Factors Initiating 

Phytoplankton Blooms 

and Resulting Effects on 

Dissolved Oxygen in 

Duwamish River Estuary 

Seattle, Washington

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1873-A

Prepared in cooperation with the 

Municipality of Metropolitan Seattle
Factors Initiating 

Phytoplankton Blooms 

and Resulting Effects on 

Dissolved Oxygen in 

Duwamish River Estuary 

Seattle, Washington

By EUGENE B. WELCH

ENVIRONMENTAL QUALITY

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1873-A

Prepared in cooperation with the 

Municipality of Metropolitan Seattle

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HIGKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 35 cents
CONTENTS

	Page

Abstract_ _____________________________________________ ________ Al

Introduction _____________________________________ _ ______________ 2

Purpose and scope_____________________________________________ 2

Description of the estuary______________________________________ 3

Background information on conditions in the estuary __________ _ __ 3

Acknowledgments. ____________________________________________ 6

Procedures and methods________________________________________ 6

Sampling____________ _________________________________________ 7

Analysis and computation____________________________________ 12

Evaluation of methods________________________________________ 15

Results____________________________________ 18

Phytoplankton distribution.____________________________________ 18

Effects of effluent nutrients_____________________________________ 24

Phosphorus._____ _______________________________________ 25

Nitrogen_____________________________________________ 28

Maximum biomass produced________________________________ 30

Effects of physical factors___________________________________ 33

Relation of phytoplankton to dissolved oxygen.___________________ 41

Distribution of dissolved oxygen____________________________ 41

Relation of phytoplankton to biochemical oxygen demand and

dissolved oxygen________________________________________ 44

Evaluation of results___________________________________________ 49

Timing of phytoplankton blooms.______________________________ 49

Effect of nutrients on maximum biomass produced_______________ 56

Relation of phytoplankton to dissolved oxygen____________________ 58

Literature cited_________________________________________________ 60

ILLUSTRATIONS

Page

FIGURE 1. Map of Duwamish River and vicinity, Washington, showing principal sampling stations and the Renton Treatment Plant_______________________-________------ A4

2-6. Graphs showing:

2. Relation of phytoplankton-productivity rate to 

chlorophyll a concentrations at four stations 

during 1966_____________ 17

3. Distribution of phytoplankton blooms during 

June 1964-September 1966, as indicated by 

chlorophyll a concentrations at five stations _ 19

m
IV CONTENTS

FIGURES 2-6. Graphs showing Continued page

4. Relation of phytoplankton activity to solar radiation, maximum water temperature, minimum 

specific conductance, and tidal-prism thickness 

at station 7.7 during the phytoplankton bloom 

in August 1966___________________ A 20

5. Distribution of phytoplankton, as indicated by cell 

centrations at five stations, June 1964-September 1966___________________ 22

6. Phytoplankton productivity at four stations, July

1964-September 1966_______________. 24 

7-9. Graphs showing nutrient content at five stations:

7. Total phosphorus, June 1964-September 1966. _ _ 26

8. Downstream changes in annual mean concentrations of ammonia nitrogen, nitrate nitrogen, 

total-phosphate phosphorus, and soluble-phosphate phosphorus during 1963-66-___________ 27

9. Soluble phosphate at high and low tide, January

1965-October 1966__________________ 29 

10-16. Graphs showing:

10. Increased assimilation of carbon-14 in samples to

which effluent nutrients were added-_________ 31

11. Water temperature, incident solar radiation, and

residual radiation during 1964-66.___________ 34

12. Mean daily fresh-water discharge at station 21.0,

May-September 1963-66______________ 36

13. Relation of phytoplankton-bloom timing to freshwater discharge and tidal-prism thickness of 

June-September 1965 and 1966____________ 37

14. Vertical profiles of specific conductance and

temperature during June-August 1966________ 38

15. Contrast between growth of phytoplankton in 

place and that of potential phytoplankton 

during August and early September 1966_____ 40

16. Relation of minimum DO concentrations to 

phytoplankton-bloom timing and to freshwater discharge, June-September 1963-66 _ 42 

17-19. Graphs showing relation of chlorophyll a, BOD, and DO, 

June-September 1966:

17. Longitudinal distribution at low tide ___________ 43

18. Longitudinal distribution at high tide ______ 44

19. Seasonal variation at low tide in the area of

minimum DO______________________------- 46

20-22. Graphs showing:

20. Monthly means of chlorophyll a and BOD in the

area of minimum DO, June-September 1966. _ 47

21. Vertical distribution of chlorophyll a, BOD, and 

DO at station 9.7 on August 9, 1966, during 

phytoplankton bloom______________-_----- 49

22. Logarithmic relationship of chlorophyll a to BOD

during the summer of 1966__________ _- 50
CONTENTS

TABLES

Page 

TABLE 1. Schedule for collection of water samples during 1963-66- A 8

2. Results of variance analyses comparing downstream 

changes in mean total-phosphate concentration before 

and after introduction of effluent from the RTF (cell 

sizes unequal)--_-___---_-___-___---_-_______-___- 28

3. Results of variance analyses comparing downstream 

changes in mean ammonia concentration before and 

after introduction of effluent from RTF (cell sizes 

unequal)________________________________________ 30

4. In vitro carbon-14 assimilation by phytoplankton inoculated into filtered river water containing different 

effluent nutrient contents_____-____-__-___--_______ 32

5. Results of variance analyses comparing downstream 

changes in mean annual chlorophyll o concentrations 

in 1965 and 1966 (cell sizes unequal) _ _ _ _ __ _ _ 33
ENVIRONMENTAL QUALITY

FACTORS INITIATING PHYTOPLANKTON BLOOMS

AND RESULTING EFFECTS ON DISSOLVED

OXYGEN IN DUWAMISH RIVER ESTUARY

SEATTLE, WASHINGTON

By EUGENE B. WELCH

ABSTRACT

Phytoplankton productivity, standing stock, and related environmental factors 

were studied during 1964-66 in the Duwamish River estuary, at Seattle, Wash., 

to ascertain the factors that affect phytoplankton growth in the estuary; a 

knowledge of these factors in turn permits the detection and evaluation of the 

influence that effluent nutrients have on phytoplankton production. The factors 

that control the concentration of dissolved oxygen were also evaluated because 

of the importance of dissolved oxygen to the salmonid populations that migrate 

through the estuary.

Phytoplankton blooms, primarily of diatoms, occurred in the lower estuary 

during August 1965 and 1966. No bloom occurred during 1964, but the presence 

of oxygen-supersaturated surface water in August 1963 indicates that a bloom 

did occur then.

Nutrients probably were not the primary factor controlling the timing of 

phytoplankton blooms. Ammonia and phosphate concentrations increased significantly downstream from the Municipality of Metropolitan Seattle's Benton 

Treatment Plant outfall after the plant began operation in June 1965, and concentrations of nitrogen and phosphorus were relatively high before operation of 

the Renton Treatment Plant and during nonbloom periods. The consistent coincidence of blooms with minimum fresh-water discharge and tidal exchange during August throughout the study period indicates that bloom timing probably 

was controlled mostly by hydrographic factors that determine retention time 

and stability of the surface-water layer. This control was demonstrated in part by 

a highly significant correlation of gross productivity with retention time (as 

indicated by fresh-water discharge) and vertical stability (as indicated by the 

difference between mean surface and mean bottom temperatures). The failure 

of a bloom to develop in 1964 is related to a minimum fresh-water discharge that 

was much greater than normal during that summer. Hydrographic factors are 

apparently important because, as shown by studies of other estuarine environments by other workers, phytoplankton production increases when the zone of 

vertical turbulent mixing is not markedly deeper than the compensation depth,

Phytoplankton cells produced in the surface waters sink, thereby contributing 

oxidizable organic matter to the bottom saline-water wedge. The maximum BOD

Al
A2 ENVIRONMENTAL QUALITY

(biochemical oxygen demand) in this bottom wedge occurs in the same section 

of the estuary and at the same time as the maximum phytoplankton biomass 

(as indicated by chlorophyll a) and minimum DO (dissolved oxygen). Other 

sources of BOD occur in the estuary, and conditions of minimum discharge and 

tidal exchange assist in reducing DO. Nonetheless, the highly significant correlation of chlorophyll a with BOD throughout the summer indicates that respiration and decomposition of phytoplankton cells is clearly an important contributor 

of BOD.

Increases in the biomass and resultant BOD of blooms because of increased 

effluent nutrients presumably would further decrease the concentration of DO. 

This possible effect of effluent nutrients was evaluated by laboratory bioassays 

and by a comparison of mean annual biomasses in the estuary. A green algal 

population in vitro did increase in response to added effluent nutrients; however, 

the available field data suggest that a 46-percent increase in effluent discharge 

between 1965 and 1966 did not increase the estuary's phytoplankton biomass 

significantly.

INTRODUCTION 

PUEPOSE AND SCOPE

A cooperative program between the Municipality of Metropolitan 

Seattle (Metro) and the U.S. Geological Survey was initiated in 1963 

to study the variation in biological populations in the Duwamish 

River estuary in relation to water quality changes due particularly 

to domestic and industrial waste-disposal and treatment practices.

To determine the influence of physical factors on phytoplankton 

in the estuary, in situ productivity and standing stock measurements 

were studied in relation to the physical variables of water stability, 

retention time, and light transmittance. These variables were indicated by measurements of (1) salinity and temperature at different 

depths in a profile, (2) volume of fresh-water discharge, (3) tidalprism thickness, (4) incident solar radiation, and (5) water transparency. To evaluate the effect of effluent nutrients on phytoplankton, 

the relation of productivity and standing stock measurements to phosphorus and nitrogen concentrations was studied in situ during periods 

of active phytoplankton growth. Further indications of the effect of 

the effluent were obtained from in vitro bioassays in which the quantity 

of added effluent was varied under controlled conditions. Phytoplankton standing stock and BOD (biochemical oxygen demand) distribution were related to the timing and location of DO (dissolved 

oxygen) minima to determine the importance of phytoplankton in 

causing oxygen depressions. This report evaluates possible influences 

of changes in water quality (especially increased nitrogen and phosphorus) on phytoplankton production and the resulting effect on 

dissolved-oxygen conditions in the estuary.
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A3 

DESCRIPTION OF THE ESTUARY

The area studied includes the lower 21 km (kilometers) of the 

Duwamish Kiver at Seattle, Wash. (fig. 1). This river section has a 

surface area of about 2.6 sq km (square kilometers) and ranges in 

depth from less than 1.0 m (meter) at the upper end to IT m near the 

mouth; its mean depth is 9.5 m. The volume is estimated to be 11.2 X 

106m3 (cubic meters). In the transition zone where the river enters the 

lower 10 km of the estuary a reach that has been dredged for navigation the velocity of flow decreases. Inorganic and organic particulate matter are deposited in this transition zone just upstream from 

the navigable reach and contribute to the depression of dissolved-oxygen concentrations. These conditions will be discussed in detail later 

in the report.

Five regular sampling stations in the river will be referred to by 

their distances 1.9, 7.7, 10.4,12.6, and 21.0 km above the mouth (fig. 

1). The effluent from Metro's ETP (Eenton Treatment Plant) enters 

the river 20.4 km above the mouth.

Fresh-water discharge ranged from 425 to 20,400 m3/min (cubic 

meters per minute) during 1963-66. The mean tidal range recorded 

6.5 km above the mouth was 2.3 m. The relatively large tidal range 

and discharge rate during most of the year result in conditions of 

great turbulent mixing and flushing, which upset the salinity and 

temperature patterns. The entire 21-km reach is affected by the tide, 

but the saline-water wedge intrudes only the lower two-thirds of the 

reach and then only at high tide and low discharge. Stoner (1967) 

has described conditions that permit saline-water intrusion to kilometer 12.6. Under these conditions saline water is always present at high 

tide when discharge is less than 1,060 m3/min, but is never present 

when the discharge is greater than 1,700 m3/min. Between these two 

discharge rates, saline intrusion depends on discharge and tide height. 

Water stability and retention time increase in the estuary as the period 

of low summer discharge is approached and during periods of low 

tidal exchange (high-low and low-high tides). Conditions of increased 

stability are indicated by increased differences in specific conductance 

and temperature with depth. During these low-discharge conditions, 

water dyed with rhodamine-B takes about 5 days to traverse the 21 km 

estuary.

BACKGROUND INFORMATION ON CONDITIONS IN THE ESTUARY

The Duwamish estuary receives waste effluents that affect its water 

quality. The upland flow into the estuary is a dilute soft water, but it 

carries some municipal, industrial, and agricultural byproducts. In ad321-832
A4 ENVIRONMENTAL QUALITY

ELLIOTT BAY 

PUGETSOUND)

4 KILOMETERS

FIGURE 1. Map of Duwamish River and vicinity, Washington, showing principal 

sampling stations (in kilometers above river mouth) and the Rentoii Treatment 

Plant.
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A5

dition, many sewers discharge untreated material into the lower estuary. In June 1965, Metro's RTP, which provides secondary treatment 

for sewage from a large area adjacent to Lake Washington, started 

discharging effluent into the Duwamish River. This plant is part of 

Metro's extensive system of waste-disposal and treatment facilities 

designed to eliminate the discharge of treated wastes into Lake Washington and of untreated wastes into Puget Sound.

Conditions of low DO have frequently occurred in the lower estuary 

during late summer (Okey, 1957; Isaac and others, 1964). There is 

concern that any further depression of DO during that period of the 

year will interfere with the migration of chinook and coho salmon. 

By about 1970, as the Metro waste-disposal system nears completion, 

all waste effluents presently discharging into the Duwamish below 

the RTP (mostly near the mouth) will be intercepted and discharged 

into Puget Sound following primary treatment. Though the treated 

effluent discharge from the RTP will continue to increase, the total 

BOD added to the estuary will ultimately be greatly reduced. In the 

meantime, the RTP effluent is superimposed on the other wastes in the 

estuary.

In 1966 the RTP operated at only about 9 percent of its ultimate 

capacity of 545,000 ms/day (cubic meters per day), and the effluent 

discharge was about 8 percent of the ultimate quantity of the minimum 

river discharge, but the efficiency of BOD removal by the RTP was 

high. The average effluent BOD during 1966 was 10.5 mg/1 (milligrams per liter), which represents only 8 percent of the influent BOD. 

The removal of mineral nutrients, however, was only 16 percent for 

ammonia and 3 percent for phosphate. The average effluent concentrations of ammonia and phosphate during 1966 were 15.5 mg N/l (milligrams of nitrogen per liter) and 9.5 mg P/l (milligrams of phosporus 

per liter), respectively. The average nitrate concentration in the effluent during 1966 was only 0.24 mg N/l (G. T. Mason, Metro, written 

commun., 1967).

Because BOD contribution by the RTP is small, the effluent was 

expected initially to have only a slight direct effect on dissolvedoxygen concentrations in the estuary. The relatively high nutrient content of the effluent, however, should result in increased nutrient concentrations downstream from the outfall. Therefore, if the effluent has 

any direct biological effect, there should be increased production of 

phytoplankton through utilization of the added nutrients. Nuisance 

blooms and eutrophication are well known problems that develop from 

increased phytoplankton production, frequently as a result of nutrients in domestic waste (Hasler, 1947; Edmondson and others, 1956; 

Bartsch, 1960; Barlow and others, 1963; Shapiro and Ribeiro, 1965; 

Oglesby and Edmondson, 1966; and Oswald and Golueke, 1966).
A6 ENVIRONMENTAL QUALITY

Physical factors such as stability, retention time, and transparency 

of the water mass are particularly critical to estuarine phytoplankton 

production. The timing of phytoplankton blooms in oceanic environments (particularly in the spring), as well as estuarine environments, 

is determined by the relations between depth of turbulent mixing, 

available light, and depth at which production equals respiration 

(termed compensation depth; Riley, 1942, Sverdrup, 1953, and Anderson and Banse, 1961). Gran and Braarud (1935) found that in waters 

which had a high tidal range and strong vertical mixing, phytoplankton were prevented from accumulating in the illuminated zone. These 

conditions were unfavorable for production even though nutrient concentrations were always high. Because the tidal range, flushing rate, 

and turbulent mixing are relatively great in the Duwamish even at 

low river discharge (Brown and Caldwell, 1958) and because nutrient 

concentrations were high before the effluent was introduced, a moderate increase in estuarine nutrient concentrations following activation of the RTF was not expected to alter bloom timing. The total 

phytoplankton biomass of a bloom might increase, however, as a result 

of a nutrient increase, thereby contribute to the BOD, and further depress the oxygen concentration in the estuary.

ACKNOWLEDGMENTS

The study was carried out under the direct supervision of J. F. Santos, Project Chief, and under the general direction of L. B. Laird, 

District Chief, Water Resources Division, U.S. Geological Survey.

The assistance provided by project personnel of the U.S. Geological 

Survey and of Metro contributed greatly to this study. In particular, 

Mr. Gary W. Isaac of Metro offered many helpful suggestions and 

furnished assistance in the collection of the data. Critical review of the 

manuscript by Doctors Max Katz, Richard Van Cleve, and Alan C. 

DeLacy of the University of Washington and Dr. Keith V. Slack of 

the U.S. Geological Survey is most appreciated. In addition, Doctors 

W. T. Edmondson and Karl Banse of the University of Washington 

kindly reviewed results on various occasions during the study.

PROCEDURES AND METHODS

Procedures for the collection and analysis of water samples varied 

during the period of study. Because this was a cooperative project 

between Metro and the U.S. Geological Survey, both agencies contributed to the collection of data. The sample-collection schedule also 

changed as the project progressed and an increased understanding was 

gained of the variability and importance of some of the measurements. 

The data which specifically relate to the study of phytoplankton are 

included in table 1.
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A7

SAMPLING

Water samples for the determination of chlorophyll a, phytoplankton cell number, specific conductance, and temperature were collected 

on a reasonably consistent schedule, particularly after 1964 (table 1). 

During 1964, samples for the determination of these four variables 

were collected about twice a month, usually at a midtide stage. During 

1965-66, samples for the determination of nutrients were collected 

monthly during the winter months and biweekly during the remaining 

time at both high and low tide. Samples were also collected from the 

saline-water wedge (1m above the bottom during 1965-66 and 5 m 

below the surface in 1964) at stations 1.9 and 7.7 for determination of 

all variables except nutrients.

The sampling procedure for nutrients varied slightly from year to 

year (table 1). Metro personnel collected samples from the five principal stations biweekly during 1963-65 and monthly during 1966, but 

no collections were made between January and April in 1963 and 1964. 

Data from the Metro collections included determinations of nitrate, 

ammonia, and total phosphorus. During 1965-66, soluble phosphate 

and, occasionally, orthophosphate were determined. Metro samples 

were collected without regard to tidal stage, except for a few times 

in 1966. In 1965, samples were collected by the Geological Survey at 

high and low tide at the same stations where phytoplankton samples 

were collected; they were analyzed for nitrate and soluble phosphate. 

In 1966, ammonia and most of the other nutrient forms were determined in samples collected at high and low tide. Samples for phosphate 

determination were "fixed" with chloroform, and all samples were 

kept on ice or were refrigerated until analyzed.

The procedures for estimating primary productivity rates differed 

between years and between stations (table 1). (For a definition of the 

terms primary, net, and gross productivity, see Strickland, 1960, p. 6- 

8.) Samples for determination of productivity and chlorophyll a were 

collected at stations 1.9, 7.7,12.6, and 21.0 bimonthly during the winter 

months, and monthly during the rest of the year. At the same time, 

Secchi-disk measurements were made at stations 1.9 and 7.7. In 1966, 

samples were collected at all stations for determination of total carbon 

dioxide. Samples were collected with a Van Dorn bottle at the surface 

and from depths of 1, 2, 3, and 4 m at the lower stations (1.9 and 7.7; 

fewer depths were sampled for productivity determinations from 

July to October 1964; see table 1) and at the surface and from a depth 

of only 1 m at the upper stations. Productivity profiles and lightattenuation estimates indicate that the depth range from the surface 

to about 4 m usually includes most of the photic zone. Measurements 

were limited to 1 m or less at the upper stations because the maximum
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FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON All

depth of the river was only about 1 m during low flow. The light- and 

dark-bottle oxygen method was used to estimate productivity at all 

stations in 1964 and 1965, but only at station T.T in 1966. Estimates 

of productivity by the carbon-14 method were made at all stations in 

1966. Duplicate light-bottle samples were collected at each depth for 

use with both methods, but duplicate dark bottles were used only with 

the oxygen method. Samples were incubated in situ for 24 hours (48 

hours in the winter) for use with the oxygen method and 4 hours for 

use with the carbon-14 method). Incubation times using the oxygen 

method were shorter (6-8 hrs) in July and August 1964. Incubation 

depths (with the exception of July-October 1964) were 0.25,1,2,3, and 

4 m at the lower two stations and 0.25 and 1 m at the upper three 

stations.

The incubating bottles were attached to a weighted line which 

hung vertically from the center of an aluminum bar suspended between polyurethane floats. This unit was attached to the bridge structure at each station in a manner that permitted free vertical movement 

with the tide. The arrangement maintained the bottles satisfactorily 

under conditions of fluctuating tidal stage and current.

Data were collected during June-September 1966 to study the relation between phytoplankton standing stock (as indicated by chlorophyll a, concentration), DO, and BOD (table 1). Water samples were 

collected, usually weekly, at high and low tide less than a meter above 

stream bottom at stations 1.9, 4.3, 6.4, T.T, 10.0, 10.2, and at 10.4 km 

above the mouth. At the same time, surface samples were collected at 

stations 1.9 and T.T. In addition, DO, BOD, chlorophyll «, nutrients, 

temperature, and specific conductance were determined in samples 

collected biweekly from the surface and bottom at station T.T.

The "potential" of water to produce phytoplankton was studied 

under controlled conditions. Duplicate surface or 1-m water samples 

were collected at station T.T during August 1966 along with scheduled 

chlorophyll samples. The additional samples were incubated in vitro 

under conditions of uniform light (about T,000 lux) and temperature 

(20°-21°C), without mixing for 5 or 6 days. After this period, chlorophyll a concentration was determined and compared with the initial 

concentration.

Surface-water samples were collected from stations T.T, 12.6, and 

21.0 on March 1 and June 3,1966, for a study of the effect that ETP 

effluent had on phytoplankton production in vitro. Samples were 

filtered through HA Millipore filters of an average 0.45/u, (micron) 

porosity. Sufficient filtered effluent was added to three 500-ml (milliliter) flasks containing water from station T.T to obtain concentrations 

of 5,10, and 25 percent effluent in a final volume of 400 ml. No effluent

321-832 O 69 3
A12 ENVIRONMENTAL QUALITY

was added to three other flasks containing filtered water from stations 

7.7, 12.6, and 21.0. Each, of these flasks was inoculated with 5 ml of 

river water from station 7.7. Sodium chloride was added in various 

amounts to filtered water in one sample series to equalize salinity between the flasks. Radioactive carbon in sodium carbonate form was 

added to each flask at a concentration of 50/*c/l (microcuries per liter) 

to measure carbon assimilation according to the procedure by Groldman and Carter (1965, p. 1056). The flasks were incubated for about 

8 days under conditions of uniform light (about 7,000 lux) and temperature (20°-21°C) with moderate mixing. Aliquots of 25 or 50 ml 

were removed from each flask daily. These were then filtered and the 

activity of the filters was determined as a relative measure of carbon 

assimilation.

Profiles of specific conductance and temperature were determined 

in situ at station 7.7 three times per month at high and low tide, from 

June to September 1966 (table 1). These data were used as an indicator of vertical stability.

Automatic water-quality monitors have operated periodically at 

stations 1.9, 7.7,12.6, and 21.0 from August 1964 through 1966. Water 

for analysis is pumped into the monitors from 0.9 m below the surface 

at all stations and from 0.9 m above the bottom at the lower two stations (1.9 and 7.7). The monitor determinations of DO (table 1), temperature, and specific conductance were used to supplement data 

obtained biweekly or monthly by manual sampling and to study daily 

changes in the variables at station 7.7. Solar radiation measured with 

an Eppley pyrheliometer was recorded continuously by the monitor at 

station 12.6.

ANALYSIS AND COMPUTATION

Nutrient concentrations were determined by Metro according to 

methods defined by the American Public Health Association and others (1965). To show seasonal variability in nutrient concentrations, 

data collected by different sampling procedures were grouped to facilitate comparison. Where determinations are available for samples collected at high and low tide, a mean of the two values is included with 

data that are collected without regard to stage of tide. To illustrate 

variability due to tide, season, and conditions of river flow, both the 

high- and low-tide values are given.

Mean concentrations of ammonia and total phosphate were compared between stations and between years both by graph and by 

analysis of variance to determine if a significant increase occurred in 

posteffluent nutrient concentrations. For this purpose, ammonia and
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A13

total phosphate data from the preeffluent period, January-April 1965, 

were used for the same months in 1963 and 1964 for which data were 

not available. The use of these data permitted comparisons between 

annual mean concentrations before and after introduction of the effluent. Nutrient content is low and relatively constant during JanuaryApril because fresh-water discharge is high. Thus, actual nutrient 

values during that period in 1963 and 1964 probably differed little 

from those in 1965.

The Alsterberg modification of the Winkler method (American Public Health Association and others, 1965) was used to determine DO in 

samples collected with the Van Dorn bottle. The concentrations (with 

corrections for salinity and temperature) recorded by the monitors 

were determined automatically by a polarographic oxygen electrode.

BOD was determined by the standard method (American Public 

Health Association, 1965). Duplicate samples were drawn from a Van 

Dorn bottle; one was analyzed for initial dissolved oxygen and the 

other incubated at 20°C for 5-6 days. In August 1966, when DO declined, duplicate samples were drawn from a Van Dorn bottle as usual, 

but one was saturated with oxygen (mechanically aerated) and the 

other was left unsaturated.

Chlorophyll a was determined by the method of Richards and 

Thompson (1952) as modified by Creitz and Richards (1955). The 

specific absorption coefficients reported by Parsons and Strickland 

(1963) were used to calculate chlorophyll a concentration. To show 

seasonal variation in chlorophyll a, the most important factor in the 

occurrence of blooms, the chlorophyll a data collected by slightly different sampling procedures were grouped to furnish a more complete 

record for comparison between the 3 years. The majority of the data 

are means of values determined from samples collected biweekly at 

both high and low tide. Also included are means of values determined 

from surface and 1-m samples collected without regard to tide (from 

productivity studies), and values determined from samples collected 

on a midtide stage (mostly in 1964).

The procedures of Strickland and Parsons (1960, p. 153) were followed in productivity determinations. When the oxygen method was 

used, estimates were expressed as gross productivity of equivalent carbon and were based on a photosynthetic quotient, AO2/ACO2, of 1.2. 

Daily productivity per square meter was estimated to a depth of 4 m 

at the two lower stations and to a depth of 1 m at the upper stations. A 

mean of each pair of productivity measurements at successive depths 

was assumed to represent the mean of the cubic meter between them; 

for example, a mean of the surface and 1-m values represents the first 

meter in the column. If fewer depths were sampled or if the incubation
A14 ENVIRONMENTAL QUALITY

periods were shorter than usual (July-October 1964), data were extrapolated to permit productivity estimates to a 4-m depth at the lower 

two stations and to a 1-m depth at the upper stations.

Anomalies in the productivity results based on the light- and darkbottle oxygen method occurred particularly during winter, when 

photosynthetic activity was low. These anomalous results were modified slightly in computing a productivity estimate per square meter. 

Negative values for gross productivity (that is, dark-bottle oxygen 

concentration exceeding that of the light bottle) were treated as zero; 

however, where a negative respiration value (dark-bottle oxygen concentration exceeding the initial concentration) at a particular depth 

in the profile was associated with positive values at other depths in the 

profile, the negative value was replaced by the mean of the positive 

values. This was done because there is no logical explanation for negative respiration (an increase in oxygen content in the dark bottle). Assuming that organisms were fairly evenly distributed in the vertical 

profile when negative respiration values occurred, the respiration values should have been nearly equal at most depths. Under such conditions, a mean of the positive respiration values represents the best estimate of true respiration.

Standardized activity of the carbon-14 stock solution was determined by Dr. C. R. Goldman, professor of zoology at the University 

of California at Davis; he (1961, p. 104) used his total-filter combustion method. A Picker sealer equipped with a thin-window gas-flow 

ft detector was used to count filter activity. Other variations in the 

methods of Strickland and Parsons (1960, p. 153) included the use 

of smaller bottles (125 ml) and smaller filters (25 mm). The estimates 

of carbon-14 productivity determined for the 4-hour incubations were 

extrapolated to daily productivity by using the ratio of total daily 

incident light to light during the incubation period.

Total inorganic carbon concentration, a figure necessary to compute 

productivity rates by the carbon-14 method, was determined gravimetrically with a carbon dioxide train (Goldman, 1961, p. 106). The 

error between duplicate samples with this method was usually less 

than ± 5 percent.

Phytoplankton cell concentrations in water samples preserved with 

Lugol's solution were counted. The counts were made by the sedimentation technique, using an inverted microscope (Lund and others, 

1958). Cells greater than 5/* in the longest dimension were included 

in the counts. Usually, at least 50 cells each of diatoms and green algae 

were counted per sample. Limited data indicate that the error between 

duplicate observations is +10-20 percent and that most of this error 

is involved with subsampling the original sample. Diatom frustules
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A15

were included unless they were noticeably damaged. This procedure 

facilitated the distinction between "live" and "dead" cells. Diatoms 

and some green algae were usually classified to the generic level, but 

no attempt was made to classify spherical and flagellated green species. 

Computations were made for means of high- and low-tide concentrations, except for the 1964 data, when samples were collected on a midtide stage.

The mean daily solar radiation for each weekly period was computed 

from continuous data recorded by the monitor at station 12.6 and from 

supplementary data supplied by the Weather Bureau at the SeattleTacoma Airport. Light attenuation with depth in the water column 

was measured at station 7.7 with a submarine photometer and was 

compared with Secclii-disk measurements. This comparison provided 

a constant of 0.73 with which attenuation coefficients (K) could be 

computed from Secchi-disk measurements (Z>) according to the formula K=Q.lS/D. These data were used to compute the depth of the 

photic zone (the depth at which light intensity is 1 percent of the 

surface intensity) and the quantity of light reaching a particular 

depth.

The daily tidal-prism thickness was approximated by computing 

the difference between the sum of the two predicted high tides and the 

algebraic sum of the two low tides for each day. Fresh-water discharge 

was recorded continuously at station 21.0.

In the regular biweekly sampling, temperature was measured in the 

field with a pocket thermometer, and specific conductance (in micromhos per centimeter at 25°C) was measured in the laboratory with 

a conductivity bridge. Profiles of temperature and specific conductance 

were also measured in situ with a salinometer at depth intervals of 

0.9m.

Analysis-of-variance tests and correlation analyses were computed 

according to BMDO5V and BMDO2R digital computer programs that 

were prepared by the Health Sciences Computing Facility, University 

of California at Los Angeles.

EVALUATION OF METHODS

A comparison of BOD results from aerated and unaerated water 

samples indicated that the BOD was partly dependent on the initial 

DO. The mean BOD concentration was 1.5 mg/1 in unaerated samples 

with an initial DO less than 4.0 mg/1 and with a postincubation concentration greater than 0.0 mg/1. The mean BOD concentration in 

duplicate water samples that were aerated before incubation was 2.6 

mg/1, or 75 percent greater. Zobell (1940) found that oxidation of 

organic matter by marine bacteria was not limited by oxygen concen-
A16 ENVIRONMENTAL QUALITY

trations between 0.3 and 12.7 mg/1. He suggested, however, that a low 

oxygen concentration could limit oxidation reactions in localized microspheres that are rich in oxidizable matter, such as decomposing plankton. This situation could occur if oxygen is consumed more rapidly 

than it is replaced by diffusion from surrounding water. Such a phenomenon may account for the apparent dependence of BOD on oxygen 

concentration since phytoplankton density was great during the period 

of low dissolved oxygen in the estuary.

Part of the variability in chlorophyll a measurements was due to 

the method of sample handling and analysis, as well as to diurnal 

variation in tidal stage. The variability due to the method was evaluated by analyzing 13 subsamples from a single river sample collected 

at station 7.7. The mean and standard errors of this series were 2.6 and 

0.08 mg/m3 (milligrams per cubic meter), respectively. Chlorophyll 

a was found to vary inversely with tidal stage in the middle part of 

the estuary, where the greatest diurnal changes in salinity occur. This 

relationship and possible causes are discussed in a paper by Welch and 

Isaac (1967).

Because of interference by detrital chlorophyll, the use of cholorophyll a concentration as an estimate of phytoplankton standing stock 

or metabolic density is sometimes questioned (Saunclers and others, 

1962, p. 47), but chlorophyll a concentration is considered a reliable 

estimate of the standing stock in the Duwamish because of the highly 

significant correlations between chlorophyll a and productivity. 

Eegression analyses of productivity (carbon-14) data and of chlorophyll a data from all stations during 1966 (fig. 2) and of similar data 

(oxygen method) from all stations during 1964 65 (Welch and Isaac, 

1967) show correlation coefficients of 0.97 and 0.96, respectively. The 

regression line intercept for both groups of data is near the origin; 

this relation indicates that very little detrital cholorophyll is present. 

The larger concentrations and higher productivity rates of chlorophyll 

a probably explain these highly significant correlations. Under these 

conditions, populations are actively growing, and in an estuary characterized by considerable flushing, such as the Duwamish, there is little time for old, inactive cells (detrital chlorophyll) to accumulate.

Great differences have been reported between productivity rates 

measured by the carbon-14 method and those measured by the oxygen 

method. Estimates of productivity by the carbon-14 method give 

values that usually are greater than net but less than gross productivity, according to Steemann Nielson and Hansen (1959). Experimental 

work by Thomas (1964) showed that the relation of carbon-14 to net 

and gross oxygen productivity depends on the growth stage and the 

supply of nutrients available to the population. Antia and others,
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A17

 95-percent confidence 

interval for the line

CHLOROPHYLL a, IN MILLIGRAMS PER CUBIC METER

FIGURE 2. Relation of phytoplankton-productivity rate to chlorophyll a concentrations at four stations during 1966. Productivity is measured J>y the 

carbon-14 method. Points are means of measurements taken at stream surface 

and 1 m below stream surface. Inset shows off-scale values. Y is the productivity and X is the chlorophyll a concentration.
A18 ENVIRONMENTAL QUALITY

(1963) found a great difference between oxygen evolution (as equivalent carbon) and carbon assimilation in a growing marine phytoplankton population isolated in a plastic sphere. The lower rate of 

carbon assimilation was attributed to excretion of dissolved organic 

matter (35 percent of the total fixed). A considerable difference between the two methods of estimating productivity rates was observed 

in the Duwamish during 1966, particularly during the bloom at station 

7.7. The gross and the net productivity determined by the oxygen 

method were 4,700 and 3,700 mg C/m2/day (milligrams carbon per 

square meter per day), respectively, while coincident productivity 

that was determined by the carbon-14 method was 1,500 mg C/m2 /day. 

At least part of the measured difference between the methods is probably due to nutrient excess and cellular excretion, which respectively 

result in increased oxygen evolution and a loss of fixed carbon from 

the cells.

RESULTS

PHYTOPLANKTON DISTBIBUTION

During much of the year, the phytoplankton standing stock is small 

in the Duwamish estuary and consists primarily of fresh-water species, 

for example, green algae, euglenophytes, desmids, and fresh-water 

pennate diatoms. During the late summer, however, the salinity is 

higher and conditions are satisfactory for dense nannoplankton blooms, 

primarily of marine diatoms. Figure 3 shows the distribution of these 

blooms, as indicated by chlorophyll «, both before and after the introduction of RTP effluent in June 1965. The standing stock was greatest 

at station 7.7. A maximum chlorophyll a concentration of 28 mg/m3 

was determined on a surface sample collected at station 7.7 at a midtide 

stage on August 18, 1965, whereas a maximum concentration of 70 

mg/m3 was determined at that station on August 16, 1966, at low tide. 

The greater chlorophyll a concentrations in 1966 may be due in part 

to more frequent sampling during the bloom period; this probably 

increased the chance of observing maximum densities.

A bloom did not occur in 1964, prior to the introduction of the RTP 

effluent. The maximum chlorophyll a concentration was only about 

3.0 mg/m3 at station 7.7 during that year. Although phytoplankton 

samples were not taken during 1963, a bloom probably did occur in 

August because DO at station 7.7 reached 92 and 152 percent of saturation in the afternoons on August 8-9 and 19-20, respectively. Single 

samples from 4 other days of the month showed percent-saturation 

maxima ranging from 88 to 109. (The daily maxima in September 

1963 ranged from 60 to 74 percent.) Maximum daily oxygen saturation 0.9 m below the water surface at station 7.7 during the bloom of 

August 1966 were greater than 100 percent (figs. 3, 4); the maxima
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A19

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J J'A'S O'N'D J'F'M'A'M'J'J'A'S'O'N'D

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1964 1965

J'F'M'A'M'J'J'A'S 

1966

FIGURE 3. Distribution of phytoplankton blooms during 

June 1964-September 1966, as indicated by the chlorophyll 

a concentrations at five stations.

321-832 O 69-
A20 ENVIRONMENTAL QUALITY

AUGUST 1966

FIGURE 4. Relation of phytoplankton activity to solar radiation, maximum 

water temperature, minimum specific conductance, and tidal-prism thickness 

at station 7.7 during the phytoplankton bloom in August 1966. Phytoplankton 

activity is indicated by percent saturation of oxygen.
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A21

during 1963 ranged from 60 to 152, therefore the saturation percentages in 1963 probably indicates a bloom during that year.

The distribution of phytoplankton during 1964-66 is further portrayed by cell counts (fig. 5). Diatoms were the most abundant organisms in the estuary, particularly during bloom periods. At station 

7.7 on August 10, 1965, the most common diatom was Skeletonema 

costatwn, which formed 82 percent of the 13,000 cells per milliliter at 

high tide. At the same station at low tide, unicellular green algae were 

more abundant; they formed 52 percent of the 9,000 cells per milliliter. 

At station 1.9 on this date, Skeletonema formed 92 and 93 percent 

of the 32,300 and 25,200 cells per milliliter at low and high tide, respectively. On August 24, however, neither Skeletonema cells nor 

total cells, were abundant at high or low tide at station 1.9 or at high 

tide at station 7.7. The low-tide sample at station 7.7 contained a 

greater number of cells than the high-tide samples, and a centric 

diatom (probably Thalassiosira sp.) constituted 63 percent of the total 

5,500 cells per milliliter. Samples for cell counts were not taken on 

August 18, when maximum chlorophyll a concentrations were determined at station 7.7; however, total cell numbers were probably greater 

on August 18 than on August 10 or 24, and the two diatom species 

certainly must have represented a large part of the biomass.

In 1966, Skeletonema costatum was scarce; however, Thalassiosira 

was again abundant at station 7.7. At low tide on August 16, when 

the annual maximum chlorophyll a concentration (70 mg/m3 ) was determined, Thalassiosira comprised 48 percent, while a green algal species, Micratinium sp., comprised 26 percent of the total 59,300 cells per 

milliliter. The total number of cells was less at high tide than low 

tide (20,600 cells per milliliter), but Thalassiosira still comprised 38 

percent of the total. Unidentified unicellular green algae and flagellates 

comprised 59 percent of the total high-tide concentration, but Micratinium was scarce. Cell numbers at station 1.9 were smaller during the 

1966 bloom period than during the same period in 1965. At low tide 

on August 16, 1966, Thalassiosira comprised 22 percent of the total 

4,200 cells per milliliter, and species of green algae (someMicratinium) 

comprised 75 percent. The total number at high tide was only 900 

cells per milliliter.

The centric diatom collected at stations 7.7 and 1.9 in 1965 was 

tentatively identified as Thalassiosira sp. because it was apparently 

limited in distribution to areas of higher salinity in the lower estuary 

and because it has the superficial characteristics of a member of that 

genus that is abundant in Puget Sound. In August 1966, however, a 

small centric diatom was observed in small numbers at the farthest 

upstream station (21.0), which is beyond the limit of salinity intrusion
A22 ENVIRONMENTAL QUALITY

2 -

j'J'A'S'O'N'D'J'F'M'A'M' J 1 j'A's'o'rro1 J'F'M'A'M' J 1 J'A'S 

1964 1965 1966
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A23

but still is subject to variation in stage by tidal action. The presence 

of this diatom so far upstream suggests that the diatom may be of 

fresh-water origin, perhaps a species of Stephanodiscus. [Eegardless, the organism flourishes in the brackish water of the lower estuary, 

and it has a wider salinity tolerance than would be expected for either 

of the two species.]

Productivity rates, like chlorophyll a concentrations, were greatest 

at station 7.7 during August 1965 and 1966 (fig. 6). In fact, productivity rates, chlorophyll a concentrations, and cell counts all show 

reasonable agreement in describing phytoplankton distributions in 

the estuary; however, cell counts during August and September 1966 

were much higher at the upper stations (especially 21.0) than the 

chlorophyll a content might indicate (fig. 5). Most of the peak cell 

population at these stations consisted of very small unicellular green 

organisms. Figures 3 and 6 show that these high counts of green algae 

contributed proportionately much less chlorophyll a and productivity 

at the upper stations than the dominantly diatom population contributed at station 7.7.

Although the productivity rates shown in figure 6 are computed to a 

depth of 4 m at the lower two stations (1.9 and 7.7) but to only 1 m 

at the upper two stations, the rates are nonetheless comparable because 

the greatest productive area is in the first meter below the surface. 

During the bloom period in 1966, productivity estimates in the top 

cubic meter at stations 12.6 and 7.7 were 43 and 127 mg C/m3/hr 

(milligrams carbon per cubic meter per hour), respectively. These 

rates are of the same magnitude relative to each other as are the rates 

shown for the stations 1.9 and 7.7 in figure 6. Yet, in that figure, the 

rate for station 7.7 represents a total production to a depth of 4 m, 

whereas the rate for station 12.6 applies to the production of only the 

top meter. Thus, regardless of sampling method, the greatest biomass 

and rate of productivity clearly occur at station 7.7.

Dense phytoplankton blooms occurred in the lower estuary downstream from the ETP outfall during both years following effluent 

introduction. To determine if the ETP effluent was instrumental in 

causing these blooms, studies were made of the physical and chemical 

factors that are known to affect phytoplankton production.

 ^ FIGTJBE 5. Distribution of phytoplankton, as indicated by 

cell concentrations at five stations, June 1964-September 

1966. Water samples were collected 1 m below stream 

surface. Curves are based on means of samples taken at 

high and low tide during 1965-66, and single samples 

taken between high and low tide during 1964. Green 

algae and diatoms are indicated by crosshatching and 

shading, respectively.
A24 ENVIRONMENTAL QUALITY

J J ASON D J F M AM J J A S OND J F M A MJJAS

1964 1965 1966

FIGURE 6. Phytoplankton productivity at four stations, July 1964-September 

1966. Productivity measured by the oxygen method gives gross values (solid 

line) and by the carbon-14 method gives values between gross and net (dashed 

line). Integrated water depths are: 4 m at stations 1.9 and 7.7 and 1 m at stations 12.6 and 21.0.

EFFECTS OF EFFLUENT NUTRIENTS

Phosphate, ammonia, and nitrate concentrations in the estuary 

before and after the introduction of effluent have been compared. 

Nutrient levels based on total weight for the various years cannot be
FACTORS INITIATING PHTTOPLANKTON BLOOMS IN WASHINGTON A25

computed and compared because accurate water-discharge measurements are not available from the lower stations in the estuary; therefore, the nutrient levels are compared by graphical and statistical 

methods. The possible effects of the addition of effluent nutrients on 

phytoplankton production are evaluated by comparing in situ nutrient 

and phytoplankton observations in the Duwamish River estuary during 1964-66 and by experimenting with in vitro nutrient bioassays at 

the ETP laboratory during 1966.

Although silicate is important for the growth of diatoms, data on 

the relationship between the two in the Duwamish River are not included in this paper. The RTF did not contribute silicate to the river, 

but during the summer of 1966 concentrations of silicate remained high, 

ranging between 4 and 9 mg Si/1 (milligrams silicon per liter) in the 

surface water at station 7.7.

PHOSPHORUS

Total-phosphate concentrations were greater downstream from the 

RTF outfall during 1965-66 than they were upstream. No such increase was obesrved at these midestuary stations during 1964, before 

effluent had been introduced (fig. 7). Some of the difference in phosphate concentrations between 1964 and 1965-66 can probably be related to fresh-water discharge, which was unusually high during the 

summer of 1964. The highest concentrations of total phosphate were 

measured about July 1, 1965, and about August 1, 1966, and both 

coincided with decreased fresh-water discharge. Because the inverse 

relationship of higher phosphate to fresh-water discharge was also 

observed above the RTF outfall at station 21.0, the differences in 

total phosphate between 1964 and 1965-66 must have been exaggerated 

by differences in discharge. However, not all phosphate differences 

before and after the introduction of effluent were due to discharge. 

A comparison of data during 1963 and 1965, when discharge conditions were similar, shows that the mean annual concentrations of total 

phosphate at stations 1.9-12.6 during 1963 were less than the comparable values during 1965. Station 21.0, which is above the RTF 

outfall, was the only exception (fig. 8). Also, figure 7 shows consistently 

higher total phosphate below the outfall during the winter of 1965-66 

than during the winter of 1964-65.

Downstream changes in the concentrations of total phosphate before 

and after introduction of the RTF effluent have been compared by 

analysis of variance (table 2). If stations and periods are considered 

fixed, the increase in total phosphate in 1965-66 (after effluent introduction) and the differences in concentration between stations are significant at the 0.5-percent level. Interaction between periods and stations, however, is also significant at the 0.5-percent level. Part of the
A26 ENVIRONMENTAL QUALITY

0.6 r-r

0.4

0.2

0.4

a:

0.2

CE 

Ld 

O.

0.4

0.2

O

I

Q_

< 0.4 

O

0.2

Station 21.0

12.6

10.4 

A

7.7

0.4

0.2

J J AS O N D J F M AM J J A SO N D J F M A M J J A S

1964 1965 1966

FIGURE 7. Nutrient content of total phosphate, expressed 

as phosphorus, at five stations, June 1964-September 1966. 

Water samples were collected 1 m below stream surface, 

usually without regard to tidal stage.
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A27

0.4

0.3

0.2

0.1

o: n 3 LJ U -° 

CL

.

< 0.2

2 0.1

z

(/ >" 

o

c/5

o

I

^ 0.2

Z 0 

0.3

0.2

0.1

Ammonia nitrogen

Nitrate nitrogen

Total-phosphate phosphorus

Soluble-phosphate phosphorus

1963 1964 1965 1966

FIGUBE 8. Downstream changes in annual mean concentrations of ammonia 

nitrogen, nitrate nitrogen, total-phosphate phosphorus, and soluble-phosphate 

phosphorus during 1963-66. Bach group of five bars represents sampling 

stations arranged in downstream order from left tt> right as follows: 21.0, 

12.6, 10.4, 7.7, and 1.9. Water samples were collected 1 m below stream 

surface. Annual mean concentrations are weighted by time.

321-832 O 69-
A28 ENVIRONMENTAL QUALITY

interaction might be due to a trend toward higher total phosphate 

downstream and lower concentrations upstream in 1964 (fig. 8). This 

trend differs from the pattern of concentration in 1965-66; after effluent was introduced, concentrations were highest at the first station 

below the outfall (12.6) and decreased progressively downstream. The 

differences between the means for the two periods (1963-64 and 1965- 

66) may not be great enough to mask the interacting effect of the 

unusual downstream trend of higher total phosphate in 1964 (fig. 8).

TABLE 2. Results of variance analyses comparing downstream changes in mean 

total-phosphate concentration before and after introduction of effluent from the 

RTF (cell sizes unequal)

Source of variance Sum of Degrees of Mean F value ]

squares freedom square

Before and after first effluent _ ___________

Downstream changes _ _ _____________ ___

Interaction between periods and stations. __ 

Error. __ ___ _________ __ _____

0.83

.75

.56 

13.09

1

4

4 

644

0.83

0. 19

0. 14 

0. 02 _.

40. 7

9.3

6.9

Total_____________________ 15.23 653

i Significant at 0.5-pereent level using fixed model. Minimum values of F necessary for the indicated 

probability level and degrees of freedom are:

1 4

F = 7.95; F = 3.76.

644 0.005 644 0.005

Soluble phosphate was not determined during 1963-64, but concentrations during 1965-66 were usually proportional to total phosphate, 

and the downstream distribution patterns were similar (fig. 8). Total 

phosphate exceeds soluble phosphate and orthophosphate in the Duwamish by factors of about 1.5 and 2, respectively.

The variation of nutrient distribution with tide is illustrated by 

soluble-phosphate concentrations at high and low tide in 1965-66 (fig. 

9). The concentrations vary most during the low-flow period at stations 7.7, 10.4, and 12.6, where variation in other tide-influenced hydrographic factors is greatest. The maximum concentrations occur at low 

tide, partly because sea-water dilution of the ETP effluent and of other 

fresh-water nutrient sources is minimum then.

NITKOGEN

Comparison of annual mean concentrations of ammonia during the 

period 1963-66 clearly illustrates a nutrient increase in the estuary 

during 1965-66, following introduction of the ETP effluent (fig. 8).
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A29

0.4

0.2

0.4

0.2

0.4

0.2

0

0.4

0.2

0.4

0.2

i i i i r i r

Station 21.0

12.6

10.4

7.7

1.9

i _ i .. i i .. i I i i .. i

i i i i i i i r

J'FM'AM'J'J'ASO'N'D JF'M'A'M'J'J'A'SO i .. i .. i I i .. i Ti I : i _ i

1965 1966

FIGURE 9. Soluble phosphate at high and low tide (solid and dashed lines, respectively) at five stations January 1965-October 1966. Water samples were 

collected 1 m below stream surface.

The difference in annual mean concentration of ammonia during the 4 

years is only slight at station 21.0, whereas the increase is obvious at 

stations below the effluent outfall during 1965-66. Nitrate concentrations, on the other hand, did not noticeably change following introduction of the effluent; this was predictable, however, owing to the low 

nitrate content of the effluent.
A30 ENVIRONMENTAL QUALITY

Downstream changes in the mean concentration of ammonia before 

and after introduction of the RTF effluent (1963-64 and 1965-66, respectively) have been compared by variance analysis (table 3). If 

stations and periods are again considered fixed, the increase in ammonia concentrations after effluent introduction, as well as the concentration differences between stations, is significant at the 0.5-percent level.

TABLE 3. Results of variance analyses comparing downstream changes in mean 

ammonia concentration before and after introduction of effluent from RTF (cell 

sizes unequal)

Source of variance Sum of Degrees of Mean F value l

squares freedom square

Before and after first effluent. __ _ __

Downstream changes __ _______ _ _ __

Interaction between periods and stations. __ 

Error. _____ _ _____ _____ _ __ _ _ __

Total. _. __ __ __ _ __ ______

1. 16

2.43

.73 

62.89

67.21

1

4

4 

606

615 .

1.16

0.61

0. 18 

0. 10 _

2 11.2

2 5.8

1.8

1 Minimum values of F necessary for the indicated probability level and degrees of freedom are:

1 4

F = 7.95; F = 3.76.

606 0.005 606 0.005

2 Significant at 0.5-percent level using fixed model.

In contrast to total phosphate, there was no significant interaction 

at the 5-percent level between period and stations for ammonia concentrations, even though the distribution pattern in 1964 of increasing 

mean ammonia concentrations downstream was similar to, but more 

pronounced than, that of total phosphate (fig. 8). The greater difference between mean ammonia concentrations before and after effluent 

introduction than between total phosphate cencentrations before and 

after effluent introduction, may have masked the interaction effect of 

ammonia between stations and periods.

MAXIMUM BIOMASS PRODUCED

In vitro nutrient bioassays suggest that the addition of effluent 

stimulates phytoplankton production. Therefore, an increase in the 

maximum biomass of the bloom might result from the addition of 

effluent, even though the timing of the bloom is not altered. Figure 10 

shows results of carbon-14 assimilation by a mixed phytoplankton 

culture in filtered river water with and without filtered RTF effluent 

added. A green alga (Scenedesnws sp.) was the most abundant organism in both culture experiments.
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A31 

100

23456 

DAYS AFTER INOCULATION

EXPLANATION

Samples without 

added effluent:

Station Symbol

21.0 n

12.6 A

7.7 o

Samples with added 

effluent (station 

7.7):

Percentage Symbol 

added

5 B 

10 A 

25(March 1) 

and 15 

(June 3)

FIGUBE 10. :Increase in assimilation of carbon-14 indicates in vitro response 

of phytoplankton to added effluent nutrients.
A32 ENVIRONMENTAL QUALITY

Assimilation was greatest in water from station 7.7 with effluent 

added, but only in the June 3 experiment was the amount assimilated 

proportional to the amount of effluent added, in fact, the assimilation 

order was reversed in the March 1 experiment. In general, assimilation 

in water without added effluent increased downstream in samples 

collected June 3, 1966, but a progressive downstream increase in assimilation was not apparent in the March 1 experiment because water 

from station 21.0 (without added effluent) produced a greater biomass 

than water from station 12.6. Nutrient concentrations, final carbon-14 

activity, and some comparable cell concentrations from the March 1 

experiment are given in table 4. Of course, trace elements and various 

organic growth stimulants, which were not determined, may account 

for some of the variability not explained by nitrogen and phosphorus 

compounds. Nevertheless, effluent addition does stimulate the assimilation rate of a green algal species (Scenedesrrws) in fresh water and 

in dilute sea water (specific conductance about 7,000 micromhos). Unsuccessful attempts were made to grow pure cultures of a marine 

diatom (Skeletonema costatwn) that is found in the blooms in lowsalinity estuarine water under the stated artificial conditions.

Annual mean concentrations of chlorophyll a in 1965 and 1966 were 

compared to determine whether the increase in mean effluent discharge 

from 34,000 myday in 1965 to 49,200 m3/day in 1966 caused a similar 

increase in total biomass. The chlorophyll concentrations in 1965 and 

1966 were not significantly different at the 10-percent confidence level, 

but station-to-station differences were significant at the 5-percent level

TABLE 4. In vitro carbon-14 assimilation by phytoplankton inoculated into filtered 

river water containing different effluent nutrient contents

[Water samples were collected Mar. 1,1966]

Station

7.7_________.______

7.7________________

7.7

7.7__________._.___

12.6 _______________

21.0 _______________

Effluent 

added 

(percent)

25

10

5

0

0

0

Initial nutrient 

concentrations

Ammonia 

plus nitrate 

(mg N/l)

4.0 

1.8 

1. 1 

.38 

.32 

.08

Phosphate 

(mg P/l)

1.8 

.77 

.43 

.08 

.05 

.05

Final C-14 

activity 

(counts per 

minute, in 

thousands) 1

62 

83 

99 

34 

5.4 

21

Final number of cells 

per milliliter 

(thousands)

50 

12 

3.7

1 8 days after inoculation.
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A33

(table 5). Statistically, the interaction between stations and years was 

not significant. The comparison of mean chlorophyll for 1965 and 1966 

shows, therefore, that the 46-percent effluent increase in 1966 did not 

increase phytoplankton production in the estuary. However, sampling 

frequency during the bloom periods may not have been sufficient to 

detect possible differences in the annual mean concentration. Maximum gross productivity in 1966 (4,680 mg C/m2/day) was greater 

than that in 1965 (3,740 mg C/m2/day), and maximum chlorophyll a 

concentrations were TO and 28 mg/ms during 1966 and 1965, respectively. Although these data indicate that phytoplankton production 

did increase in 1966 compared to 1965, the observations themselves 

do not indicate this; however, the observations may not represent the 

production maxima that actually occurred during the 2 years. More 

frequent sampling during the bloom period in 1965 might have shown 

a higher .maximum.

TABLE 5. Results of variance analyses comparing downstream changes in mean 

annual chlorophyll a concentrations in 1965 and 1966 (cett sizes unequal) 

[Mean effluent discharge averaged 34,000 m'/day in 1966 and 49,000 m»/day in 1966.]

Source of variance Sum of Degrees of Mean F value l

squares freedom square

1965 versus 1966__ __________ _____

Downstream changes. _____ _ _ ___ __

Interaction between periods and stations. 

Error. _ ______________________________

0.51

130. 34

7.36 

3, 288. 20

1

4

4 

251

0.51

32.59

1.84 

13. 10

0.039

3 2.49

0. 14

Total_________________ 3,426.41 260

1 Minimum values of F necessary for the indicated probability level and degrees of freedom are:

1 4 

F = 2.73; F = 2.42 

251 0.10 251 a 05

2 Significant at 5-percent level using fixed model.

EFFECTS OP PHYSICAL FACTORS

Seasonal and year-to-year variations of incident light intensity and 

estimated residual intensity at a depth of 2 m are shown in figure 11. 

Total insolation during August of each year was about the same 

(13.5X103 , 12.8 X103 , and 15.5 XlO3 langleys in 1964, 1965, and 1966, 

respectively). A comparison of incident and residual light indicates 

that conditions in August 1964 would probably have been sufficient for 

a phytoplankton bloom to develop if other environmental factors also 

had been favorable. The relation between light penetration and the 

depth and extent of mixing in the surface-water layer probably has an 

important influence on phytoplankton production because the depth
A36

10,000

ENVIRONMENTAL QUALITY

UJD 

C3Z

Qco

1000

200

2965" -

MAY JUNE JULY AUGUST SEPTEMBER

FIGURE 12. Mean daily fresh-water discharge at station 21.0, May-Sept ember 

1963-1966. Discharges in 1964 that exceeded those of 1963 and 1965-66 are 

emphasized by shading.

Further evidence of the effect of these hydrographic factors on the 

timing of blooms is shown by the relation of the chlorophyll a concentration that was determined at station 7.7 to fresh-water discharge and 

tidal-prism thickness during the summers of 1965 and 1966 (fig. 13). 

During both years, the blooms occurred when the volume of discharge 

and the tidal-prism thickness were at their minimums; this timing indicates that water-retention time and stability in the photic zone were 

probably at their maximums. These conditions apparently promoted 

rapid phytoplankton growth by retaining organisms in the zone of 

optimum light intensity within a few meters of the surface. Thus, net 

production was high because the organisms were not carried below 

the compensation depth (usually between 2 and 3 m) into zones where 

respiration exceeds photosynthesis. The standing stock thereby increased rapidly under stratified conditions, as a result of the increased 

net production.

The occurrence of low discharge earlier in 1965 than in 1966 may 

partly explain the more gradual increase of the standing stock in 1965. 

Discharge remained relatively high during the early summer of 1966, 

but declined rapidly and reached the 550-m3/min level during the first 

week in August, the week having the most favorable tides. Thus, conditions became ideal for bloom development rather suddenly in 1966, 

and a sudden outburst resulted.

The stability of the photic zone at station 7.7 increased with decreasing discharge, as indicated by profiles of temperature and specific conductance for June-August 1966 (fig. 15). Stability seemed to be greatest when the tidal-prism thickness and discharge approached minimums in early August. The estuary was stratified earlier, as shown by
CHLOROPHYLL a IN MILLIGR

!- t>0 t~0 t>0 

O1 O -P» 00 O 

I

I

I

3

TIDAL-PRISM THICKN

"- "-1 o 

O tn 

o o 

o o 

FRESH-WATER DISCHARGE, IN C
A38 ENVIRONMENTAL QUALITY 

JUNE JULY AUGUST

 

24

_ <f> QL 

CLZH

TEMPERATURE, IN DEGREES CELSIUS 

20 0 10 20 0 10 20 30

I I I

0 20 40 60

SPECIFIC CONDUCTANCE, IN MICROMHOS PER CM AT 25°C X10~3

FIGURE 14. Vertical profiles of specific conductance (solid line) and temperature (dashed line) at station 7.7 near low tide during June-August 1966 in 

relation to fresh-water discharge and tidal-prism thickness.

a bloom. The maximum observed differences of specific conductance 

and temperature between the surface water and water at a depth of 3 m 

occurred August 11 when tidal-prism thickness was at a minimum and 

discharge was 550 mYmin. The fact that these conditions coincided 

with maximum phytoplankton productivity (figs. 4,13) strongly sug-
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A39

gests that the factors affecting stability and retention time also indirectly control the timing of phytoplankton blooms.

Conditions of low fresh-water discharge and minimum tidal-prism 

thickness occurred again during September of 1965 and 1966, but 

blooms did not occur. During both those years, however, incident light 

and temperature decreased greatly in September compared to the prebloom levels of about August 1 (fig. 12). Although conditions of discharge and tide were favorable during the two Septembers, light intensity may not have been sufficient to promote the photosynthetic rate 

necessary for a bloom to develop or to raise the temperature of the 

surface water high enough to maintain stability.

Control of the timing of phytoplankton blooms by physical factors 

was further indicated by comparing in situ chlorophyll a concentrations with the "potential" concentrations cultivated in water collected 

on the same date but incubated under artificial conditions (fig. 15). 

The increases in biomass that were grown under artificial conditions 

during short periods before and after the bloom indicate that the water 

could have supported growth. Thus, the lack of bloom then suggests 

that the physical environment in situ was not suitable to sustain the 

growth over a longer period. The failure of "potential" chlorophyll 

a, concentrations to exceed in situ values during the bloom was probably 

the result of suboptimal intensities of artificial light.

Conditions of reduced vertical turbulence and greater stability, 

which occur during periods of minimum tidal-prism thickness and 

minimum discharge, are also indicated by greater minimum specific 

conductance near the surface at station 7.7 (fig. 4). Maximum specific 

conductances do not change appreciably with tidal-prism thickness, so 

periods of thin prism thickness are characterized by greater minimum 

conductivities as well as less diurnal change in specific conductance. 

If light intensity is great during these periods, surface temperatures 

will increase. According to Eiley (1942), vertical differences in temperature are due to the combined effect of heating by solar radiation 

and submergence of warmer surface water by vertical turbulence; 

therefore, the difference between surface- and bottom-water temperatures (A°C) during the year indicates the extent of stratification or vertical turbulence. Adequate data are not available for the more precise 

variables that describe stratification and retention time; therefore, the 

importance of these two factors in controlling phytoplankton productivity is examined using a multiple regression analysis with A°C and
A40 ENVIRONMENTAL QUALITY

SEPTEMBER

FIGURE 15. Contrast between growth of phytoplankton in place (solid line) 

and "potential" phytoplankton (dashed line) during August and early 

September 1966. Both types of samples were collected at low tide 1 m 

below stream surface at station 7.7. Growth is indicated by chlorophyll 

a concentrations. "Potential" concentrations were determined after 5-6 days 

of static1 incubation under artificial conditions (light intensity about 7,000 

lux, temperature about 20°-21°C).

fresh-water discharge as independent variables and gross productivity 

as the dependent variable. Fourteen monthly or biweekly observations 

from May to October during 1965-1966 were used in the analysis. The 

temperatures were measured biweekly, usually about a week before 

productivity measurements. The A°C values are the difference between 

the mean of high- and low-tide surface determinations and the mean 

of high- and low-tide bottom determinations. The computed prediction 

equation for the relationship is

log #=4.113+ 0.243Z"!-0.729 log Z2 , 

where T is gross productivity, in milligrams carbon per square meter
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A41

per day; Xi is A°C (difference between surface and bottom water temperature) ; and X2 is discharge, in cubic meters per minute. Correlation coefficient, 0.71 for temperature change (A°C) only and 0.78 for 

both A°C and discharge together are significant at the 1-percent level. 

The upper and lower 95-percent confidence intervals for the mean 

productivity are 610 and 222 mg C/m2/day, respectively. Temperature 

and discharge presumably accounted for 61 percent of the variation 

in productivity, even though they were not specific measurements of 

stability and retention time.

RELATION OF PHYTOPLANKTON TO DISSOLVED OXYGEN 

DISTRIBUTION OF DISSOLVED OXYGEN

Minimum concentrations of DO are present near the bottom in the 

saline-water wedge between stations 7.7 and 10.4 during periods of 

minimum fresh-water discharge and of minimum tidal-prism thickness. The available determinations of daily minimum DO in samples 

from less than 1 m above stream bottom at station 7.7 are illustrated 

in figure 16. Minimum concentrations of DO observed during preeffluent conditions in 1963 were nearly as low as those observed during 

effluent conditions in 1966; however, concentrations remained below 

3.0 mg/1 for a longer period in 1966 than in 1963. Summer DO data 

from 1965 are inadequate because the automatic monitoring equipment 

at station 7.7 was inoperative. Nevertheless, limited data from manual 

sampling show concentrations below 3.0 mg/1 at that station during 

late August and early September.

During the summer of 1964, when fresh-water discharge was never 

less than 760 m3/min, observed concentrations of DO were below 3.0 

mg/1 on only 2 days and were considerably greater than 3.0 mg/1 

during most of the summer.

The timing and duration of minimum DO concentrations are greatly 

dependent on fresh-water discharge and tidal-exchange conditions. 

The cyclic nature of minimum concentrations in 1963 and 1966 (fig. 

16) is directly related to tidal-prism thickness, whereas the long-term 

trend of the minimum concentration within each cycle seems to be related to discharge. Lowest minimum concentrations usually are not 

reached until discharge falls below about 600 m3/min. Thus, minimum 

concentrations remained relatively high in 1964 when discharge was 

always greater than 760 m3/min.

The minimum DO concentration also seems to be related to the occurrence of phytoplankton blooms. Very low DO values followed 

blooms during the summers of 1963, 1965, and 1966 (fig. 16). In contrast, daily DO minimums remained relatively high during the summer
A42 ENVIRONMENTAL QUALITY

6 -

JUNE JULY AUGUST SEPTEMBER

FIGURE 16. Relation of minimum DO concentrations at station 7.7 to phytoplankton-bloom timing and to fresh-water discharge, June-September 1963-66. 

Water samples were collected during low tide less than 1 m above stream 

bottom. The minimum concentrations were usually recorded during continuous 

monitoring throughout a tidal cycle. Method of sampling was as follows: 

Manual, large circle; automatic, small circle. Fresh-water discharge is indicated as follows : Less than 600 nis/nrin, solid horizontal bar; 600 to 760 ma/min, 

open bar ; more than 760 m3/min, no bar. Approximate periods of phytoplankton 

bloom are based on chlorophyll a concentrations greater than 4.0 mg/m3. Dissolved-oxygen concentrations less than 3 ing/1 are emphasized by shading.

of 1964, when there was 110 bloom. Further, the relationship between 

phytoplankton biomass and minimum DO is inverse in water samples 

collected less than 1 m above stream bottom from an eight.-station series 

(between kilometers 1.9 and 10.4) throughout the summer of 1966 

(figs. 17, 18). Maximum concentrations of chlorophyll a in the estuary
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A43

12 r^

10

O Z

Q; <

O

_j 

I 

O

i i i i i r T r

4 6 8 10 

DISTANCE UPSTREAM FROM RIVER MOUTH, IN KILOMETERS

12

FIGURE 17. Longitudinal distribution of chlorophyll a, BOD, and DO in the 

estuary at low tide. Water samples were collected less than 1 m above stream 

bottom during June-September 1966. Number of observations: 22 at station 

7.7 and 14 each at remaining stations. Mean concentrations are weighted by 

time.

consistently occur between the point of minimum low-tide DO (station 

7.7) and the point of minimum high-tide DO (station 10.4 or above).

Concentrations of DO in the saline-water wedge decrease during late 

summer apparently because retention time of that water increases as 

discharge and consequent flushing rate decrease. Water in the part of 

the wedge that is farthest upstream has been retained in the estuary 

for the longest time without reaeration. This is true in a salt-wedge 

estuary because there is a net upstream movement of bottom water, and 

because the only escape for that water is through mixing and entrainment within the brackish surface-water layer. As retention time of the 

bottom water increases, the DO concentration decreases because the 

BOD has a longer time to act.

The timing of the DO minima is related to fresh-water discharge 

and tidal-prism thickness, as is the timing of phytoplankton blooms.
A44 ENVIRONMENTAL QUALITY

4 6 8 10 

DISTANCE UPSTREAM FROM RIVER MOUTH, IN KILOMETERS

12

FIGURE 18. Longitudinal distribution of chlorophyll a, BOD, and DO in the 

estuary at high tide. Water samples were collected less than 1 m above stream 

bottom during June-September 1966. Number of observations: 20 at station 

7.7 and 11 each at remaining stations. Mean concentrations are weighted by 

time.

The similar timing of blooms and minimum DO with hydrographic 

conditions might suggest that these two events are not directly related 

to each other. A decrease in oxygen concentration, however, is necessarily caused by an oxygen demand, and phytoplankton organic matter serves as a possible source for an important part of the total in situ 

BOD.

RELATION OF PHYTOPLANKTON TO BIOCHEMICAL OXYGEN DEMAND AND

DISSOLVED OXYGEN

In the estuary, the longitudinal relation between BOD, chlorophyll 

a, and DO indicates that the BOD contributed by phytoplankton is 

consistently greatest in the area of minimum DO. Figures 17 and 18 

show that the maximum mean BOD determined in the saline-water 

wedge less than 1 m above stream bottom occurred at station 9.7 at 

both high and low tide, and that maximum concentrations of chlorophyll a usually coincide with maximum BOD. In contrast, the minimum DO concentrations determined at low tide usually occurred at 

station 7.7, and the minimum high-tide concentrations occurred at 

station 10.4. Station 10.4 was the point farthest upstream at which 

samples were collected during these studies; hence, even lower oxygen 

concentrations might have been present above this station at high tide.
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A45

However, no such concentrations were detected by the automatic monitor at station 12.6 farther upstream. The relationships depicted in 

figures 17 and 18 show that between stations 1.9 and 10.4 in the lower 

estuary, BOD and chlorophyll a are related to each other, and each is 

negatively correlated with DO. Also, the water with maximum concentrations of BOD and chlorophyll a usually occurs at station 9.7 

regardless of tide stage.

The part of the estuary where the maximum concentrations of BOD 

and chlorophyll a occur near the bottom is a transitional area between 

the shallower upriver reach and the widened and dredged lower reach. 

Extensive organic deposition occurs in this transition area. This is 

indicated by the relatively greater oxygen demand of bottom deposits 

in the transitional area compared to that of deposits both upstream 

and downstream (G. W. Isaac, Metro, written commun., 1964). The 

great number of bloom-causing diatoms in the particulate matter in 

water samples collected less than a meter above stream bottom at stations 7.7 and 9.7 following phytoplankton blooms also indicates the 

deposition of organic material. A sample collected near the bottom at 

station 7.7 on August 10, 1965, had a chlorophyll a concentration of 

4.6 mg/m3 and a total cell number of 3,900 per milliliter. Skeletonema 

costatum was abundant in the surface water at this station and comprised 22 percent of the cells in the bottom water. A chlorophyll concentration of 5.5 mg/m3 was determined in a bottom-water sample 

collected from station 9.7 on August 26, 1966. Cells in the sample 

totaled 11,300 per milliliter, of which Thalassiosira comprised 40 percent. These data indicate that particulate matter from phytoplankton 

blooms near the surface becomes an important component of the highly 

organic bottom deposits in this area.

Figure 19 shows the seasonal relationships between concentrations 

of chlorophyll a, BOD, and DO at low tide in the area between stations 7.7 and 10.0. Although chlorophyll and BOD increased slightly 

in June 1966, the maximum concentrations occurred during the bloom 

in August. The seasonal relationship is also indicated by concentrations of chlorophyll a and BOD in the reach between stations 7.7 and 

10.0. Mean concentrations of chlorophyll a and BOD in this section of 

the estuary were greatest during August, when the bloom occurred 

(fig. 20). The seasonal timing of maximum chlorophyll a and BOD 

in the saline-water wedge, as well as their correlated distribution 

throughout the estuary, strongly indicates that the organic matter 

contributed by sinking phytoplankton cells during blooms is an important source of BOD in the Duwamish.
A46 ENVIRONMENTAL QUALITY

28

24

C 20

So: / « ^ <£ loo Ij

BS

0:0. 

£wi6

in < 

2o: 

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JUNE JULY AUGUST SEPTEMBER

1966

FIGURE 19. The seasonal variation of chlorophyll a, BOD, and DO in the 

area of minimum DO during the summer of 1966. Water samples were 

collected at low tide less than 1 m above stream bottom at stations 10.0, 

9.7, and 7.7. Each point on graph is a mean value for the three stations.

A pronounced decrease in DO, to concentrations less than 1.0 mg/1, 

occurred in the bottom water at stations 7.7 and 9.7 in late August 

1966. The decrease closely followed the observed maximum concentrations of BOD and chlorophyll a (figs. 16 and 19). Part of the decrease 

may have been initiated by hydrographic conditions, because it 

coincided with a period of minimum tidal-prism thickness. However, 

the DO minimums were only slightly greater during the period of 

thicker tidal prism in early September; this fact suggests that the
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A47

20

16

~ hQQ ~,

OL 0.

£ 1 12

^ K.

< C3

^ 2 

Z -

o z

o: < 

O

I 

o

JUNE JULY AUGUST SEPTEMBER

1966

FIGURE 20. Monthly means of chlorophyll a and BOD in the area of minimum 

DO, June-September 1966. Water samples were collected at low tide less 

than 1 m above stream bottom at stations 10.0, 9.7, and 7.7. Each point on 

graph is a monthly mean (weighted by time) for the three stations. Vertical 

bars indicate standard error.

increased BOD was contributing significantly to DO depression. Furthermore, minimum DO concentrations at the start of the August 

bloom were considerably higher during the period of thin prism, about 

August 11-15. than they were during a similar period in late August 

and early September. This was true even though fresh-water discharge was about the same during both periods. The minimum DO 

concentration increased in response to hydrographic conditions about 

September 20, when the tidal prism was thickest, before dropping 

again to low levels in late September. The timing and persistence of 

DO is largely related to discharge and tidal-prism thickness, both 

of which affect water-retention time. However, increased BOD from 

the bloom seems to interact in this relationship to some extent and
A48 ENVIRONMENTAL QUAMTY

produces lower and more persistent DO minimums than might otherwise be expected during conditions of low discharge and tidal action.

The increases in chlorophyll a concentration and BOD during 

August 1966 were not of the same magnitude (fig. 19). As previously 

suggested, the disproportionately lower BOD values could be due to 

the dependence of oxygen demand rates on DO concentration. Also, a 

large proportion of cells were living during this period, and the BOD 

of living cells probably would be less than that of dead cells, since 

only a small part of the cell carbon would be available for oxidation. 

Conversely, when the concentration of chlorophyll a decreased following the bloom, more of the cells probably were dead; thus suggesting 

a relatively high BOD in a low-chlorophyll sample could actually 

be attributed to organic matter from phytoplankton cells that were 

dead and therefore lacked chlorophyll. If some of the chlorophyll a 

were destroyed by decomposition, the amount present would not constitute a valid measure of oxidizable cell carbon (BOD).

A close proportional relationship between chlorophyll #, BOD, and 

DO is shown by a vertical profile of samples collected at station 9.7 

during the bloom in 1966 (fig. 21). The profile also illustrates the 

significance of the phytoplankton bloom as a source of BOD. The 

amount of oxygen produced by this large biomass of phytoplankton 

can be reversed to indicate the amount of oxygen consumed by respiration and decomposition when the biomass sinks below the photic zone. 

The sharp stratification shows that the productive zone was limited 

primarily to the upper 2 m.

A comparison of all low-tide observations of chlorophyll a and 

BOD throughout the estuary showed that the relationship between 

these variables was curvilinear, that is, high concentrations of chlorophyll a were not accompanied by proportionately high BOD values, 

but transformation of the data into logarithms resulted in linearity 

(fig. 22). The equation for the calculated line is

log r=0.015 + 0.46 log X,

where T is the predicted BOD concentration and X is the chlorophyll 

a concentration. The coefficient of correlation, 0.82, is highly significant, and the upper and lower 95-percent confidence limits for the 

mean BOD concentration are, respectively, 1.9 and 1.6 mg/1. The 

interception of the regression line with the ordinate (BOD) indicates that other BOD sources exist in addition to phytoplankton cells 

containing chlorophyll a; as noted earlier, one source may be dead
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A49

cells without chlorophyll ((. This analysis shows the importance 

of phytoplankton organic matter as a source of BOD in the estuary.

100

CHLOROPHYLL a, IN MILLIGRAMS PER CUBIC METER, 

AND OXYGEN, IN MILLIGRAMS PER LITER X10

FIGURE 21. Vertical distribution of chlorophyll a, BOD, and DO at station 9.7 on August 9, 1966, during the phytoplankton bloom.

EVALUATION OF RESULTS

TIMING OF PHYTOPLANKTON BLOOMS

Concentrations of ammonia and phosphate increased significantly 

in the Duwamish estuary during 1965-66, following introduction of
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FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A51

the RTF effluent, and blooms occurred in the lower estuary in August 

of both years, during the period of maximum nutrient concentrations. In contrast, no bloom occurred in 1964, when nutrient concentrations were comparatively low before the introduction of effluent. 

The maximum chlorophyll a concentrations observed during each of 

the 3 years (1964-66) were 3, 28, and 70 mg/m3 , respectively.

Considering only the summer periods, the lowest mean concentration of total phosphate during 1964-66 (0.10 mg P/l) occurred in 1964, 

and it coincided with the minimum concentration of chlorophyll a 

(3 mg/m3 ). The mean quantities of soluble phosphate and orthophosphate during that summer may have been about 0.07 and 0.05 

mg P/l, respectively; this figure is based on an average ratio among 

the three phosphate forms in the Duwamish.

The comparison of nutrient concentrations and phytoplankton 

activity during 1964-66 suggests that the blooms in 1965-66 resulted 

from nutrients added by the ETP effluent. The addition of nutrients, 

however, is probably not the sole cause; the following points also 

deserve consideration:

1. Mineral nutrient concentrations during 1964 were relatively high 

compared to concentrations in other marine areas which support 

large blooms.

2. Nutrient concentrations were greater at station 7.7 during blooms 

in 1965 and 1966 than during a similar period in 1964, yet the 

blooms declined in both 1965 and 1966. Nutrients probably were 

not limiting during the blooms: there was no apparent reduction 

in situ in nutrient concentration that was related to phytoplankton growth, and the assimilation rate per unit chlorophyll a was 

high, as shown by the steep slope of the productivity-chlorophyll 

a regression line (fig. 2).

3. In spite of relatively high nutrient concentrations during most of 

the 1965-66 summer periods (fig. 7, 9), phytoplankton blooms 

occurred only during August of each year.

4. No appreciable change in phosphate concentration was apparent 

during 1964-66 at station 1.9 (figs. 7, 8, and 9), yet phytoplankton blooms occurred there in 1965 and 1966, but not in 1964.

5. The occurrence of supersaturated oxygen in August 1963 indicated 

the presence of bloom activity prior to completion of the ETP.
A52 ENVIRONMENTAL QUALITY

The phosphate concentrations in the Duwamish during 1964 

probably exceeded quantities that would limit the production of 

phytoplankton in most similar estuaries. Because the effect of various 

nutrient concentrations on in situ production under constant physical 

conditions is not known in the Duwamish, the possibility that limiting 

nutrient concentrations are higher there than in other estuaries should 

be considered. A lower limiting concentration in the Duwamish, however, would seem more reasonable because of the relatively great turbulence and exchange rate of the Duwamish water; these two factors 

would tend to increase the effective contact time between existing 

nutrients and phytoplankton cells. Therefore, comparison with the 

nutrient content in other productive but probably less turbulent estuaries seems justified as one approach to evaluating the possible 

limiting effect of nutrient concentrations in the Duwamish.

Because relatively large phytoplankton blooms develop in other 

estuaries with nutrient contents at least as low as those in the Duwamish, the difference between the phytoplankton standing stock in the 

Duwamish before and after effluent introduction can not be attributed 

solely to the difference in nutrient concentrations. Pratt (1965) has 

described diatom blooms in Narragansett Bay, Rhode Island, in which 

Skeletonema costatum frequently exceeded 50,000 cells per milliliter. 

Inorganic phosphate concentrations there ranged between 0.03 and 

0.05 nig P/l at the beginning of rapid phytoplankton growth, but 

reached very low levels, along with nitrate and silicate, at the bloom 

maximum. In describing the nutrient distribution in Raritan Bay, 

Jeffries (1962, p. 24) showed that the seasonal mean concentrations of 

inorganic phosphate ranged from 0.06 to 0.14 mg P/l. Maximum 

chlorophyll a concentrations in excess of 100 mg/m3 are also reported 

from Raritan Bay (Patten, 1961, p. 372). According to Patten, Mulford, and Warinner (1963, p. 5, 7), chlorophyll a reached a maximum 

concentration of 22 mg/m3 in Lower Chesapeake Bay, where inorganic 

phosphate ranged from 0.07 mg P/l to barely detectable. The mean 

concentrations at all stations ranged from 0.007 to 0.12 mg/1, but the 

authors found no clear relationship between nutrients and abundance 

of phytoplankton. The inorganic phosphate concentrations in the English Channel were comparatively low at the beginning of the spring 

bloom, ranging between 0.014 and 0.022 mg P/l (Raymont, 1963, p. 

148). Thus, nutrient concentrations in the Duwamish during 1964 

were high (at least 0.10 mg/1) compared to those necessary for 

development of large phytoplankton blooms in other estuaries.

Other evidence also suggests that nutrient concentrations in 1964 

were adequate for development of a bloom similar to those in 1965 

and 1966. That no nutrient depletion was apparent at station 7.7 during
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A53

the blooms may be explained in part by the high nutrient concentrations and the relatively constant water exchange and nutrient income, 

but it also implies that nutrient limitation was not the cause for bloom 

declines in 1965-66. In estuaries where nutrients are considered to limit 

production, the nutrient concentrations decrease greatly and approach 

zero at the bloom maximums (Pratt, 1965, p. 174,175). In the absence 

of a nutrient reduction during the blooms in 1965 and 1966, the production probably should have continued to increase rather than decline, 

if in fact the nearly two-fold increase in nutrient concentrations after 

effluent introduction was the sole cause of bloom development.

The high assimilation rate per unit of chlorophyll a in the Duwamish estuary, as indicated by the steep slope of the regression line for 

productivity and chlorophyll a (fig. 2), also suggests that nutrients 

were not a limiting factor during the blooms. Stress and Stottlemeyer 

(1965, p. 139) concluded the same thing in the Patuxent estuary, Maryland, on the basis of similar evidence.

The control of bloom timing other than by nutrient content is further 

indicated by the following points:

1. Nutrient concentrations were relatively high during most of the lowflow period of 1965 and 1966, yet blooms existed for only about 20 

days during August of each year.

2. The sudden outburst in August 1966 caused concentrations of chlorophyll a to increase from less than 5 mg/m3 to more than 30 mg/m3 

in less than 5 days, even though soluble-phosphate concentrations 

had ranged between 0.1 and 0.2 mg P/l during the two previous 

months.

3. No appreciable difference in the mean total-phosphate concentration was observed at station 1.2 in the three summers studied 

(1964-66) in spite of blooms there in 1965 and 1966 but not in 1964.

4. Finally, a phytoplankton bloom probably occurred under preeffluent nutrient levels during August 1963, as indicated by supersaturated oxygen concentrations in the surface water at station 

7.7. All these observations indicate that the nutrient increases 

attributable to introduction of effluent from the RTF probably 

was not the primary cause for the blooms in 1965-66. 

Hydrographic conditions, rather than nutrient content, apparently 

control the timing of blooms in the Duwamish. A minimum fresh-water 

discharge of about 550 m3/min during a period of minimum tidal exchange and high light intensity increases the photic-zone stability and 

water-retention time. These conditions in turn favor phytoplankton 

blooms because the standing stock is apparently held in the shallow 

photic zone (above the compensation depth), where production greatly 

exceeds respiration. The compensation depth in the Duwamish is generally between 2 and 3 meters, which is not unusual for an estuary.
A54 ENVIRONMENTAL QUALITY

Hull (1963, p. 604) found that the compensation depth in Baltimore 

Harbor ranged between 0.6 and 2.4 meters, with a mean of 1.7 meters. 

The importance of hydrographic factors in controlling bloom timing 

is shown by a significant correlation of productivity at station 7.7 

during 1965-66 with measurements of relative stability (A°C) and 

retention time (fresh-water discharge).

The increase in temperature of the surface water probably has a 

greater effect on the timing of blooms by indirectly increasing stability 

in the photic zone than by directly influencing the photosynthetic rate. 

The observed decrease in daily maximum surface temperature of 4°C 

in 4 days at station 7.7 during the 1966 bloom, (fig. 4) apparently did 

not directly influence the photosynthetic rate. On days when the daily 

maximum temperature was lowest, the daily maximum percent saturation of oxygen was greatest; this relationship indicates that photosynthetic oxygen evolution is not related to a temperature change of 

this magnitude. An increase in the vertical temperature gradient in the 

surface water would indirectly increase the stratification and also 

would reflect the extent of turbulent mixing. A vertical gradient in 

specific conductance might not always indicate stability when water 

from the salt wedge is continually added to the surface water of an 

estuary by strong tidal action and turbulent mixing. Under such conditions, an instantaneous specific-conductance profile might show a 

gradient, even though in reality a considerable exchange was occurring 

which would reduce the temperature gradient. During periods of less 

severe tidal action and turbulent mixing, the temperature gradient 

would tend to increase as the surface water was retained under high 

light intensity. Although correlation of the bloom periods with surface-to-bottom differences in temperature probably does not indicate 

a direct effect of temperature on photosynthetic rate, it does show the 

influence of surface-water stability on bloom development.

The conditions which permit bloom development are evidently too 

critical to be predicted solely by tidal-prism thickness and discharge. 

Favorable tidal-prism thicknesses and fresh-water discharges apparently occurred also during September of 1965 and 1966, but a 

second bloom did not develop in either year. However, light intensity 

and temperature in the surface water during these September periods 

were much lower than on about each August 1, preceding the bloom. 

Thus, while conditions of tidal action and discharge in these Septembers were favorable, the combination of reduced light and temperature may have resulted in insufficient stability, as well as 

insufficient light intensity, for the production of a bloom.

To facilitate prediction of the timing of blooms in the Duwaniish, 

more frequent and specific measurements of stability related to pro-
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A55

ductivity should be made, particularly during the summer months, 

to provide a closer correlation. Vertical profiles of water velocity 

recorded at peaks of ebb and flood tides would probably correlate well 

with productivity because such measurements would tend to integrate 

the factors of vertical turbulence and retention time.

Hydrographic conditions during the summer of 1964 probably best 

explain why a phytoplankton bloom failed to develop then. The minimum fresh-water discharge of 760 m3 /min in August 1964, compared 

to about 550 m3 /min during the blooms in 1965 and 1966, was the 

highest minimum flow of record since the start of data collections 

in 1937. The greater minimum flow in 1964 probably created conditions of greater turbulent mixing in the photic zone and of shorter 

retention times; both conditions could have prevented bloom development. Also the mean temperatures in the surface water were lower 

during the summer of 1964 than in 1965 and 1966. Thus, available 

light, turbulent mixing, and retention time probably were not as 

favorable to phytoplankton growth during 1964 as they were during 

1965 and 1966.

The importance of stability and compensation depth in controlling 

the timing of phytoplankton blooms is well known in nearshore environments. Riley (1942, p. 78) found that although nutrient concentrations are abundant in north-temperate waters during the winter, the 

spring phytoplankton blooms do not occur until March or April, when 

stability increases. The rate of increase in the phytoplankton population on Georges Bank, Mass., varied directly with the reciprocal of the 

depth of vertical-turbulence when the photic-zone thickness and the 

rates of photosynthesis and respiration were constant. Stability was 

also important in promoting bloom conditions in Block Island Sound, 

off the Connecticut-Rhode Island coast (Riley, 1952). Gran and 

Braarud (1935) found that stability controlled the timing of blooms in 

the nutrient-rich waters of the Bay of Fundy. Although nutrient concentrations were always high, strong vertical turbulence did not allow 

the phytoplankton cells to remain in the illuminated zone long enough 

to utilize the nutrients. A bloom resulted when the zone of vertical turbulence was shallower than the photic zone, a condition that permitted 

production to exceed respiration. Smayda (1957, p. 355) pointed out 

that although the maximum winter phytoplankton population in Narragansett Bay occurs during a period of unstable conditions, the shallowness and weak currents characteristic of the water body are sufficiently confining for population development. Gilmartin (1964, p. 530) 

found a close correspondence between phytoplankton production and 

vertical stability in a British Columbia fjord.
A56 ENVIRONMENTAL QUALITY

Although the timing of the spring bloom in north-temperate oceanic 

waters is determined primarily by light availability, compensation 

depth, and depth of vertical turbulence, the magnitude of the bloom is 

generally limited by nutrient supply (Anderson and Banse, 1961, p. 

62-63). Pratt (1965, p. 182) concluded that the maximum biomass attained during the diatom flowering in Narragansett Bay was controlled 

by the nitrate and silicate content. Growth was observed to continue 

after exhaustion of the nitrate during most years, but never after depletion of the silicate. The foregoing observations help to establish that 

the increased nutrients in the Duwamish estuary probably would tend 

to increase the maximum biomass attained during a bloom rather than 

to alter its timing.1

EFFECT OF NUTRIENTS ON MAXIMUM BIOMASS PRODUCED

Limited experimental evidence indicates that effluents added to 

Duwamish River water stimulate the rate of nutrient assimilation by 

green algae. This suggests that the biomass attained in a bloom or in 

the total annual production might increase in response to increased 

nutrients from the RTF; however, field data during 1965-66 indicate 

that this did not occur. The annual mean biomass (chlorophyll a concentration) in 1966 was not significantly greater than the mean biomass 

in 1965 even though the mean effluent discharge was 46 percent greater 

in 1966. Nevertheless, this conclusion is only tentative because sampling 

frequency during the bloom may not have been adequate to define the 

total biomass produced.

Results of the in vitro bioassay cannot be automatically interpreted 

as evidence that added nutrients in the effluent will increase the bloom 

biomass because the response measured was from green algae, not from 

the diatoms that were present in the blooms. Further, the experiments 

were performed in the spring rather than during the bloom period; 

as Founder (1966, p. 18) pointed out, the physiological response of 

phythoplankton communities to nutrient enrichment varies with the 

season and the physiological state of the community (that is, its position on the theoretical growth curve).

Examples of phytoplankton production that increased as a result of 

nutrient increments from domestic wastes are largely for water bodies 

in which preeffluent nutrient concentrations were much lower than 

those in the Duwamish. Edmondson, Anderson, and Peterson (1956, 

p. 50) reported maximum phosphate concentrations of 0.022, 0.020, 

and 0.038 mg P/l in the hypolimnion of Lake Washington during the 

summers of 1933, 1950, and 1955, respectively. The minimum mean

1 -Silicate is not one of the limiting nutrients in the Duwamish estuary because it is always 

an abundant constituent of the natural fresh-water inflow.
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A57

value for the hypolimnion in 1933 was about 0.005 nig P/l. The increase 

in phosphate during 1950-55 was attributed to an increased inflow of 

treated domestic waste and was accompanied by greater productivity 

in the lake. Sawyer (1952) concluded from a study of several Wisconsin lakes that an inorganic phosphate concentration of 0.01 mg/1 was 

sufficient to promote bloom conditions. Goldman and Carter (1965, p. 

1058) found in bioassay studies that phytoplankton in the highly oligotrophic Lake Tahoe responded to very small additions of treated domestic waste effluent. The maximum concentrations of phosphate and 

nitrate in the lake were very low (0.007 nig P/l and 0.015 mg N/l, 

respectively).

Preeffluent nutrient concentrations in the Duwamish were much 

greater than those that reportedly promote accelerated eutrophication 

in lakes. A decline in nutrient concentrations to limiting levels was not 

apparent during the blooms; however, additional effluent nutrients 

in excess of the present (1966) relatively large quantities might increase the bloom biomass. Ryther, Yentsch, Hulbert, and Vaccaro 

(1958) studied a phytoplankton bloom in a small tidal creek tributary 

to Long Island Sound. They found that a large population, in which 

chlorophyll a concentration ranged from 116 to 245 mg/m3 , seemed to 

be limited in growth by ambient nitrate concentrations of 0.05 mg N/l 

and phosphate concentrations of 0.12 mg P/l. The possibility that 

these nutrient concentrations limited growth was indicated by the comparison of the high photosynthetic rate of the population (5 g C/m2/ 

day) with the rate of phosphorus and nitrogen uptake that would be 

necessary for growth to occur; the ratio of 100:15:1 for C: N: P was 

used. The necessary daily supply exceeded the amounts of the two 

nutrients available. Shapiro and Ribeiro (1965) found that although 

phosphate was relatively high in the Potomac River (0.007-0.15 mg 

P/l) upstream from an outfall for domestic waste, effluent phosphate 

added to river water that initially contained 0.037 mg P/l increased 

the in vitro growth of green and blue-green algae. The increase in 

growth was proportional to the added concentrations of phosphate, 

which ranged from 0.295 to 2.10 mg P/l. These results from the 

Potomac were generally similar to those found in the in vitro bioassay 

studies with Duwamish water; however, extrapolation of experimental

data of this type to predict field conditions would have only limited 

value at best. As pointed out by Tailing (1962, p. 747), algae grown in 

cultures often require much greater concentrations of nitrogen and 

phosphorus than algae growing under field conditions partly because 

cultures have more rapid growth rates than natural-growing algae 

and higher cell densities. Thus, ambient concentrations of phosphate 

which are not limiting to algae growing in an estuary may well become
A58 ENVIRONMENTAL QUALITY

limiting to algae growing in water confined within a flask. To predict 

more reliably the effect of future increases in E.TP effluent on total 

phytoplankton production in the Duwamish estuary, in situ nutrient 

addition experiments are recommended. These should be made in large 

polyethylene test cells during periods of active growth, according to 

procedures outlined by Goldman and Carter (1965, p. 1048).

BELATION OF PHYTOPLANKTON TO DISSOLVED OXYGEN

If total phytoplankton production in the Duwamish increases as a 

result of nutrients added by the RTF effluent, the late summer depression of DO will probably increase in extent and persistence as a result 

of the added BOD from the phytoplankton. Minimum DO occurs in 

the upstream end of the saline-water wedge presumably because increased retention of this unaerated water mass during periods of 

minimum discharge and tidal-prism thickness allows more time for the 

existing BOD to act. Water with maximum BOD near stream bottom 

was observed consistently at station 9.7 regardless of tide stage even 

though the water with minimum DO occurred downstream from that 

point on low tide and upstream on high tide. This implies a mechanism 

by which oxidizable particulate matter is continually being resuspended and redeposited in this transition area as the bottom water 

moves upstream and downstream with the high and low tide. The 

BOD has ample time to act during periods of prolonged retention; 

the bottom water probably traverses the area many times without 

reaeration before it is finally removed from the estuary by entrainment in the surface water.

A large amount of the BOD in this bottom water originated from 

phytoplankton that sank from the near-surface waters. This fact 

was shown by a significant correlation between BOD and phytoplankton biomass (chlorophyll a, concentration) in all low-tide samples 

collected throughout the summer of 1966, as well as by close correlations of these two variables longitudinally in the estuary and seasonally at stations in the transition area where the maximum values were 

observed.

Because the regression line for chlorophyll a and BOD intercepts 

the BOD ordinate (fig. 22), some of the BOD probably was not due 

to chlorophyll a. Undoubtedly organic particulate matter from sources 

other than phytoplankton settles in the transition area and contributes 

to the BOD. A BOD, however, can be exerted by phytoplankton 

organic matter lacking chlorophyll &, as was pointed out earlier. 

Organic detritus from upstream and solids from the numerous but 

relatively small organic waste effluents in the area are probable sources 

of BOD. The amount of BOD contributed from various sources is
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A59

unknown, however, and the organic carbon produced by phytoplankton alone represents a BOD source capable of extensive DO depletion. 

During the bloom in 1966, the mean of the gross productivity rate 

measured by the oxygen method and by the carbon-14 method was 

about 3.0 g/C/m2 /day. The conditions of chlorophyll a concentrations greater than 50 mg/m3 in the bottom water and cell concentrations there of about 11,000 per milliliter when chlorophyll a is comparatively low (5.5 mg/m3 ) indicate that a large proportion of cells 

produced in the surface water sink into the bottom-water layer. Even 

if phytoplankton sink in the reach, toward the bay, some could ultimately be deposited in the transition area because the net flow of the 

bottom water is upstream (J. D. Stoner, U.S. Geol. Survey, written 

commun., 1967). Another indication that most of the phytoplankton 

organic matter is deposited in the upper transition area is the comparatively low concentration of phytoplankton in the lower estuary 

(station 1.9), however, sea-water dilution may have reduced the concentration by as much as 50 percent. About two-thirds of the organic 

matter produced in the surface layers (or about 2 mg C/m2 /day) 

probably sinks into the bottom-water layer. Twenty days (the approximate duration of the bloom in 1966) of production at this rate would 

result in an equivalent oxygen consumption of 120 gm 02/m2 (for 5 

days, the value would be 30 g 02/m2 ). The saline-water wedge has an 

estimated mean depth of about 5 m at station 7.7 for a complete tidal 

cycle, and the mean depth undoubtedly is much less at station 9.7. 

Assuming an even distribution of organic matter in a 5-m column, the 

20-day BOD would be 24 mg/1 (24 g 02 /m3 ), and the 5-day BOD would 

be 6 mg/1. The distribution would not be uniform, however, because the 

particulate matter, and hence the BOD, would probably be concentrated near the bottom. The farthest upstream segment of the salinewater wedge would probably be the oldest water in the estuary, and its 

volume, upon which the particulate BOD could act, would be the 

smallest. Thus, the low DO at low tide can be explained by the respiration and decomposition of large phytoplankton concentrations in a 

rather small isolated volume of water in the estuary.

The effect of large concentrations of decomposing algae on DO 

has been shown by workers in other areas. Sinking organic matter 

produced by phytoplankton blooms results in a hypolimnetic oxygen 

deficit in lakes; this deficit in turn is a measure of the productivity and 

the degree of eutrophication (Edmondson and others, 1956). Barlow, 

Lorenzen, and Myren (1963, p. 251, 261) observed similar conditions 

in the Forge Eiver, Long Island estuary. There, planktonic organic 

matter that was produced locally because of relatively great nutrient 

supplies exceeded the organic matter inflow from other sources, thereby 

resulting in anerobic conditions in the bottom mud below the photic
A60 ENVIRONMENTAL QUALITY

zone. In other fresh-water environments, Bartsch (1960) has shown 

that the BOD from algae produced as a result of added effluent nutrients has exceeded the demand of the original waste. Nash (1947, p. 160) 

measured minimum DO in the bottom water of the Patuxent River 

estuary, Maryland, during maximum stratification in May and June. 

(Although he suggests that bacterial respiration and increased stratification of the bottom layer caused the DO to decrease, he did not 

mention the potential BOD in the spring phytoplankton bloom, which 

occurred in April and May preceding the oxygen decrease.)

Further depression of DO minimums in the Duwamish estuary seems 

likely if a substantial increase in phytoplankton production results 

from the anticipated increase in effluent nutrients. Although the timing 

of phytoplankton blooms seems to be strongly controlled by hydrographic factors, nutrient increases may in turn increase the maximum 

biomass produced in the bloom. This condition has not happened 

yet; moreover a comparison of the mean annual biomass of 1965 to 

that of 1966 shows no significant difference despite a 46-percent increase in effluent discharge. However, the conclusion-that nutrient 

increases may effectively increase biomass production is only tentative, 

because the number of individual measurements of maximum productivity and chlorophyll a was much greater in 1966 than in 1965. In 

order to define the total biomass produced, increased sampling frequency is certainly warranted during the bloom periods. The increase 

in effluent discharge, if the discharge is cool enough to mix well with 

the river water, may also have an inhibiting effect on phytoplankton 

productivity by increasing turbulence in the photic zone. Such an 

effect would probably require a substantial increase in effluent discharge, which is not expected in the near future.

LITERATURE CITED

American Public Health Association and others, 1965, Standard methods for the 

examination of water and waste water [12th ed.] : New, York, Am. Public 

Health Assoc., Inc., p. 769.

Anderson, G. 0., and Banse, Karl, 1961, Hydrography and phytoplankton production, in Doty, M. S., ed., Proceedings of the conference on primary productivity measurement, marine, and fresh-water: Washington, D.C., U.S. Atomic 

Energy Oomm. Pub. TID-7633, p. 61-71.

Antia, N. J., McAllister, C. D., Parson, T. R., Stephens, K., and Strickland, J. D. 

H., 1963, Further measurements of primary production using a large-volume 

plastic sphere: Limnology and Oceanography, v. 8, no. 2, p. 166-183.

Barlow, J. P., Lorenzen, C. J., and Myren, R. T., 1963, Eutrophication of a tidal 

estuary: Limnology and Oceanography, v. 8, no. 2, p. 251-262.

Bartsch, A. F., 1960, Algae in relation to oxidation processes in natural waters, 

in Tryon, C. A., Jr., and Hartman, R. T., ed., The ecology of algae: Univ. 

Pittsburgh [Pittsburgh, Pa.], Pymatuning Lab. of Field Biology, Spec. Pub. 

1, p. 56-71.
FACTORS INITIATING PHYTOPLANKTON BLOOMS IN WASHINGTON A61

Brown and Caldwell, 1958, Metropolitan Seattle sewerage and drainage survey:

San Francisco, Calif., Brown and Caldwell, Civil and Chemical Engineers,

546 p. 

Creitz, G. I., and Richards, F. A., 1955, A note on the use of "Millipore" membrane

filters in the estimation of plankton pigments, pt. 3 of the estimation and

characterization of plankton populations by pigment analysis; Jour. Marine

Research, v. 14, no. 2, p. 211-216. 

Edmondson, W. T., Anderson, G. C., and Peterson, D. R., 1956, Artificial eutrophication of Lake Washington: Limnology and Oceanography, v. 1, no. 1,

p. 47-53. 

Fournier, R. O., 1966, Some implications of nutrient enrichment on different

temporal stages of a phytoplankton community: Chesapeake Sci., v. 7, no. 1,

p. 11-19. 

Gilmartin, Malvern, 1964, The primary production of a British Columbia fjord:

Fisheries Research Board of Canada Jour., v. 21, no. 3, p. 505-538. 

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A62 ENVIRONMENTAL QUALITY

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