing bloom crashes or impeding blooms 

from rebounding in August: (1) light limitation from cloudy 

weather, (2) a reduction in the plankton inocula or “seed” 

entering the lower river from upstream sources, (3) increased 

summer streamflows, (4) changes in the dominant sources 

of flow as the percentage of flow augmentation and WWTF 

discharges have increased, (5) zooplankton grazing, 

and (6) low concentrations of bioavailable phosphorus 

(<0.015 milligram per liter). All of these hypotheses are 

supported in some fashion by the available data and statistical 

analyses. Zooplankton grazing, short-term declines in 

photosynthesis from cloudy weather, total flow as it affects 

residence time, and the dominant source of flow are primary 

factors responsible for the low-DO events caused by declines 

in algae in the lower Tualatin River during late summer. 

Cloudy weather and increased turbidity are known to 

inhibit algal growth in the Tualatin River, and slight increases 

in turbidity in recent years may be a problem. Upstream 

sources of algae are critical in determining the characteristics 
Introduction 3

and size of downstream populations, as illustrated by the 

Wapato Lake bloom in 2008, but more data are needed from 

upstream to fully define the importance of this connection. 

The sources of flow, through their differential contribution 

of plankton inocula (quality and amount), were, at times, 

important factors affecting phytoplankton populations. While 

SRP concentrations were often most highly correlated with 

phytoplankton species community, the bioavailability of 

phosphorus is still somewhat unknown and there are several 

sources to consider. Preliminary bioassay tests suggested 

that while treated wastewater effluent may stimulate algae 

at 30 percent concentrations, negative effects (or decreased 

stimulation) on Chl-a and DO production may occur at 

concentrations of 50 percent. Targeted data collection and 

future research will be needed to further understand the 

importance of these factors on Tualatin River phytoplankton.

While the data and analysis completed for this report 

provide insights into future research and monitoring that 

would be useful to continue, additional monitoring of turbidity, 

Chl-a, and plankton abundance and species composition in 

the upper part of the basin would enhance our understanding 

of plankton dynamics and factors affecting phytoplankton 

abundance in the lower river. Assessment of the key upstream 

sources of algal inocula via surveys of the major flow sources 

as well as tributaries and wetlands would provide useful 

information for the management of river water quality. Other 

studies that could prove useful for developing management 

strategies include targeted experiments to evaluate the 

bioavailability of phosphorus from a variety of sources. New 

research on phytoplankton–zooplankton interactions, and 

studies of planktivorous fish, might also provide insight about 

food web dynamics and potential “top‐down” effects of fish 

predation on the plankton communities. In addition, further 

development of neural-network or other water-quality models 

would help to evaluate management strategies and provide 

forecasts of water-quality conditions. Finally, periodic future 

reassessments of the available data with the multivariate 

statistical tools used in this study would be helpful to assess 

whether and how plankton communities are changing, and to 

continue to shed light on the importance of factors shaping 

the plankton. Although certain types and sizes of algal blooms 

are undesirable, minimum phytoplankton populations are 

an important part of aquatic food webs and are needed to 

maintain healthy levels of DO in the river. By understanding 

the sources, characteristics, causal factors, and responses of 

the plankton communities, management strategies can be 

developed to improve DO conditions in the lower Tualatin 

River during the important summer low-flow period.

Introduction

The Tualatin River drains a 712-square-mile (mi2) basin 

west of Portland in northwestern Oregon (fig. 1). Although the 

river descends from the Coast Range as a mountainous stream, 

it cuts and meanders as a relatively slow moving river across a 

fertile, fine-sediment filled valley for most of its 80-mile (mi) 

length. The Tualatin River has a history of algal blooms and 

associated water-quality problems dating back to the 1960s 

(table 1). At that time, blooms flourished in the sewage- and 

nutrient-enriched, warm, and slow-moving waters during 

summer. Algal blooms were still an aesthetic nuisance decades 

later, causing high pH and low concentrations of dissolved 

oxygen (DO) following bloom crashes, sometimes to levels 

that violated State of Oregon water-quality standards (Oregon 

Department of Environmental Quality, 2001). These problems 

are most severe in the pooled “reservoir reach” located from 

river mile (RM) 33.3 to RM 3.4, the end of which is marked 

by a low-head weir, the Oswego Dam (fig. 2). The average 

width in this reach increases to about 150 feet (ft) and the river 

becomes lake-like during the summer low-flow period, taking 

as long as 14 days for water to transit this slow-moving pool 

(Rounds and others, 1999).

The dominant types of algae in the Tualatin River during 

summer, based on a limited number of past studies, included 

planktonic diatoms (Stephanodiscus, Cyclotella, Aulacoseira, 

Melosira, and others), Chlamydomonas and other green 

algae, and occasional blooms of potentially toxic blue-green 

algae, including Anabaena and Microcystis (Carter and 

others, 1976; Rinella and others, 1981; Oregon Department of 

Environmental Quality and Unified Sewerage Agency, 1982; 

Doyle and Caldwell, 1996).

In 1972, the Oregon Department of Environmental 

Quality (ODEQ) listed the Tualatin River as “water-quality 

limited” because of impacts to aesthetics caused by algal 

blooms. In 1988, regulatory pollution limits, or Total 

Maximum Daily Loads (TMDLs), were established for 

ammonia and total phosphorus that were fueling excessive 

algal growth and causing problems associated with low DO 

and high pH levels (Oregon Department of Environmental 

Quality, 2001). Although nutrient controls in the early 1990s 

reduced the severity of algal blooms, phytoplankton Chl-a

concentrations continued to exceed ODEQ’s target upper limit 

of 15 μg/L to prevent nuisance algal conditions.
4 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig01

123°30 123°15' 123° 122°45' 122°30'

45°45'

45°37'30"

45°30'

45°22'30"

0 10

10

20 MILES

0 20 KILOMETERS

EXPLANATION

Base modified from U.S. Geological Survey and Metro digital data sets (1:24,000)

Projection: Oregon Lambert, North American Datum of 1983

OREGON

Study

Area

Lake

Oswego

Tualatin

Sherwood

Portland

Tigard

Beaverton

Hillsboro

Cornelius

Forest

Grove

Cherry

Grove

ROCK

CREEK

DURHAM

WILLAMETTE

TUALATIN

RIVER

RIVER

RIVER

COLUMBIA

Hagg

Lake

Wapato

Lake

Jackson

Bottom

Wetland

Barney

Reservoir

217

26

5

5

5

205

205

84

WASHINGTON

COLUMBIA

TILLAMOOK

YAMHILL

CLACKAMAS

MULTNOMAH

East

Fork

Dairy

Gales

Creek

Scoggins

Creek

McKay

Creek

Rock

Creek

TUALATIN MOUNTAINS

CHEHALEM MOUNTAINS

COAST RANGE

Fanno

Creek

Designated urban growth boundary (2009)

Basin boundary

Wastewater treatment facility

Plankton sampling site

Plankton sampling and continuous water-quality monitoring site

Streamflow gage

River mile

Trask River

West

Linn

RM

60 RM

70

RM

RM 30

RM

40

RM

50

RM

10

20

RM

0

RM

30

Figure 1. Tualatin River basin, Oregon, showing the location of selected sampling sites, streamflow gages, and 

continuous water-quality monitors.
Introduction 5

tac12-0780_fig02

0 5

5

10 MILES

0 10 KILOMETERS

EXPLANATION

Sampling site and map number 1

1 Tualatin River at Weiss Bridge

2 Tualatin River at West Linn

3 Tualatin River at Oswego Dam

4 Tualatin River downstream of Stafford Road

5 Tualatin River at Boones Ferry

6 Tualatin River downstream of Fanno Creek and Durham WWTF

7 Tualatin River downstream of Cook Park

8 Tualatin River at Jurgens Park

9 Tualatin River at Highway 99W Bridge

10 Tualatin River at Elsner (Roy Rogers Rd)

11 Tualatin River at River Mile 24.5

12 Tualatin River at Rood Bridge

13 Tualatin River at Highway 219

14 Tualatin River at Spring Hill Pump Plant

15 Wapato Creek at Gaston Road

Base modified from U.S. Geological Survey and Metro digital data sets (1:24,000)

Projection: Oregon Lambert, North American Datum of 1983

Designated urban growth boundary (2010)

Wastewater treatment facility (WWTF)

River mile

123°05' 123° 122°55' 122°50' 122°45' 122°40'

45°30'

45°25'

45°20'

12

15

13

14

11

10

9

8

7

6

5

2

4

3

1

ROCK

CREEK

WWTF

Lake

Oswego

West

Linn

Durham

Tualatin

Portland

King

City

Tigard

Beaverton

Farmington

Hillsboro

Forest Cornelius

Grove

Dilley

WILLAMETTE

RIVER

TUALATIN RIVER

TUALATIN

RIVER

Rock

Beaverton

Creek

Creek

Creek

Fanno

Gales

Creek

Summer Creek

217

26

5

5

5

205

DURHAM

WWTF

Wapato

Lake

26

Scogn. C.

TUAL.

Oswego

Canal

OREGON

Study

Area

RM 60

RM 30

RM 20

RM 10

RM 40

RM

50

RM 0

RM 10

Christensen Creek

Burris Creek

Creek

McFee

Baker Creek

Butternut Creek

Gordon 

 Creek

Figure 2. Tualatin River basin, Oregon, showing the location of water-quality and plankton sampling sites.
6 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Table 1. History of water-quality conditions and management actions in the Tualatin River basin, Oregon.

[References: Carter and others (1976); Oregon Department of Environmental Quality and Unified Sewerage Agency (1982). Abbreviations: ODEQ, 

Oregon Department of Environmental Quality; TMDL, Total Maximum Daily Load; USA, Unified Sewerage Agency; USGS, U.S. Geological Survey; BOD, 

biochemical oxygen demand; WWTF, wastewater treatment facility; RM, river mile; mL, milliliter]

Time period Description

1960s Raw waste in river; river often blanketed with floating algae during summer.

1966 Melosira dominant with toxic blue-green algae (Microcystis) during low flow.

1969 Countywide ban on new construction in urban areas until new plan developed for wastewater treatment.

1970 Unified Sewerage Agency formed to consolidate and manage wastewater treatment plants. Eldon Mills Dam (Barney 

Reservoir) completed on Middle Fork of North Fork Trask River with releases to Tualatin River for drinking water.

1972 Clean Water Act enacted. ODEQ lists Tualatin River as water-quality limited [303(d) listed] for aesthetics and swimming.

1972–73 Nutrient enriched, but not organically polluted; 51 genera of pollution-tolerant algae.

1975 Scoggins Dam completed; summertime releases from Henry Hagg Lake begin. Improvements in temperature, conductance, 

and BOD, but not in other parameters (Oregon Department of Environmental Quality and Unified Sewerage Agency, 1982).

1976 Durham Advanced Wastewater Treatment Facility comes on line replacing 14 small treatment plants. Decreased 

carbonaceous biochemical oxygen demand, total suspended sediment, total phosphorus, Wetter and cooler year; nutrient 

enriched, but not organically polluted; 31 genera of organic pollution-tolerant algae (Stephanodiscus/Melosira dominated).

1978 Rock Creek Advanced Wastewater Treatment Facility comes on line replacing six small treatment plants. Decreased 

carbonaceous biochemical oxygen demand, total suspended sediment, total phosphorus.

1977–79 Blue-green algae dominant, Aphanizomenon flos-aquae was major species present.

1970–80 Overall, Tualatin River showed improvements in water quality despite a 40 percent increase in population.

1980 Severe Aphanizomenon bloom.

1984 Tualatin River listed as water-quality limited [303(d) list] for low dissolved oxygen and nuisance algal blooms.

1986 Tualatin River listed as water-quality limited [303(d) list] for low dissolved oxygen and nuisance algal blooms. Northwest 

Environmental Defense Center filed suit against the U.S. Environmental Protection Agency to require that TMDLs be 

established for the Tualatin River.

1987 Unified Sewerage Agency (now Clean Water Services) started actively managing their share of releases from Henry Hagg 

Lake.

1988 ODEQ established TMDLs for ammonia-nitrogen and total phosphorus for the Tualatin River and its largest tributaries.

1990 Clean Water Services becomes the stormwater management utility for the urbanized portion of Washington County; USGS 

and Clean Water Services begin long-term collaboration on monitoring and research.

1991–93 USGS conducts surveys of phytoplankton, zooplankton, and water quality.

1994 All major upgrades completed at Rock Creek and Durham WWTFs to remove ammonia and phosphorus (some completed 

earlier); largest pH problems solved.

1998 First releases of flow augmentation water from newly expanded Barney Reservoir (expanded from 4,000 to 20,000 acre‐feet). 

Tualatin River listed for temperature and bacteria; tributaries for same plus dissolved oxygen.

2001 ODEQ issues a revised Tualatin Basin TMDL, adding bacteria and temperature. Dissolved oxygen TMDL issued for 

tributaries. Anabaena flos-aquae and A. planktonica detected in Hagg Lake in June and July, peaking in late August 

(348 colonies per mL).

2003 In July, a loss of algal growth resulted in lower than normal dissolved oxygen levels when conditions for growing algae 

appeared favorable. First year Clean Water Services released water from Hagg Lake specifically for temperature trading.

2005 Aphanizomenon and Anabaena present during July–September.

2006 Diatoms and cryptomonads dominate. Low abundance of Anabaena in June near Scholls, RM 24.5.

2007 Bloom of the diatom Stephanodiscus binderanus and Cryptomonas erosa in the lower river in July.

2008 Oregon Department of Human Services issues a human health advisory for a bloom of Anabaena flos-aquae associated with 

the draining of the Wapato Lake agricultural area in July.
Introduction 7

Before 1975, the Tualatin River basin had no water 

storage reservoirs, and the mean flow for July and August 

during 1929–74 averaged just 45 cubic feet per second 

(ft3/s) at the West Linn stream gage (RM 1.8; figs. 1 and 2) 

during the dry summer period. This provided phytoplankton 

a considerable amount of time (possibly several weeks) to 

develop in the lower river. Completion of Henry Hagg Lake 

on Scoggins Creek in 1975 substantially increased flows in 

the Tualatin River, resulting in a July–August mean of about 

200 ft3/s for 1975–2010 (fig. 3). 

In 1987, Clean Water Services (CWS) began actively 

managing releases from Hagg Lake (12,618 acre-feet (acre-ft)) 

to improve water-quality conditions during the critical summer 

low-flow periods. In 1998, releases from an expansion of 

Barney Reservoir (fig. 1), located in the adjacent Trask River 

watershed west of the Tualatin River basin, started contributing 

flow to the Tualatin River for water-quality augmentation and 

as a source of municipal drinking water. Barney Reservoir 

releases to the Tualatin River currently range from 0 to 

40 ft3/s during summer depending on demand and the need for 

flow augmentation.

Although flow augmentation has been an effective 

management tool for diluting some sources of nutrients, 

reducing water temperatures, and lessening the severity of 

algal blooms (CH2M Hill, 1992; Unified Sewerage Agency, 

1992), this practice may hamper healthy phytoplankton 

development. The higher flows decrease the time available for 

phytoplankton to develop and have the potential to abruptly 

purge the system, effectively “flushing out” the algae (Rounds 

and others, 1999). Further, the large reductions in discharges 

of ammonia and phosphorus from the two largest wastewater 

treatment facilities (WWTFs) in the early 1990s greatly 

reduced the occurrence and severity of algal blooms in the 

river. Ironically, although these improvements have reduced 

the size of algal populations and associated problems with 

high pH, the same factors can have deleterious implications 

for DO concentrations in the river as figure 4 illustrates. 

DO concentrations closely track phytoplankton Chl-a such 

that when algal populations decline, the DO can decrease to 

undesirable levels, sometimes for extended periods of time. 

The correlation between Chl-a and DO (fig. 5) also confirms 

the importance of photosynthesis in maintaining minimum 

DO levels in the river, substantiating previous findings that 

some moderate level of algal photosynthesis is beneficial and 

necessary for the river to overcome the steady loss of DO from 

sediment oxygen demand (SOD) created as aerobic bacteria 

decompose organic-rich bottom sediments (Rounds and Doyle, 

1997; Rounds and others, 1999).

Simultaneously balancing the desire to prevent high 

phytoplankton biomass and harmful algal blooms with the 

need to maintain minimum DO concentrations in the river, 

however, is challenging. Given the importance of algal 

photosynthesis in maintaining a healthy DO level for the river, 

it is critical to understand the factors that control the size 

and, possibly, the composition of phytoplankton populations, 

especially in the lower river. That information will enable the 

designated management agencies to better understand how to 

balance these competing goals most effectively and plan for 

water-quality improvements.

Figure 3. Average flow in the Tualatin River at West Linn (river mile [RM] 1.8), 

Oregon, July and August 1929–2010.

0 

50 

100 

150 

200 

250 

300 

350 

400 

Year

Streamflow, in cubic feet per second

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 

USGS station 14207500

Start of Hagg Lake water releases 

for flow augmentation
8 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig04

0 

2 

4 

6 

8 

10 

12 

14 

0 

20 

40 

60 

80 

100 

0 

2 

4 

6 

8 

10 

12 

14 

0 

20 

40 

60 

80 

100 

0 

2 

4 

6 

8 

10 

12 

14 

0 

20 

40 

60 

80 

100 

0 

2 

4 

6 

8 

10 

12 

14 

0 

20 

40 

60 

80 

100 

0 

2 

4 

6 

8 

10 

12 

14 

0 

20 

40 

60 

80 

100 

0 

2 

4 

6 

8 

10 

12 

14 

0 

20 

40 

60 

80 

100 

0 

2 

4 

6 

8 

10 

12 

14 

0 

20 

40 

60 

80 

100 

0 

2 

4 

6 

8 

10 

12 

14 

0 

20 

40 

60 

80 

100 2001

2002

2003

2004

2005

2006

2007

2008

Chlorophyll-a

15-micrograms-per-liter chlorophyll-a action level

Minimum daily dissolved oxygen 

6.5-milligrams-per-liter dissolved oxygen standard

EXPLANATION

Chlorophyll-a, in micrograms per liter Chlorophyll-a, in micrograms per liter 

Minimum daily dissolved oxygen concentration, in milligrams per literMinimum daily dissolved oxygen concentration, in milligrams per liter

July August July August

Month Month

Figure 4. Time series of phytoplankton chlorophyll-a and dissolved oxygen concentrations in the lower Tualatin River, Oregon, 

July and August 2001–08. For reference, the 6.5 milligrams per liter dissolved oxygen standard is shown, but the standard is 

calculated as the 30-day mean concentration, not the daily minimum values as shown. Chlorophyll-a for Stafford Road, RM 5.5, 

provided by Clean Water Services. Dissolved oxygen data from U.S. Geological Survey continuous monitor at Oswego Dam, 

RM 3.4.
Introduction 9

0 

20 

40 

60 

80 

100 

120 

140 

4 6 8 10 12 14 16 

ChlorophyllDissolved oxygen concentration, in milligrams per liter

a concentration, in micrograms per liter

R2 = 0.55

Figure 5. Chlorophyll-a and dissolved oxygen concentrations in the 

Tualatin River at Stafford Road (river mile 5.5), Oregon, July and August, 

1991–2009. R2

, correlation coefficient. Data from Clean Water Services 

ambient monitoring program.

Patterns in Longitudinal and Seasonal 

Phytoplankton Abundance

Although phytoplankton populations in the Tualatin River exhibit some 

general longitudinal and seasonal patterns, much year-to-year variation 

occurs in the timing and size of blooms owing to streamflow and other 

factors that affect growing conditions. In the 1990s, when sampling for 

algae included sites in both the upper and lower river, Chl-a concentrations 

typically increased from an average of about 1 μg/L at Cherry Grove at RM 

68, upstream of Scoggins Creek where Hagg Lake water enters the Tualatin 

River, to 3.5–4.5 μg/L in the reach between Golf Course Road and Rood 

Bridge, RMs 51.4 to 38.4 (fig. 6). At Farmington (RM 33.3), average Chl-a

concentrations decreased to about 2.5 μg/L 

during summer because of dilution by 

discharges from the Rock Creek WWTF, 

which enters the river at RM 38.1 (fig. 2). 

The Chl-a concentrations increased in the 

reach between Farmington and Scholls, 

RMs 33.3 to 26.9, and continued to 

increase downstream to Elsner Bridge, 

RM 16.2, where the largest biomass 

values often occur. Downstream of Elsner 

and beyond, algal biomass is sometimes 

higher, but highly variable. At times, Chla concentrations increase steadily to the 

Oswego Dam at RM 3.4 (see photograph 

1 [plankton tow samples] p. 10), while 

at other times, biomass levels decline, 

sometimes abruptly, at one or more of 

these sites during summer.

The average phytoplankton Chl-a

concentrations in the Tualatin River for 

the June to August peak algae growing 

seasons, 1991–2009, are shown in 

figure 7. This plot excludes samples from 

the atypical 2001 and 2008 years, when 

severe drought (2001) and an unusual 

algal bloom resulting from Wapato Lake 

discharges (2008; see section, “Wapato 

Lake Algal Bloom”) occurred. A seasonal 

increase in phytoplankton biomass occurs 

in spring; from April to May, Chl-a levels 

increased to over 10 μg/L at Elsner, 

RM 16.2 (fig. 7). Previous studies of 

Tualatin River phytoplankton in 1991–93 

(Rounds and others, 1999) found the 

highest growth rates to occur in June or 

early July, lower rates in mid-summer, 

and slightly higher rates in late August 

or September, based on carbon uptake 

experiments. While growth rates show 

this trend, algal Chl-a typically peaks in 

July, when streamflows are lower, days 

are long, light is abundant, and water 

temperatures are warm. Chl-a levels 

then decline in August and September 

in response to shorter days, lower light 

availability, higher flows, or other factors.

Lower Chl-a concentrations in 

July and August are clearly evident in 

data from 2002 and later, denoted with 

dashed lines in figure 7. This decline in 

algal abundance is one of the reasons for 

conducting this study and is discussed in 

more detail later in the report.

0 

10 

20 

30 

40 

50 

60 

Chlorophyll60 50 40 30 20 10 0 a concentration, in micrograms per liter

River mile

Figure 6. Longitudinal pattern in average chlorophyll-a concentrations 

in the Tualatin River, Oregon, May–October 1991–2009. Average 

chlorophyll-a concentration ±1 standard deviation for May–October. Data 

from the atypical 2001 (drought) and 2008 (Wapato event) years were not 

included. Data from Clean Water Services.
10 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

1 

10 

100 

80 70 60 50 40 30 20 10 0 

April 1991-2000

April 2001-2009

May 1991-2000

May 2002-2009

June 1991-2000 

June 2002-2009

July 1991-2000

July 2002-2009

August 1991-2000

August 2002-2009 

River mile

Average chlorophyll-a concentration, in micrograms per liter

EXPLANATION Elsner, river mile 16.2

Figure 7. Changes in the average monthly phytoplankton chlorophyll-a in the Tualatin River, Oregon, highlighting the 

1991–2000 and 2002–2009 periods. Note y-axis log scale. Data from the atypical 2001 (drought) and 2008 (Wapato event) 

years were not included. Data provided by Clean Water Services.

Photograph 1: Plankton tow sample vials showing longitudinal changes in plankton 

abundance and composition. Photograph by Kurt Carpenter, June 6, 2006.

38.4 24.5 10.8 9.5 9.2 5.4 3.4

Main-stem river mile

Primary Factors Controlling 

Phytoplankton Growth

Previous USGS studies determined the 

primary factors controlling phytoplankton 

populations and water-quality conditions in the 

Tualatin River. Water-quality models calibrated 

for 1991–93 (Rounds and others, 1999) and 

1991–97 (Rounds and Wood, 2001) indicated that 

streamflow, light conditions, water temperature, 

and phosphorus concentrations were most 

influential in determining the onset, duration, 

and magnitude of phytoplankton blooms in the 

Tualatin River. With these four variables, the 
Introduction 11

models can simulate, with acceptable accuracy, the onset, 

duration, and general magnitude of phytoplankton blooms. 

While zooplankton grazing of algal cells occasionally was 

found to be an important loss mechanism for phytoplankton 

populations, it was limited to the lowermost reaches of the 

Tualatin River and only occurred during certain periods 

(Rounds and others, 1999).

Although the USGS Tualatin River water-quality 

models can predict many important constituents much of 

the time, errors in the simulated DO and SRP concentrations 

are largely due to inaccuracies in the size of the modeled 

phytoplankton population, at times accounting for 50 percent 

of the errors (Rounds and Wood, 2001). This uncertainty in 

the modeled phytoplankton populations arises, in part, from 

the models’ oversimplification of the population to just one 

algal type. In addition, the large degree of annual variability 

in algal populations (figs. 4 and 6) and lack of a consistent 

successional pattern in the algal species composition (Doyle 

and Caldwell, 1996) has made it difficult to incorporate 

interspecies differences in buoyancy, nutrient requirements, 

growth rates, or susceptibility to grazing by zooplankton, for 

example, into the models. 

Study Background, Objectives, Scope, 

and Approach

During the 1990s, phytoplankton populations in the 

Tualatin River would typically grow throughout the summer 

and decline when flows increased or light decreased such 

that algal growth was no longer favorable. These conditions 

occurred most often during the later part of the growing 

season (late August or early September). Algal populations in 

the lower river were mostly sustained above 25 μg Chl-a/L 

in July and August and DO concentrations generally stayed 

above the State DO standard for the river—a three-level 

tiered standard—based on (1) the 30-day mean concentration 

of 6.5 mg/L, (2) the 7-day average of the daily minimum 

concentration of 5 mg/L, and (3) the absolute minimum 

concentration of 4 mg/L. While episodes of lower DO would 

occasionally occur when blooms crashed or algal populations 

subsided, this typically occurred at the end of the growing 

season, in late summer or early fall (Jan Miller, retired Clean 

Water Services Water Resources Program Manager, oral 

commun., 2010). Beginning in 2002 or 2003, however, Chl-a

levels have been notably lower, particularly in August, and the 

frequency of periods when the DO has not met one or more of 

the State standard has increased (fig. 8).

0

10

20

30

40

50

60

70

80

90

100

0 110

10

20

30

40

50

60

70

80

90

100

110

1991199219931994

19951996

199719981999

2000

2001

20022003200420052006200720082009

Chlorophyll-a concentration, in micrograms per liter

Year

Percentage of time dissolved oxygen standard violated

EXPLANATION

Chlorophyll-a

Any dissolved oxygen criteria

30-day dissolved oxygen criteria 

Figure 8. Trends in the average August chlorophyll-a concentration and the percentage of time in August that the 

minimum dissolved oxygen (DO) standard was violated in the lower Tualatin River, Oregon, 1991–2009. Chlorophyll 

data are from the Stafford Road site (river mile 5.5); DO data are from the Oswego Dam site (RM 3.4). The dark blue 

bars represent days when any of the three DO criteria were violated (instantaneous, 7-day minimum mean, 30-day 

mean); the light blue bars are for days when only the 30-day mean DO criteria was violated. The 6.5-milligram per 

liter State standard is based on the 30-day mean concentration.
12 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Since about 2003, the simple model of residence time, 

light, temperature, and phosphorus has been insufficient to 

explain the absence of even moderate algal populations in 

the river for some periods in July and August. Phytoplankton 

populations in recent years often are smaller or terminate 

earlier than in years past, despite apparently favorable flow, 

light, and nutrient conditions. To exacerbate matters, the large 

decline in phytoplankton abundance now often coincides with 

the seasonal peak in water temperature in late July or early 

August, which because of the lower DO solubility at higher 

temperatures, can lower DO concentrations even further.

In response to these declines in phytoplankton abundance 

and episodes of low DO, the USGS partnered with CWS to 

investigate the relative importance of the factors influencing 

phytoplankton and formulate and evaluate hypotheses to 

explain the causes of such declines. Because DO is such 

an important indicator of river health, and because DO is 

needed to offset oxygen demands in the river, it is critical that 

the causes of such reductions in phytoplankton abundance 

be identified.

The study objectives were to characterize the 

phytoplankton populations during the 2006−08 growing 

seasons; examine how phytoplankton respond to factors 

such as streamflow, sources of flow, weather, zooplankton, 

and selected water-quality parameters; and identify those 

factors contributing to the recent declines in phytoplankton 

populations. Some of the questions that this study was 

designed to answer included:

1. What is the current composition of the 

phytoplankton community in the Tualatin River 

during summer?

2. What are the most important environmental variables 

influencing phytoplankton species composition?

3. Why does phytoplankton abundance decline abruptly 

(typically in July or early August) and not rebound 

when streamflow, sunlight, and other conditions 

otherwise appear to be favorable?

To address these questions, both new and existing data 

on plankton and water quality were analyzed. New plankton 

and water-quality data were collected in the Tualatin River at 

2- to 3-week intervals during the growing season, typically 

May−September for 3 years. Longitudinal sampling of the 

river extended from either the Rood Bridge site (RM 38.4) or 

the sampling station near Scholls (RM 24.5) to the Oswego 

Dam (RM 3.4) at a half-dozen sites per trip on 5−6 occasions 

each year (table 2, fig. 2). Other data used in the analyses 

included streamflow and water-quality data from continuous 

monitors, and results from discrete water-quality samples from 

the CWS routine watershed monitoring program. In certain 

instances, the scope was expanded beyond the intensive 

2006–08 time period to include more data from 1991 to 2009 

to frame the larger issues. In particular, ten bloom/crash cycles 

during 2003–08 were examined in detail to help identify 

specific causal factors. The resulting dataset was large and 

complex, and multivariate and other statistical techniques were 

required to weigh the importance of the various environmental 

factors influencing phytoplankton assemblages. In 2008, 

in-stream bioassay experiments were conducted to test the 

effect of WWTF effluent on phytoplankton biomass (Chl-a) 

and DO production, and phosphorus additions were used to 

examine the potential for phosphorus to limit algal growth.

Hydrology and Study Area Description

The hydrology of the Tualatin River and surrounding 

landscapes have been modified through water withdrawals for 

municipal, agricultural, and other uses, as well as discharges 

from tile drains, reservoir releases, treated wastewater, and 

stormwater runoff. Each modification has had an effect on 

water quality. This section provides a brief overview of the 

basin and the important hydrologic conditions that affect 

phytoplankton populations; more information is available 

from Rounds and others (1999) and other reports listed in the 

section, “References Cited.”

The Tualatin River can be divided into four reaches 

based on stream gradient and geomorphology (Rounds and 

others, 1999; Bonn, 2008), and the different characteristics 

of these reaches are critical in influencing the abundance of 

phytoplankton. In the headwater reach upstream of RM 55, the 

river is narrow and relatively steep, particularly in the Coast 

Range. The Spring Hill Pump Plant withdraws water in this 

reach to supply the Tualatin Valley Irrigation District (TVID) 

as well as the Joint Water Commission (JWC), which provides 

drinking water to many cities in the western part of the basin.

Downstream from RM 55, the Tualatin River meanders 

through a 25-mi section where the average gradient decreases 

to just 1.3 ft/mi., then beginning around RM 30, the river takes 

on a reservoir-like character that extends to the Oswego Dam 

at RM 3.4 (see photographs 2–5, p. 14). This “reservoir reach” 

has an average gradient of 0.1 ft/mi, is wide enough (>150 

ft) to minimize shading from streamside vegetation, averages 

about 10–15 ft of water depth, and includes a few atypical 

pools that reach maximum depths of 25–30 ft (Rounds, 

2002). This reach is deep enough and has a high enough 

concentration of colloidal and suspended particles, that light 

does not reach the river bottom except at the margins (Bonn 

and Rounds, 2010). As a result, algal production is dominated 

by phytoplankton that float with the water and grow mainly in 

the top 5–10 ft of the water column; the river can be thought 

of as a conveyor belt that provides a certain amount of time 

for algae to grow before transporting them downstream, to be 

replaced by new populations from upstream—provided there 

is ample inocula.
Introduction 13

Table 2. Sampling sites and data collection activities in the Tualatin River basin, Oregon, 2006–08.

[Bolded sites were sampled for the majority of the longitudinal plankton surveys. For a more complete listing of river mile (RM) indexes, refer to Bonn (2008). 

Abbreviations: USGS, U.S. Geological Survey; WWTF, wastewater treatment facility]

Location Main-stem 

river mile

Plankton 

sampling

Waterquality 

sampling

Continuous 

waterquality 

monitor

Streamflow 

gage

Inflow Outflow

Tualatin River at Weiss Bridge 0.2 X

Tualatin River at West Linn 1.8 X

Tualatin River at the Oswego Dam 3.4 X X X

Tualatin River downstream of Stafford Road Bridge 15.4/5.5 X X

Tualatin River at the Oswego Canal headgates 6.7 X

Tualatin River at Boones Ferry 8.7 X

Tualatin River downstream of Fanno Creek and 

Durham WWTF

9.2 X X

Tualatin River at Durham WWTF Release / Fanno Creek 9.3 X

Tualatin River downstream of Cook Park 9.7 X X

Tualatin River at RM 9.9 near Tualatin 9.9 X

Tualatin River at RM 10.2 10.2 X

Tualatin River at RM 10.6 10.6 X

Tualatin River at Jurgens Park 10.8 X X

Tualatin River downstream from the Highway 99W Bridge 11.1 X

Tualatin River at Highway 99W Bridge 11.6 X X

Tualatin River at RM 13.5 13.5 X

Tualatin River at Elsner (Roy Rogers Rd) 16.2 X

Tualatin River at RM 17.3 17.3 X

Tualatin River at RM 17.9 17.9 X

Tualatin River at RM 20.4 20.4 X

Tualatin River at RM 21.1 21.1 X

Tualatin River at RM 23.2 23.2 X

Tualatin River at River Mile 24.5 24.5 X X X

Tualatin River at Scholls Bridge 26.9 X

Tualatin River at Farmington 33.3 X X

Tualatin River at Rock Creek WWTF Release 38.1 X

Tualatin River at Rood Bridge 38.4 X X X

Tualatin River at Golf Course Road 51.5 X

Tualatin River at JWC/TVID Spring Hill Pump Plant Intake 56.1 X

Tualatin River at Dilley 58.8 X

Scoggins Creek Below Henry Hagg Lake – 2 X X

Scoggins Reservoir (Hagg Lake) Release – 3 X

Tualatin River at Wapato Improvement District Return Flow 60.1 X X

Tualatin River at Wapato Improvement District Headgate 61.9 X

Tualatin River at Cherry Grove 67.8 X

Tualatin River at City of Hillsboro Haine’s Falls Intake 73.3 X

Barney Reservoir Aqueduct Outfall 78 X

1

 USGS plankton site is located 0.1 mile downstream from the bridge where Clean Water Services samples.

2

 Scoggins Creek site is located 0.1 mile downstream from Hagg Lake.

3

 Hagg Lake outfall is located at RM 5.1 on Scoggins Creek.
14 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Photograph 2: Tualatin River at the water-quality monitoring station 

near Scholls (RM 24.5). Photograph by Kurt Carpenter, July 19, 2007.

Photograph 3: Tualatin River at the Highway 99W Bridge put-in 

(RM 11.6). Photograph by Kurt Carpenter, July 19, 2007.

Photograph 4: Tualatin River downstream of Fanno Creek (RM 9.2). 

Photograph by Kurt Carpenter, August 9, 2005.

Photograph 5: Tualatin River at the Oswego Dam (RM 3.4).

Photograph by Kurt Carpenter, August 9, 2005.

A growth curve of increasing phytoplankton populations 

with downstream distance in the reservoir reach (as in fig. 6) may 

be shifted downstream if flows increase, and variations in algal 

biomass and DO production occur in response to the growing 

conditions, primarily driven in summer by the amount of sunshine 

and magnitude of flow. Because of these spatial and temporal 

effects, algal populations at any given location are governed by 

the particular suite of upstream conditions integrated over several 

days to a couple of weeks or longer. Events that increase flow are 

especially influential as they shorten the time available for algae to 

grow before they are transported out of the system downstream. 

Residence times in the reservoir reach during summer have 

historically been long enough, as much as 10–14 days or longer, 

for phytoplankton populations to develop into substantial blooms. 

Previous studies (Rounds and others, 1999) found that flows less 

than about 300 ft3/s were required to allow enough residence 

time for phytoplankton populations to become substantial at the 

Oswego Dam water-quality monitoring site (fig. 2). Additional 

factors affecting residence time in the reservoir reach included 

large water withdrawals (50 ft3/s) from the Tualatin River at 

the Oswego Canal (RM 6.7) through 1995, as well as the use of 

plywood flashboards on the concrete weir of the Oswego Dam 

to raise water levels in the river 1–2 ft; this facilitated water 

withdrawals at the Oswego Canal via gravity into Lake Oswego 

(fig. 2). Oswego Canal withdrawals decreased substantially 

after 1995 but continued to average about 10–15 ft3/s through 

2003, after which the withdrawal rate decreased to 1 ft3/s or 

less. The use of flashboards was common until the late 1990s 

when fewer boards were placed on the weir. Variable use of the 

flashboards continued through about 2005, but their use was 

discontinued thereafter.

Beyond the Oswego Dam, from RM 3.4 to the Tualatin 

River’s mouth, is the lowermost “riffle reach” of the Tualatin 

River. The river gradient in this reach increases to 13 ft/mi and the 

river passes through a narrow gorge before joining the Willamette 

River south of Portland. Monitoring sites in this reach include the 

USGS streamflow gaging station at West Linn (RM 1.8) and the 

CWS water-quality sampling station at Weiss Bridge (RM 0.5).
Sample Collection and Data Analysis Methods 15

Sample Collection and Data 

Analysis Methods

Water-Sample Collection and Processing

Water samples were collected by USGS for analyses of 

total and dissolved nutrients, Chl-a, and selected major ions 

and metals with clean 2-liter (L) polyethylene bottles using 

clean techniques, and samples stored on ice prior to laboratory 

analysis. Samples collected during the USGS longitudinal 

surveys in 2006−08 were grab samples taken from just 

beneath the water surface, typically at the center of the stream 

cross section, although at two stations, the Tualatin River 

at RM 24.5 and at Jurgens Park (RM 10.8), samples were 

collected from a shore-based floating dock, when a canoe was 

not available. The Oswego Dam site (RM 3.4) was sampled 

near the fish ladder along the left bank. 

Water samples collected by CWS were grab-integratedcomposite (GRIC) samples collected at five points across 

the river width, and integrated over the depth of the river or 

as much as 10 feet, whichever was less, and five subsamples 

were composited in a churn splitter. Water samples were 

processed and analyzed by the CWS Water-Quality Laboratory 

in Hillsboro, Oregon, using methods published in their 

watershed monitoring plan (Clean Water Services, 2006). 

Water samples for dissolved constituents were passed through 

0.45-micron syringe filters prior to analysis, and subsamples 

for Chl-a were collected onto glass fiber filters and analyzed 

fluorometrically (Clean Water Services, 2006). 

Chl-a samples for the WWTF effluent experiments 

were analyzed at the USGS Oregon Water Science Center 

(ORWSC) in Portland, Oregon, using a Turner Designs 

fluorometer. Filters were ground in 90 percent acetone and 

fluorescence values were obtained before and after the 

addition of acid to account for the presence of phaeopigments 

according to methods described by the American Public 

Health Association (1992).

Field Parameters and Streamflow

Instantaneous field parameters (water temperature, DO, 

pH, specific conductance, turbidity, and Chl-a) were measured 

with a Yellow Springs Instruments (YSI) multiparameter 

sonde during USGS longitudinal surveys, and these discrete 

measurements complimented continuous data from nearby 

water-quality monitors located at RM 24.5, Cook Park 

(RM 9.9), and Oswego Dam (RM 3.4) (fig. 2). Continuous 

streamflow data were obtained from an Oregon Water 

Resources Department (OWRD) gage located in the Tualatin 

River at Farmington (RM 33.3) and the USGS gage at West 

Linn (RM 1.8). Flow data for Scoggins Creek downstream 

of Henry Hagg Lake were obtained from the Bureau of 

Reclamation. Releases from Barney Reservoir and Hagg 

Lake for flow augmentation were obtained from the Tualatin 

River Flow Management Technical Committee (Bonn, 2006; 

2007; 2008). In this report, natural flow is defined as any 

flow not derived from upstream reservoirs or WWTFs, and 

flow augmentation water is flow derived from both Barney 

Reservoir and Hagg Lake minus all known water withdrawals.

Plankton-Sample Collection, Processing, 

and Identifications

Phytoplankton grab samples were collected from just 

below the river surface into 250-mL polyethylene bottles and 

placed on ice. Zooplankton samples were collected with a 

12-in. diameter, 80-μm mesh plankton net which was hand 

tossed from a canoe or from shore, capturing approximately 

10 ft of towing distance per sample for a total volume of 

approximately 222 L, or 0.222 cubic meter (m3) per sample. 

The sample was washed down the net toward the “cod” end 

with repeated rinses, into a 20-mL plastic vial. Phytoplankton 

samples were preserved with 2.5 mL of Lugol’s solution 

(1 percent final concentration), and zooplankton samples 

were preserved with isopropyl alcohol (25 percent final 

concentration, by volume). Additional tows of “net plankton” 

were collected at each site for microscopic observation of the 

unpreserved plankton community.

Preserved phytoplankton samples were shipped to 

Aquatic Analysts, Inc., in Friday Harbor, Washington, for 

identification and enumeration. Permanent microscope slides 

were prepared for each sample by filtering an appropriate 

aliquot of the sample through a 0.45-μm membrane filter 

(APHA Standard Methods, 1992). A section of filter was cut 

out and placed on a glass slide with immersion oil added to 

make the filter transparent. A cover slip was placed on top, 

with nail polish applied to the periphery for permanency. Most 

algae were identified by cross-referencing several taxonomic 

sources. A minimum of 100 algal units, defined as discrete 

particles (either cells, colonies, or filaments), were counted 

along a measured transect on a microscope slide with a Zeiss 

standard microscope using 1000X magnification. Only algae 

with intact chloroplast and believed to be alive at the time 

of collection were counted. Average biovolume estimates of 

each species were obtained from calculations of microscopic 

measurements of each alga type in each sample analyzed. The 

number of cells per colony or the length of a filament was 

recorded to determine the biovolume per unit-alga conversion 

factors, which were used to calculate the total biovolume 

per taxon for each sample. In this report, biovolume is used 

along with Chl-a (continuous measurement at the two waterquality monitors and discrete measurement of water samples 

analyzed in the laboratory) to estimate algal abundance in the 

water column.
16 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Zooplankton samples were shipped to ZP’s Taxonomic 

Services (Lakewood, Washington), for identification and 

enumeration. Zooplankton densities, in number of organisms 

per cubic meter, were determined for each sample by counting 

a minimum target of 400 organisms or, if fewer organisms 

were present, the entire sample.

Plankton Bioassay Experiments

Streamside experiments were conducted using 300-mL 

glass BOD bottles to examine the potential effects of 

WWTF effluent (0, 30, and 50 percent) and soluble reactive 

phosphorus (SRP) additions of 20 and 50 μg/L on Chl-a and 

DO production during incubations lasting 6–24 hours. 

Water for the experiments was collected from the 

Tualatin River at Rood Bridge (RM 38.4, upstream of the 

Rock Creek WWTF discharge location), and transferred to 

an 18-L churn splitter. WWTF effluent was collected from 

the Rock Creek treatment facility just prior to its discharge 

to the river. Varying amounts of river water and WWTF 

effluent were dispensed into BOD bottles with a graduated 

cylinder to obtain a range of effluent concentrations, either 

0 and 50 percent, or 0, 30, and 50 percent, depending on 

the experiment. The starting conditions in each bottle were 

either measured (for DO) or estimated from measurements 

of Chl-a and nutrient concentrations in the water sources 

used to prepare the initial samples (unamended native water, 

plankton-amended water, and treated effluent), taking into 

account the proportions of each source. Ending conditions 

were determined by directly analyzing the contents of each 

sample bottle.

On the morning of each experiment, one large 20-L 

carboy was filled with native water collected from the Tualatin 

River at Rood Bridge and one 10-L carboy of treated effluent 

was obtained. The filled carboys were suspended in the river 

to attain temperature equilibrium, and then they were removed 

to a shady streamside location where the native water carboy 

was purged for about a minute with gaseous N2 to reduce 

the DO concentration to approximately 75−95 percent of 

saturation. Purging with N2 did not alter the pH appreciably 

(by removing CO2), and gave the phytoplankton some “room” 

to photosynthesize. Despite this effort, production of excessive 

DO sometimes occurred within the bottles, forming oxygen 

bubbles in the supersaturated conditions. Because this gas 

(DO) tends to escape from the BOD bottle when the stopper is 

removed, some of the DO measurements likely underestimated 

actual DO production.

“Plankton-amended” water was created by adding several 

20-mL vials of concentrated plankton collected from the 

Tualatin River at Jurgens Park (RM 10.8) using an 80-μm 

mesh net to river water collected from the Rood Bridge site. 

The plankton-amended samples contained 0.3−1.9 times the 

amount of phytoplankton biovolume compared with algal 

biovolume in the Tualatin River at Jurgens Park on the day 

of the experiment. Because samples were collected with 

a plankton net, however, the zooplankton densities in the 

amended samples were much higher—containing as much 

as 24 times the amount of zooplankton. During the July 

15−17, 2008 experiment, the zooplankton density attained 

in the amended sample bottles was within the range of 

zooplankton abundance at some of the downstream sites, and 

was more representative of actual conditions compared with 

earlier experiments. 

Experiments were set up such that a range of WWTF 

effluent and phosphorus concentrations were tested. To test for 

potential phosphorus limitation, some bottles were spiked with 

a concentrated solution of KH2PO4 using a micropipette to 

boost SRP concentrations by 20 μg/L (low dose) or 50 μg/L 

(high dose). 

Bottles were topped off with a small volume of river 

water from Rood Bridge to allow proper removal of air 

bubbles and stoppered. Once all bottles were filled, the DO 

and water temperature were measured using a Yellow Springs, 

Inc. Clark-type DO probe equipped with an electric stirrer. 

The bottles were randomly arranged in two metal wire baskets 

placed in the river on the upstream side of the Oswego Dam 

next to the fish ladder, in an exposed location that receives 

afternoon sunshine. Bottles were incubated at a depth of about 

one foot in an area with sufficient flow to ensure that bottles 

were maintained at river temperature. 

Following incubation, the DO in each bottle was 

measured on site, bottles were placed on ice in a cooler, 

and samples were transported to the laboratory for further 

processing. Chl-a samples were collected on 47-mm diameter 

0.7-μm GF/F glass fiber filters and frozen prior to analysis at 

the USGS ORWSC. Samples for dissolved nutrients—nitriteplus-nitrate (NO2

-

 + NO3

-

), ammonia-plus-ammonium (NH3 

+ NH4

+), hereafter referred to as ammonia, and SRP—were 

filtered through 25-mm diameter 0.45-μm AcrodiskTM filters 

using a 60-mL syringe, refrigerated, and then transported to 

the CWS Water-Quality Laboratory in Hillsboro, Oregon, for 

analysis. In this report, all constituent concentrations are given 

in atomic units (as N or P, for example).

Quality Assurance Data

All of the CWS data used in this study, including 

nutrients, Chl-a, turbidity, and field parameters (water 

temperature, pH, DO, specific conductance), were subjected 

to a comprehensive and rigorous quality assurance (QA) 

procedure by the CWS Water-Quality Laboratory. CWS 

laboratory methods and protocols have been reviewed by 

the USGS Branch of Quality Systems and were determined 

to be suitable. Field methods in use by CWS, including the 

collection of samples using depth- and width-integrating 

techniques and the use of churn splitters for subsampling, 
Sample Collection and Data Analysis Methods 17

are consistent with USGS procedures. In addition, the CWS 

laboratory has been certified by the National Environmental 

Laboratory Accreditation Conference (NELAC), participates 

in a monthly-to-quarterly QA program with the USGS 

ORWSC for nutrients and Chl-a, and participates in many 

laboratory performance tests such as the twice-a-year USGS 

Standard Reference Sample (SRS) program, a national interlaboratory comparison study (U.S. Geological Survey, 2013).

SRS results from many years of participation in the program 

have shown that the CWS laboratory consistently produces 

high quality data that are sufficiently accurate for all the 

parameters used in this study.

Results from quality-assurance samples collected 

specifically for this study are shown in appendix A (tables A-1

to A-3), and the results are summarized below. Replicate 

water-quality samples collected during this study (table A-1) 

revealed no major problems. Replicate samples for Chl-a, 

nitrite-plus-nitrate, and SRP had percent relative differences 

(PRD) of less than 5 percent. The PRD for replicate dissolved 

ammonia analyses was higher (28 percent), but at these low 

concentrations (0.014 and 0.018 mg/L), such variability 

in replicates is not uncommon and does not affect the 

interpretations in this report.

Replicate samples for phytoplankton identification and 

enumeration showed some variations in species composition 

and identifications (table A-2), but for the most part, the 

dominant taxa were the same or similar, and overall, the 

reproducibility was deemed acceptable. More variability was 

found in the zooplankton counts (table A-3), and there was 

a greater amount of variability in the replicate zooplankton 

tow-net samples compared with the phytoplankton samples. 

This may have been due to the greater heterogeneity of 

zooplankton in surface waters, to larger spatial scales—a 10-ft 

transect compared with grab samples of surface water—or 

longer amount of time (and thus more movement across the 

water) for repeat zooplankton sample collections compared 

with phytoplankton. In any case, the reproducibility in the 

zooplankton taxa identifications was acceptable for the 

dominant taxa.

Data Analysis Methods

Plankton, flow, water-quality, and meteorological 

data (solar radiation and rainfall) were analyzed using 

a combination of standard regression and multivariate 

statistical methods. Longitudinal, time-series, and other plots 

were generated to examine patterns in the data. Spearman 

rank correlation, Principal Components Analysis (PCA), 

and a variety of multivariate statistical analyses were 

performed using the computer software package PRIMER 

(Plymouth Routines In Multivariate Ecological Research), 

version 6 (Clarke and Gorley, 2006). Environmental data 

were log transformed and standardized (or “normalized” 

in PRIMER) prior to their use in the multivariate analyses. 

Species composition data were analyzed using nonmetric 

multidimensional scaling (NMDS) ordinations and 

other methods in PRIMER including BEST, ANOSIM, 

and SIMPER. NMDS ordination analyses were used to 

understand patterns in the phytoplankton species data, and 

to identify potential subsets of samples for further analysis. 

Bio-Env+Stepwise (BEST) analyses were performed to 

test the relative importance of various factors in explaining 

variations or patterns in the phytoplankton species 

composition. The strength of each variable, or combination 

of variables, is represented by the rho value and associated 

P value. PRIMER also tests the statistical significance 

of the selected combination of variables (global rho (R) 

statistic) or individual rho value using a Monte-Carlo 

permutation simulation. Analysis of similarity (ANOSIM), the 

nonparametric equivalent to analysis of variance (ANOVA), 

was used to test for significant differences among selected 

pre‐defined groups of samples, such as those collected on 

different dates or during different flow or phytoplankton 

biomass (Chl-a) condition. Then the similarity percentages 

(SIMPER) procedure was used to calculate the average species 

abundance in each group to identify algal taxa that might 

explain any differences.

Many of the water-quality and flow variables were 

autocorrelated, with suites of variables explaining similar 

sources of variation within the data. Chloride, for example, 

was highly correlated with specific conductance, nitrate, 

and percent flow from WWTF, owing to the relatively high 

conductance and nitrate concentrations in WWTF effluent. 

In addition, because the percentages for each flow source 

(WWTFs, reservoir augmentation, and natural flow) are not 

independent of each other, they often were correlated. In these 

cases, one “best” representative or “surrogate” variable was 

selected for the final BEST analysis, based on the strength of 

its individual correlation with the phytoplankton assemblage 

data. The removal of redundant variables did not reduce the 

explanatory power of the analysis substantially, and had the 

benefit of simplifying the interpretation. Through an iterative 

process, unimportant and redundant variables were removed 

until a final solution of 1–5 variables was obtained. In cases 

where removing a flow variable from the model resulted in a 

much lower overall correlation, multiple flow variables were 

permitted in the final solution.

NMDS ordinations of phytoplankton samples were 

constructed from Bray-Curtis similarity matrices based on 

square-root transformed algal biovolume data. The NMDS 

ordination routines in PRIMER work iteratively to optimize 

a solution whereby samples having higher similarity are 

plotted close together and samples with lower similarity are 

plotted farther apart. The underlying similarity matrix used 

to construct the ordinations was correlated with each of the 

environmental variables to identify those variables with the 

most significant correlations (rho values). 
18 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

In several cases, water chemistry data from CWS were 

used to fill gaps in the data set. For this purpose, CWS data 

from the Tualatin River at Scholls Bridge (RM 26.9) were 

used for the USGS sampling site at RM 24.5; data from Elsner 

(RM 16.2) were used for the Highway 99W/Jurgens Park sites 

(RMs 11.6/10.8); data from Boones Ferry (RM 8.7) were used 

for the site downstream of Fanno Creek (RM 9.2); and in 2006 

only, data from Weiss Bridge (RM 0.5) were used to fill gaps 

at the Oswego Dam site (RM 3.4). In most cases, the substitute 

data were collected by CWS on the same day, and always 

within 2–3 days of the plankton sample collections.

Several flow variables were included in the final 

multivariate environmental dataset, including total streamflow, 

natural streamflow (total and percent of streamflow without 

WWTF inputs or flow augmentation), flow augmentation 

level (total and percent of reservoir releases minus any 

withdrawals), and WWTF effluent flow (total and percent). 

Flow variables were calculated for each sampling site and 

adjusted to the nearest day to account for (a) travel time 

between a streamflow gage and the sampling sites, and (b) the 

distance between inputs of water from Barney Reservoir, 

Hagg Lake, or inputs of treated effluent from the Rock Creek 

and Durham WWTFs, to the downstream sampling sites. 

Time lags for streamflow measurements also were applied 

to calculate streamflow for each of the sampling sites on 

the day of sampling. Effluent discharge data were obtained 

from Bonn (2006, 2007, 2008). Streamflow data from the 

Tualatin River at Farmington gage were obtained from OWRD 

and used to calculate the flow and effluent percentages for 

samples collected at sites from Rood Bridge to Cook Park 

(RMs 38.4–9.7). Total streamflow, flow augmentation, and 

WWTF effluent percentages for sites downstream of Cook 

Park, which are affected by inputs from the Durham WWTF 

and Fanno Creek at RM 9.3, were calculated using streamflow 

data from the USGS gage at West Linn (RM 1.8).

Travel times for the 2006−08 “summer” period 

(May–September) were estimated from a number of sources, 

including measured and modeled travel times published by 

Rounds and others (1999), from results of dye tracer studies 

conducted by USGS on September 14–15, 1992 (Lee, 1995), 

and from modeled results from other dye tracer studies. Water 

velocities and estimated travel times were assigned according 

to whether sampling occurred during three categories of flow 

conditions (190–220, 240–275, and 285–310 ft3/s; table 3). 

The flow ranges were determined by evaluating break points 

in the distribution of flow data at West Linn for summer 

months (June–September) for 2006–08. These streamflow 

ranges correspond to the majority of the flow conditions 

during the USGS sampling, although higher flows did occur 

in May 2008: 567 ft3/s at West Linn. The timing of flow 

augmentation from Barney Reservoir, located to the west in 

the Trask River basin (fig. 1), was estimated by adding 2 days 

of travel time to the estimated travel time from Hagg Lake to 

each sampling site (Bonn, 2008).

Climate, Streamflow, and 

Water-Quality Conditions

To put the results from this 2006–08 study into 

perspective, selected climatological, streamflow, Chl-a, and 

other water-quality variables were compared to a longer 

historical record going back to 1991 (fig. 9). Although 

conditions were variable among the 3 study years, they were 

not atypical for many basic parameters, including rainfall, 

solar radiation, water temperature, or specific conductance. 

The nitrate levels for 2006–08, however, were among the 

highest during 1991–2009.

Streamflow and Sources of Flow

The magnitude and source of streamflow can affect 

phytoplankton through mechanisms including water chemistry, 

temperature, travel time, and other factors. Changes in 

the source of flow may alter the amount or composition 

of plankton inoculum entering the river, thereby affecting 

downstream algal communities.

Differences in winter precipitation, and the timing of 

snowmelt and late-spring rains, resulted in higher late-May 

and early-June flows in 2006 and 2008 compared to 2007, 

but by late June or early July, flows were similar among 

the 3 years. Compared with the 1991–2010 average flow 

at West Linn of 215 ft3/s in July and August, flows of 215, 

245, and 261 ft3/s in 2006–08 were average or somewhat 

above average.

Summertime flow in the Tualatin River originates from 

four sources: (1) natural flow from the headwaters, wetlands, 

tributaries, and groundwater; (2) flow augmentation from 

two upstream reservoirs; (3) discharges from the two largest 

WWTFs (Rock Creek and Durham); and (4) discharges and 

return flows from agricultural areas and managed wetlands 

in the basin. These sources maintained streamflow levels 

Table 3. Estimated average water velocities used to estimate 

travel times in four reaches of the Tualatin River, Oregon, over a 

range of summer streamflows.

Reach

(length)

Streamflow in the Tualatin 

River at West Linn 

(river mile 1.8)

(cubic feet per second)

190–220 240–275 285–310

Water velocity

(miles per hour)

Hagg Lake to Rood Bridge (26.6 miles) 1 1 1

Rood Bridge to RM 24.5 (13.9 miles) 0.30 0.40 0.50

RM 24.5 to Elsner (8.3 miles) 0.14 0.18 0.22

Elsner to Oswego Dam (12.8 miles) 0.10 0.12 0.14

Oswego Dam to Weiss Bridge (2.9 miles) 0.70 0.85 1.00
Climate, Streamflow, and Water-Quality Conditions 19

tac12-0780_fig09

0 

20 

40 

60 

80 

100 

120 

140 

160 

Number of days maximum chlorophyll-a greater than 20 micrograms 

per liter (May–October) (Oswego Dam, river mile [RM] 3.4) 

0 

5 

10 

15 

20 

25 

30 

35 

Max snowpack, snow-water equivalent, in inches

(January–June) (Saddle Mountain) 

0 

20 

40 

60 

80 

100 

120 

140 

Number of days maximum solar greater than 1,250 microeinstein per 

square meter per second (May–October) (Durham wastewater 

treatment facility [WWTF]) 

16.5 

17.0 

17.5 

18.0 

18.5 

19.0 

19.5 

20.0 

Average of daily maximum water temperature, in degrees Celsius

(May–October) (Oswego Dam, RM 3.4) 

2 

0 

4 

6 

8 

10 

12 

Total rainfall, in inches

(May–October) (Durham WWTF) 

50 

0 

100 

150 

200 

250 

300 

350 

Average of daily maximum specific conductance, in microsiemens 

per centimeter (May–October) (Oswego Dam, RM 3.4) 

0 

100 

200 

300 

400 

500 

600 

700 

800 

1991 1992 1993 

1994 

1995 1996 1997 1998 1999 2000 2001 2002 2003 

2004 

2005 2006 2007 2008 2009 

Year 

Average of daily median streamflow, in cubic feet per second

(May–October) (Tualatin River at West Linn, RM 1.8) 

0 

0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

3.5 

4.0 

1991 1992 1993 

1994 

1995 1996 1997 1998 1999 2000 2001 2002 2003 

2004 

2005 2006 2007 2008 2009 

Year 

Median nitrite-plus-nitrate concentration, in milligrams per liter

(July) (Tualatin River at Elsner, RM 16.2) 

Study years 2006–08

EXPLANATION

Figure 9. Annual patterns in snowpack, solar radiation, streamflow, water-quality, and algal conditions in the Tualatin 

River, Oregon, 1991−2009.

near or above 200 ft3/s at West Linn during 2006–08. Flow 

measurements from various locations and knowledge of 

travel time through the river system is routinely used to 

estimate proportions of these flow sources at key locations to 

understand how they influence water-quality conditions (Bonn, 

2006; 2007; 2008; see fig. 10). 

Natural flow accounted for 70–80 percent of the total 

flow at Farmington and West Linn in early summer (June) 

in 2006–08, declining to as low as 38–40 percent later in 

summer as reservoir releases for irrigation, water supply, and 

flow augmentation, and treated WWTF-effluent percentages 

increased (fig. 11). 

Patterns in flow augmentation were similar during 

2006–08, with most releases beginning in earnest in late June 

and accounting for roughly 20–30 percent of the streamflow at 

West Linn in late summer. The releases were somewhat more 

consistent in 2006, in contrast to 2007 and 2008 when short 

periods of reduced releases from reservoirs occurred because 

rainfall decreased the need for flow augmentation (fig. 11). 

Note that flow augmentation was calculated as reservoir 

releases minus withdrawals from the Cherry Grove, Patton, 

Wapato, TVID Spring Hill, and JWC Spring Hill intakes, and 

sometimes not all of the reported withdrawals are made, so 

flow augmentation values may, at times, be higher.
20 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig10 Flow, in cubic feet per second

Farmington (2006) West Linn (2006)

Farmington (2007) West Linn (2007)

Farmington (2008) West Linn (2008)

Month

May June July Aug. Sept. Oct. Nov.

Month

May June July Aug. Sept. Oct. Nov.

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Services

Farmington flow 

Hagg Lake

Barney Reservoir

Rock Creek wastewater 

treatment facility

Durham wastewater 

treatment facility

Flow without Clean Water 

Services

West Linn flow

EXPLANATION EXPLANATION

Figure 10. Natural flow, wastewater treatment facility discharges, and flow augmentation in the Tualatin River at Farmington 

(river mile [RM] 33.3) and West Linn (RM 1.8), Oregon, 2006–08. Figures modified after Bonn (2006, 2007, 2008).
Climate, Streamflow, and Water-Quality Conditions 21

tac12-0780_fig11

0 

100 

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300 

400 

500 2006 

0 

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300 

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Streamflow at West Linn, in cubic feet per second

Flow augmentation, in cubic feet per second 

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Total flow augmentation

Streamflow in the Tualatin River 

at West Linn

USGS plankton sampling 

EXPLANATION

Month

May June July Aug. Sept.

Figure 11. Streamflow and flow augmentation in the lower Tualatin River, Oregon, showing the timing of longitudinal 

samplings, 2006–08. Flow augmentation includes reservoir releases minus withdrawals from Hillsboro-Cherry Grove, Patton, 

Wapato, TVID Spring Hill, and JWC Spring Hill intakes.
22 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Effluent discharged to the Tualatin River from the Rock 

Creek WWTF contributed as much as 25–31 percent of the 

total streamflow at Farmington during May–October of 2006, 

2007, and 2008 (fig. 12); downstream of the Durham WWTF, 

treated effluent accounted for as much as 33–38 percent of the 

total streamflow at West Linn (fig. 12). Discharges from the 

WWTFs have increased over the last 20 years, particularly 

from the Rock Creek WWTF (fig. 13) as population growth 

in that part of Washington County increased. As a result, the 

percentage of treated effluent in the river downstream of the 

Rock Creek WWTF also has increased over the years.

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Month

May June July Aug. Sept. Oct. Nov.

Month

May June July Aug. Sept. Oct. Nov.

Rock Creek wastewater treatment facility

Hagg Lake

Barney Reservoir

Natural flow 

EXPLANATION EXPLANATION

Durham wastewater treatment facility

Rock Creek wastewater treatment facility

Hagg Lake

Barney Reservoir

Natural flow

Percent of total flow

Figure 12. Percentage of natural flow, flow augmentation, and wastewater treatment facility effluent in the Tualatin River at 

Farmington (river mile [RM] 33.3), and West Linn (RM 1.75), Oregon, 2006−08. Flow augmentation includes reservoir releases minus 

withdrawals from Hillsboro-Cherry Grove, Patton, Wapato, TVID Spring Hill, and JWC Spring Hill intakes.
Climate, Streamflow, and Water-Quality Conditions 23

tac12-0780_fig13

0 

20 

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100 

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Treated effluent discharge, in cubic feet per second

Year

Rock Creek wastewater treatment facility

Durham wastewater treatment facility

Figure 13. Daily treated effluent discharges from the Rock Creek and 

Durham wastewater treatment facilities, Oregon, May–October 1991–2008. 

Data from Clean Water Services.

The quality of treated effluent from these two WWTFs 

is shown in table 4. The WWTFs convert ammonia to nitrate 

during summer when temperatures are warm enough to 

support populations of nitrifying bacteria in the treatment 

plants. As a result, WWTF effluent during summer is 

characterized by low concentrations of ammonia but high 

concentrations of nitrate. Phosphorus removal is required to 

meet TMDL limits during May through October, resulting 

in relatively low median SRP concentrations ranging from 

0.013 to 0.018 mg/L in 2006–08 (table 4). It is not clear 

how much of this phosphorus is available to phytoplankton; 

studies in other treatment facilities that utilize alum for 

tertiary phosphorus removal seem to indicate that much of the 

phosphorus may not be bioavailable (Li and Brett, 2010).

Bioavailable Nutrients

Dissolved nutrient concentrations in the lower Tualatin 

River (fig. 14) were generally high enough to support 

algal growth during most of the year. Patterns in nutrient 

concentrations reflected the seasonal pattern in streamflow 

variations (source and amount) and patterns in algal growth. 

Nitrogen concentrations in the river are controlled mainly 

by discharges from the Rock Creek WWTF (RM 38.1). 

Nitrification of ammonia during treatment results in high 

concentrations of nitrate-nitrogen in effluent, causing 

concentrations to increase to 4–5 mg/L during summer at sites 

downstream (note the large increase between Rood Bridge and 

Scholls in fig. 14). Concentrations of ammonia, on the other 

hand, tend to be low and occasionally decline during algal 

blooms to less than 0.005 mg/L at Stafford Road (RM 5.5). 
24 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Table 4. Quality of treated effluent from the Rock Creek and Durham wastewater treatment facilities, Oregon, May–October 2006–08.

[Data from Clean Water Services. Abbreviations: °C, degrees Celsius; WWTF, wastewater treatment facility; mg/L, milligram per liter]

Parameter Units

Rock Creek WWTF Durham WWTF

2006 2007 2008 2006 2007 2008

Effluent temperature °C Minimum 16.7 17.8 16.7 15.7 16.5 15.4

Median 21.3 21.3 21.3 21.2 20.3 20.2

Maximum 23.3 23.3 23.5 23.0 22.4 23.3

Dissolved oxygen mg/L Minimum 7.3 7.1 7.5 7.3 6.3 7.4

Median 8.5 8.6 8.7 8.9 8.9 9.1

Maximum 9.7 9.8 11.1 10.3 10.2 11.7

Chloride mg/L Minimum 40.8 36.7 42.6 42.0 40.2 43.5

Median 51.3 51.6 51.2 55.1 59.0 51.7

Maximum 55.6 56.2 57.7 61.0 65.9 59.9

Ammonia mg/L Minimum 0.01 0.01 0.01 0.03 0.02 0.03

Median 0.03 0.03 0.03 0.07 0.04 0.05

Maximum 11.20 9.51 13.00 1.33 1.18 9.86

Nitrite+nitrate mg/L Minimum 9.5 10.6 8.9 6.8 6.0 3.7

Median 14.5 14.4 15.4 10.2 9.7 10.0

Maximum 18.8 18.7 19.7 12.6 14.8 14.1

Total Kjeldahl nitrogen mg/L Minimum 0.90 0.83 0.84 0.93 0.96 1.01

Median 1.2 1.2 1.3 1.5 1.5 1.6

Maximum 12.2 11.3 13.8 2.4 2.7 11.2

Soluble reactive phosphorus mg/L Minimum 0.005 0.005 0.005 0.005 0.005 0.005

Median 0.018 0.014 0.013 0.016 0.016 0.013

Maximum 0.66 0.66 0.78 0.32 0.75 0.76

Total phosphorus mg/L Minimum 0.025 0.025 0.034 0.025 0.025 0.035

Median 0.072 0.060 0.078 0.088 0.080 0.092

Maximum 0.80 0.88 1.03 0.50 0.89 1.01

Despite the fact that ammonia may be a preferred source 

of nitrogen for most algae, the nitrate in the river should be 

available for algal growth; therefore, it appears unlikely that 

nitrogen would limit the growth of most phytoplankton species 

in the river downstream of the Rock Creek WWTF.

Periods of high phytoplankton abundance often 

corresponded to sharp declines in ammonia and SRP 

concentrations. These dissolved nutrients are taken up and 

converted into algal biomass, which resulted in negative 

relations between concentrations of Chl-a and these nutrients 

each summer, particularly in 2007 (fig. 15). The ammonia and 

SRP concentrations were sometimes below detection during 

algal-bloom periods when Chl-a concentrations were highest. 

Some unusually high concentrations of SRP were measured at 

Rood Bridge in July 2008 (fig. 14). Additional water testing 

verified the source to be drainage water discharged from the 

Wapato Lake agricultural area, where a levee failure in the 

previous winter caused this low-lying area to remain ponded 

until early summer (Bonn, 2008). The elevated ammonia 

concentration at Stafford Road in early July 2008 was due to 

a discharge from the Rock Creek WWTF (Jan Miller, Clean 

Water Services, written commun., 2011).
Climate, Streamflow, and Water-Quality Conditions 25 tac12-0780_fig14

0

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Tualatin River at Rood Bridge (river mile 38.4)

0.60 0.50 0.40 0.30 0.20 0.10 0.00 

5 4 3 2 1 0 

Ammonia and soluble reactive phosphorus concentration, in milligrams per liter

Nitrite plus nitrate concentration, in milligrams per liter

Nitrite plus nitrate concentration, in milligrams per liter

Ammonia and soluble reactive phosphorus concentration, in milligrams per liter

Ammonia and soluble reactive phosphorus concentration, in milligrams per liter

Nitrite plus nitrate concentration, in milligrams per liter Tualatin River near Scholls (river mile 26.9) 0.40 0.30 

0.20 0.10 0.00 

5 

4 

3 2 1 0 

2006

Tualatin River at Stafford Road (river mile 5.4) 

Tualatin River at Rood Bridge (river mile 38.4)Tualatin River near Scholls (river mile 26.9) Tualatin River at Stafford Road (river mile 5.4) 

Tualatin River at Rood Bridge (river mile 38.4)Tualatin River near Scholls (river mile 26.9) Tualatin River at Stafford Road (river mile 5.4) 

EXPLANATION

May June Sept. July Aug. May June Sept. July Aug. May June Sept. July Aug.

Nitrite-plus-nitrateAmmoniaSoluble reactive phosphorus

Figure 14. Time series of dissolved nitrite-plus-nitrate, ammonia, and soluble reactive phosphorus concentrations at select sites in the Tualatin River, Oregon, 2006–08. 

Data from Clean Water Services.
26 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig15

0

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Soluble reactive phosphorus concentration,

in milligrams per liter

0

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in milligrams per liter

July 5

July 13

July 19

August 3

August 13

2007

EXPLANATION

Chlorophyll-a concentration, in micrograms per liter 

B

A

Figure 15. Chlorophyll-a and (A) ammonia and (B) soluble reactive phosphorus in the lower Tualatin River, 

Oregon, 2007. Lower Tualatin River includes the Highway 99W and Jurgen’s Park sites (river miles 11.6 and 

10.8, respectively) downstream to Oswego Dam (river mile 3.4).
Patterns in Plankton Populations 27

Table 5. Taxa richness and abundance (biovolume) of major algal groups (divisions) in the main-stem 

Tualatin River, Oregon, 2006–08.

[Data from 117 phytoplankton samples collected from the main-stem Tualatin River]

Algal division Total number

of taxa

Percent of total 

number of taxa

Total

biovolume

Percent of

total biovolume

Bacillariophyta (diatoms) 93 65 42,910,833 38.5

Cryptophyta (cryptophyte algae) 2 1 42,586,213 38.2

Cyanobacteria (blue-green algae) 6 4 22,039,143 19.8

Chlorophyta (green algae) 28 20 3,044,019 2.7

Chrysophyta (golden algae) 6 4 437,434 0.4

Pyrrophyta (dinoflagellates) 3 2 382,223 0.3

Euglenophyta (euglenoid algae) 3 2 67,243 0.1

Unknown classification 2 1 64,113 0.1

Total 143

Patterns in Plankton Populations

Phytoplankton

Annual, longitudinal, and seasonal patterns in 

phytoplankton Chl-a reveal much about the cycles and 

dynamics of Tualatin River algal communities (figs. 4, 6, 

and 7). Substantial phytoplankton populations developed 

each year; concentrations at Stafford Road peaked well 

above the 15-μg/L action level (fig. 4). Chl-a concentrations 

peaked in July at 80, 70, and 50 μg/L in 2006, 2007, and 

2008, respectively, and then declined to below 5–10 μg/L in 

late July or early August. In 2006, a June–July bloom was 

followed by a rapid decline, but the population then rebounded 

to similar concentrations only to decline again at the end of 

July. In 2007, a single bloom occurred, peaking in mid-July 

and declining a week later. In 2008, a bloom also formed 

in mid-July (a result of Wapato Lake discharges, discussed 

below), and after that the bloom declined slowly, eventually 

producing the lowest concentrations of DO observed during 

this study—less than 4 μg/L at the Oswego Dam (fig. 4).

Phytoplankton populations in the Tualatin River are 

composed of a variety of species, including diatoms, green 

and blue-green algae, and small algal flagellates. Although the 

algal assemblages are dynamic and diverse, just a few groups 

made up the majority of the total algal biovolume in the 

lower river during much of the growing season in 2006–08. 

A total of 143 algal taxa were identified in 117 main-stem 

samples in 2006–08 (table 5; appendix B). The dominant 

algal taxa during summer included filamentous centric 

diatoms Stephanodiscus binderanus, S. hantzschii, and several

Cyclotella and Aulacoseira (formerly Melosira) species, small 

flagellated green algae, Chlamydomonas sp., cryptophytes 

including Cryptomonas erosa and Rhodomonas minuta, and 

occasional but sometimes large blooms of blue-green algae, 

mainly Anabaena flos-aquae and Aphanizomenon flos-aquae

(see photographs 6–16, p. 27–28).

Photographs 6–8: Stephanodiscus 

binderanus, a fiamentous centric 

diatom. Photographs courtesy of Rex 

Lowe, Bowling Green State University.

Photograph 9: The centric diatom Cyclotella 

meneghiniana. Photograph courtesy of the 

Philadelphia Academy of Natural Sciences.
28 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Photograph 10: Melosira sp., a centric filamentous diatom. Photograph 

by Kurt Carpenter, June 26, 2006.

Photograph 11: Asterionella formosa, a colonial 

planktonic diatom. Photograph by Kurt Carpenter, 

July 16, 2008.

Photograph 12: Fragilaria crotonensis, a colonial 

planktonic diatom. Photograph by Kurt Carpenter, 

July 7, 2006.

Photograph 13: Cryptomonas 

erosa, a Cryptophyte unicellular 

flagellate. Photograph by Kurt 

Carpenter, June 30, 2008.

Photograph 14: Pediastrum, 

a colonial Green algae. 

Photograph by Kurt Carpenter, 

July 24, 2006.

Photograph 15: Anabaena flos-aquae, a colonial 

blue-green algae. Photograph by Kurt Carpenter, 

July 15, 2008.

Photograph 16: Aphanizomenon flos-aquae, a colonial blue-green 

algae. Photograph by Kurt Carpenter, July 16, 2008.
Patterns in Plankton Populations 29

Diatoms and green algae comprised 85 percent of the 

total number of algal taxa (93 and 28 taxa, respectively), and 

blue-green and golden algae contributed 6 taxa each. While 

most of the algae identified in Tualatin River were planktonic 

and characteristic of lake phytoplankton (Porter, 2008), 

some were facultatively planktonic, meaning that they can 

reside on the bottom and migrate to the water column when 

conditions permit (Wehr and Sheath, 2003). Colony-forming 

algae were common, including the blue-green algae Anabaena

and Aphanizomenon, green algae Scenedesmus quadricauda,

Pediastrum, and Actinastrum hantzschii, and the diatoms 

Asterionella formosa and Fragilaria crotonensis. A few 

benthic taxa also were a part of the phytoplankton assemblage, 

tending to occur most often at the upstream sites (Rood Bridge 

and RM 24.5 near Scholls), at relatively low abundances, 

especially once flows declined.

Most of the algal taxa, and the majority of the most 

abundant taxa, are considered “eutrophic” and either require 

or prefer elevated nutrient concentrations (table 6), which 

is consistent with the relatively high concentrations of 

nitrate-nitrogen and moderate SRP concentrations in the river 

most of the time. Although several taxa were consistently 

abundant in samples collected during all 3 years, about one 

third, or 53 of the 143 algal taxa, occurred in just one of 

three years, highlighting the annual variability in species 

composition. Most (29) of these unique taxa were identified 

in 2008, whereas 10–15 taxa were unique to 2006 and 2007. 

Just 7 of the 29 taxa unique to the 2008 growing season were 

identified in drainage water from Wapato Lake (see section, 

“Wapato Lake Algal Bloom”). These data indicate that 

although there is some consistency in phytoplankton species 

composition from year to year, particularly among the most 

abundant species, considerable variations in abundance and 

character occur.

The most abundant algal taxa in 2006–08 was 

Cryptomonas erosa, a relatively small (520 μm3) flagellated 

alga which comprised 33 percent of the overall total 

algal biovolume for all sites over the 3-year period (see 

photograph 13, p. 28). This taxa occurred in 97 percent 

of phytoplankton samples (table 6). Only the red algae 

Rhodomonas minuta, another relatively small flagellate, 

was more frequently detected (99 percent of samples). 

Other important taxa included two Stephanodiscus

taxa (S. binderanus [see photographs 6–8, p. 27], and 

S. hantzschii), which comprised 21 percent of the overall 

total biovolume, the blue-green Anabaena flos-aquae, which 

contributed 11 percent (nearly all in 2008), and the green algal 

flagellate Chlamydomonas sp. contributed 7 percent. These 

5 taxa accounted for 71 percent of the total phytoplankton 

biovolume among all samples.

Blue-green algae, either Aphanizomenon, Anabaena, or 

both (see photographs 15–16, p. 28) were identified in the 

Tualatin River during USGS longitudinal samplings each 

summer in 2006–08, but their abundance was highly variable 

(appendix B). Microcystis, another type of blue-green algae 

that often was reported to be present in the Tualatin River 

decades earlier (Oregon Department of Environmental Quality 

and Unified Sewerage Agency, 1982), was only identified 

in July 2008 during the large Anabaena bloom, and only in 

fresh samples observed microscopically; none were identified 

in preserved samples. These blue-green algae species tend 

to cloud the water or form surface scums during quiescent 

conditions, and many strains produce potent liver or neural 

toxins that can be a threat to public health. 

A comparison of the phytoplankton species identified 

in the Tualatin River during 2006–08 with previous USGS 

samples collected in 1991–93 showed that the overall 

assemblages were quite similar, with 78 percent of the taxa 

detected previously. In addition, nearly half (49 percent) 

of the algal species identified during the current study also 

were identified in samples from Hagg Lake between 2000 

and 2006 (Rounds and others, 1999; Bonn, 2006). Two taxa, 

Stephanodiscus binderanus and Cyclotella pseudostelligera, 

were absent from the 1991–93 collections from the Tualatin 

River, whereas they were abundant in 2006–08, especially S. 

binderanus. According to Kipp and others (2013), this species 

was imported from Eurasia to the Great Lakes in ballast water 

in 1938 and is considered nonindigenous. S. binderanus

occurs in eutrophic conditions, tolerates a wide range of water 

temperature and osmotic pressure, and can be a nuisance by 

causing taste and odor problems in drinking-water supplies 

and by clogging filters.

The phytoplankton species composition in the Tualatin 

River is dynamic and shows a general seasonal succession. 

Despite year-to-year variation, the filamentous centric diatom 

Stephanodiscus binderanus dominated or co-dominated 

the early summer bloom each year during 2006–08 along 

with other diatoms as river flow was decreasing. The initial 

bloom of S. binderanus typically was followed by a bloom 

of small flagellates (Cryptomonas and Chlamydomonas), 

then by a different centric diatom (Melosira granulata or 

Stephanodiscus hantzschii) in 2006–07, or in 2008 by a 

large bloom of the blue-green algae Anabaena flos-aquae 

(appendix B).
30 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon Table 6. Dominant algal taxa in the Tualatin River, Oregon, 2006–08. [Includes top 20 taxa by percent occurrence and top 20 taxa by total biovolume for all samples. Abbreviations: RM, river mile; 99W, Highway 99 west; d/s, downstream; ?, unknown autecology] Scientific name Species code Division Growth habit Trophic indicator

Samples

(percent)

Number of 

detections

Average biovolume (cubic microns per milliliter)

Wapato 

pump 

discharge1

Spring 

Hill pump 

station1

Tualatin River sites

2006

2007

2008

Wapato Rood Bridge RM 24.5 99W/ Jurgens d/s Cook Park d/s Fanno Creek Stafford Road Oswego Dam

Rhodomonas minuta RHodminu Cryptophyte Planktonic ? 99 34 37 54 2 14,365 2,148 2,199 1,251 25,280 16,364 11,817 32,192 15,089Cryptomonas erosa CRYerosa Cryptophyte Planktonic Eutrophic 97 33 37 52 2 4,311,115 418,786 59,371 74,646 285,659 361,073 216,529 476,395 629,238Stephanodiscus 

hantzschii

SThant Diatom Planktonic Eutrophic 97 32 37 51 1 72,160 15,463 14,918 11,255 71,264 78,922 60,516 77,700 122,838

Cyclotella 

pseudostelligera

CYpsdste Diatom Planktonic Eutrophic 93 31 36 47 1 38,110 1,745 1,083 3,642 38,863 45,173 42,533 33,901 44,922

Chlamydomonas sp. CHLsp Green Planktonic Eutrophic 82 26 33 46 2 570,014 52,348 14,658 14,583 63,779 63,256 63,030 142,110 66,275Ankistrodesmus falcatus ANKfalca Green Planktonic Eutrophic 82 23 27 54 2 61,386 4,027 979 4,153 3,679 2,668 2,229 3,379 2,286Stephanodiscus 

binderanus

STbinder Diatom Planktonic Eutrophic 75 25 32 34 0 0 0 4,535 5,675 146,384 168,135 139,399 256,172 136,818

Selenastrum minutum SELminut Green Planktonic Eutrophic 70 16 29 42 0 0 0 505 917 1,135 1,200 1,271 585 483Unidentified flagellate XXsp Unknown Planktonic ? 59 26 22 27 2 1,774,045 5,369 449 893 528 1,162 3,389 0 0Scenedesmus 

quadricauda

SCEquadr Green Planktonic Eutrophic 55 12 15 45 2 385,981 65,784 5,051 10,330 12,611 12,631 12,259 14,417 6,808

Cyclotella meneghiniana CYmenegh Diatom Planktonic Eutrophic 53 21 16 30 1 57,127 10,201 2,228 6,004 24,490 20,182 21,218 6,384 20,863Chromulina sp. CHRsp Chrysophyte Planktonic ? 53 16 19 30 0 0 0 0 232 431 510 917 607 961Nitzschia acicularis NZacicul Diatom Planktonic Eutrophic 41 13 10 29 1 77,951 7,517 5,591 3,698 3,601 6,445 2,579 4,925 20,244Gomphonema 

angustatum

GOangstt Diatom Benthic Oligotrophic 35 6 13 28 0 0 4,832 4,589 1,259 1,127 709 1,045 795 5,188

Melosira distans 

alpigena

MEdist-alp Diatom Planktonic ? 38 11 15 20 0 0 0 692 2,175 4,951 5,322 6,203 10,965 10,613

Sphaerocystis schroeteri SPHschro Green Planktonic Eutrophic 36 11 9 25 1 77,951 22,550 631 5,887 6,722 14,206 10,587 7,276 5,359Achnanthes lanceolata AClanc Diatom Benthic Eutrophic 29 5 12 20 0 0 0 5,464 1,311 1,451 1,800 249 271 1,345Glenodinium sp. GLenosp Dinoflagellate Planktonic ? 30 13 13 10 0 0 0 3,332 0 19,714 14,017 8,849 17,076 14,702Crucigenia quadrata CRUquad Green Planktonic Eutrophic 28 8 4 23 1 12,778 2,282 332 666 1,581 555 1,437 2,628 3,972Achnanthes minutissima ACminu Diatom Benthic Eurytrophic 25 7 7 20 0 0 1,342 1,507 428 486 97 301 58 120Stephanodiscus astraea 

minutula

Stas-mi Diatom Planktonic ? 25 12 8 10 0 0 0 692 307 4,325 1,973 3,569 14,143 10,968

Anabaena flos-aquae ABFA Bluegreen Planktonic Eutrophic 17 0 0 26 2 287,248 89,932 167,588 32,051 235,648 111,138 94,823 48,178 0Aphanizomenon flosaquae AFNfloaq Bluegreen Planktonic Eutrophic 15 2 3 18 1 87,694 1,691 164,383 3,964 5,391 22,019 29,526 2,107 92,859Melosira granulata MEgran Diatom Planktonic Eutrophic 14 7 5 5 0 0 0 0 1,075 1,036 4,204 542 30,741 61,093Synedra ulna SYulna Diatom Benthic Eurytrophic 8 3 1 9 0 0 0 30,760 2,492 0 0 2,946 0 14,159Actinastrum hantzschii AThn Green Planktonic ? 6 0 1 8 1 267,259 0 0 0 10,177 15,825 7,959 16,443 7,159Melosira ambigua MEambigu Diatom Planktonic Eutrophic 6 5 1 1 0 0 0 0 0 4,791 16,373 0 36,894 1,460Fragilaria crotonensis FRcroton Diatom Planktonic Mesotrophic 4 4 1 0 0 0 0 0 1,056 78,228 0 0 0 8,240

1 In 2008, two samples were collected from the Wapato pump discharge and one sample from the Spring Hill pump station.
Patterns in Plankton Populations 31

Small flagellates, particularly Cryptomonas erosa, were 

secondary dominant taxa during the early part of the summer, 

when other taxa such as S. binderanus and two eutrophic 

Melosira taxa (M. ambigua and M. italica) were dominant. 

Large peaks in biovolume were measured for Melosira 

ambigua (at Stafford Road, July 2006) and Stephanodiscus 

binderanus (several sites, July 2007 and June 2008). These 

blooms were followed by sharp declines in algal biovolume 

between late July and early August at Stafford Road (RM 5.4) 

in 2006 and 2007, mostly from declines in the abundance 

of diatoms (fig. 16), especially Stephanodiscus binderanus, 

whereas the relative biovolume of Cryptomonas erosa

increased from 5−25 percent to over 80 percent. During this 

late July to early August period, some colonial forms of green 

algae such as the spined Scenedesmus quadricauda became 

proportionally more abundant, although its overall abundance 

declined along with that of the other types of algae.

Conditions were considerably different in 2008, when 

the draining of Wapato Lake (near RM 60) provided a rich 

inoculum of blue-greens and other algae, copepods, and high 

levels of bioavailable nutrients—particularly phosphorus—

that supported a large bloom of Anabaena flos-aquae

downstream. It is interesting that the decline in diatoms, which 

was observed during all three summers, occurred 2–3 weeks 

earlier in 2008 than in 2006−07. The diatom decline may 

have been precipitated by competition from other types of 

phytoplankton (blue-green algae) and/or enhanced grazing 

pressures from copepods and other zooplankton that were 

discharged from the Wapato Lake agricultural area to the 

upper river. 

Wapato Lake Algal Bloom

During July 2008, a large bloom of Anabaena flos-aquae

(primarily) and Aphanizomenon flos-aquae occurred in the 

lower Tualatin River, prompting the Oregon Health Authority 

(formerly the Department of Human Services) to issue a public 

health advisory for recreational water contact on July 12 for 

the reach extending from Jurgens Park (RM 10.8) to the river 

mouth (Bonn, 2008). This bloom formed a thick surface scum 

along the margins and backwater areas for several miles of the 

lower Tualatin River. Anabaena can produce potent nerve and 

liver toxins (anatoxin-a and microcystin), and although tests 

were not conducted for the more toxic anatoxin-a, microcystin 

was detected at concentrations ranging from 0.14 to 2.4 μg/L. 

Although low levels of these potentially toxic blue-green algae 

were observed in the river each year, cell counts in 2006 and 

2007 were much lower than those in 2008.

The July 2008 bloom was quickly traced upstream to 

the Wapato Lake agricultural area located adjacent to and 

southeast of Gaston, Oregon (fig. 2). Because of a breach in 

one of the levees during a high-flow event on December 2–3, 

2007, the low-lying area was flooded to a much greater extent 

than normal and could not be drained until much later than 

normal in spring and summer 2008. Drainage water entering 

the Tualatin River near RM 60 was elevated in ammonia 

and SRP and contained a rich inoculum of algae including 

Anabaena flos-aquae and Aphanizomenon flos‐aquae, several 

types of flagellates, including Trachelomonas volvocina, 

Cryptomonas erosa, Chlamydomonas sp., green algae 

including Scenedesmus, Actinastrum, and Sphaerocystis, 

and many types of diatoms, including planktonic Cyclotella

meneghiniana and C. pseudostelligera. Drainage water 

from Wapato Lake also contained very high abundances 

of zooplankton (appendix C), primarily copepods, which 

continued to thrive in the lower Tualatin River during the 

bloom, and possibly affecting the algal population through 

selective grazing.

Zooplankton

The diverse phytoplankton assemblage in the lower river 

sometimes supports sizable populations of zooplankton—

including protozoans, rotifers, copepods, and cladocerans. 

These organisms feed on a wide variety of foods such as algae, 

bacteria, detritus, and other zooplankton, by selective and 

indiscriminant filter feeding (cladocerans and rotifers), and by 

direct grazing (copepods) (Wetzel, 1983). 

The most abundant zooplankton identified in Tualatin 

River samples included early life stages of copepods (nauplii 

and cyclopoid copepodites), the cladocerans Bosmina 

longirostris and Chydorus sphaericus, and the copepod 

Diacyclops thomasi (table 7) (see photographs 17–19, p. 34). 

These taxa were identified in 70−100 percent of samples. 

The most abundant zooplankton taxa, by far, was Bosmina 

longirostris, which made up nearly 28 percent of the total 

density considering all samples collected during the study and 

was particularly abundant in the lower reaches of the river. 

This taxon reached its highest density in late June 2007, when 

the density at Stafford Road exceeded 60,000 individuals per 

cubic meter (appendix C).

Total zooplankton densities in the Tualatin River were 

much higher in 2008 than in 2006−07 for sites extending 

from Rood Bridge (RM 38.4) to Highway 99W/Jurgens 

Park (RM 11.6−10.8). Water discharged from the Wapato 

Lake agricultural area in 2008 contained high abundances of 

zooplankton and was an important source of zooplankton to 

the river that year. 

Zooplankton populations in the Tualatin River exhibited 

important longitudinal and seasonal patterns during 2006−08. 

Zooplankton densities showed a distinct longitudinal increase, 

particularly downstream of Highway 99W (RM 11.6), about 

20 mi downstream of where the reservoir reach begins 

(fig. 17). A seasonal increase in zooplankton abundance was 

observed in early summer (fig. 18) just as or shortly after 

the phytoplankton population began to increase in the river 

(fig. 19). These patterns, especially for the lower river sites, 

are a typical result consistent with phytoplankton being a 

major food source for zooplankton.
32 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig16

0 

500,000 

3,500,000 3,000,000 2,500,000 2,000,000 1,500,000 1,000,000 

0 

80 60 40 20 

100 

Tualatin River at Highway 99W/Jurgen's Park (river mile 11.5-10.8) 

0 

5,000,000 4,000,000 3,000,000 2,000,000 1,000,000 

0 

80 60 40 20 

100 Tualatin River at Stafford Road (river mile 5.4) 

2008

0 

500,000 

3,500,000 3,000,000 2,500,000 2,000,000 1,500,000 1,000,000 

0 

80 60 40 20 

100 

Tualatin River at Highway 99W/Jurgen's Park (river mile 11.5-10.8) 

0 

5,000,000 4,000,000 3,000,000 2,000,000 1,000,000 

0 

80 60 40 20 

100 Tualatin River at Stafford Road (river mile 5.4) 

2007

0 

500,000 

3,500,000 3,000,000 2,500,000 2,000,000 1,500,000 1,000,000 

0 

80 60 40 20 

100 

Tualatin River at Highway 99W/Jurgen's Park (river mile 11.5-10.8) 

Total algal biovolume, in cubic microns per milliliter

Total algal biovolume, in cubic microns per milliliter

Total algal biovolume, in cubic microns per milliliter

Percent of total algal biovolume 

Percent of total algal biovolume 

Percent of total algal biovolume 

0 

5,000,000 4,000,000 3,000,000 2,000,000 1,000,000 

0 

80 60 40 20 

100 Tualatin River at Stafford Road (river mile 5.4) 

2006

0 

500,000 

2,000,000 1,500,000 1,000,000 

0 

80 60 40 20 

100 

0 

50,000 

200,000 150,000 100,000 

0 

80 60 40 20 

100 

0 

50,000 

200,000 150,000 100,000 

0 

80 60 40 20 

100 Tualatin River near Scholls (river mile 24.5) Tualatin River near Scholls (river mile 24.5) Tualatin River near Scholls (river mile 24.5 ) 

EXPLANATION

CryptophytesBlue-green algaeEuglenoidsDiatoms

ChrysophytesGreen algaeDinoflagellatesTotal algal biovolume 

June July Aug. June July Aug. June July Aug.

Figure 16. Time series of total algal biovolume and percentage of major phytoplankton groups (divisions) at selected sites in the Tualatin River, Oregon, 2006

−08.
Patterns in Plankton Populations 33 Table 7. Dominant zooplankton taxa in the Tualatin River, Oregon, 2006–08. [Includes top 20 taxa by percent occurrence and top 20 taxa by total density for all samples. Abbreviations: RM, river mile; 99W, Highway 99 West; d/s, downstream] Zooplankton taxa Group

Samples 

(percent)

Number of 

occurrences

Average density (number per cubic meter)

Wapato 

pump 

discharge1

Spring Hill 

pump 

station1

Tualatin River sites

2006

2007

2008

Wapato Rood Bridge RM 24.5 99W/ Jurgens Park d/s Cook Park d/s Fanno Creek Stafford Road Oswego Dam

Copepod nauplii Copepoda 100 24 33 44 2 36,076 0 293 554 586 1,897 667 933 1,853

Cyclopoid copepodites Copepoda 97 22 34 42 2 400,148 6,780 87 160 2,259 2,824 1,067 512 687

Bosmina longirostris Cladocera 94 23 32 40 2 3,867 0 18 49 414 2,693 1,771 6,290 4,937

Chydorus sphaericus Cladocera 80 18 28 35 1 704 0 105 4 37 83 58 113 55

Chironomidae sp. Other zooplankters 79 24 24 32 0 0 10 10 15 26 35 21 25Diacyclops thomasi Copepoda 70 12 24 35 2 277,737 6,780 34 22 130 431 331 95 75

Euchlanis dilatata Rotifera 66 15 20 32 0 0 15 13 30 9 20 31 21

Ceriodaphnia dubia Cladocera 65 14 23 29 0 0 6 5 22 256 105 234 1,078Synchaeta sp. Rotifera 60 17 18 26 0 0 5 1 2,844 75 39 509 124

Brachionus angularis Rotifera 53 4 19 31 1 877 0 15 45 64 272 84 43 30

Scapholeberis armata Cladocera 50 14 14 23 0 0 1 2 70 170 39 78 225

Philodina sp. Rotifera 47 6 19 22 0 0 3 2 2 7 10 14 6

Harpacticoid copepods Copepoda 46 7 17 22 0 0 5 2 6 14 12 195 4

Diaptomus reighardi Copepoda 45 5 20 20 1 4,930 0 4 2 21 32 26 212 55

Diaptomus sp. copepoidites Copepoda 45 7 21 17 0 0 3 2 24 31 9 64 39

Microcyclops varicans Copepoda 45 11 18 16 0 0 12 4 2 15 9 4 36Difflugia sp. Other zooplankters 42 8 14 20 2 9,489 1,695 5 6 81 31 12 9 22

Polyarthra vulgaris Rotifera 42 11 10 21 0 0 78 33 11 1 1 20 18Brachionus havanensis Rotifera 38 12 19 7 0 0 5 4 54 1,573 295 60 37

Brachionus quadridentata Rotifera 30 0 15 15 0 0 0 253 1,322 590 179 92 21

1In 2008, two samples were collected from the Wapato pump discharge and one sample from the Spring Hill pump station.
34 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

0 

20,000 

40,000 

60,000 

80,000 

100,000 

120,000 

140,000 

40 30 20 10 0 

River mile 

May 19

June 11

June 17

June 30

July 1

July 9

July 15–16

Aug. 5–6

Aug. 19

2008

EXPLANATION

0 

20,000 

40,000 

60,000 

80,000 

Density, in number per cubic meter

June 27

July 5

July 13

July 19

Aug. 13

Sept. 6

2007

EXPLANATION

0 

3,000 

6,000 

9,000 

12,000 

15,000 

June 26

July 7

July 24

Aug. 7

Aug. 31

2006

EXPLANATION

Figure 17. Longitudinal pattern in total zooplankton density in the Tualatin River, Oregon, 2006−08. Note the 

variable y-axes scales.

Photograph 17: Bosmina longirostris, 

a cladoceran. (Photograph by Kurt 

Carpenter, June 26, 2006.) Photograph 18: Chydorus sphaericus, a 

cladoceran. (Photograph by WIM VAN 

EGMOND/VISUALS UNLIMITED, INC. /SCIENCE 

PHOTO LIBRARY.)

Photograph 19: The copepod Diacyclops 

thomasi (Photograph by Kurt Carpenter, 

August 6, 2008.)
Patterns in Plankton Populations 35

tac12-0780_fig18

0 

500 400 300 200 100 

0 

90 80 70 60 50 40 30 20 10 

100 Tualatin River near Scholls (river mile 24.5) 

0 

5,000 4,000 3,000 2,000 1,000 

0 

90 80 70 60 50 40 30 20 10 

100 

Total zooplankton denstiy, in number per cubic meter Tualatin River at Highway 99W/Jurgen's Park (river mile 11.5-10.8) 

0 

9,000 6,000 3,000 

15,000 12,000 

0 

90 80 70 60 50 40 30 20 10 

100 Tualatin River at Stafford Road (river mile 5.4) 

Tualatin River near Scholls (river mile 24.5) Tualatin River at Highway 99W/Jurgen's Park(river mile 11.5-10.8) Tualatin River at Stafford Road (river mile 5.4) 

Tualatin River near Scholls (river mile 24.5) Tualatin River at Highway 99W/Jurgen's Park(river mile 11.5-10.8) Tualatin River at Stafford Road (river mile 5.4) 

0 

500 

2,000 1,500 1,000 

0 

90 80 70 60 50 40 30 20 10 

100 

0 

6,000 5,000 4,000 3,000 2,000 1,000 

0 

90 80 70 60 50 40 30 20 10 

100 

Percent of total zooplankton density 

Percent of total zooplankton density 

Total zooplankton denstiy, in number per cubic meter

Total zooplankton denstiy, in number per cubic meter

Percent of total zooplankton density 

0 

80,000 60,000 40,000 20,000 

0 

90 80 70 60 50 40 30 20 10 

100 

0 

8,000 6,000 4,000 2,000 

0 

90 80 70 60 50 40 30 20 10 

100 

0 

50,000 40,000 30,000 20,000 10,000 

0 

90 80 70 60 50 40 30 20 10 

100 

0 

9,000 6,000 3,000 

15,000 12,000 

0 

90 80 70 60 50 40 30 20 10 

100 

CladoceransCopepodsRotifersTotal zooplankton 

EXPLANATION

June July Aug. Sept. June July Aug. Sept. June July Aug.

2006 2007 2008

Figure 18. Seasonal pattern in zooplankton abundance and percentage of major groups at select sites in the Tualatin River, Oregon, 2006

−08. Note the variable y-axes 

scales.
36 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig19

0 

5,000 

30,000 25,000 20,000 15,000 10,000 

0 

500,000 

2,500,000 2,000,000 1,500,000 1,000,000 

0 

5,000 

30,000 25,000 20,000 15,000 10,000 

0 

500,000 

2,500,000 2,000,000 1,500,000 1,000,000 

0 

5,000 

30,000 25,000 20,000 15,000 10,000 

0 

500,000 

2,500,000 2,000,000 1,500,000 1,000,000 

0 

5,000 

20,000 15,000 10,000 

0 

5,000,000 4,000,000 3,000,000 2,000,000 1,000,000 

0 

5,000 

20,000 15,000 10,000 

0 

5,000,000 4,000,000 3,000,000 2,000,000 1,000,000 

Total zooplankton density, in number per cubic meter

Total zooplankton density, in number per cubic meter

Total algal biovolume, in cubic microns per milliliter

Total algal biovolume, in cubic microns per milliliter

Total zooplankton density, in number per cubic meter

Total algal biovolume, in cubic microns per milliliter

0 

5,000 

20,000 15,000 10,000 

0 

5,000,000 4,000,000 3,000,000 2,000,000 1,000,000 

0 

80,000 70,000 60,000 50,000 40,000 30,000 20,000 10,000 

0 

8,000,000 6,000,000 4,000,000 2,000,000 

12,000,000 10,000,000 

0 

80,000 70,000 60,000 50,000 40,000 30,000 20,000 10,000 

0 

8,000,000 6,000,000 4,000,000 2,000,000 

12,000,000 10,000,000 

0 

80,000 70,000 60,000 50,000 40,000 30,000 20,000 10,000 

0 

8,000,000 6,000,000 4,000,000 2,000,000 

12,000,000 10,000,000 

Tualatin River downstream of Fanno Creek/Durham wastewater treatment facility (river mile 9.2) Tualatin River at Stafford Road (river mile 5.4) Tualatin River at Oswego Dam (river mile 3.4) 

Tualatin River downstream of Fanno Creek/Durham wastewater treatment facility (river mile 9.2) Tualatin River at Stafford Road (river mile 5.4) Tualatin River at Oswego Dam (river mile 3.4) 

Tualatin River downstream of Fanno Creek/Durham wastewater treatment facility (river mile 9.2) 

Tualatin River at Stafford Road (river mile 5.4) 

Tualatin River at Oswego Dam (river mile 3.4) 

2006 2007 2008

EXPLANATION

Total algal biovolumeTotal zooplankton density 

July June Aug. Sept. June Sept. July Aug. June May July Aug.

Figure 19. Time series of algal biovolume and zooplankton density at select sites in the Tualatin River, Oregon, 2006

−08. Note the variable y-axes scales.
Multivariate Analyses of Phytoplankton Assemblages and Environmental Data 37

The seasonal successional pattern in zooplankton 

indicates that cladocerans tended to follow the major peak in 

the total zooplankton density, decreasing in abundance after 

the decline in algal populations. After that point, when the 

overall density of zooplankton decreased, copepods became 

the dominant zooplankton in the river.

Multivariate Analyses of Phytoplankton 

Assemblages and Environmental Data

A combination of multivariate data-analysis techniques 

were performed to discern patterns in the phytoplankton 

species data, to test the relative importance of environmental 

factors in explaining variations or patterns in the 

phytoplankton species composition, and to identify which 

species were most important in explaining differences between 

selected groups of samples.

Nonmetric Multidimensional Scaling 

(NMDS) Ordinations

The 2006−08 plankton dataset consisted of 

143 phytoplankton and 99 zooplankton taxa. Considering 

only those taxa occurring in more than 3 samples resulted 

in a total of 78 phytoplankton taxa. NMDS ordinations of 

the phytoplankton assemblage data were used to graphically 

represent the patterns among samples; this identified four 

subsets of samples for further analysis (described below). By 

first determining the patterns in the data and how samples may 

be related, a better understanding of the factors that influence 

the composition, abundance, and dynamics of phytoplankton 

communities in the Tualatin River can be obtained.

The ordination analysis first suggested a longitudinal 

separation of sites, with samples from the two uppermost 

Tualatin River sites (Rood Bridge at RM 38.4 and RM 24.5 

near Scholls) separating from the downstream sites in the 

reservoir reach (fig. 20A). Algal abundance (Chl-a and total 

biovolume) at these sites tended to be lower than sites farther 

downstream, with relatively greater abundances of benthic 

diatoms including Gomphonema angustatum, Achnanthes 

lanceolata, and Synedra ulna (table 6). Water temperatures 

also were almost always lower at these upstream sites 

compared with the downstream sites.

Phytoplankton samples collected in late June and early 

July 2008 during the Anabaena bloom associated with the 

draining of Wapato Lake formed another clustered group of 

samples in the NMDS ordination (fig. 20B). This separation 

was determined largely by a greater abundance of blue-green 

algae (Anabaena sp. and Aphanizomenon flos-aquae), green 

algae (Scenedesmus quadricauda, Scenedesmus acuminatus, 

Chlamydomonas sp., and Sphaerocystis schroeteri), and the 

high-phosphorus-indicating diatom Cyclotella meneghiniana

in the Wapato-affected samples. Algal taxa with higher 

abundances in the non-Wapato-affected sample group 

included two eutrophic centric diatoms, Stephanodiscus 

binderanus and Cyclotella pseudostelligera. Algal taxa 

having relatively high abundance in both groups included 

Cryptomonas erosa, Rhodomonas minuta, and Stephanodiscus 

hantzschii. These three taxa were nearly ubiquitous in the 

Tualatin River, occurring in 97−100 percent of samples at 

varying abundances. 

After removing the more upstream and Wapato-affected 

samples, the remaining lowermost-basin samples separated 

along the first ordination axis (fig. 20C), which reflects the 

seasonal change in the phytoplankton species composition 

from May–July to August–September. Filamentous centric 

diatoms Stephanodiscus binderanus and S. hantzschii were 

more abundant in May–July, whereas small flagellates 

(Cryptomonas erosa and Chlamydomonas sp.) and a number 

of green algal taxa were relatively more abundant in August 

and September (table 8).

Separation of these late summer samples into distinct 

groups based on algal community data coincided with 

several changes in river conditions from July to August. 

Phytoplankton populations tended to decline sharply during 

this time interval, and zooplankton densities, which were 

relatively high in July, plummeted in early August (fig. 19), 

resulting in a “clear water phase” for a period lasting 

3–5 weeks. This also is the time period when natural flows 

reached their minimum for the season, and flow augmentation 

and WWTF effluent were at their highest percentage (fig. 12).

Spearman Rank Correlations, PCA, and 

BEST Analyses

Spearman rank correlations and Principal Components 

Analyses (PCA) were used to identify sources of variation 

in the datasets to better understand the underlying data 

structure. The Spearman rank correlation matrix for the full 

suite of environmental variables listed in table 9 and for 

groups of algae and zooplankton (for non-Wapato-affected 

lower Tualatin River samples over the 3-year study period) 

is given in table 10. Although many of the correlations in 

these tables are significant, care should be exercised before 

drawing firm conclusions from these tables because such 

correlations do not necessarily indicate a cause-and-effect 

relation between variables. Nevertheless, many of the 

variables, including biovolumes of individual algal divisions 

and various components of flow, had significant correlations 

with important water-quality variables, including total flow 

augmentation (total reservoir releases minus withdrawals) and 

DO concentrations (table 10).
38 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig20

-1.5 

-1 

-0.5 

0 

0.5 

1 

1.5 

A

-1.5 

-1 

-0.5 

0 

0.5 

1 

1.5 

-1.5 

-1 

-0.5 

0 

0.5 

1 

1.5 

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 

B

C

Multidimensional scaling ordination axis 2

Multidimensional scaling ordination axis 1

A

 Upper river (river mile 38.4–24.5)

 Lower river (river mile 11.5–3.4)

B

 Non-Wapato event samples

 Wapato event samples

C

 May-July samples

 August-September samples 

EXPLANATION

Figure 20. Phytoplankton samples from the Tualatin River, Oregon, 2006–08, 

highlighting (A) the two upstream sites, (B) the group of samples collected 

during and after the Anabaena flos-aquae bloom associated with the draining 

of Wapato Lake, and (C) downstream sites with Wapato-affected samples 

removed, showing separation of samples collected in May–July versus 

August–September.
Multivariate Analyses of Phytoplankton Assemblages and Environmental Data 39

Table 8. Average phytoplankton percent biovolume for

non-Wapato affected lower river samples for May–July and 

August–September 2006–08.

[Results from Primer-E SIMPER analysis showing species that were 

important for separating the May–July samples from August–September 

samples]

Algal taxa Algal 

division

May–July

(percent)

August–

September

(percent)

Higher in May–July

Stephanodiscus binderanus Diatom 25 7

Stephanodiscus hantzschii Diatom 10 3

Higher in August–September

Cryptomonas erosa Cryptophyte 35 44

Less than 5 percent change

Cyclotella pseudostelligera Diatom 6 5

Aphanizomenon flos-aquae Bluegreen 1 0

Melosira ambigua Diatom 1 0

Nitzschia acicularis Diatom 1 0

Chlamydomonas sp. Green 6 8

Cyclotella meneghiniana Diatom 1 4

Actinastrum hantzschii Green 0 3

Sphaerocystis schroeteri Green 0 2

Ankistrodesmus falcatus Green 0 1

Crucigenia quadrata Green 0 1

Melosira distans alpigena Diatom 1 2

Melosira granulata Diatom 1 2

Scenedesmus quadricauda Green 1 2

Synedra ulna Diatom 0 1

Tetrastrum staurogeniaforme Green 0 1

Rhodomonas minuta Cryptophyte 2 3

BEST analyses were performed to determine which 

combination of environmental variables listed in table 9 could 

explain the greatest amount of variation in the phytoplankton 

assemblage structure. Nine significant (P<0.001) BEST 

solutions were produced: three for the entire 3-year study 

period and six for the individual years (table 11). 

Often, the environmental variables that were most highly 

correlated to algal assemblages were variables that were 

dependent on algal conditions (biomass and metabolism), 

including Chl-a, algal biovolume, DO, DO percent saturation, 

and pH. Because the objective of the BEST analysis is 

to understand the independent factors that shape algal 

assemblages, these algal-dependent biomass and metabolism 

parameters were removed from the pool of variables for the 

analysis. A similar dependency exists between algae and 

dissolved nutrients, particularly ammonia and SRP. Although 

previous studies have identified the importance of phosphorus 

in regulating phytoplankton populations in the Tualatin 

River, and enriched nutrient concentrations help to explain 

the prevalence of eutrophic taxa in the river (table 6), uptake 

of nutrients by algae is great enough that the lowest nutrient 

concentrations during summer occur during the largest algal 

blooms. Therefore, in the cases where SRP was included in 

a BEST solution, additional BEST analyses were performed 

without SRP to determine whether other variables emerged as 

also being significant (table 11).

The individual variables selected for each BEST analysis 

were determined by the rho value for each variable, and 

combinations of these variables were selected to maximize the 

overall rho score. Taken together, the BEST solutions indicate 

that many of the flow variables—total flow and individual 

source of flow, including natural flow, flow augmentation, 

and WWTF effluent—had the most significant influence over 

algal assemblages in the Tualatin River. Other variables that 

were sometimes important included chloride or conductance, 

rotifer and copepod abundance, nitrite-plus-nitrate, and water 

temperature; and several of those variables were highly 

correlated with some of the flow variables such as percentage 

WWTF effluent. 

Separating the datasets by year, BEST analyses identified 

many of the same variables as being important in shaping 

phytoplankton assemblages as those included in the BEST 

solutions for the all-year analysis. In 2006, total flow, nitriteplus-nitrate, rotifer abundance, percent WWTF effluent (or 

chloride), and water temperature produced the best solution, 

whereas in 2007, natural flow and flow augmentation, both as 

a percentage of total flow, were identified as most important, 

along with SRP, SC, and rotifer abundance. In 2008, total and 

percentage of natural flow were most important, along with 

SRP and copepod abundance (table 11). 

The similarity percentages (SIMPER) procedure in 

PRIMER (Clarke and Gorley, 2006) was used to identify 

the dominant algae during four different algal conditions, 

including early season periods (Chl-a < 10 μg/L); growth 

phase (Chl-a 10–20 μg/L); bloom periods (Chl-a > 20 μg/L); 

and late season (Chl-a < 10 μg/L) (table 12). Stephanodiscus 

binderanus and S. hantzschii were dominant during the early 

season, whereas Cryptomonas erosa, S. binderanus, and 

Chlamydomonas sp. were dominant during the algal “growth 

phase”, when populations were increasing. Two of these taxa, 

Cryptomonas erosa and S. binderanus, and Anabaena flosaquae were dominant during bloom periods, and Cryptomonas

erosa was dominant during the late season, when Chl-a

concentrations were less than 10 μg/L.

The pairwise analysis of similarity (ANOSIM) tests 

showed that the largest difference (those with the highest 

rho values) between one sampling period to the next 

occurred between the last sample collected in July and the 

first sample collected each August (table 13), affirming the 

previous observation of altered algal communities at that 

time. Recall that the SIMPER analysis of May–July versus 

August–September samples identified several phytoplankton 

taxa that were more abundant in each time period (table 8), 

and the NMDS ordinations also showed a clear distinction 

in the grouping of phytoplankton samples between these two 

time periods (fig. 20C).
40 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Table 9. Summary statistics for environmental variables included in the multivariate analysis of phytoplankton assemblages in the 

Tualatin River, Oregon.

[Data for 105 main-stem Tualatin River samples collected during 2006–08. Abbreviations: WWTF, wastewater treatment facility; μg/L, micrograms per liter; 

mg/L, milligrams per liter; NTU, nephelometric turbidity units; μS/cm, microsiemens per centimeter; °C, degrees Celsius; %, percent; cfs, cubic feet per second 

[ft3/s]; PAR, photosynthetically active radiation; (μE/m2)/s, microeinsteins per square meter per second]

Variable Definition Minimum Median Maximum Units

Chl-a Chlorophyll-a 1.2 9.2 134 μg/L

Pheo-a Pheophytin-a 0.1 2.7 25 μg/L

Pheo-a: Chl-a Pheophytin-a : chlorophyll-a ratio 0.0 0.4 3.5 ratio

DIN Dissolved inorganic nitrogen 

(nitrite+nitrate+ammonia)

0.4 3.1 4.8 mg/L

NH3 Dissolved ammonia 0.01 0.027 0.567 mg/L

NO2+NO3 Nitrite+nitrate 0.3 3.1 4.8 mg/L

TKN Total Kjeldahl nitrogen 0.2 0.6 2 mg/L

TN Total nitrogen (TKN+nitrite+nitrate) 0.6 3.8 6 mg/L

SRP Soluble reactive phosphorus 0.006 0.037 0.082 mg/L

TP Total phosphorus 0.1 0.1 0 mg/L

DIN:SRP Ratio of DIN to SRP 5.7 83 574 ratio

SRP:TP Ratio of SRP to TP 0.1 0.4 0.7 ratio

TSS Total suspended sediment 2.4 6.4 14 mg/L

Turbidity Turbidity 0.3 6.0 14 NTU

Temp Water temperature 12.2 21.0 25 °C

SC Specific conductance 92 254 335 μS/cm

DO Dissolved oxygen 4.3 7.9 13.7 mg/L

DO% Dissolved oxygen percent saturation 49 84 163 %

pH pH 6.3 7.1 8.9 standard units

Cl Chloride 3.7 17.1 24 mg/L

Total_Flow_cfs Total streamflow 0 221 587 cfs

Natural_Flow_cfs Total flow minus reservoir and WWTF releases 0 98 533 cfs

Flow_augmentation_cfs Total flow from Barney Reservoir and Hagg Lake 

minus withdrawals1

0 69 128 cfs

WWTF_cfs Total flow from WWTF 0 48 78 cfs

%Flow_augmentation Percent of flow from Barney Reservoir and Hagg 

Lake minus withdrawals1

0 34 66 percent

%Natural_Flow Natural flow as percentage of total flow 0 43 100 percent

%WWTF WWTF discharges as percentage of total flow 0 24 38 percent

PARmax Maximum daily solar radiation at the Durham WWTF 1,170 1,680 1,850 (μE/m2)/s

1

 Includes withdrawals from the Hillsboro-Cherry Grove, Patton, Wapato, Tualatin Valley Irrigation District Spring Hill, and Joint Water Commission Spring 

Hill intakes.
Multivariate Analyses of Phytoplankton Assemblages and Environmental Data 41 Table 10. Spearman rank correlation matrix for environmental variables and select groups of algae and zooplankton for the lower Tualatin River with Wapato-affected samples removed. [See table 9 for variable definitions. Rho values in red are significant at the P<0.001 level; Rho values in blue are significant at the P<0.01 level; Rho values in bold are significant at the P<0.05 level; Includes 65 samples collected during the 2006–08 growing season from river mile 11.5 to 3.4. Abbreviation: <, less than] Variable

Total algal biovolume

a Chla Pheoa :Chl- a PheoDIN

NH3

NO2+NO3

TKN

TN

SRP

TP

DIN:SRP

SRP:TP

TSS

Turbidity

Temp

SC

DO

DO%

pH

Cl

Total_Flow_cfs

Natural_Flow_cfs

Flow_augmentation_

cfs

WWTF_cfs

%Flow_augmentation

%Natural_Flow

%WWTF

PARmax

Total zooplankton 

density

Cladocerans

Copepods

Rotifers

Blue-green algae

Chrysophyte algae

Cryptophyte algae

Diatoms

Centric diatoms

Filamentous diatoms

Dinoflagellates

Euglenoids

Green algae

Chl-a 0.70Pheo-a 0.37 0.33

Pheo-a:Chl-a -0.12 -0.36 0.76

DIN -0.18 -0.11 0.02 0.10NH3 -0.51 -0.65 -0.04 0.41 0.05

NO2+NO3 -0.16 -0.09 0.03 0.09 1.00 0.02

TKN 0.17 0.34 0.36 0.12 0.32 0.11 0.31

TN -0.11 -0.02 0.10 0.11 0.98 0.03 0.98 0.48

SRP -0.66 -0.81 -0.41 0.15 0.07 0.45 0.06 -0.37 -0.02

TP -0.00 -0.02 0.17 0.18 0.13 0.14 0.13 0.34 0.19 0.22

DIN:SRP 0.56 0.72 0.40 -0.10 0.32 -0.41 0.33 0.47 0.40 -0.92 -0.16

SRP:TP -0.69 -0.84 -0.47 0.12 0.05 0.44 0.04 -0.47 -0.06 0.96 -0.04 -0.89

TSS 0.18 0.29 0.14 -0.07 0.05 -0.14 0.05 0.25 0.10 -0.18 0.35 0.19 -0.28

Turbidity 0.19 0.09 0.05 -0.02 0.29 -0.11 0.30 0.22 0.32 0.03 0.34 0.08 -0.07 0.47

Temp 0.29 0.32 0.29 0.06 0.29 -0.38 0.30 0.21 0.32 -0.33 -0.01 0.42 -0.32 -0.08 0.08

SC -0.18 -0.08 0.17 0.22 0.76 -0.07 0.77 0.32 0.77 0.02 0.03 0.27 0.03 0.00 0.11 0.53

DO 0.64 0.54 0.18 -0.20 -0.28 -0.51 -0.26 0.13 -0.21 -0.56 0.02 0.42 -0.58 0.20 0.02 0.08 -0.21

DO% 0.67 0.59 0.22 -0.18 -0.23 -0.56 -0.21 0.16 -0.16 -0.60 0.02 0.48 -0.63 0.18 0.03 0.23 -0.13 0.99

pH -0.07 0.11 -0.14 -0.22 -0.27 0.04 -0.27 0.01 -0.25 -0.20 -0.16 0.08 -0.16 0.06 -0.28 -0.37 -0.25 0.18 0.12

Cl 0.02 0.10 0.18 0.10 0.77 -0.21 0.78 0.28 0.78 -0.08 -0.02 0.37 -0.07 0.03 0.10 0.48 0.87 -0.04 0.03 -0.28

Total_Flow_cfs -0.04 -0.26 -0.27 -0.09 -0.50 0.19 -0.50 -0.15 -0.50 0.18 -0.01 -0.36 0.18 -0.16 -0.36 -0.53 -0.43 0.18 0.09 0.17 -0.45

Natural_Flow_cfs 0.22 0.04 -0.29 -0.32 -0.52 0.06 -0.52 -0.15 -0.52 -0.08 -0.11 -0.13 -0.07 0.04 -0.30 -0.47 -0.49 0.30 0.22 0.43 -0.44 0.76

Flow_augmentation_cfs -0.23 -0.02 0.17 0.18 0.50 -0.08 0.50 0.22 0.51 0.00 0.08 0.20 -0.02 -0.21 0.01 0.49 0.55 -0.32 -0.23 -0.28 0.52 -0.45 -0.61

WWTF_cfs 0.04 -0.02 0.19 0.20 0.23 0.01 0.23 0.23 0.25 -0.03 -0.04 0.12 -0.02 0.05 -0.03 0.08 0.49 0.13 0.13 -0.19 0.53 0.24 -0.01 0.04

%Flow_augmentation -0.31 -0.11 0.23 0.31 0.61 0.01 0.61 0.24 0.61 0.09 0.12 0.15 0.08 -0.13 0.22 0.52 0.60 -0.40 -0.31 -0.36 0.51 -0.67 -0.85 0.87 -0.05

%Natural_Flow 0.31 0.14 -0.28 -0.37 -0.52 -0.01 -0.52 -0.17 -0.52 -0.15 -0.12 -0.06 -0.14 0.12 -0.25 -0.43 -0.51 0.34 0.26 0.45 -0.43 0.63 0.98 -0.65 -0.10 -0.87

%WWTF 0.05 0.19 0.39 0.25 0.57 -0.15 0.58 0.29 0.59 -0.19 -0.03 0.40 -0.17 0.14 0.22 0.49 0.73 -0.04 0.04 -0.30 0.78 -0.61 -0.65 0.41 0.62 0.50 -0.61

PARmax 0.45 0.36 0.13 -0.12 0.02 -0.52 0.05 -0.05 0.04 -0.24 -0.02 0.24 -0.27 0.07 0.13 0.30 0.00 0.36 0.40 -0.17 0.23 -0.16 -0.11 0.05 0.04 0.01 -0.07 0.17

Total zooplankton density 0.35 0.39 0.32 0.05 0.15 -0.22 0.16 0.33 0.21 -0.38 0.13 0.42 -0.45 -0.04 0.07 0.41 0.19 0.03 0.09 -0.15 0.26 -0.18 -0.08 0.45 0.00 0.28 -0.07 0.15 0.21

Cladocerans 0.38 0.35 0.38 0.13 0.06 -0.20 0.07 0.29 0.11 -0.33 0.09 0.34 -0.39 -0.03 0.07 0.41 0.15 0.01 0.08 -0.24 0.23 -0.07 -0.05 0.39 0.14 0.22 -0.05 0.16 0.15 0.93

Copepods 0.22 0.35 0.22 -0.02 0.21 -0.14 0.21 0.41 0.27 -0.35 0.15 0.42 -0.42 -0.03 0.03 0.31 0.21 -0.01 0.05 -0.10 0.26 -0.19 -0.10 0.47 -0.01 0.32 -0.09 0.15 0.15 0.93 0.79

Rotifers 0.56 0.60 0.29 -0.13 0.13 -0.46 0.15 0.19 0.18 -0.58 0.02 0.60 -0.62 -0.03 0.09 0.43 0.07 0.25 0.31 -0.02 0.18 -0.23 -0.02 0.26 -0.15 0.13 0.04 0.07 0.43 0.81 0.68 0.72

Blue-green algae 0.21 0.07 -0.08 -0.12 -0.16 -0.13 -0.16 -0.10 -0.17 0.02 -0.07 -0.08 0.03 0.13 -0.07 -0.26 -0.16 0.15 0.11 0.03 -0.10 0.37 0.38 -0.39 0.27 -0.47 0.37 -0.11 0.04 -0.21 -0.09 -0.29 -0.18

Chrysophyte algae -0.26 -0.26 -0.02 0.16 0.29 0.26 0.28 0.05 0.26 0.21 -0.03 -0.09 0.22 -0.20 -0.17 0.21 0.43 -0.27 -0.24 -0.20 0.35 -0.02 -0.12 0.17 0.26 0.19 -0.16 0.25 -0.07 0.08 0.07 0.08 -0.02 -0.18

Cryptophyte algae 0.75 0.52 0.33 -0.03 0.12 -0.48 0.14 0.19 0.17 -0.39 0.06 0.41 -0.44 0.14 0.26 0.52 0.19 0.33 0.40 -0.28 0.35 -0.33 -0.12 0.02 0.16 0.05 -0.03 0.38 0.41 0.40 0.40 0.29 0.48 0.12 -0.07

Diatoms 0.82 0.60 0.19 -0.23 -0.29 -0.41 -0.28 0.01 -0.25 -0.60 -0.00 0.46 -0.62 0.26 0.07 0.07 -0.32 0.64 0.64 0.13 -0.15 0.15 0.43 -0.39 -0.06 -0.51 0.51 -0.18 0.40 0.16 0.18 0.03 0.42 0.27 -0.35 0.36

Centric diatoms 0.80 0.61 0.18 -0.25 -0.29 -0.44 -0.27 0.01 -0.25 -0.60 -0.02 0.46 -0.62 0.23 0.06 0.05 -0.32 0.67 0.66 0.16 -0.13 0.16 0.44 -0.39 -0.07 -0.51 0.51 -0.20 0.42 0.17 0.16 0.05 0.46 0.26 -0.36 0.36 0.97Filamentous diatoms 0.74 0.54 0.14 -0.23 -0.35 -0.41 -0.33 -0.04 -0.32 -0.57 -0.08 0.41 -0.56 0.15 0.01 0.08 -0.32 0.62 0.62 0.14 -0.16 0.22 0.43 -0.36 -0.05 -0.48 0.48 -0.23 0.35 0.14 0.19 0.00 0.40 0.24 -0.35 0.30 0.93 0.96

Dinoflagellates 0.25 0.18 0.29 0.16 -0.06 -0.19 -0.06 0.20 -0.02 -0.16 -0.07 0.13 -0.16 -0.19 0.10 0.32 0.07 0.06 0.11 -0.24 0.05 -0.12 -0.20 0.21 -0.04 0.25 -0.21 0.07 0.10 0.30 0.32 0.20 0.30 -0.27 0.06 0.34 -0.01 0.01 0.05

Euglenoids -0.02 0.06 -0.03 -0.07 -0.08 0.01 -0.08 0.01 -0.08 -0.06 -0.04 0.02 -0.06 -0.06 -0.03 0.15 -0.06 -0.05 -0.02 0.13 -0.17 -0.15 -0.06 0.08 -0.29 0.13 -0.04 -0.12 -0.21 0.01 -0.02 -0.01 0.02 -0.06 -0.06 -0.03 -0.01 -0.02 0.01 0.07

Green algae 0.70 0.55 0.37 -0.01 -0.06 -0.29 -0.06 0.38 0.03 -0.46 0.09 0.41 -0.53 0.10 0.36 0.25 -0.11 0.43 0.46 -0.18 -0.02 -0.18 -0.03 -0.11 -0.05 -0.06 0.03 0.09 0.17 0.32 0.30 0.28 0.43 0.07 -0.22 0.60 0.41 0.40 0.33 0.22 0.08

Unidentified algae 0.19 0.00 0.27 0.27 0.01 0.01 0.01 0.04 0.03 0.00 0.12 0.00 -0.05 -0.03 0.01 0.06 0.02 -0.03 -0.02 -0.14 0.11 -0.01 0.02 0.18 0.10 0.04 0.02 0.08 0.20 0.18 0.24 0.10 0.12 0.14 -0.11 0.17 0.19 0.15 0.16 0.03 0.10 -0.01

An oversized version (11 × 17) of this table is available for download at http://pubs.usgs.gov/sir/2013/5037.
42 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Table 11. Summary of BEST analyses listing the top environmental variables explaining patterns in the phytoplankton species 

composition in the Tualatin River, Oregon, 2006–08.

[See table 9 for varible definitions. Abbreviations: BEST, Bio-Env STepwise multivariate analysis; SRP, soluble reactive phosphorus; %, percent; cfs, cubic feet 

per second (ft3/s)]

Time period Sites BEST variables

Individual 

variable 

correlation

BEST solution sample statistics

Overall 

Rho

Significance 

level

Number of 

samples

Multi-year

2006–08 May–September Lower SRP 0.242 0.368 P<0.046 65

with SRP Non-Wapato %Flow_augmentation 0.215

Cl 0.169

2006–08 May–September Lower %Flow_augmentation 0.215 0.27 P<0.001 65

without SRP Non-Wapato Natural_Flow_cfs 0.214

Rotifers 0.172

2006–08 May–September Lower %Natural_Flow 0.248 0.248 P<0.001 65

without SRP Non-Wapato

2006–08 May–September All non-Wapato %WWTF 0.376 0.494 P<0.001 71

Temp 0.318

pH 0.191

TSS 0.145

Cladocerans and Copepods 0.105

Individual years

2006 May–August Lower Total_Flow_cfs 0.423 0.574 P<0.001 20

NO2+NO3 0.371

Rotifers 0.325

%WWTF 0.218

Temp 0.179

2006 May–August Lower Total_Flow_cfs 0.423 0.561 P<0.001 20

without %WWTF NO2+NO3 0.371

Rotifers 0.325

Cl 0.226

Temp 0.179

2007 June–September Lower %Natural_Flow 0.402 0.539 P<0.001 29

%Flow_augmentation 0.368

SRP 0.289

SC 0.264

2007 June–September Lower %Natural_Flow 0.402 0.462 P<0.001 29

without SRP %Flow_augmentation 0.368

SC 0.264

Rotifers 0.226

Total_Flow_cfs 0.188

2008 May–August Lower %Natural_Flow 0.546 0.698 P<0.001 29

Natural_Flow_cfs 0.544

SRP 0.425

2008 May–August Lower Natural_Flow_cfs 0.544 0.604 P<0.001 29

without SRP %Natural_Flow 0.546

Copepods 0.354
Multivariate Analyses of Phytoplankton Assemblages and Environmental Data 43

Table 12. Average percent biovolumes of select algal taxa grouped according to chlorophyll-a and seasonal growth phase 

in the Tualatin River, Oregon.

[Chlorophyll-a (Chl-a) concentrations in micrograms per liter. Abbreviations: <, less than; >, greater than; sp., species]

Algal taxa Algal

division

Algal biomass condition, in relative biovolume (percent) Average change in 

relative biovolume 

from bloom to late 

season (percent)

Chl-a <10 

(Early eason)

Chl-a 10–20 

(Growth phase)

Chl-a >20 

(Bloom)

Chl-a <10 

(Late season)

Anabaena flos-aquae Bluegreen 6 2 26 0 -26

Aphanizomenon flos-aquae Bluegreen 1 1 6 0 -6

Stephanodiscus binderanus Diatom 28 10 14 10 -4

Melosira ambigua Diatom 0 0 2 0 -2

Rhopalodia gibba Diatom 0 0 1 0 -1

Sphaerocystis schroeteri Green 0 2 1 0 -1

Actinastrum hantzschii Green 0 2 1 0 -1

Cryptomonas erosa Cryptophyte 9 36 29 37 8

Cyclotella pseudostelligera Diatom 4 5 1 5 4

Stephanodiscus hantzschii Diatom 21 8 3 5 2

Fragilaria crotonensis Diatom 0 0 0 2 2

Melosira distans alpigena Diatom 1 1 0 2 2

Rhodomonas minuta Cryptophyte 0 2 1 3 2

Cyclotella meneghiniana Diatom 1 3 1 3 2

Chlamydomonas sp. Green 1 8 4 6 2

Glenodinium sp. Dinoflagellate 0 1 1 2 1

Melosira varians Diatom 1 0 0 1 1

Gomphonema angustatum Diatom 1 0 0 1 1

Achnanthes lanceolata Diatom 2 0 0 1 1

Eudorina elegans Green 2 0 0 1 1

Melosira granulata Diatom 0 2 1 2 1

Synedra ulna Diatom 4 1 0 1 1

Ankistrodesmus falcatus Green 0 1 0 1 1

Scenedesmus quadricauda Green 2 3 1 2 1

Table 13. Pairwise analysis of similarity (ANOSIM) for phytoplankton samples collected 

each year by sequential sampling date, lower Tualatin River (river mile 11.5–3.4), Oregon.

[Abbreviations: Global R, Global Rho value; R, Rho value; P, probability]

Year

Pairwise significance Global significance

Global R Level Pairwise tests R statistic Level

2006 0.851 P<0.001 06-26-06, 07-07-06 0.96 P<0.05

07-07-06, 07-24-06 0.98 P<0.05

07-24-06, 08-07-06 1.00 P<0.05

08-07-06, 08-31-06 0.20 P<0.01

2007 0.736 P<0.001 06-27-07, 07-05-07 0.21 P<0.2

07-05-07, 07-13-07 0.86 P<0.01

07-13-07, 07-19-07 0.82 P<0.01

07-19-07, 08-13-07 0.86 P<0.01

08-13-07, 09-06-07 0.61 P<0.05

2008 0.737 P<0.001 06-17-08, 07-01-08 0.32 P<0.05

07-01-08, 07-09-08 0.62 P<0.05

07-09-08, 08-06-08 0.89 P<0.01

08-06-08, 08-19-08 0.89 P<0.05
44 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Bioassay Experiment Results

The bioassay experiments were designed to assess the 

potential effects of phosphorus concentrations and treated 

effluent from the Rock Creek WWTF on the plankton 

assemblages. Experimental results were evaluated by 

measuring changes in Chl-a (growth) and dissolved oxygen 

(photosynthetic activity) over the course of each incubation 

period. Although many experiments were conducted, and 

under a variety of river conditions, results presented here 

focus on the two earliest experiments conducted May 19 

and June 11, 2008, before the Wapato event caused unusual 

plankton and water chemistry conditions, and on the June 30 

experiment that was conducted at the onset of the Wapato 

event but under conditions where blue-green algae were not 

identified at the Rood Bridge site.

Phytoplankton populations in these experiments 

were dominated by diatoms (see fig. 21 for the specific 

dominants). Although some algal activity was noted during 

the first bioassay in May, the DO production was small and 

gains were just 2–6 percent of saturation in all light bottles 

owing to the low algal abundance. Gains in Chl-a and DO 

were smaller in the 50 percent wastewater sample, but the 

difference was not large. Similarly, addition of SRP had no 

clear or repeatable effect on either Chl-a or DO production 

in the May experiments. On June 11, the 30 percent effluent 

sample produced a 12 percent increase in Chl-a compared 

with just 2.9 percent for the 0 percent effluent sample; Chl-a

concentrations in the 50 percent sample declined 15 percent. 

On June 30, Chl-a gains of 44–58 percent were observed over 

the course of the experiment, with the smallest gains occurring 

in the 50 percent effluent treatment (fig. 21). Any conclusions 

from these initial bioassays should be tempered by the limited 

range of conditions tested and the lack of sufficient replicate 

samples. Regardless, these results suggest that, at least under 

certain conditions, effluent concentrations of 30 percent may 

stimulate algal growth, whereas 50 percent may either inhibit 

(June 11) or stimulate growth to a lesser degree (June 30). The 

subject bears further study because of the large fraction of 

treated effluent present in the Tualatin River during summer, 

but these bioassays did not show a repeatable positive or 

negative effect of treated effluent or added phosphorus on 

algal growth or photosynthetic activity.

Declines in Phytoplankton Populations

A notable decline in phytoplankton abundance in the 

Tualatin River in late July and August began in 2002, after 

the 2001 drought year, and continued from 2003 to 2009, 

resulting in lower DO concentrations and more frequent 

occurrence of DO standard violations (fig. 8). Although 

Chl-a levels upstream of the reservoir reach of the Tualatin 

River are generally low (less than 6 μg/L), the size of the 

population entering that reach appears to be a key determinant 

of population levels downstream. As lower concentrations of 

algae come into the reservoir reach, the blooms do not reach 

abundances as high as in years past.

The cause of this decline in Chl-a is unknown, but 

several factors could contribute, as suggested by the BEST 

analyses. Increased reservoir releases in July for flow 

augmentation, for example, coincide with these trends in 

decreased upstream algae populations (fig. 22). Another factor 

that will require more study, and which might be correlated 

with increased reservoir releases, is the possibility of slight 

increases in minumum turbidity levels (figs. 23, 24, and 25). 

Increased turbidity during the summer of 1996 was known to 

be an important factor holding down algal populations that 

year (fig. 25), suggesting that algal production in the Tualatin 

River is susceptible to light limitation. Water-quality modeling 

supports that conclusion (Rounds and others, 1999). No 

observable trend in water temperature was found to explain 

lower algal abundances in recent years, based on analysis of 

continuous data from RM 24.5 for the month of August going 

back to 1997.

A reduction in the upstream algal seed source, or 

“inoculum,” has important implications because the maximum 

algal biomass in the lower river is strongly dictated by its 

initial population size entering the reservoir reach, which is 

now considerably lower at Rood Bridge, RM 38.4 (figs. 7 and 

23). Even small reductions in the inoculum could translate 

into reductions at downstream sites. Ratios of measured Chla at Elsner to those upstream at Rood Bridge (here termed 

the “chlorophyll-a growth ratio”) are useful in evaluating 

algal growth in this reach of the river over time. Historically, 

this reach was characterized by a rapidly increasing algal 

population, but that has typically not been the case in recent 

years. In the past, the population at Elsner was often 10 times 

larger than at Rood Bridge, whereas growth ratios in this 

reach in recent years are much smaller, and often less than 1, 

indicating times such as August 2004 when the Chl-a actually 

declined in this reach (fig. 26).
Declines in Phytoplankton Populations 45

tac12-0780_fig21

-25 25 0 75 125 175 225 

+ plankton 50% WWTF +P high

+ plankton 50% WWTF +P high

+ plankton 0% WWTF +P high

+ plankton 0% WWTF +P high

+ plankton 50% WWTF

+ plankton 50% WWTF

+ plankton 0% WWTF

+ plankton 0% WWTF

+ plankton 50% WWTF +P high

+ plankton 50% WWTF +P high

+ plankton 50% WWTF +P low

+ plankton 50% WWTF +P low

+ plankton 0% WWTF +P high

+ plankton 0% WWTF +P high

+ plankton 0% WWTF +P low

+ plankton 0% WWTF +P low

+ plankton 50% WWTF

+ plankton 50% WWTF

+ plankton 0% WWTF

+ plankton 0% WWTF

unamended 50% WWTF

unamended 0% WWTF

A

0 2 4 6 8 10 

-20 -10 0 10 20 

unamended 50% WWTF

unamended 30% WWTF

unamended 0% WWTF

Light bottles B

0 2 4 6 8 10 

0 20 40 60 80 

unamended 50% WWTF

unamended 30% WWTF

unamended 0% WWTF

Light bottles C

Light bottles

Dark bottles

0 2 4 6 8 10 

Percent change in chlorophyll-a Percent change in dissolved oxygen saturation

Figure 21. Changes in chlorophyll-a and dissolved oxygen concentrations during 

bioassay experiments on (A) May 19, (B) June 11, and (C) June 30, 2008, Tualatin 

River, Oregon. On May 19, unamended samples were dominated by Synedra, 

Stephanodiscus, Cymbella, and other diatoms; amended samples dominated by 

Asterionella, Stephanodiscus, and Melosira. On June 11, unamended samples 

dominated by Anabaena and Stephanodiscus. On June 30, Cryptomonas erosa, Eunotia 

pectinalis, and Melosira varians dominated unamended samples. Abbreviations: 

WWTF, wastewater treatment facility effluent; P, phosphorus; %, percent; DO, 

dissolved oxygen.
46 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig22

10 

20 

30 

40 

50 

60 

70 

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 

0 

Year

Daily mean reservoir releases for flow augmentation, 

in cubic feet per second

July releases

August releases 

EXPLANATION

Figure 22. Time series of water releases from Barney Reservoir and Hagg Lake, Oregon, by Clean Water Services for 

flow augmentation, July and August, 1991–2009. Data from Clean Water Services.

0 

3 

6 

9 

12 

15 

0 

3 

6 

9 

12 

15 

Chlorophyll-a concentration,

in micrograms per liter Turbidity, in nephelometric turbidity units

1992 1991 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Year

Figure 23. Time series of (A) 

chlorophyll-a concentration and (B) 

turbidity in the Tualatin River at Rood 

Bridge (river mile 38.4), Oregon, July–

August 1992–2009. Data from Clean 

Water Services.
Declines in Phytoplankton Populations 47

Figure 24. Time series of turbidity in the Tualatin River at Cherry Grove (river mile 

67.8), Oregon, June–August 1991–2009. Data from Clean Water Services.

Figure 25. Time series of chlorophyll-a concentrations and turbidity in the 

Tualatin River at Elsner Road (river mile 16.2), Oregon, July and August 1991–2009. 

Data from Clean Water Services.

Figure 26. Trend in the chlorophyll-a growth ratio in the reach between Rood 

Bridge (river mile [RM] 38.4) and Elsner Road (RM 16.2), Tualatin River, Oregon, 

during August, 1991–2008. The growth ratio is the chlorophyll-a concentration at 

Rood Bridge divided by the chlorophyll-a concentration at Elsner Road. Data from 

tac12-0780_fig24 Clean Water Services.

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 

Year

0 

2 

4 

6 

8 

10 

Turbidity, in nephelometric turbidity units

June

July

August 

EXPLANATION

0 

5 

10 

15 

20 

25 

0 

20 

40 

60 

80 

100 

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 

Chlorophyll-a concentration, in micrograms per liter

Turbidity, in nephelometric turbidity units

Year

EXPLANATION

Chlorophyll-a concentration

Turbidity

0 

5 

10 

15 

20 

25 

Chlorophyll-a growth ratio

Year

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 
48 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Declines in the Chl-a growth ratios 

in the Rood to Elsner reach suggest that 

something is affecting algal growth or 

accumulation of biomass. Although it 

is possible that factors such as flow, 

nutrient levels, or the percentage of flow 

from reservoir releases and WWTFs also 

contribute to the declines, higher turbidity 

may be able to explain the reduced 

algae levels because algae require light 

for photosynthesis. Increased turbidity 

levels, accompanied by a reduced 

upstream inoculum, might combine to 

limit algal growth in the reservoir reach. 

Historical July–August turbidity and Chl-a

concentrations at Elsner are plotted in 

figure 27. No clear threshold of turbidity 

is indicated in that plot as a necessary 

condition to limit algal growth. Some of 

the scatter in that plot is caused by higher 

turbidity at higher algal abundance, which 

is expected; regardless, the bulk of the 

highest Chl-a concentrations occur when 

the turbidity values are less than about 7 nephelometric 

turbidity units (NTUs), thus offering more evidence suggestive 

of light limitation on algal growth. Apparent increases in 

summer turbidity minima in recent years at Rood Bridge 

(fig. 23) and Elsner (fig. 25), at around 4–8 NTUs, may be 

high enough to begin exerting a growth limitation that, along 

with other factors, is suppressing algal populations in July 

and August.

Changes in River Hydrology and 

Wastewater Management, 1991–2008

To understand why algal populations have generally 

declined in July and August since 2003, it is important 

to quantify and assess the changes in hydrology and 

improvements in wastewater treatment that have occurred 

over the past two decades, as those factors have an influence 

on algal populations. Nutrient reductions from the WWTFs, 

especially lower SRP levels, have greatly reduced the 

magnitude and duration of algal blooms, by design. Moreover, 

tac12-0780_fig27

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

0 5 10 15 20 25 30 35 

Tubidity, in nephelometric turbidity units

Chlorophyll-a concentration, in micrograms per liter

Figure 27. Relation between turbidity and chlorophyll-a concentrations 

in the Tualatin River at Elsner Road (river mile 16.2), Oregon, May–August 

1992–2009. Data from Clean Water Services.

water releases during summer, mostly from Hagg Lake, and 

now also from Barney Reservoir, greatly increase streamflow 

during critical low-flow periods. In the upper reaches of 

the Tualatin River at Cherry Grove, flows in August have 

increased (fig. 28) owing to releases from Barney Reservoir. 

Increased summer flow augmentation is especially evident in 

the lower river at West Linn (fig. 29), where the frequency and 

duration of lower flows (darker blue colors) have decreased 

over time, while the frequency of higher flows (lighter blue 

colors) has increased in summer, especially in August. The 

particularly low flows at West Linn in 1991–95 were partly 

due to large (50 ft3/s) withdrawals at the Oswego Canal 

(RM 6.7), which decreased to 10–15 ft3/s during 1996–2003 

and 1 ft3/s or less thereafter. Close examination of the data 

reveals short pulses of higher flows in August (Julian days 

213 to 243), indicated by the red bars in the black box of 

figure 29. Flow augmentation (and decreased withdrawals) has 

essentially raised the baseflow condition for the river (fig. 30). 

By shortening the residence time and reducing phosphorus 

concentrations, the direct consequence is a decrease in algal 

populations in July and August. 
Changes in River Hydrology and Wastewater Management, 1991–2008 49

tac12-0780_fig28

0 

50 

100 

150 

200 

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 

Year

Streamflow, in cubic feet per second

Figure 28. Time series of streamflow in the Tualatin River at Cherry Grove (river mile 67.8), Oregon, August 

1991–2009. Data from Clean Water Services.

Figure 29. Time series of streamflow in the Tualatin River at West Linn (river mile 1.8), Oregon, 1991–

2011. Black box indicated by arrow is the month of August.

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

0 30 60 90 120 150 180 210 240 270 300 330 360

Day of year

Year

Tualatin River at West Linn, Oregon (14207500)

Daily mean streamflow, in cubic feet per second

January 1, 1991 to December 31, 2011

Daily mean streamflow, in cubic feet per second

100

150

175

200

225

250

275

300

325

350

500

1,000

5,000

no data
50 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig30

0 

100 

200 

300 

400 

500 

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 

July

August 

Streamflow, in cubic feet per second 

Year 

EXPLANATION

Figure 30. Time series of streamflow in the Tualatin River at West Linn (river 

mile 1.8), Oregon, July and August, 1991–2011.

Higher flows in the river also result from higher 

discharges of treated effluent from the Rock Creek WWTF 

(fig. 13), which serves a population that has grown faster than 

the population in the lower basin, which was developed earlier 

and is served by the Durham WWTF. Greater percentages 

of WWTF effluent and flow augmentation releases from 

upstream reservoirs result in decreased percentages of natural 

flow in the river in summer. This is important because the 

natural flow contains a greater amount of both Chl-a and 

diatoms (fig. 31). Conversely, flow augmentation and WWTF 

effluent as a percentage of the total flow were negatively 

correlated with these algal indicators at the RM 24.5 and 

38.4 sites. 

Although the importance of Chl-a in maintaining 

minimum levels of DO in the Tualatin River is well 

established (Rounds and others, 1999), the types of algae 

occurring in the river also have an influence on the health 

of this ecosystem. Diatoms and other algae make important 

contributions to the river’s food web, supporting seasonal 

populations of zooplankton, which, in turn, are food for 

planktivorous fish. From a management standpoint, because of 

their high fatty acid content, diatoms are nutritious and much 

preferred over blue-green algae (Caramujo and others, 2008), 

which can form surface blooms, and sometimes produce 

harmful toxins. Identification of potential management options 

that contribute to conditions supportive of diatoms might also 

lessen competition from blue-green algae and the problems 

associated with their blooms.
Changes in River Hydrology and Wastewater Management, 1991–2008 51tac12-0780_fig31 0 80 60 40 20 100 0 50,000 100,000 150,000 200,000 0 80 60 40 20 100 0 1 2 3 4 5 0 1 2 3 4 5 0 50,000 100,000 150,000 200,000 0 1 2 3 4 5 0 50,000 100,000 150,000 200,000 R2=0.82 R2=0.70 R2=0.53 R2=0.44 R2=0.67 R2=0.62 Natural flow Natural flow Flow augmentation WWTF effluent Flow augmentation WWTF effluent A. Chlorophyll-a concentration, in micrograms per liter B. Diatom biovolume, in cubic microns per milliliter

Percent

Figure 31. Relations between the percentage of natural flow, flow augmentation, and wastewater treatment facility (WWTF) effluent and (A) 

chlorophyll-a and (B) diatom biovolume in the Tualatin River, Oregon, at river miles 38.4 and 24.5. Two outlier samples collected during blooms in July 2007 and July 2008 with chlorophyll-a concentrations greater than 10 micrograms per liter were not included.
52 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Case Studies of Low Dissolved Oxygen 

Events and Bloom Crashes in 2003–08

Although phytoplankton blooms still occur, they 

tend to be shorter in duration and terminate earlier than in 

years past. In previous years, Chl-a concentrations would 

remain high enough to sustain the DO throughout most or 

all of summer, at least through late August; in recent years, 

while blooms sometimes decline gradually, they often crash 

abruptly in mid-summer. This earlier onset of algal declines 

often coincides with peak water temperatures of 24–25°C in 

late July or early August, which can contribute to low DO 

conditions because DO solubility decreases with increasing 

temperature and sediment oxygen demand (SOD) increases 

with increasing temperature. To augment the general analysis 

of algal declines in July-August and to better determine the 

factors leading to algal declines, an examination of specific 

algal bloom-decline events during 2003–08 was undertaken.

Low Dissolved Oxygen Events

Daily minimum DO measurements at the Oswego Dam 

in 2003–08 show 9 periods, labeled A to I in figure 32, when 

concentrations were less than 6.5 mg/L, the State standard 

based on the 30-day mean DO concentration. The occurrence 

of low-DO conditions during summer was directly related 

to declines in algal populations and Chl-a levels. Specific 

timeframes, duration, flow, light, and turbidity conditions, and 

possible causes of each of the events are listed in table 14. 

Although short-term declines in DO may occur during summer 

in response to (1) clouds and rain that temporarily disrupt 

photosynthesis or shorten the residence time via streamflow 

increases, or (2) rare discharges of high ammonia levels from 

the WWTFs, the longest duration low-DO conditions always 

occurred after the decline of an algal bloom. During these 

times, the daily minimum DO concentrations often were less 

than 6.5 mg/L for as many as 77 days (table 14 and fig. 4), and 

sometimes less than the absolute minimum State standard of 

4 mg/L for shorter durations.

Table 14. Low dissolved oxygen events in the lower Tualatin River at the Oswego Dam, Oregon, 2003–08, and potential contributing 

factors.

[Low dissolved oxygen event: See figure 32. Low dissolved oxygen (DO) events resulting in DO concentrations less than 6.5 mg/L. The 6.5-mg/L State 

standard is based on the 30-day mean concentration. Abbreviations: mg/L, milligrams per liter; WWTF, wastewater treatment facility; ft3/s, cubic feet per 

second]

Low 

dissolved 

oxygen 

event

Year

Dates when dissolved 

oxygen was 

less than 6.5 mg/L

Approximate 

duration of daily 

minimum dissolved 

oxygen less than 

6.5 mg/L (days)

Minimum 

dissolved 

oxygen

Potential cause / Contributing factors

A 2003 July 11–November 1 77, intermittent 3.8 mg/L on August 10 Algal population decline; high water temperature 

B 2004 July 26–Sept 11 48 4.3 mg/L on August 23 Algal population decline; high water temperature

C 2005 August 8–Sept 22 46 4.6 mg/L on August 30 Algal population decline 

D 2006 May 22–26 4 4.8 mg/L on May 23 0.87 inch of rain on May 21 preceded this event

E 2006 July 29–Oct 7 53, intermittent 5.4 mg/L on July 30 Algal population decline; high water temperature 

F 2007 July 19–Sept 22 57, intermittent 4.7 mg/L on July 23 Low solar radiation and 0.5 inch of rain; high 

water temperature

G 2008 May 21–26 6 6.0 mg/L on May 26 Ammonia release from the Rock Creek WWTF; 

multi-day 1-inch rain event

H 2008 July 1–9 9 5.2 mg/L on July 5 Algal decline; 0.4 inch of rain 2 days prior

I 2008 August 12–October 12 60 3.8 mg/L on August 20 Crash of large algal population; increase in 

flow augmentation up to 125 ft3/s; zooplankton 

grazing; rainfall
Case Studies of Low Dissolved Oxygen Events and Bloom Crashes in 2003–08 53

tac12-0780_fig32

0 

5 

10 

15 

20 

25 

30 

2 

4 

6 

8 

10 

12 

14 

2003 2004 2005 2006 2007 2008 2009 

Water temperature, in degrees Celsius

Dissolved oxygen concentration, in milligrams per liter

Year

Daily minimum dissolved oxygen

Daily maximum water temperature 

1–10 Algal blooms

A–I Low dissolved oxygen events

EXPLANATION

2

9

3

5

1

4

6

7

10 8

A 

C 

E 

D 

H 

B 

G 

F 

I 

Figure 32. Dissolved oxygen (DO) concentrations and water temperature in the Tualatin 

River at Oswego Dam (river mile 3.4), Oregon, 2004–08, highlighting algal blooms and low-DO 

events. Shaded areas indicate periods when DO was less than 6.5 milligrams per liter. See 

tables 14 and 15 for information regarding the conditions leading up to the low-DO event and 

potential contributing factors in bloom declines.
54 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Specific Bloom Crashes 

A combination of algae, water-quality, weather, and flow 

data were analyzed for 2003–08 to determine some of the 

factors that may have triggered the 10 bloom crashes listed in 

table 15. For this analysis, data from the continuous monitor 

at the Oswego Dam, CWS discrete data for the mainstem, and 

qualitative (in 2005) and quantitative (in 2006–08) plankton 

conditions were utilized. Total photosynthetically active 

radiation (PAR) and rainfall data from the Durham WWTF 

monitoring site also provided weather data to help discern 

whether clouds or rain played a role in any bloom crashes.

2003

In 2003, algal growth started in late May, leading to a 

bloom that peaked in late June and early July. No species 

composition data are available to determine the type of 

algae that caused a 70 μg/L peak in Chl-a concentration at 

Stafford Road (RM 5.5). Although low SRP concentrations 

at Boones Ferry (0.01 mg/L, RM 8.7) may have contributed 

to or triggered the bloom crash in the second week of July, 

cloudy conditions also may have contributed (fig. 33 and 

table 15). The low SRP concentrations probably resulted from 

the combined effect of lower than normal concentrations of 

SRP in treated effluent being discharged from the Durham 

WWTF (0.012 mg/L on July 13–14), and uptake of SRP by 

phytoplankton, which had peaked at 64 μg Chl-a/L the week 

before at Boones Ferry, about a half mile downstream from the 

Durham WWTF (fig. 2), but declined to 9 and 4 μg Chl-a/L, 

respectively, on the next two samplings.

A second bloom in 2003 peaked on July 25, followed by 

a second crash (fig. 33) that also occurred when SRP levels 

at Elsner were quite low (0.01 mg/L) and turbidity increased 

slightly. For 77 days after the crash, the daily minimum DO 

was intermittently less than 6.5 mg/L at the Oswego Dam, 

reaching a minimum DO of 3.8 mg/L on Aug 10 (table 14). 

Chl-a levels rebounded slightly to 20 μg/L on August 1, but a 

rainfall event prevented further growth by increasing flow and 

decreasing both available light and the residence time (fig. 33).

2004

In 2004, a bloom peaked at 60 μg Chl-a/L on July 7 at 

Stafford Road, decreasing the SRP to 0.01 mg/L at Boones 

Ferry. The bloom declined slowly thereafter, rebounded at 

times, but finally crashed on July 22 (table 14). The cause of 

this crash appears to be due to limitation by phosphorus as 

well as flow factors, as this was about the time when the total 

percentage of WWTF effluent reached its seasonal peak. From 

July 15 to July 22, the percentage of WWTF effluent in the 

river at Boones Ferry increased from about 41 to 48 percent. 

Whatever the cause, for 48 days from July 26 to September 11, 

the daily minimum DO was below 6.5 mg/L. The lowest 

DO value of 4.3 mg/L was observed on Aug 23, when the 

maximum water temperature was about 23°C (table 14). 

2005

A small and steady population of Aulacoseira developed 

in June 2005, but was interrupted twice by two approximately 

0.35-in. rain events in late June and early July. The first rain 

event produced a streamflow at West Linn of 453 ft3/s (greater 

than the 300 ft3/s level below which algal growth can typically 

be sustained) and a turbidity of 16 NTU. These conditions 

were followed by a moderate population of blue-green algae 

(Aphanizomenon and Anabaena) that developed in the lower 

river at Elsner (RM 16.2) and produced the peak Chl-a

concentration of the season, 40 μg/L at the Oswego Dam on 

July 18. Thereafter, algal biomass was lower and declined 

slowly to less than 10 μg/L at the end of July. Zooplankton, 

primarily cladocerans, were observed in samples from 

Boones Ferry to the Oswego Dam at this time, suggesting 

the possibility that grazing may have played a role in the 

algal biomass decline. The lack of algal photosynthesis was 

followed by a decline in the DO to a minimum concentration 

of 4.6 mg/L on August 30. The 6.5-mg/L DO State standard 

(based on the 30-day mean concentration) was not met for 72 

consecutive days, from August 12 to October 22.

2006

In 2006, an early July bloom was followed by a rapid 

crash; the population rebounded only to decline again at 

the end of July. The first crash coincided with a period of 

cloudy weather, but the algae rebounded once light conditions 

improved. At the end of July, this second bloom declined, 

coinciding with a small decline in PAR and a slight increase 

in turbidity at the Oswego Dam of 9 formazin nephelometric 

units (FNUs) (fig. 34). The daily minimum DO concentration 

was less than 6.5 mg/L at the Oswego Dam for 53 of the 71 

days from July 29 to October 7, reaching a minimum of 5.4 

mg/L on July 30 (table 14).

A marked shift in the phytoplankton species composition 

also occurred during this time period in 2006: the biovolume 

of diatoms, mostly Stephanodiscus binderanus, declined from 

70 to less than 15 percent of the total biovolume, whereas the 

biovolume of Cryptophytes, mostly Cryptomonas erosa and 

Rhodomonas minuta, increased from less than 25 to almost 

80 percent from July 7 to August 7 (fig. 34). This change 

coincided with an increase in releases from Hagg Lake, and a 

further decrease in the percentage of natural flow, as well as 

higher populations of zooplankton, all of which seem to be 

correlated with algal declines.
Case Studies of Low Dissolved Oxygen Events and Bloom Crashes in 2003–08 55 Table 15. Algal bloom–crash sequences in the Tualatin River, Oregon, during 2003–08, and possible contributing factors.

[Algal blooms: See figure 32. Favorable conditions for phytoplankton growth defined as: greater than 1,250 (μE/m2)/s daily maximum photosynthetically active radiation at the Durham WWTF, daily median 

turbidity values less than 7 FNU at the Oswego Dam, SRP greater than or equal to 0.015 mg/L, and streamflow less than 300 ft3/s at West Linn. Water temperature (daily maximum) from the Oswego Dam site. 

Abbreviations: (μE/m2)/s, microeinsteins per square meter per second; WWTF, wastewater treatment facility; FNU, formazin nephelometric units; SRP, soluble reactive phosphorus; mg/L, milligrams per liter; 

ft3/s, cubic feet per second; μg/L, micrograms per liter; NTU, nephelometric turbidty units; DO, dissolved oxygen; m3, cubic meters]

Algal 

blooms Year Algal conditions

Peak 

chlorophyll-a, 

location

Growing conditions for phytoplankton in lower river Streamflow at West Linn during crash

Possible trigger or contributing 

factors Solar

radiation

Water

temperature Turbidity SRP Streamflow 

Daily mean 

flow on 

first day

Average 

during 

crash

1 2003 Growth started in late May; 

first bloom peaks on June 30, then crashes from July 7–13

70 μg/L, Stafford Road

Unfavorable 23–25°C Favorable

Unfavorable, 0.01 mg/L

Favorable 183 187 Cloudy weather conditions; low 

SRP (0.01 mg/L) at Boones Ferry on June 30

2 2003 Second bloom peaks on 

July 25, then crashes from July 26–29

50 μg/L, Oswego Dam

Unfavorable 22–23°C Unfavorable, 

6–9 NTU

Unfavorable, 0.01 mg/L

Sometimes favorable

150 150 Cloudy weather conditions; 

Somewhat higher turbidities in the lower river may have limited light availability; Low SRP (0.01 mg/L) at Elsner; subsequent growth limited by higher flows and less inocula coming into the reservoir reach from upstream

3 2004 Algal bloom peaked about 

July 7, then declined for 9 days with minor rebounds; final crash on July 22

60 μg/L, Stafford Road

Some clouds, but mostly favorable

23–25°C Favorable Unfavorable, 

0.01 mg/L

Favorable 211 183 Low SRP (0.01 mg/L) at Boones 

Ferry on July 6; 0.012 mg/L at Stafford Road on July 12; 10 and 19 percent increase in WWTF discharge from the Rock Creek and Durham WWTFs increases the proportion of WWTF to 48 percent of the total flow

4 2005 Small population of 

Aulacoseira develops in June

~10 μg/L, Oswego Dam

Unfavorable 20–21°C Briefly 

unfavorable, up to 16 NTU

Favorable Unfavorable 340 407 0.35-inch rain event interrupted 

the first bloom

5 2005 Blooms of Aulacoseira, then 

mixed with Aphanizomenonand Anabaena peaks on July 18, and then declines

40 μg/L, Oswego Dam

Favorable 24°C Favorable Favorable Favorable 173 184 Zooplankton (cladocerans 

and copepods) abundant [not quantified], so crash may have been due to grazing losses

6 2006 Cryptomonas erosa, 

Chlamydomonas sp., and several diatoms (Aulacoseira and Stephanodiscus) dominate assemblage; bloom peaks 

on July 3, declines, then rebounds

80 μg/L, Stafford Road

Unfavorable 24°C Favorable Unfavorable, 

0.011 mg/L

Favorable 197 217 Low light from cloudy 

conditions; Low SRP ( 0.011 mg/L) at Stafford on July 3; flow augmentation increases to 98 ft3/s, zooplankton (mostly 

cladocerans) peak at about 15,000 per m3
56 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon Algal blooms Year Algal conditions Peak chlorophyll-a, location Growing conditions for phytoplankton in lower river Streamflow at West Linn during crash Possible trigger or contributing factors Solar radiation Water temperature Turbidity SRP Streamflow Daily mean flow on first day Average during crash

7 2006 A second bloom of 

flagellates peaks on 

July 24, then declines

80 μg/L, Stafford Road

Favorable 24°C Possibly 

unfavorable, up to 9 FNUs

Unfavorable,0.011 mg/L

Favorable 224 210 Low SRP (0.011 mg/L) on July 

24; Flow augmentation increases to 122 ft3/s; natural flow makes up 

less than 50 percent of total starting on July 20; Low light on July 30; cladoceran density 12,000 per m3 at 

Oswego Dam on July 24

8 2007 Stephanodiscus and 

Cryptomonas erosa and dominate assemblage, which peaks on July 16 then crashes

68 μg/L, Stafford Road

Unfavorable 22–24°C Favorable Unfavorable, 

0.005 mg/L

Sometimesfavorable

224 261 0.5-inch rain event; Low SRP 

at Stafford Road (0.005 mg/L); Increase in flow augmentation up to 96 ft3/s; high zooplankton densities 

(25,000-50,000 cladocerans per m3 at Stafford and Oswego 

Dam) suggest grazing may have contributed to the algal decline

9 2008 Stephanodiscus dominates 

algal assemblage, which peaks on June 27, then declines

20 μg/L, Oswego Dam

Unfavorable 22°C Favorable Favorable Favorable 257 258 0.4-inch rain event

10 2008 Anabaena, 

Aphanizomenon, and Cryptomonas erosa blooms from July 10 to August 4, then declines by August 11

50 μg/L on July 10 at Oswego Dam; 83 μg/L on July 24 at Elsner

Favorable 22–23°C Favorable Favorable Favorable 250 275 Inocula ended; Increase in flow 

augmentation up to 125 ft3/s; high 

desity of zooplankton (88,000 per m3 downstream from Elsner 

on July 16) suggests that grazing losses might also have played a role in the crash or in keeping the bloom from rebounding.

Table 15. Algal bloom–crash sequences in the Tualatin River, Oregon, during 2003–08, and possible contributing factors.—Continued[Algal blooms: See figure 32. Favorable conditions for phytoplankton growth defined as: greater than 1,250 (μE/m2)/s daily maximum photosynthetically active radiation at the Durham WWTF, daily median 

turbidity values less than 7 FNU at the Oswego Dam, SRP greater than or equal to 0.015 mg/L, and streamflow less than 300 ft3/s at West Linn. Water temperature (daily maximum) from the Oswego Dam site. 

Abbreviations: (μE/m2)/s, microeinsteins per square meter per second; WWTF, wastewater treatment facility; FNU, formazin nephelometric units; SRP, soluble reactive phosphorus; mg/L, milligrams per liter; 

ft3/s, cubic feet per second; μg/L, micrograms per liter; NTU, nephelometric turbidty units; DO, dissolved oxygen; m3, cubic meters]
Case Studies of Low Dissolved Oxygen Events and Bloom Crashes in 2003–08 57

tac12-0780_fig33

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450 

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Streamflow, in cubic feet per second 

Chlorophyll-a, in micrograms per liter,

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Maximum daily solar availability,

 in microeinsteins per square meter per second 

Daily rainfall, in inches 

Rainfall total

Solar maximum

Chlorophyll-a at Oswego Dam

Turbidity

Daily mean streamflow at Dilley 

(river mile 58.8)

Daily mean streamflow at West Linn

EXPLANATION

EXPLANATION

Month

June July Aug. Sept. Oct.

Figure 33. Time series of (A) chlorophyll-a, turbidity, and streamflow in the Tualatin River and (B) daily 

rainfall and solar maximum at the Durham wastewater treatment facility, Oregon, June–October 2003. 
58 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig34

0 

20 

40 

60 

80 

100 

0 

40 

80 

120 

160 

200 

240 

280 

Daily maximum values for 

chlorophyll-a concentration, in micrograms per liter.

Dissolved oxygen, in percent saturation.

Daily streamflow at Dilley, in cubic feet per second.

Diatom and Cryptophye biovolume, in percent.

Daily average wastewater treatment facility effluent, in percent.

Month

Daily maximum turbidity, in nephelometric turbidity units.

Chlorophyll-a

Streamflow at Dilley

Percent wastewater treatment facility effluent

Dissolved oxygen saturation

Turbidity

Cryptophyte algae

Diatoms

June July Aug.

EXPLANATION

Figure 34. Time series of chlorophyll-a, streamflow, and water-quality conditions in the 

Tualatin River, Oregon, June–August 2006. The percent biovolume of diatoms and Cryptophyte 

algae at Stafford Road also are indicated. Chlorophyll-a, dissolved oxygen, and turbidity data 

from the continuous monitor at the Oswego Dam, RM 3.4.

2007

In 2007, persistent cloudy periods and rain resulted in 

unfavorable conditions for phytoplankton populations in 

late June and mid-July. A single bloom occurred, peaking 

in mid-July (fig. 35A) and crashing a week later from more 

rain and cloudy weather (fig. 35B). Although small rebounds 

in Chl-a occurred later in the summer, more clouds and rain 

again reduced algal biomass. The largest crash coincided with 

a pulse of increased flow from 209 to 312 ft3/s on July 20 at 

West Linn, a level that is higher than the general 300 ft3/s ruleof-thumb flow threshold above which the residence time tends 

to be insufficient to grow an appreciable algal population. The 

daily minimum DO was less than 6.5 mg/L at the Oswego 

Dam for 57 of the 66 days between July 19 and September 22, 

declining to 4.7 mg/L on July 23 (table 14).

2008

The first of three low-DO events in 2008 occurred in 

late May (see event G, fig. 32), when an ammonia release 

occurred after an interruption of the nitrification process at 

the Rock Creek WWTF and treated effluent concentrations 

were approximately 11 mg/L. Clouds and rain for several days 

also contributed to this low-DO event, which lasted for 6 days 

and produced a minimum DO concentration of 6.0 mg/L on 

May 26 (table 14).

In late June, a population of Stephanodiscus developed 

in the lower river. After reaching about 20 μg Chl-a/L at the 

Oswego Dam, unfavorable growing conditions caused by rain 

and cloudy weather caused the bloom to decline from June 

29 into the first week of July, and Chl-a declined to 7 μg/L 

(fig. 36). The decline in Chl-a precipitated another low-DO 

event (daily minimum DO less than 6.5 mg/L) that started on 

July 1 and lasted 9 days, reaching a minimum DO of 5.2 mg/L 

on July 5 (table 14). By this time, the Wapato Lake blue-green 

algae bloom in the upper river had already started and was 

moving downstream. 
Case Studies of Low Dissolved Oxygen Events and Bloom Crashes in 2003–08 59

tac12-0780_fig35

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50 

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150 

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0.4 

Chlorophyll-a concentration, in micrograms per liter.

Turbidity, in formazin nephelometric units.

Percent biovolume diatoms

and Cryptophyte algae.

Percent WWTF effluent Daily total rainfall, in inches

Streamflow and discharges from WWTFs, 

in cubic feet per second

Month

Month

A

B

Chlorophyll-a

Percent Cryptophyte algae

Percentage of wastewater 

treatment facility effluent 

at West Linn

Discharges from wastewater 

treatment facilities 

Percent Diatoms

Turbidity

Flow at West Linn 

EXPLANATION

EXPLANATION

Rainfall total

Solar maximum 

June July Aug.

June July Aug.

Figure 35. Time series of (A) chlorophyll-a and turbidity in the Tualatin River at the Oswego Dam (river mile 

[RM] 3.4), and percent biovolume of diatoms and cryptophyte algae at Stafford Road (RM 5.4), and (B) rainfall 

and solar radiation at the Durham wastewater treatment facility (WWTF), Oregon, June–August 2007. Daily 

average chlorophyll-a and turbidity from the Oswego Dam monitor (RM 3.4). 
60 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig36

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0 

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80 C

Chlorophyll-a concentration, in micrograms per liter.

Turbidity, in nephelometric turbidity units.

Percent wastewater treatment facility effluent Daily rainfall, in inches Percent biovolume

Streamflow, in cubic feet per second

Daily maximum solar radiation,

in microeinsteins per square meter per second

Diatoms

Cryptophyte algae

Blue-green algae

EXPLANATION

EXPLANATION

Daily maximum turbidity

Streamflow at West Linn 

Daily maximum chlorophyll-a

Percent wastewater treatment 

 facility effluent 

EXPLANATION

Rainfall total

Solar maximum 

Month

June July Aug.

Figure 36. Time series of (A) chlorophyll-a and turbidity in the Tualatin River at Oswego Dam (river mile 

[RM] 3.4), streamflow at West Linn (RM 1.8), and percentage of wastewater treatment facility (WWTF) 

effluent; (B) rain and solar radiation at the Durham WWTF; and (C) biovolume of diatoms, cryptophyte 

algae, and blue-green algae at RM 24.5 near Scholls, Oregon, June–August 2008.
Evaluation of Hypotheses to Explain Phytoplankton Declines 61

In the first week of July, Chl-a concentrations in the 

upper river at Highway 219 (RM 44.4), upstream of Rood 

Bridge, sharply increased from 2.1 to 6.7 μg/L, and by 

then concentrations had also sharply increased—to 65 μg/L 

at Elsner downstream. By the following week, Chl-a

concentrations had increased to 24 μg/L at Highway 219 

and 83 μg/L at Elsner. By July 28, the Chl-a concentration 

at Highway 219 was 46 μg/L, but after that date it declined 

over a period of about 6 weeks. The bloom dissipated once 

pumping from Wapato ended and the inocula associated 

with those discharges stopped entering the river. After the 

bloom ended, the daily minimum DO concentration at the 

Oswego Dam declined to less than 6.5 mg/L for 60 of the 

next 62 straight days, falling to 3.8 mg/L on August 20. By 

the time diatoms made a bit of a comeback in late August, 

more rain and clouds ended the blooms. Given the severity 

of the July blue-green algae bloom, the low DO values in 

August probably resulted from a combined reduction in algal 

photosynthesis and DO demand from bacterial decomposition 

of senescing algal cells overlaid on the background SOD. Not 

since 2003 had the DO concentration been that low (table 14).

Evaluation of Hypotheses to Explain 

Phytoplankton Declines

Based on the evaluation of the bloom–crash sequences 

for 2003–08 and the multivariate analyses of the 2006–08 

data, six factors are hypothesized to cause bloom crashes or 

prevent blooms from rebounding in August. These include: 

(1) light limitation, (2) a reduction in the inocula, or amount 

of phytoplankton entering the lower river from upstream 

sources, (3) increased summer streamflows, (4) changes in 

the dominant sources of flow, as an increasing percentage 

of flow augmentation and WWTF effluents has decreased 

the percentage of natural flow, (5) zooplankton grazing, and 

(6) low concentrations of bioavailable phosphorus. Each of 

these six proposed hypotheses is evaluated below.

Light Limitation

Hypothesis 1: Reductions in phytoplankton biomass and 

bloom crashes are brought about by light limitation caused 

by clouds or high turbidity.

Light is a required factor for algal photosynthesis, and 

light may become a limiting factor for phytoplankton growth 

during cloudy periods or when high turbidity limits light 

penetration into the water column. Rain and cloudy weather 

was a possible contributing factor in 6 of the 10 blooms 

evaluated for 2003–08 (table 15). Support for this hypothesis 

comes from numerous observations of algal declines during 

inclement weather, especially since monitoring has included 

continuous data for DO, pH, Chl-a, solar radiation, and 

rainfall that greatly facilitates the tracking of blooms and 

deciphering the causes of declines or crashes (fig. 35). Light 

limitation has been known as an important factor affecting 

algal blooms in the Tualatin River since the early 1990s and 

has been verified as an important factor not only through the 

available data but also through modeling analyses (Rounds 

and others, 1999; 2001). Often during spring and early 

summer, even partly cloudy days can have a marked effect on 

algal photosynthesis and DO production, but once sunny days 

return, the algae typically rebound, Chl-a levels increase, and 

DO production resumes. 

Light limitation is also suggested at some level from the 

pattern in Chl-a and turbidity observed at Elsner (RM 16.2; 

fig. 27), which shows an increase in the algal biomass when 

turbidity is less than about 10 NTUs, with the bulk of the 

highest values occurring at even lower turbidity levels. This 

may help to explain potential decreasing trends in Chl-a

concentrations in the upper part of the Tualatin River basin 

(fig. 7); more research and data are needed to clarify turbidity 

trends in these upstream areas. Elevated turbidity has been 

known to limit algal activity in the Tualatin River, as the data 

from 1996 at Elsner show (fig. 25); in that year, algal growth 

was suppressed to such an extent that maximum pH levels 

in the lower river were far lower than in the several years 

preceding or subsequent.

The apparent influence of turbidity on algal Chl-a means 

that if some small population of algae is needed to help 

maintain minimum DO levels, then erosion control becomes 

more important in this silt-laden watershed, especially during 

spring and summer when algal populations are developing. 

Unfortunately, this is also the period when some wetlands 

are drained and many other processes that may generate 

turbidity occur. Higher turbidity in the Tualatin River occurs 

from rain storms and urban runoff mobilizing materials from 

the tributaries, from drainage of wetlands for agricultural 

use, from higher flows associated with flow augmentation 

and reservoir releases, and from other sources. Even small 

reductions in algal populations in the upper part of the basin 

may lead to large algal declines in the lower river, which can 

result in low DO concentrations. In summary, this hypothesis 

seems to have some merit and basis in fact, and future 

research and monitoring of issues related to this topic would 

be valuable.
62 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Reduced Algal “Seed Source” to Inoculate 

the River

Hypothesis 2: Reductions in upstream algal inocula 

“seed sources” help to account for the decline in algal 

populations.

For phytoplankton populations to develop in the river, 

an initial seed population of viable algal cells must be present 

and delivered to the river. This occurs either by germination of 

algal cells from river bottom sediments (Stoermer and Julius, 

2003), or by immigration of an inoculum from upland sources 

such Barney Reservoir, Hagg Lake, small ponds, tributaries, 

wetlands, drained agricultural areas (Wapato Lake, for 

example), and/or other instream or off-channel habitats that 

support algal growth. These inputs may enter the mainstem 

through tributary discharges, by pumpage, or by resuspension. 

Centric diatoms, for example, can live among bed sediments 

in backwater habitats or on submerged wood, where they 

develop long filaments. Such filaments fragment easily, 

become entrained into the water column when disturbed, and 

may be an important source of algal inocula for the river.

A rich diversity of algae was available during summer 

to seed the lower river as far upstream as RM 24.5, where the 

highest algal species richness for any site (91 taxa) occurred 

during the 3-year study. In comparison, downstream sites 

had lower algal taxa richness, ranging from 58 to 70 taxa. 

Many of the taxa found only at RM 24.5, however, were 

benthic diatoms such as Nitzschia, Navicula, Caleonis, 

Pinnularia, Amphora, and others, which would not be 

expected to proliferate in the lower-gradient reservoir reach 

downstream. The next upstream sample site (Rood Bridge, 

at RM 38.4) had fewer taxa (67) but species similar to the 

sites downstream from RM 24.5. This increase in algal taxa 

richness between RM 38.4 and RM 24.5 could be attributed 

to inputs from any number of tributaries, including Rock 

Creek (RM 38.1), Gordon Creek (RM 37.4), Butternut Creek 

(RM 35.7), Christensen Creek (RM 31.9), Burris Creek (RM 

31.6), Jackson Creek (RM 30.8), and Baker and McFee Creeks 

(RM 28.2). Many of these creeks drain areas with ponds 

that could serve as important sources of inocula to the lower 

Tualatin River; an examination of those sources might be a 

fruitful topic for future study.

The degree to which algal populations develop in the 

upper river has a direct effect on the eventual size of the 

population downstream. Large inputs of algal seed sources 

can provide the initial materials that produce large blooms 

downstream, depending on the location, flow rate, and other 

factors, as the 2008 Wapato Lake event demonstrated. In that 

case, elevated concentrations of algae, zooplankton, SRP, 

and ammonia initiated and fueled a large bloom of bluegreen algae in the lower river; this was the largest bloom in 

recent years. This situation is not unique: algal blooms in 

the Sacramento River, California, are enhanced partly by 

the injection of algal inocula and nutrient rich water from 

upstream agricultural drains (Stringfellow and others, 2009).

Disruption of the seed source, if it occurs, would make 

it unlikely for algal populations in downstream reaches to 

rebound until another seed population enters the system 

from upstream, which could take time. A good example of 

this occurred at the end of the Wapato Lake event, when a 

reduction in the amount of algae coming into the upper river 

was the beginning of the end of the bloom (Bonn, 2008). 

Declines in algal populations in the lower Tualatin River 

during 2006–07 were preceded by declines in Chl-a upstream 

at Rood Bridge, where concentrations dropped from 3–5 μg/L 

to 1–2 μg/L prior to the decline in algal biomass downstream. 

In all, it took about 6 weeks for the 2008 Wapato 

bloom to dissipate once the upstream source was cut off. 

Future targeted sampling of algae in the water column and 

bed sediments in the upper basin at key times and places 

could help to identify how the river is seeded with algae so 

that a better understanding of the dominant processes can 

be identified.

Higher Summer Streamflows

Hypothesis 3: Higher summer streamflows have reduced 

algal growth by lowering residence time and possibly also 

lowering water temperatures.

It is well established in the Tualatin River (and for 

many other rivers worldwide) that streamflow has a large 

effect on phytoplankton, primarily by affecting the amount 

of time available for algal populations to develop, grow, and 

reproduce. This effect is evident in the Tualatin River (fig. 37). 

In general, flows less than about 300 ft3/s at West Linn (RM 

1.8) result in a long enough residence time (7−10 days) 

through the pooled reservoir reach for algae to multiply into 

substantial populations, provided that growing conditions are 

favorable. Streamflow in the Tualatin River during summer 

can be as low as 100−200 ft3/s, which provides as many as 

14−17 days for phytoplankton to develop into a large bloom. 
Evaluation of Hypotheses to Explain Phytoplankton Declines 63

Withdrawals through the 

Oswego Canal at RM 6.7 decreased 

streamflow by about 50 ft3/s during 

summer prior to 1996, resulting in 

substantially increased residence 

times in the several miles upstream 

of Oswego Dam; in recent years, 

such withdrawals are largely 

inconsequential, resulting in higher 

flows and less time for algal growth. 

In addition, the use of flashboards 

at the Oswego Dam, which was 

commonplace in the 1980s and early 

1990s but not in recent years, raised 

the water level approximately 10 in. 

in the 6-mi reach upstream of the 

dam, which further increased the 

residence time. Because this practice 

was discontinued, residence time has 

been further reduced in recent years. 

If flows are high enough, 

algal populations may be “washed 

out” (Reynolds, 1990). This was 

demonstrated in September 1993, 

when a release from Hagg Lake, 

conducted as part of an experiment, 

increased flows and greatly lowered 

Chl-a concentrations (fig. 38). 

Increased flows in July and especially 

August (figs. 29 and 30) in recent 

years have reduced travel time such 

that algae now have less time to grow. 

The discontinued use of flashboards 

in the last 10 years also has the effect 

of reducing the residence time, but it 

is probably not as big a factor as the 

increased flow. 

tac12-0780_fig37

0 

10 

20 

30 

40 

50 

60 

70 

80 

100 200 300 400 500 600 700 800 900 1,000 

July–August (2001–05)

July–August (2006–07) 

July–August (2008)

Other dates 

Daily median chlorophyll-a concentration,

in micrograms per liter

Daily median streamflow, in cubic feet per second

EXPLANATION

Figure 37. Streamflow and chlorophyll-a in the lower Tualatin River at the 

Oswego Dam (river mile 3.4), Oregon, 2001–08.

0 

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25 

30 

Chlorophyll-a concentration, in micrograms per liter

Month

Streamflow, in cubic feet per second

Chlorophyll-a at Elsner Road

Streamflow at Farmington 

EXPLANATION

Flow augmentation

experiment

June July Aug. Sept. Oct.

Figure 38. Effect of the September 1993 Hagg Lake flow augmentation 

experiment on chlorophyll-a concentrations in the Tualatin River at Elsner Road

(river mile [RM] 16.2), Oregon. Streamflow data from Farmington (RM 33.3).
64 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Changes in the Dominant Sources of Flow

Hypothesis 4: Changes in the dominant source of flow 

during July and August may contribute to declines in algal 

populations.

Changes in the dominant source of flow in the Tualatin 

River, which include greater contributions from reservoir flow 

augmentation and discharges from the WWTFs, especially the 

Rock Creek facility, might also partly explain the reduction 

in algal growth in recent years. In 2006–08 the percentage of 

natural flow in the river declined from about 70–80 percent 

of the total flow in June to 38–40 percent later in summer 

(fig. 12) in response to reservoir releases. This reduction in 

0 

20 

40 

60 

80 

100 

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Year

Percentage of natural flow at Farmington

Figure 39. Percentage of natural flow in the Tualatin River at Farmington (river mile 33.3), Oregon, June–August 

1991–2008.

natural flow is anticipated to reduce algal populations by 

dilution and, thereby, decrease the concentration of algal 

inoculum reaching the lower river (see hypothesis 2). Given 

that both Chl-a and diatom biovolume tend to increase with 

higher percentages of natural flow at the two uppermost 

sites sampled, RM 38.4 and 24.5 (fig. 31), a change toward 

less natural flow might be partly responsible for algal 

declines. There is, however, no clear trend in the percentage 

of natural flow over the past twenty years (fig. 39), which 

is not entirely unexpected as natural flows are governed 

by multiannual patterns in precipitation, runoff, and 

groundwater discharges, as well as year-to-year variations in 

flow-augmentation management.
Evaluation of Hypotheses to Explain Phytoplankton Declines 65

The importance of flow was clearly evident in the 

multivariate BEST models (table 11), which identified total 

flow and the source of flow (including flow augmentation, 

natural flow, and WWTF effluent as a percentage of total 

flow) as the most significant factors that define patterns in 

the phytoplankton assemblages. That the dominant source of 

flow was important makes sense given the potentially large 

differences among these sources in terms of the amount and 

type of algae they would be expected to contribute, and the 

differences in the quality of each source. 

It is not known, but certainly possible that the amount 

of algae in the natural component of the flow coming from 

upstream also has decreased along with the amount of natural 

flow. Even small reductions in Chl-a levels far upstream could 

give rise to lower populations downstream, especially if travel 

times are shortened by higher flows during summer, which 

has occurred in recent years, from periodic pulses of flow 

augmentation water or WWTF discharges (figs. 29, 30).

Differences in the flow source had notable effects 

on the relative biovolume of just a few key taxa, namely 

Stephanodiscus binderanus and S. hantzschii, which show 

increased prevalence at higher natural flows and decreased 

prevalence at higher levels of flow augmentation, whereas 

Cryptomonas erosa and Chlamydomonas sp. were higher 

during augmentation (fig. 40). Note, however, that although 

the relative biovolume of these two Stephanodiscus taxa 

increased with higher amounts of natural flow, the overall 

biovolume of algae was reduced with higher flow. The highest 

biovolume for these two taxa occurred when flows were 

between 200 and 300 ft3/s. Note also that the levels of natural 

flow and flow augmentation are highly correlated with the 

time of year, length of day, and other factors, so these results 

may not necessarily represent true cause and effect, and 

additional research and analysis is needed.

Water Storage Reservoirs and Flow 

Augmentation

Water releases from Hagg Lake (primarily) and Barney 

Reservoir (fig. 1) have a marked effect on the river’s flow and 

quality. By increasing the rate of flow, the travel time through 

the reservoir reach (RM 33–3.4) decreases, reducing the 

amount of time algae have to grow. 

The decline in Chl-a during the 1993 flow augmentation 

experiment, and also at the upper sampling sites (Rood Bridge 

[RM 38.4] or RM 24.5) in late July during this study, suggests 

that upstream reservoir releases probably are not a significant 

source of inocula in mid- to late-summer. Releases from Hagg 

Lake in summer are from an outlet located at a depth of about 

70 ft at full pool (Sullivan and Rounds, 2005), well below the 

thermocline from late spring through mid- to late-summer, 

which is an important factor that might limit the export 

of plankton from the lake. Although it is possible that the 

reservoirs help seed the Tualatin River with algae during the 

growing season—about half of the algal taxa found in the river 

also have been identified in Hagg Lake—this possibility has 

not been investigated. Alternatively, releases from upstream 

reservoirs could dilute the algal inocula from other sources to 

such an extent that downstream populations do not thrive, as 

the negative correlation between flow augmentation and Chl-a

suggests (fig. 31).

The effect of flow augmentation by itself was highly 

variable, although algal biovolume was notably lower when 

flow augmentation was greater than about 95–100 ft3/s. The 

biovolume of diatoms at the two uppermost sites, for example, 

was negatively correlated with flow augmentation (fig. 31), 

whereas higher relative biovolumes of Cryptomonas erosa and 

Chlamydomonas sp. occurred with greater amounts of flow 

augmentation (fig. 40). This was not the case in 1993, when 

the abundance of Tabellaria fenestrata (a colonial diatom) 

and Melosira varians increased during the experimental flow 

release, whereas Cryptomonas erosa abundance remained 

unchanged. Although it is likely that algal assemblages 

in the lower Tualatin River are affected by upstream seed 

sources including releases of phytoplankton from Hagg Lake, 

additional data are needed to quantify the seasonal inputs of 

algae from all potential sources before this hypothesis can be 

sufficiently evaluated.

WWTF Discharges

The proportion of treated wastewater in the Tualatin 

River has increased over the past couple decades (fig. 13), and 

despite its high quality, that increase may be contributing to 

algal declines. The BEST analysis identified the percentage 

of WWTF effluent to be important in determining patterns in 

algal assemblage, although percent WWTF effluent was not 

as important as total flow, and water temperature also was 

included in the solution (table 11). In nearly all cases, the 

total algal biovolume measured downstream from the Durham 

WWTF outfall location was lower than that measured just 

upstream of these inputs (fig. 41), particularly when biomass 

levels were high. This difference can be partly attributed to 

dilution from WWTF effluent discharges and small inputs 

from Fanno Creek, but may also be due to some particular 

quality of the treated effluent.

At times when flow from the WWTFs is greatest as 

a percentage of total river flow (as much as 38 percent in 

2006–08), there was an accompanying increase in the relative 

biovolume of blue-green algae and a decrease in the relative 

biovolume of two Stephanodiscus species (fig. 42). Because 

the percentage of WWTF effluent is always highest in late 

summer when flows are low, it is impossible to determine 

with the available data whether these observations were 

due to WWTF effluent, from natural seasonal succession in 

the plankton, or changes in upstream inocula, zooplankton 

grazing, reservoir releases, temperature, or some other factor. 
66 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig40

EXPLANATION

B. Response to flow augmentation 

less than 20 20-59 60-103 greater than 104 

Flow augmentation category, in cubic feet per second 

 0-33 34-73 80-115 120-157 greater than 160 

Natural flow category, in cubic feet per second 

A. Response to natural flow

0 

5 

10 

15 

20 

0 

10 

20 

30 

40 

0 

10 

20 

30 

40 

50 

60 

0 

5 

10 

15 

20 

Stephanodiscus binderanus

Stephanodiscus hantzschii

Chlamydomonas sp. 

Cryptomonas erosa

Relative biovolume, in percent

Figure 40. Relative biovolume of select algal taxa showing potential (A) responses to natural flow and (B) flow 

augmentation in the Tualatin River, Oregon, 2006–08. Natural flow is not derived from reservoirs or WWTFs. Flow 

augmentation includes reservoir releases from Hagg Lake and Barney Reservoir minus estimated withdrawals.
Evaluation of Hypotheses to Explain Phytoplankton Declines 67

tac12-0780_fig41

0 

500,000 

1,000,000 

1,500,000 

2,000,000 

2,500,000 

3,000,000 

3,500,000 

Upstream of Durham wastewater treatment 

 facility and Fanno Creek

Downstream of Durham wastewater treatment 

 facility and Fanno Creek 

0 

500,000 

1,000,000 

1,500,000 

2,000,000 

2,500,000 

3,000,000 

3,500,000 

0 

500,000 

1,000,000 

1,500,000 

2,000,000 

2,500,000 

3,000,000 

3,500,000 

Month

EXPLANATION

2006

2007

Total algal biovolume, in cubic microns per milliliter

2008

June July Aug. Sept.

Figure 41. Comparison of total algal biovolume in the Tualatin River upstream and 

downstream from the Durham WWTF and Fanno Creek, Oregon, 2006–08.
68 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig42

0 

5 

10 

15 

20 

25 

30 

Anabaena flos-aquae

Aphanizomenon flos-aquae 

0 

5 

10 

15 

20 

0-19 20-28 29-38 

Percent wastewater category 

Stephaodiscus binderanus

Stephanodiscus hantzschii 

A

B

EXPLANATION

EXPLANATION

Average percent relative biovolume

Figure 42. Average percent relative biovolume of select taxa showing potential 

(A) positive responses and (B) negative responses to percentage of wastewater 

treatment facility (WWTF) effluent in the Tualatin River, Oregon, 2006–08.

The limited bioassay experiments conducted for 

this study were inconclusive; while some stimulation of 

phytoplankton growth was observed at 30 percent WWTF 

effluent concentration, effluent concentrations of 50 percent 

sometimes increased and other times decreased Chl-a and 

DO production during the experiments. Because most of the 

bioassays were conducted during the unusual Wapato Lake 

discharge event, results from only three experiments were 

presented here—with no or limited replication in treatments. 

Data from these initial pilot experiments should not, therefore, 

be considered in any way definitive.

Overall, the hypothesis of a connection between algal 

declines and the source of flow can neither be accepted nor 

rejected at this point. The available data certainly point to 

some interesting correlations of algae with the source of flow, 

but definitive cause and effect has not yet been confirmed. The 

possibility that WWTF effluents could be decreasing algal 

populations remains unanswered, and the simple dilution of 

the natural flow could certainly contribute to the observed 

declines in Chl-a, but other causes are also likely. The inocula 

hypothesis (2), for example, is supported by multiple lines of 

evidence, and the source of flow (this hypothesis) may act to 

reinforce the processes involved in the inocula hypothesis.
Evaluation of Hypotheses to Explain Phytoplankton Declines 69

Zooplankton Grazing

Hypothesis 5: Zooplankton grazing accounts for some 

algal declines.

Although not measured directly in this study, zooplankton 

grazing can have a strong effect on phytoplankton abundance 

and species composition in lakes and rivers (Wetzel, 1983; 

Thorp and Covich, 2001). Previous modeling of Chl-a in the 

Tualatin River (Rounds and others, 1999) found that, at times, 

the simulated biomass was higher than that measured in the 

river, and attributed this to losses from zooplankton grazing. A 

comparison of zooplankton taxa from 2006–08 with previous 

data from 1991–93 (Doyle and Caldwell, 1996) showed 

that many of the same Genera (Daphnia, Ceriodaphnia, and 

Bosmina) were dominant in all years.

In spring or summer, before zooplankton populations 

are significant, phytoplankton growth occurs without much 

control by zooplankton. Zooplankton abundance increases as 

flows decline and food resources become more abundant. As 

zooplankton densities increase, grazing losses can begin to 

surpass the reproduction rate of the phytoplankton, causing 

algal populations to decline. With fewer food particles 

available, sharp reductions in the zooplankton density occur, 

tracking or lagging declines in the phytoplankton (fig. 19). 

By August, with fewer phytoplankton and zooplankton in 

the water column, a clear-water phase can occur, similar to 

that exhibited in many temperate lakes during late summer 

(Wetzel, 1983). In the Tualatin River at Stafford Road, for 

example, water transparency (Secchi disk depths) increased by 

about 2 ft during these late-season periods—especially in 2006 

and 2007, years when zooplankton grazing was suggested 

by data or observations of high amounts of zooplankton in 

net tows. 

Supporting evidence for this hypothesis comes from 

several observations, including high overall density of 

zooplankton in the lower river at certain times (fig. 17). In 

2006 and 2007, zooplankton abundance increased downstream 

of RM 24.5 with notably higher densities at and downstream 

of the Highway 99W/Jurgens Park sites near RM 11. In 2008, 

as a result of discharges from Wapato Lake, large densities 

of copepods developed in the river, resulting in much higher 

densities at RM 24.5 (fig. 17).

More convincing evidence is found in the relationship 

between zooplankton density and total algal biovolume 

(fig. 19), which suggests zooplankton grazing may, at times, 

contribute to algal declines. Without specific measurements 

of grazing rates, however, it is difficult to infer how grazing 

affects phytoplankton abundance or species composition. 

Many of the zooplankton taxa found in the Tualatin River 

have been shown in other studies to graze on many of the 

phytoplankton taxa that occur in the Tualatin River. For 

example, laboratory experiments by Fulton (1988) showed that 

filamentous centric diatoms (Melosira granulate) were readily 

consumed by cladocerans including Bosmina longirostris, 

which was one of the dominant zooplankton species in the 

Tualatin River in 2006–08, occurring in 94 percent of samples 

(table 7). Descy (1993) documented a sharp 10-fold decrease 

in phytoplankton abundance in the River Moselle (France) 

that was attributed to zooplankton grazing. Some of the most 

heavily grazed diatoms in that study (Stephanodiscus and

Cyclotella) were often abundant in the Tualatin River.

Algal types vary in their susceptibility to zooplankton 

grazing, and many have adaptations that make them less 

palatable. For example, many of the diatoms in the Tualatin 

River form colonies, spines, or other structures that may deter 

grazing. Diatoms are generally a preferred food source over 

most blue-green algae, which are commonly regarded as a 

poor-quality food source for zooplankton (Gulati and DeMott, 

1997). While diatoms are rich in fatty acids, blue-greens are 

considered to be less desirable to grazers because of their 

larger size, presence of a mucilaginous coating, and potential 

for toxin production (Caramujo and others, 2008). Similar 

to diatoms, small naked flagellates, including Cryptomonas 

erosa, also are considered to be a higher-quality food 

compared to blue-green algae (Chen and Folt, 1993). 

While zooplankton grazing certainly exerts a negative 

effect on algal populations, the strength of the grazing 

hypothesis needs to be tested with experimental approaches 

that elucidate and quantify the grazer-algal relations in the 

Tualatin River. Ideally, such experiments would also include 

planktivorous fish, as they may have an important influence on 

the abundance or composition of zooplankton.

Phosphorus Limitation

Hypothesis 6: Phytoplankton bloom declines are caused by 

limited availability of bioavailable phosphorus.

Phosphorus limitation may be a contributor to algal 

declines and the crashes of certain algal blooms, as 

concentrations of SRP often were reduced to very low levels, 

less than 0.01–0.15 mg/L, during high biomass peaks. As just 

one example, SRP concentrations were decreased to 0.01 mg/L 

at Boones Ferry (RM 8.7) in July 2004 during an algal bloom 

(fig. 43); similarly low concentrations of SRP occurred in the 

lower river during 6 of the 10 bloom crashes listed in table 15. 

In addition, the near linear relation between Chl-a and SRP 

demonstrates this negative or “uptake” pattern (fig. 15B). 
70 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

tac12-0780_fig43

0.01 

0.02 

0.03 

0.04 

0.05 

0.06 

0.07 

0.08 

0.09 

0.1 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

Chlorophyll-a

Soluble reactive phosphorus 

Chlorophyll-a concentration,

in micrograms per liter

Soluble reactive phosphorus concentration,

 in milligrams per liter

Month

EXPLANATION

0 

June July Aug.

Figure 43. Pattern in chlorophyll-a and soluble reactive phosphorus 

concentrations in the Tualatin River at Boones Ferry (river mile 8.7), 

Oregon, June–August 2004.

Previous studies and modeling (Rounds and others, 

1999) found that phosphorus concentrations essentially place 

a cap on the size of the blooms, which is consistent with 

these results, but they also found that typical SRP levels were 

sufficient to initialize a bloom, and so low SRP does not 

always explain why algal growth remains low. The importance 

of SRP to phytoplankton populations was clear from the 

BEST analyses, which identified SRP as an important factor in 

several of the solutions, and the most important factor (highest 

rho value, 0.242) in the solution for the combined 2006–08 

years that was significant at P<0.05 (table 11). 

Although phosphorus concentrations typically may be 

sufficient to start a bloom, algal uptake can decrease SRP 

concentrations to levels that may become limiting. Despite 

the fact that many of the algal taxa in the Tualatin River are 

considered eutrophic (table 6), lab experiments (VanDonk 

and Kilham, 1990) showed that some of the same diatom 

species in the Tualatin River (Stephanodiscus hantzschii, 

Asterionella formosa, and Fragilaria crotonensis) are able to 

assimilate SRP at very low concentrations and have relatively 

low half-saturation growth constants for phosphorus, ranging 

from 0.001 to 0.003 mg/L, which makes these taxa highly 

competitive during periods of low SRP availability. The 

presence of these taxa could indicate that the phosphorus 

supply, while perhaps limiting to other algae, may not 

limit the growth of these algal species if the measured SRP 

concentrations are an accurate measure of bioavailable P.

Measured summer SRP concentrations in the Tualatin 

River were greatly decreased as a result of major upgrades at 

the WWTFs in 1990–1992 and subsequent improvements in 

treatment processes. Median SRP concentrations in treated 

effluent for the May−October period in 2006−08 ranged 

from 0.013 to 0.018 mg/L (table 4), much lower than typical 

concentrations occurring in the Tualatin River. As a result, 

SRP concentrations can be quite low in the river in late 

summer when WWTF effluent comprises a higher proportion 

of the flow or during phytoplankton blooms when uptake rates 

are high. 

Whether the SRP in the river is bioavailable, however, 

continues to be an important question. Results from Tualatin 

River studies in the early 1990s showed that phosphorus 

could form a co-precipitate with colloidal iron and silica, 

but the solubility and bioavailability of that coprecipitate is 

unknown (Mayer and Jarrell, 1995). Future studies of the 

forms and bioavailability of phosphorus in the Tualatin River 

and in treated effluent would provide useful information. For 

example, Li and Brett (2010) found that much of the SRP in 

the Spokane River, Washington, another river that receives 

substantial amounts of municipal wastewater, was not readily 

available to at least some types of algae tested in bioassays. 

While phosphorus reductions are an important management 

strategy to reduce the occurrence, frequency, and magnitude 

of algal blooms, and potentially toxigenic harmful algal 

blooms, such aggressive treatment may be starving potentially 

beneficial algal populations and affecting them or their 

environment in ways that are not clearly understood.
Conclusions and Implications for River Management 71

Conclusions and Implications for 

River Management

The results of this study indicate that algal populations 

in the Tualatin River are influenced by many factors that are 

important at various times and locations. These factors include 

the magnitude and sources of streamflow, available light, 

instream turbidity, upstream inocula, grazing by zooplankton, 

and the amount and bioavailability of phosphorus. Although 

substantive algal blooms occur nearly every year, these factors 

help to determine when blooms form, how large they become, 

how long they last, when and how the bloom declines or 

ceases altogether, and which algae species are dominant. Many 

of these factors have changed since 1991 because of imposed 

regulations, population growth, expansion of an upstream 

reservoir, and alterations in flow management, to name just 

a few influences. The combined effect of these factors on 

algae generally has been to decrease algal populations, but 

particularly in late July to September and to such an extent 

that the loss of photosynthetic activity has led to problematic 

DO concentrations, especially since 2003. 

Several possible causes were identified to explain the 

decreased phytoplankton abundance in the lower river in 

recent years. First, the level of algae in the river, estimated 

from phytoplankton chlorophyll-a (Chl-a) concentrations, has 

declined in the upper river. While such a decline alone would 

almost certainly reduce the size of downstream populations, 

the simultaneous dilution from increased reservoir releases for 

flow augmentation and higher discharges from the Rock Creek 

wastewater treatment facility (WWTF) also work to reduce 

algal populations. In addition, a slightly higher turbidity in 

the upper river may be important in causing an increased 

occurrence of light limitation. Soluble reactive phosphorus 

(SRP) concentrations, the percentage of streamflow from 

various sources (natural flow, flow augmentation, and WWTF 

effluent), and zooplankton grazing all were identified by the 

multivariate BEST analyses as important factors for explaining 

variations in the phytoplankton species composition; all these 

were listed as possible factors contributing to bloom crashes 

in 2006–08.

It is clear from data and modeling that streamflow exerts 

an important influence on phytoplankton abundance in the 

Tualatin River by affecting residence time, but the source of 

that flow also is important. The fact that flow variables were 

the most important factors in the top two multivariate BEST 

models suggests that it is the quality of flow, not simply the 

magnitude of flow (and its effect on residence time), that 

shapes phytoplankton assemblages. This influence could be 

due to a change in the amount or quality of phytoplankton 

inoculum entering the lower reservoir reach, the turbidity 

or SRP level of the water, or some other property. All such 

characteristics could have implications for downstream 

algal populations, photosynthesis and dissolved oxygen 

(DO) production.

Algal growth limitations due to low phosphorus 

concentrations probably did not account for the general algal 

declines observed in the Tualatin River since 2003 unless 

phosphorus bioavailability also has changed. As previous work 

(Rounds and others, 1999) has demonstrated, decreased SRP 

concentrations, at a minimum, can place a cap on the size of 

the blooms. Important questions remain, however, regarding 

the extent to which the SRP in the river and in WWTF effluent 

is bioavailable. 

This study demonstrated the value of a long-term, 

basin-wide, routine monitoring program in helping to 

decipher longitudinal patterns in river conditions and trends 

over time. Such data will likely continue to advance an 

understanding of how the Tualatin River functions with 

respect to its plankton communities, and the data may help 

explain future changes or potential causes of water-quality 

problems. Only by considering data over the long-term can 

patterns in year-to-year variability be understood, and unusual 

years be identified—such as the 2001 drought or the 2008 

“Wapato Event.” Without such data, many more questions 

would remain regarding the annual cycles in phytoplankton 

populations in the river and reasons for the recent declines. 

The results of this study can be used to refine future 

monitoring to confirm and further develop the hypotheses put 

forth here. Using that information, well-designed experiments 

and targeted sampling before, during, and after bloom 

crashes at key locations in the lower and upper reaches of the 

watershed should prove helpful in further understanding the 

sources, development, and dynamics of algal communities in 

the river. If phytoplankton photosynthesis is to be relied upon 

for maintaining minimum DO levels in the river, then a better 

understanding of algal sources and dynamics in the upper river 

is clearly needed.

If algal populations cannot provide sufficient DO to offset 

oxygen demands in midsummer under future flow conditions, 

then strategies aimed at reducing the sediment oxygen demand 

(SOD) might be the only other option—short of aerating 

the river or greatly increasing streamflow. A study using 

stable isotopes to characterize the sources of organic matter 

to bed sediments of the Tualatin River (Bonn and Rounds, 

2010) determined that terrestrial plants and soils were the 

most important sources of organic matter to river sediments, 

implying a largely terrestrial source that sustains the SOD. 

Erosion control in a basin with large loads of fine sediment, 

however, can be difficult. New strategies might be needed to 

control erosion and stabilize streambanks. A study examining 

the sources and transport of organic matter in the Fanno Creek 

basin is using multiple techniques, including new fluorescence 

methods that may prove useful for identifying carbon sources 

in the watershed that might contribute disproportionately to 

the river SOD. 

Building on results from this and other studies, a great 

deal of research still can be done on the factors affecting 

phytoplankton growth and population dynamics in the river, 

and the degree to which they are affected by flow quantity 

and sources, phosphorus availability, zooplankton grazing, 

temperature, and food web dynamics.
72 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

Possible Future Studies, Monitoring, 

and Research

While this study has identified several factors that appear 

to influence phytoplankton growth in the Tualatin River, 

there are additional areas of research that would deepen our 

understanding of how plankton assemblages in the Tualatin 

River function and respond. The following section provides 

some ideas and direction for future studies, monitoring, and 

research to fill data gaps, refine our understanding of plankton 

dynamics, and inform and enhance water-quality models that 

can be used by water managers to develop strategies that 

improve DO conditions in the lower Tualatin River.

Monitoring Needs

The current monitoring program implemented by Clean 

Water Services (CWS) and the U.S. Geological Survey 

(USGS) and other entities includes little monitoring of Chl-a

or plankton communities upstream of Rood Bridge. Certain 

factors identified in this study, however, connect declines in 

phytoplankton in the lower river to conditions occurring in 

the upper river—such as the amount and characteristics of 

the algal population, the importance of various streamflow 

sources, and the amount and sources of turbidity. Additional 

biological sampling (Chl-a and algal identification and 

enumeration) at a few key sites in the upper basin would be 

useful for discerning the effects of the various flow sources 

in the basin and for monitoring the potential sources of algal 

inocula to the lower river. Additional sampling in the upper 

basin might also be necessary to identify the possible presence 

and causes of elevated turbidity that could be contributing 

to lower algal abundance in recent years. The current river 

monitoring program has, in the past, included algae and 

zooplankton sampling in the lower Tualatin River at Stafford 

Road (RM 5.5); that type of monitoring data should prove 

useful in characterizing plankton communities in that part of 

the reservoir reach. Other stations such as Elsner (RM 16.2) 

or Cook Park (RM 9.9) could be added to further enhance the 

monitoring program. These additions would generate data to 

help test or refine some of the hypotheses discussed in this 

report and strengthen our understanding of the factors that 

control phytoplankton populations in the upper and lower 

Tualatin River.

Periodic and targeted monitoring of plankton species 

and Chl-a (discrete or continuous) at sites in the upper basin 

such as Gaston or Cherry Grove, Wapato Creek, Scoggins 

Creek, and the Tualatin River between Dilley and Golf 

Course Road would provide critical information on the type 

and amount of algal inocula during the early, middle, and 

later parts of summer. This information could be used in an 

expanded multivariate data analysis or used to enhance waterquality models of the river in order to explore the effects of 

management strategies such as changes in the schedules of 

flow augmentation.

In addition to evaluating upstream reservoirs as sources 

of algal inocula, other sources including farm ponds, 

tributaries, and wetland drainage from areas including 

Wapato Lake, Fernhill Wetlands, and Jackson Bottom could 

be examined for their potential to contribute plankton to the 

Tualatin River. Periodic surveys conducted during spring 

and summer, for example, would be useful to document 

contributions of algae (and zooplankton) from these sources. 

The large effect of draining the Wapato Lake agricultural 

area in 2008 highlights the need to monitor this and other 

sources of upstream plankton inocula. Sampling of the 

Tualatin River at or near the Spring Hill Pump Plant, a site 

that is downstream of many important inputs such as Wapato 

Creek, Barney Reservoir, and Hagg Lake, could be used to 

initiate targeted sampling of these individual upstream sources 

when warranted. 

Bioassay Experiments

Bioassay experiments, similar to the ones conducted in 

this study, could be used to answer questions regarding the 

effect of factors such as elevated turbidity, flow augmentation 

from various sources, and WWTF effluent on phytoplankton 

abundance. Such experiments can also be used to determine 

the level of phosphorus limitation, or to determine specific 

growth rates and other parameters used for input to waterquality models. While this study found that moderate levels 

of WWTF effluent may stimulate algal growth, higher 

concentrations (50 percent) may have either a reduced 

stimulatory effect or a negative effect. Additional bioassay 

tests with finer resolution of wastewater percentages could be 

used to verify and refine these interpretations.

Bioassays also could be used to determine whether 

SRP from various sources is available for algal growth. 

Although abundant data are available to determine the SRP 

levels in the river, it remains unclear whether that phosphorus 

is bioavailable (Mayer and Jarrell, 1995; Li and Brett, 

2010). Detailed experiments that analyze the geochemical 

characteristics of the phosphorus, such as those used by 

Simon and others (2009), might be helpful. In addition, 

bioassay experiments using smaller increments of phosphorus 

to test samples collected during specifically targeted 

periods—such as when it appears that phosphorus might 

be limiting algal growth—might be useful for estimating 

phosphorus bioavailability.
Acknowledgments 73

Studies of Zooplankton—Phytoplankton 

Interactions and Fish Predation

Future studies of zooplankton–phytoplankton 

interactions, the effect of planktivorous fishes on zooplankton, 

and the cascade of effects that dictate the abundance and 

photosynthetic rates of the phytoplankton and their production 

of DO in the lower Tualatin River would be beneficial. 

Such experiments might also measure respiration rates of 

zooplankton and phytoplankton to gage their direct effect on 

DO. Future studies of zooplankton–phytoplankton interactions 

in the Tualatin River could provide a greater understanding 

of how plankton interact. It has been difficult to disentangle 

the effects of flow management and zooplankton grazing in 

some of the historical data; targeted studies of the zooplankton 

and their grazing effects would help to fill this gap in 

our understanding.

Little is known about the abundance of non-game fishes 

or their size/age distributions in the Tualatin River, but 

several fish species in the river are planktivorous when they 

are young, particularly nonnative species such as largemouth 

bass, bluegill sunfish, black crappie, pumpkinseed, yellow 

perch, and warmouth, which could have an important effect 

on zooplankton populations in the Tualatin River. Native 

planktivorous species include redside shiner, peamouth, and 

speckled dace. Previous USGS studies in the Tualatin River 

have found that small fish (less than 150 millimeters in length) 

were feeding on zooplankton. Future studies could examine 

the effect of planktivorous fish predation on zooplankton and 

the resulting effects on phytoplankton. A better understanding 

of these complexities could greatly improve existing or future 

water-quality models and river management strategies.

Artificial Neural Network Models

Although the artificial neural network (ANN) model 

developed by Rounds (2002) to predict DO concentrations at 

the Oswego Dam based on streamflow, sunlight, rainfall, and 

air temperature predicts DO with reasonable accuracy, the 

ANN model might be improved with additional inputs such 

as the proportions of natural flow, flow augmentation, and 

WWTF effluent, as well as ammonia loads from the WWTFs. 

Calibrating the ANN model with data on the size of the algal 

inocula entering the reservoir reach might also improve the 

accuracy of modeled phytoplankton population sizes. Other 

inputs to the model might include a seasonal component 

to the modeled zooplankton population, to better predict 

algal crashes and DO concentrations. Currently, the model 

performance is least successful during these periods.

ANN models are particularly well suited for problems 

in which large datasets—such as the available continuous 

water‐quality, streamflow, and meteorological data (solar 

radiation and rainfall)—contain complicated nonlinear 

relations among many different inputs. Although mechanistic 

models that capture the essence of a system’s instream 

processes often can be used to great advantage in evaluating 

management strategies, the complexities of the algal 

community in the Tualatin River and its ties to various flow 

sources and upstream inocula are somewhat limiting for such 

models. ANN models, on the other hand, can utilize a wealth 

of data and are not tied to any particular instream mechanism 

or factor; the key is in collecting the right kind of data and 

including those data in the ANN analysis. ANN models have 

been used successfully to predict the timing and magnitude 

of Chl-a in the Nakdong River (Korea) using similar types of 

data (Jeonga and others, 2001). An updated and more refined 

ANN model could be constructed for the Tualatin River to 

further evaluate and refine the hypotheses put forth here, or to 

develop new ones in future years.

Periodic Revisitation of the 

Multivariate Analyses

The multivariate statistical analyses of the combined 

biological, chemical, and streamflow datasets in this study 

proved useful in identifying several factors affecting 

the presence, abundance, seasonal patterns, and other 

characteristics of the algal community in the Tualatin River. 

As additional datasets are collected in the future, either 

through routine monitoring or targeted studies, it might be 

beneficial to revisit these multivariate analyses to determine 

whether the patterns and factors identified here are still 

important, identify any new influences, and detect trends or 

community shifts that might be tied to management strategies.

Acknowledgments

The authors wish to thank Jan Miller, Mark Poling, 

Robert Baumgartner, and Raj Kapur of Clean Water Services 

for their insights into the water quality of the Tualatin 

River and helpful comments on data interpretations. We are 

especially thankful to Jan Miller for her enthusiastic support 

for this project, and to Robert Baur for providing treated 

effluent for the bioassay experiments and insights into facility 

operations. We also appreciate the high-quality data produced 

by the Clean Water Services Water-Quality Laboratory and 

want to acknowledge and thank Steve Thompson and his staff 

for their dedication and willingness to analyze extra water 

samples from the USGS sampling trips. We thank Bernie 

Bonn for sharing her knowledge of the basin and providing 

data files on flows, reservoir releases and withdrawals used 

in the annual reports of the Tualatin River Flow Management 

Technical Committee. This analysis benefited from discussions 

with several people, including Peter Schmidt from the Tualatin 
74 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

River National Wildlife Refuge and Tom Murtagh from the 

Oregon Department of Fish and Wildlife. We also want to 

recognize and thank several USGS staff, including Tammy 

Wood and Annett Sullivan, for sharing their knowledge of the 

basin, and Micelis Doyle, Michael Sarantou, Jami Goldman, 

and Kiara Smith (formerly with USGS) for helping with 

sample collection. Matt Johnston and Micelis Doyle are 

primarily responsible for operating the main-stem continuous 

water-quality monitors for the last 20 years, and their efforts to 

provide such a complete dataset is much appreciated. Lastly, 

we thank the staff of the USGS Portland Field Office and 

the Oregon Water Resources Department for maintaining the 

continuous streamflow gages in the Tualatin River basin.

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76 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon
Appendixes 77

Appendixes

Appendixes A–C are presented as Microsoft© Excel spreadsheets. They can be accessed and downloaded at 

http://pubs.usgs.gov/sir/2013/5037/

Appendix A. Quality-Assurance Data for Water Quality, Chlorophyll-a, Phytoplankton, and 

Zooplankton from the Tualatin River, Oregon, 2006–08

Appendix B. Algal Taxa Biovolume for Samples Collected from the Tualatin River, Oregon, 2006–08

Appendix C. Zooplankton Taxa Counts for Samples Collected from the Tualatin River, Oregon, 

2006–08
78 Plankton Communities and Summertime Declines in Algal Abundance in the Tualatin River, Oregon

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Publishing support provided by the U.S. Geological Survey 

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For additional information contact:

Director, Oregon Water Science Center

U.S. Geological Survey

2130 SW 5th Avenue

Portland, Oregon 97201

http://or.water.usgs.gov
38.4 24.5 10.8 9.5 9.2 5.4 3.4

Main-stem river mile

Carpenter and Rounds— Algal Abundance Associated with Low Dissolved Oxygen in the Tualatin River, Oregon—Scientific Investigations Report 2013–5037