Cover photographs, clockwise from lower left: 

Laboratory apparatus used to determine sediment-phosphorus release rates from 

Upper Klamath Lake sediments 

Sediment sample from Upper Klamath Lake 

Closeup of acrylic column used in the laboratory experiments 
U.S. Department of the Interior 

U.S. Geological Survey 

Effect of Water-Column pH on Sediment-Phosphorus 
 
 

Release Rates in Upper Klamath Lake, Oregon, 2001
 
 

By LAWRENCE H. FISHER and TAMARA M. WOOD 

Water-Resources Investigations Report 03–4271 

Prepared in cooperation with 

the Bureau of Reclamation 

Portland, Oregon: 2004 
____________________________________________________________________________ 

U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary 

U.S. GEOLOGICAL SURVEY 

CHARLES G. GROAT, Director 

The use of trade, product, or firm names in this publication is for 

descriptive purposes only and does not imply endorsement by 

the U.S. Government. 

For additional information: Copies of this report may be 

purchased from: 

District Chief 

U.S. Geological Survey USGS Information Services 

10615 S.E. Cherry Blossom Dr. Box 25286, Federal Center 

Portland, OR 97216-3159 Denver, CO 80225-0286 

E-mail: info-or@usgs.gov Telephone: 1-888-ASK-USGS 

Internet: http://oregon.usgs.gov 

Suggested citation:
 
 

Fisher, L.H., Wood, T.M., 2004, Effect of water-column pH on sediment-phosphorus release rates in Upper Klamath
 
 

Lake, Oregon, 2001: U.S. Geological Survey Water-Resources Investigations Report 03–4271, 25 p.
 
 

ii 
CONTENTS
 
 

Abstract ...................................................................................................................................................................................... 1
 
 

Introduction ................................................................................................................................................................................ 1
 
 

Objectives............................................................................................................................................................................ 4
 
 

Acknowledgments............................................................................................................................................................... 4
 
 

Methods and Materials ............................................................................................................................................................... 4
 
 

Sample Collection ............................................................................................................................................................... 4
 
 

Experimental Design........................................................................................................................................................... 5
 
 

Stable Interface Study ............................................................................................................................................ 5
 
 

Resuspended Sediment Study ................................................................................................................................ 6
 
 

Phosphorus Fractionation Study............................................................................................................................. 7
 
 

Analytical Methods ............................................................................................................................................................. 8
 
 

Results ........................................................................................................................................................................................ 9
 
 

Stable Interface Study ......................................................................................................................................................... 9
 
 

Resuspended Sediment Study ........................................................................................................................................... 11
 
 

Phosphorus Fractionation Study ....................................................................................................................................... 12
 
 

Discussion ................................................................................................................................................................................ 13
 
 

Summary .................................................................................................................................................................................. 17
 

References Cited ...................................................................................................................................................................... 19
 

Appendix A: Summary of Data Quality................................................................................................................................... 23
 
 

Preliminary Column Tests ................................................................................................................................................ 23
 
 

Method Detection Limit.................................................................................................................................................... 23
 
 

Quality Assurance/Quality Control................................................................................................................................... 23
 

Internal Standard Reference Samples .................................................................................................................. 23
 
 

External Reference Samples................................................................................................................................. 23
 
 

FIGURES 

1. Map of Upper Klamath Lake with sampling site identification and location ................................................................. 2
 
 

2. Graph showing total phosphorus, soluble reactive phosphorus, chlorophyll a, and pH as measured in 
 
 

biweekly samples collected at Shoalwater Bay, Upper Klamath Lake, Oregon, in 2001 by the Klamath Tribes ......... 3
 

3. Schematic diagram of a column used in the Stable Interface Study designed to have the same sediment 
 
 

surface area to water column volume ratio that is observed in the lake ......................................................................... 5
 
 

4. Diagram showing the steps followed for analysis of phosphorus fractions.................................................................... 8
 
 

5. Graphs showing soluble reactive phosphorus concentrations divided into three phases for each column
 
 

during the Stable Interface Study ................................................................................................................................... 9
 
 

6. Graphs showing total phosphorus concentrations divided into three phases for each column during the
 
 

Stable Interface Study................................................................................................................................................... 11
 
 

7. Graphs showing soluble reactive phosphorus concentrations in each column during the Resuspended
 
 

Sediment Study ............................................................................................................................................................. 12
 
 

TABLES 

1. Column pH adjustments, sediment additions, and other treatments for the Resuspended Sediment Study ................... 6
 
 

2. Column pH, and soluble reactive and total phosphorus release rates by phase for the Stable Interface Study............ 10
 
 

3. Column pH and release rates by phase for the Resuspended Sediment Study ............................................................. 13
 
 

4. Resuspended Sediment Study column pH, soluble reactive phosphorus concentration, and percent of the total
 
 

phosphorus as dissolved organic phosphorus, dissolved acid-hydrolyzable phosphorus, soluble reactive
 
 

phosphorus, and total suspended phosphorus ............................................................................................................... 13
 
 

5. Experimentally determined areal release rates of phosphorus exchange between sediments and water...................... 16
 
 

iii 
CONVERSION FACTORS 

[SI = International System of units, a modernized metric system of measurement] 

Factors for converting SI metric units to inch/pound units 

Multiply By To obtain

 Length 

micrometer (μm) 0.00003937 inch (in) 

millimeter (mm) 0.03937 inch 

centimeter (cm) 0.3937008 inch 

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

 Area 

square centimeters (cm2) 

square meters (m2) 

square kilometers (km2) 

0.1550003 

10.76391 

0.386 

square inch (in2) 

square foot (ft2) 

square miles (mi2)

 Mass 

microgram (μg) 0.00000003527 ounce (oz avoirdupois) 

milligram (mg) 0.00003527 ounce gram (g) 0.03527 ounce kilogram (kg) 2.205 pound (lb) 

Temperature 

degrees Celsius (°C) (1) degree Fahrenheit (°F) 

Concentration, By Volume 

micrograms per liter (μg/L) 1 parts per billion (ppb) 

milligrams per liter (mg/L) 1 parts per million (ppm) 

Concentration, By Mass 

milligrams per kilogram (mg/kg) 1 parts per million (ppm) 

Concentration, By Area 

milligrams per square meter (mg/m2) 0.000003277 ounce per square foot (oz/ft2) 

Specific Conductance 

microsiemens per centimeter (μS/cm) 0.00254 micromho per inch (μmho/in) 

Rate 

kilometer per hour (km/hr) 0.6213712 mile per hour (mi/hr statute) 

micrograms per liter per day (μg/L/day) 

milligrams per square meter per day (mg/m2/day) 

1 

0.000003277 

parts per billion per day (ppb/day) 

ounce per square foot per day (oz/ft2/day) 

1

Temperature °F = 1.8 (Temperature °C) + 32 
MISCELLANEOUS ABBREVIATIONS 

A. flos-aquae Aphanizomenon flos-aquae 

BOR Bureau of Reclamation 

BQA USGS Branch of Quality Assurance 

CO2 carbon dioxide 

CRA CO2-reduced air 

DAHP dissolved acid-hydrolyzable phosphorus 

DMA designated management agency 

DOP dissolved organic phosphorus 

DUP dissolved unreactive phosphorus 

HP horsepower 

K cell constant 

KH2PO4 potassium dihydrogen phosphate 

MDL method detection limit 

N normality 

NaOH sodium hydroxide 

NaOH-nrP nonreactive fraction of NaOH extractable phosphorus 

NaOH-rP reactive fraction of NaOH extractable phosphorus 

P phosphorus 

PPM parts per million 

QA/QC quality assurance/quality control 

SRP soluble reactive phosphorus 

SRS standard reference sample 

TDP total dissolved phosphorus 

TP total phosphorus 

TSP total suspended phosphorus 

USGS U.S. Geological Survey 

v 
Effect of Water-Column pH on Sediment-Phosphorus Release 

Rates in Upper Klamath Lake, Oregon, 2001 

By Lawrence H. Fisher and Tamara M. Wood 

Abstract 

Sediment-phosphorus release rates as a 

function of pH were determined in laboratory 

experiments for sediment and water samples 

collected from Shoalwater Bay in Upper Klamath 

Lake, Oregon, in 2001. Areal release rates for a 

stable sediment/water interface that is representative of the sediment surface area to water column 

volume ratio (1:3) observed in the lake and volumetric release rates for resuspended sediment 

events were determined at three different pH 

values (8.1, 9.2, 10.2). Ambient water column 

pH (8.1) was maintained by sparging study 

columns with atmospheric air. Elevation of the 

water column pH to 9.2 was achieved through 

the removal of dissolved carbon dioxide by 

sparging with carbon dioxide-reduced air, partially simulating water chemistry changes that occur 

during algal photosynthesis. Further elevation of 

the pH to 10.2 was achieved by the addition of 

sodium hydroxide, which doubled average 

alkalinities in the study columns from about 1 

to 2 milliequivalents per liter. Upper Klamath 

Lake sediments collected from the lake bottom 

and then placed in contact with lake water, either 

at a stable sediment/water interface or by resuspension, exhibited an initial capacity to take up 

soluble reactive phosphorus (SRP) from the water 

column rather than release phosphorus to the water 

column. At a higher pH this initial uptake of 

phosphorus was slowed, but not stopped. This 

initial phase was followed by a reversal in which 

the sediments began to release SRP back into the 

water column. The release rate of phosphorus 30 

to 40 days after suspension of sediments in the 

columns was 0.5 μg/L/day (micrograms per liter 

per day) at pH 8, and 0.9 μg/L/day at pH 10, 

indicating that the higher pH increased the rate of 

phosphorus release by a factor of about two. The 

highest determined rate of release was approximately 10% (percent) of the rate required to 

explain the annual internal loading to Upper 

Klamath Lake from the sediments as calculated 

from a lake-wide mass balance and observed in 

total phosphorus data collected at individual 

locations. 

INTRODUCTION 

Upper Klamath Lake (fig. 1) is a large, shallow 

lake in south central Oregon with a surface area of 

about 208 km2 (square kilometers) and a mean depth 

of about 3 m (meters) when the lake is at full pool. The 

lake receives drainage from 9,800 km2 of phosphorusrich volcanic soils. A paleolimnological study has 

shown that nutrient and sediment loading to the lake 

has increased over the last century and that recent 

bottom sediments have become enriched with phosphorus (Eilers et al., 2001). Activities in the basin, 

including logging, grazing, and agriculture, can 

mobilize phosphorus-rich soils. In addition, over 

120 km2 of wetlands in the basin have been drained, 

allowing organic soils to decompose and act as a large 

external source of phosphorus to Upper Klamath Lake 

(Snyder and Morace, 1997). 

1 
Figure 1. Upper Klamath Lake with sampling site identification and location. 

2
 
As a result of these changes, Upper Klamath 

Lake, which had been a highly productive, eutrophic 

lake for at least the last 1,000 years (Eilers et al., 2001), 

has become hypereutrophic, and now experiences 

annual occurrences of near-monoculture blooms of the 

blue-green alga Aphanizomenon flos-aquae (A. 

flos-aquae). This algal bloom corresponds to a large 

seasonal increase in total phosphorus (TP) in the lake 

each spring that cannot be accounted for by external 

loading estimates (Kann and Walker, 1999). Figure 2 

shows spring increases in TP, chlorophyll a, and pH, as 

well as changes in soluble reactive phosphorus (SRP), 

that were measured in grab samples collected by the 

Klamath Tribes at Shoalwater Bay in 2001 (data 

obtained from Lawrence Dunsmoor of Klamath Tribes 

Natural Resources, Chiloquin, Oregon, 2001). Internal 

loading of phosphorus from lake sediment, occurring 

on seasonal cycles, can drive the increase in TP and 

feed the algal blooms. During this TP increase SRP is 

released, but concentrations remain low due to algal 

uptake. SRP concentrations begin to climb as the algal 

biomass decreases, seen in figure 2 as a decrease in 

chlorophyll a concentration. 

Figure 2. Total phosphorus, soluble reactive phosphorus, 

chlorophyll a, and pH as measured in biweekly samples collected at 

Shoalwater Bay, Upper Klamath Lake, Oregon, in 2001 by the 

Klamath Tribes. 

Because the lake is relatively shallow and rarely 

becomes anaerobic near the bottom sediments, this 

seasonal internal loading process appears to occur 

under primarily aerobic conditions. High pH values 

observed in Upper Klamath Lake have been associated 

with algal photosynthesis during the rapid early growth 

phase of the A. flos-aquae blooms; median June pH 

values are often greater than 9.5 (Wood, et al. 1996). 

Under aerobic conditions, internal phosphorus loading 

in the lake has been described primarily as a geochemical phenomenon in which phosphorus is 

mobilized from bottom sediments into a water column 

under high pH conditions. Kann (1998) postulated that 

under high pH, hydroxide ions compete with phosphate 

ions for sorption sites on iron hydroxides, thereby 

releasing phosphorus into the water column. Additional 

support for this release mechanism in Upper Klamath 

Lake is provided by a correlation between pH and 

whole-lake mass balance calculations of internal 

phosphorus loading (Walker, 2001; Welch and Burke, 

2001). Whole-lake mass balance calculations for the 

years 1991–99 have shown that the rate of internal 

loading required to account for measured increases in 

TP concentration peaks between 15 and 25 mg/m2/day 

(milligrams per square meter per day) (Walker, 2001). 

Several laboratory studies have demonstrated the 

efficacy of this high pH mechanism in releasing 

phosphate ions from bottom sediments. However, the 

interpretation of some studies is confounded by the fact 

that pH was increased with the addition of strong base, 

which increased the ionic strength and alkalinity of the 

water markedly (Andersen, 1975; Jacoby et al., 1982; 

Drake and Heaney, 1987; Appan and Ting, 1996). In 

natural lake systems, the photosynthetic removal of 

dissolved CO2 raises the pH but does not increase the 

alkalinity. In many hard water lakes, alkalinity 

decreases with the precipitation of calcite during algal 

photosynthesis. Boers (1991) concluded that enhanced 

phosphate release when NaOH was used to raise pH 

was an artifact of increasing the alkalinity in the water 

column from 6.1 to 19.4 meq/L (milliequivalents per 

liter) with the addition of enough NaOH to reach pH 

9.5. 

In addition to pH considerations, it has been 

reported that resuspension of lake sediment can 

increase the internal phosphorus loading in lakes. 

Resuspension of lake sediments in the wind-exposed 

Lake Arreso, Denmark, resulted in phosphorus loading 

20–30 times greater than loading from undisturbed lake 

sediment (Sondergaard et al., 1992; Reddy, et al., 

1996). Laenen and LeTourneau (1996) estimated 

that the suspended sediment concentration in Upper 

Klamath Lake could be as high as 200 mg/L 

(milligrams per liter) as a result of a 16 km/hr 

(kilometers per hour) wind out of the northwest lasting 

2 hours. Estimates of the TP concentration associated 

with resuspended sediments during this wind event 

3 
ranged from 360 to 2,000 μg/L (micrograms per liter). 

Thus, a wind event strong enough to resuspend 

sediments has the potential to enhance mechanisms for 

mobilizing phosphorus in the bottom sediments by 

virtue of both the increased sediment surface for 

phosphorus desorption processes and the large mass of 

phosphorus that bottom sediments can carry into the 

water column. 

This report presents data on phosphorus uptake 

and release from stable and suspended Upper Klamath 

Lake sediments under varying pH regimes. The study 

was funded by the Bureau of Reclamation in Klamath 

Falls, Oregon. Sample and data collection began in 

April of 2001 and continued through January of 2002. 

Objectives 

The intent of the investigation was to determine 

whether the phosphorus release from Upper Klamath 

Lake sediments, at the photosynthetically elevated pH 

values measured in the lake, could account for the rapid 

increase in total phosphorus concentration observed 

each spring. The study was designed to investigate both 

the areal phosphorus release rates from sediments at a 

stable sediment-water interface and the desorption of 

phosphorus from suspended sediments that would 

occur during a wind-induced sediment resuspension 

event. The former investigation is referred to as the 

Stable Interface Study; the latter is referred to as the 

Resuspended Sediment Study. 

An additional objective incorporated into the 

experimental design was to elevate pH in a manner that 

would simulate the water chemistry changes that occur 

during algal photosynthesis. This was then followed by 

the addition of a small amount of strong base to elevate 

the pH in the column further for comparison. 

Additional experimental design objectives were 

to incorporate a ratio of sediment surface area to water 

column volume that is characteristic of the lake into the 

Stable Interface Study, and to use a sediment concentration representative of an extreme, but not rare, wind 

event in the Resuspended Sediment Study. Because of 

the nature of column studies in the laboratory, many 

study designs incorporate an unrealistically large 

sediment surface area to water column volume ratio 

when investigating processes at the sediment/water 

interface and an unrealistically high sediment concentration when investigating resuspension events. 

Acknowledgments 

The authors thank Mark Buettner (Bureau of 

Reclamation, Klamath Falls, Oregon) for supporting 

this work and coordinating this effort between the 

Bureau of Reclamation and the U.S. Geological 

Survey (USGS), Michael Berg (Bureau of Reclamation, Klamath Falls, Oregon) for his assistance in the 

collection of samples, Lawrence Dunsmoor (Klamath 

Tribes Natural Resources, Chiloquin, Oregon) for 

providing water-quality data sets, Micelis C. Doyle 

(U.S. Geological Survey, Portland, Oregon) for 

providing quality assurance/quality control samples, 

Joe Rinella and Dennis Lynch (U.S. Geological Survey, 

Portland, Oregon) for their technical input, and Dr. 

Patricia L. Toccalino (Oregon Graduate Institute 

School of Science and Engineering at Oregon Health 

and Science University, Portland, Oregon) for 

providing analytical equipment. 

METHODS AND MATERIALS 

Sample Collection 

Sediment samples and water samples were 

collected twice in the spring of 2001. The first samples 

were collected on May 25, 2001, for preliminary 

experiments at the established monitoring site, 

Midnorth (fig. 1). Preliminary experiments were 

conducted to establish methods and protocols for 

the study, and are not presented here. A second set 

of samples were collected June 21, 2001, from the 

established monitoring site in Shoalwater Bay in the 

northwestern part of Upper Klamath Lake (fig. 1). 

Review of historical Shoalwater Bay data indicates that 

samples collected in June would typically represent 

conditions prior to the TP peak. However, the samples 

used in this study were collected after the 2001 spring 

increase in TP, chlorophyll a, and pH (fig. 2). 

All sample water was collected from the surface 

of the lake and filtered in the field through a 353 μm 

(micrometers) mesh sieve. Bottom sediment samples 

were collected at the same site at a lake depth of 3 m 

(meters) with an Eckman dredge. The top layer of 

sediment from three dredge samples was collected, 

avoiding the deeper, anoxic sediments, and then 

combined and homogenized into a single sample to be 

used in the laboratory study. All samples were iced 

4 
during transport to the laboratory where they were 

stored at 4oC (degrees Celsius) until used in the 

experiments. 

Experimental Design 

Both the Stable Interface Study and the 

Resuspended Sediment Study were conducted in the 

USGS Oregon District Laboratory with four identical 

acrylic columns 51 cm (centimeters) tall with a 25 cm 

inside diameter (fig. 2). When filled to approximately 

48 cm, the column contained a sample volume of 23 L. 

Each column was constructed with a sealed bottom and 

a removable airtight lid. The lid was constructed with 

ports that allowed for a deep vessel pH probe, a 

temperature probe, a sampling tube (or pipette), some 

tubing for the sparging gas, and a one-way bubble-type 

exhaust valve. All ports were kept sealed between 

sampling, which allowed all exhaust gases to exit 

through the one-way exhaust valve. Sparge gas flow 

rates for each column were set using a Dwyer Rotameter ball-type flow gauge with a flow-metering valve. 

During the experiments, the columns were kept dark 

with aluminum foil to minimize algal growth. 

Stable Interface Study 

For the Stable Interface Study, acrylic cups 2.5 

cm tall with an inside diameter of 9.5 cm were constructed to hold a sediment sample at the bottom of 

each study column (fig. 3). The dimensions of the cup 

were such that sediment within the cup had a surface 

area of 71 cm2 (square centimeters), resulting in a 

sediment surface area to volume ratio in the column of 

3.1 cm2

/L. This is the same ratio that would be found in 

a water column 3.1 m in depth with a 1 m2

 (square 

meter) sediment surface area. Sample withdrawals over 

the course of the experiment increased the sediment 

surface area to water volume ratio by about 10% 

(percent). 

The experiment was conducted in triplicate in 

columns denoted A, B, and C; the fourth column 

(denoted D) was the study control. Prior to filling the 

columns with lake water, three sediment cups were 

filled with approximately 182 g (grams) of wet sediment and one cup was placed in the bottom of each of 

A, B, and C; an empty cup was placed in column D 

(control). A subsample of the sediment was analyzed 

for moisture content, which was determined graviFigure 3. A column used in the Stable Interface Study designed to 

have the same sediment surface area to water column volume ratio 

that is observed in the lake. The same column design was used for 

the Resuspended Sediment Study, except that, the sediment cup 

was removed and a large stir magnet was placed in the bottom, 

which was spun with a heavy duty stir plate situated under the 

column. The schematic shown is not to scale. 

5 
metrically to be 89.4%. The lake water was filtered 

through a 0.45 μm filter and slowly added to each 

column, taking care to minimize disturbance of the 

sediment. As the filtered lake water crested the top of 

the sediment cups, a small amount of sediment was 

entrained into the water column. After the column was 

filled with 23 L of filtered lake water, the sediment was 

allowed to settle to the bottom of the column. The 

displaced sediment was removed with a glass tube 

connected to a peristaltic pump with silicon tubing. 

This left no visible “loose” sediment around the 

sediment cup. 

The Stable Interface Study was conducted in 

three phases. During phase I, the columns were 

maintained at ambient pH for 2 weeks by sparging with 

atmospheric air (approximately 350 ppm [parts per 

million] carbon dioxide [CO2]) supplied by a Dayton 

Speedaire 1 HP diaphragm pump. During phase II the 

pH was first elevated to approximately 9.1 by sparging 

with “synthetic” compressed air that was manufactured 

by mixing relatively pure nitrogen with oxygen in 

appropriate concentrations to obtain 20.9% oxygen. 

The columns were maintained at pH 9.1 for 2 weeks. 

After 2 weeks, the synthetic compressed air gas stream, 

which had a CO2 concentration of 1–2 ppm, was passed 

though a “scrubber train” that consisted of two flasks 

of 1N NaOH, a column of soda lime CO2 absorbent, 

and a column of Ascarite II CO2 absorbent. The 

“scrubbing” delivered a CO2-reduced air (CRA) for 

sparging that elevated the pH by an additional 0.1–0.2 

units. Delivery of the CRA to the columns continued 

for another 2 weeks, which completed phase II. A pH 

of 9.2–9.3 was the maximum achievable in the columns 

with CO2 removal alone. At the beginning of phase III 

approximately 25 mL of 1N NaOH was added to each 

column to raise the pH to 10.2. Prior to titration, the 

alkalinity averaged 1.0 meq/L. The addition of strong 

base increased the alkalinity by 1.1 meq/L and the 

specific conductance by 168 μS/cm (microsiemens per 

centimeter). Phase III lasted for 2 weeks, during which 

the columns continued to be sparged with CRA. 

Resuspended Sediment Study 

The Resuspended Sediment Study was designed 

to investigate the desorption of phosphorus from 

suspended sediment during simulated wind-induced 

resuspension events. During such an event, any 

limitation on the rate of phosphorus release from 

sediments that would be in effect at a stable sedimentwater interface would be removed. The columns 

previously described were used without the sediment 

cups. Each column was placed on a Corning model 611 

heavy-duty stir plate and resuspension of the sediment 

was achieved by means of a magnetic stir bar inside the 

column. In order to avoid overheating the motor, each 

stir plate’s power cord was connected to a digital time 

controller programmed to turn the stir plates on for 3 

hours followed by 1 hour of shutdown. This was 

repeated throughout the duration of the study. During 

the 1-hour periods of no stirring, the only mechanism 

for mixing was the scrubber-gas stream. Under these 

conditions, some of the sediment settled, but the finer 

sediment particles remained suspended, leaving a 

turbid water column. While the stir plate was operating, 

the sediment was mixed well throughout the column. 

Each column (except controls) received approximately 

33 g of wet sediment with a moisture content of 89.4%, 

resulting in a suspended sediment concentration of 

approximately 150 mg/L. 

During the Resuspended Sediment Study, each 

treatment column was used to investigate desorption 

from the sediments under differing conditions 

(summarized in table 1). Column A was used to 

investigate desorption at photosynthetically elevated 

pH and received filtered lake water, sediment addition, 

and pH adjustments. Initial pH adjustments were made 

by sparging with CRA for approximately 7 days until 

a pH of 9.1 was reached. This was followed by the 

addition of 33 g of wet sediment, after which the 

column was sparged with CRA for another 19 days 

(phase I). The pH of the column was then adjusted to 

pH >10 with 26.8 mL of 1N NaOH, and the column 

was sparged with CRA for an additional 21 days (phase 

II). The immediate increase in alkalinity after titration 

was 1.0 meq/L; specific conductance increased by 

123 μS/cm. 

Table 1. Column pH adjustments, sediment additions, and other 

treatments for the Resuspended Sediment Study 

pH Sediment 
 


Column adjustmenta additionb Other 
 


A yes yes -
 B yes yes preconditioning 
 

phase added 

C no yes - D yes yes titrated to pH 

10.2 prior to 

sediment addition 

E yes no -

a Column pH was incrementally adjusted to 9.1 and >10 following 

phosphorus release assessments at ambient pH. 

b

Addition of 33 grams (wet weight) of Klamath Lake sediment. 

6 
Column B was used to determine if an SRP 

concentration gradient was inhibiting sedimentphosphorus release. The column underwent a 

preconditioning phase that removed SRP from the 

water column prior to raising the pH while undergoing 

the usual resuspension events. Column B was filled 

with 23.2 L of unfiltered Upper Klamath Lake water, 

then received 33 g of wet sediment while sparging with 

ambient air for about 6 days and maintaining a pH of 

7.9 ± 0.1. The column was then filtered through a 0.45 

μm filter to remove the sediment, followed by another 

addition of 33 g of wet sediment. The column was 

again sparged with ambient air for 8 days maintaining a 

pH of 7.9 ± 0.0 before filtering again as before. During 

this preconditioning, SRP was monitored daily (with 

the exception of some weekends). At the end of this 

preconditioning the SRP concentration in the column 

was less than 2 μg/L. The column was then sparged 

with CRA for 7 days until a pH of 9.1 was reached. At 

that point 33 g of wet sediment was added, and the 

column continued to be stirred and sparged with CRA 

for another 15 days (phase I). The pH in Column C was 

then titrated to pH>10 with 20.0 mL of 1N NaOH and 

the column was sparged with CRA for another 17 days 

(phase II). The immediate increase in alkalinity after 

titration was 0.9 meq/L; specific conductance increased 

by 144 μS/cm. An additional 3.5 mL 1N NaOH was 

added 9 days into phase II to keep the pH greater than 

10. 

Column C was used to investigate the effect of 

photosynthetically elevated pH on the desorption and 

received filtered lake water and sediment addition, but 

had no pH adjustments. The column was first sparged 

with ambient air for approximately 7 days. Then 33 g 

of wet sediment were added to the column, after which 

it was stirred and sparged with ambient air for another 

40 days, corresponding to phases I and II in the other 

columns. 

Column D was used to provide a comparison 

with previous lab studies that have raised the pH by the 

addition of strong base. Due to a limited supply of 

sample lake water, three-fourths of the column was 

filled with 0.45 μm filtered Upper Klamath Lake water, 

while the remaining one-fourth was filled with 

deionized water. The column received 33 g of wet 

sediment and was titrated immediately to pH >10 with 

11.5 mL of 3N NaOH. The column was then sparged 

with ambient air for 10 days. During this time, a total 

of 20.0 mL of 6 N NaOH was needed to maintain the 

pH above 10. The alkalinity after all strong base was 

added was 6.6 meq/L; specific conductance was 668.8 

μS/cm. This is a change in alkalinity of approximately 

5.5 meq/L; specific conductance increased by about 

558 μS/cm. 

Column E was a control column that received no 

sediment, but it received filtered lake water and the 

same pH adjustments as Column A. During phase II, 

the pH was adjusted to greater than 10 with 21.1 mL of 

1N NaOH. The immediate increase in alkalinity after 

titration was 1.0 meq/L; specific conductance increased 

by 140 μS/cm. 

Phosphorus Fractionation Study 

After the Resuspended Sediment Study was in 

progress, additional data was collected to determine if 

SRP was the dominant form of phosphorus in the water 

column. These samples were collected from columns 

A, C, and D at the beginning of phase III of the 

Resuspended Sediment Study, and from column B just 

prior to and just after the start of phase III of the same 

study. Figure 4 shows a flow chart that helps explain 

how the phosphorus fractions were determined. 

Unfiltered samples were analyzed for TP. Additional 

samples were collected and filtered through a 0.45 μm 

filter. Colorimetric analysis of this filtrate yields the 

biologically available SRP in the sample, plus a small 

amount of condensed phosphates that are unavoidably 

hydrolyzed during the analysis. Hydrolysis of the 

filtrate with hot acid yields the SRP fraction plus the 

acid-hydrolyzable fraction. By subtracting the SRP 

concentration, the dissolved acid-hydrolyzable 

phosphorus (DAHP) fraction can be determined. The 

DAHP fraction consists mostly of condensed inorganic 

phosphates that did not react to direct colorimetric 

analysis. Ammonium persulfate digestion of the filtrate 

followed by colorimetric analysis yields total dissolved 

phosphorus (TDP). TDP minus the SRP and DAHP 

fractions yields the dissolved organic phosphorus 

(DOP) fraction. The difference between TP measured 

on the unfiltered water sample and TDP is the total 

suspended phosphorus (TSP) fraction. This fraction of 

phosphorus is associated with organic and inorganic 

particles that are removed during the 0.45 μm filtration 

process. 

7 
Figure 4. Steps followed for analysis of phosphorus fractions. 

Analytical Methods 

An ammonium bicarbonate extraction (schedule 

515 for benthic phosphorus) was conducted on a 

subsample of the collected sediment by the USGS 

National Water Quality Laboratory in Denver, 

Colorado, in order to determine phosphorus content. 

This resulted in a sediment-phosphorus content of 

1,027 mg/kg of dry sediment. Therefore, 33 g of wet 

sediment added to columns during the Resuspended 

Sediment Study had the potential to contribute about 

155 μg/L of P to the water column. 

Water samples were taken from the center of 

each column, 27 cm above the bottom. Water samples 

for TP were collected using a 50-ml glass pipette. 

Water samples for SRP (50 ml) were collected using a 

section of 8 mm diameter glass tubing attached to a 60 

ml disposable syringe via silicon tubing. During the 

Stable Interface Study, SRP and TP samples were 

collected during each sample event. During the 

Resuspended Sediment Study, phosphorus samples 

were collected while the stir plate was engaged in order 

to maintain a homogeneous mixture during sample 

collection. For this study, SRP was collected during 

each sample event, and occasional samples were 

collected for TP in order to check the assumption of 

constant TP concentration. The sample collection 

interval was as short as 12 hours at the beginning of 

each phase, and as long as 6 days when column pH was 

stable and no rapid changes in concentration were 

expected. Samples for alkalinity and conductivity were 

collected prior to, and after, addition of strong base to 

the water column for phase III. 

Samples collected for SRP determinations were 

filtered through a 0.45 μm filter prior to analysis. 

Samples collected for TP analyses were digested using 

the persulfate digestion method as outlined in 4500-P, 

B5 in Standard Methods (Clesceri et al., 1998). 

Samples for DAHP were treated with nitric and sulfuric 

acid and heated to boiling. Phosphorus analysis was 

conducted by the ascorbic acid method as detailed in 

4500-P, E in Standard Methods (Clesceri, et al. 1998). 

Absorbance readings were made on a Milton Roy 

8 
Spectronic 401 spectrophotometer using a 10 cm 

cylindrical cuvette cell. A summary of the data quality 

is presented in Appendix A, and includes results from 

preliminary column tests, determinations of method 

detection limits, and data quality assurance/quality 

control. 

Temperature, pH, and sparge gas flow rates were 

recorded during each subsample collection. Measurements of pH were obtained using an Orion Research 

model SA250 digital pH meter equipped with a 

Corning deep vessel pH probe and temperature 

probe for temperature compensation. Alkalinity was 

measured on filtered samples by digital count titration 

with 0.1600N H2SO4. Alkalinity was calculated using 

the inflection point method. Determinations of conductivity were corrected for temperature and measured 

with a YSI model 32 bench top conductivity meter 

(K=1). 

RESULTS 

Stable Interface Study 

Patterns of both SRP and TP in replicate 

columns A, B, and C are nearly identical (fig. 5). In 

each column, the SRP concentration declined rapidly 

from the initial concentration of about 20 μg/L to 

between 1 and 3 μg/L 4 days after the initial filling 

of the column. The trend in concentration then reversed 

and over the next 10 days the concentration increased 

in all three columns back up to about 15 μg/L. During 

phase I of the study, therefore, SRP was largely 

removed from the water column and then replaced, 

while pH was maintained between 8 and 8.2. 

At the beginning of phase II, pH in the three 

columns increased by about 1 pH unit within 24 hours 

with no apparent affect on SRP concentration (fig. 5). 

During phase II of the study pH was maintained 

between 8.9 and 9.3 by sparging with CRA. SRP 

concentrations in all three columns showed small and 

consistent oscillations around a linear trend line that 

indicated an average (of the three replicates) SRP 

release rate of 1.4 mg/m2

/day (milligrams SRP per 

square meter of sediment surface area per day) (table 2) 

during this 28-day phase. 

At the beginning of phase III, pH in the three 

columns increased by approximately 1.1 pH units 

within 14 hours with no apparent immediate effect on 

SRP concentration (fig. 4). During phase III of the 

study pH was maintained between 10.1 and 10.4 by 

sparging with CRA. After a single addition of NaOH, 

SRP concentrations in all three columns showed small 

and consistent oscillations around a linear trend line 

that indicated an average (of the three replicates) SRP 

release rate of 1.8 mg/m2

/day (table 2) during this 

14-day phase. 

Figure 5. Soluble reactive phosphorus (SRP) concentrations divided 

into three phases for each column during the Stable Interface 

Study. During phase I, all columns were sparged with ambient air. 

During phase II, all columns were sparged witn CO2-reduced air 

(CRA). At the beginning of phase III, all columns were titrated to a 

pH of 10.2 or 10.3. Columns A, B, and C were treated identically, and 

column D had no sediment in the sediment cup. 

SRP concentration in the control column D 

initially decreased as in the treatment columns, but 

at a slower rate and did not reach as low a minimum 

(fig. 5). This trend appears to have reversed prior to 

beginning phase II of the study, but the column did not 

regain as much of the lost SRP as did the treatment 

columns. The result was that the overall decrease in 

SRP concentration by the end of phase I was greater in 

the control column than in the treatment columns. 

9 
Table 2. Column pH, and soluble reactive and total phosphorus release rates by phase for the Stable Interface Study 

[μg/L, micrograms per liter; mg/m2, milligrams per square meter] 

Total Phosphorus 

Soluble reactive phosphorus release rate Release Rate 

Phase Column Median pH μg/L/daya mg/m2/dayb μg/L/daya mg/m2/dayb 

A, B, C 

averaged 8.1 ± 0.2 -0.3 ± 0.1 -1.0 ± 0.2 -0.5 ± 0.2 -1.7 ± 0.7 

I D 8.1 ± 0.2 -0.7 -2.4 -0.3c -0.9 

A, B, C 

correctede - 0.4 1.4 -0.2 -0.8 

A, B, C 

averaged 9.2 ± 0.1 0.4 ± 0.0 1.4 ± 0.1 0.3 ± 0.1 1.0 ± 0.3 

II D 9.2 ± 0.1 0.1 0.4 -0.3 -0.9 

A, B, C 

correctede - 0.3 1.0 0.6 1.9 

A, B, C 

averaged 10.3 ± 0.1 0.6 ± 0.1 1.8 ± 0.3 0.6 ± 0.1 2.0 ± 0.3 

III D 10.3 ± 0.1 0.3 1.1 	 0.2d 0.7 

A, B, C 

correctede - 0.3 0.7 0.4 1.3 

a

 Release rates were derived from the difference between the beginning and the ending phosphorus concentration divided by the number of days. 

b

 Release rates were derived from the difference between the beginning and the ending phosphorus concentration divided by the cross sectional area of 

the sediment cup and the number of days. 

c

 The initial TP concentration was 38.5 μg/L and increased to 46.7 μg/L in 25.5 hours. It is believed that the increase was due to contamination of the 

column. The release rate was calculated from data after this initial contamination. 

d

 At the end of the Stable Sediment Interface Study Column D had flocculate that was resuspended and sampled for SRP and TP. The release rate was 

calculated from data prior to this resuspension of flocculate. 

e

 The rate for the control column (no sediment) was subtracted from the average of columns A, B, and C. 

Like the treatment columns, the control column phase II. When the control column rates are normalized 

oscillated around a linear trend line through phases II to the same sediment area used with the treatment 

and III, and overall gained SRP through the rest of the columns, the changes in TP in the control column can be 

experiment. When the control column rates are expressed in terms of a release rate equal to -0.9, -0.9, 

normalized to the same sediment area used with the and 0.7 mg/m2/day for phases I, II, and III, respectively 

treatment columns, the increase in SRP in the control (table 2). Thus, TP concentration in the control declined 

column can be expressed in terms of a “release rate” during phase II while the SRP concentration increased. 

equal to -2.4, 0.4, and 1.1 mg/m2/day for phases I, II, Interpretation of the TP concentration during phase I 

and III, respectively (table 2). When these calculated is complicated by a rapid increase of approximately 

release rates in the control column were subtracted from 7 μg/L during day 1 of the experiment (fig. 6). The 

the treatment columns, the average release rates of SRP only logical explanation for this increase was a slight 

were 1.4, 1.0, and 0.7 mg/m2/day for phases I, II, and contamination of the control column. The change in TP 

III, respectively (table 2). 	 	 through phase I, therefore, was calculated using 46.7 

μg/L as the initial concentration, which resulted in a TP in the treatment columns decreased during decrease of 0.9 mg/m2

/day. phase I, then increased during phases II and III, but with 

a different magnitude from the SRP release rates (fig. 6). The decrease in TP in column D (control) 

TP declined by an average of 1.7 mg/m2/day during through phases I and II indicates that phosphorus was 

phase I, increased by an average of 1.0 mg/m2

/day lost from solution. During phase III column D gained 

during phase II, and increased by an average of 2.0 mg/ TP, suggesting that some of the lost TP returned to 

m2/day during phase III (table 2). In the control column solution. All columns, including D, exhibited visible 

D, TP concentration decreased through phase I and settling of a light colored flocculent precipitate. 

10 
Figure 6. Total phosphorus (TP) concentrations divided into three 

phases for each column during the Stable Interface Study. During 

phase I, all columns were sparged with ambient air. During phase II, 

all columns were sparged with CO2-reduced air (CRA). At the 

beginning of phase III, all columns were titrated to a pH of 10.2 or 

10.3. Columns A, B, and C were treated identically, and column D 

had no sediment in the sediment cup. a = increase most likely due to 

contamination, b = intentional resuspension of flocculate. 

It is believed that the phosphorus lost from solution 

was incorporated into this flocculent. The composition 

of the precipitate was not determined, but two possibilities are (1) incorporation of phosphates into large 

molecular weight organic molecules such as fulvic and 

humic acids that precipitate and settle out, or (2) 

inorganic coprecipitation of phosphate with calcite or 

iron. In either case it would seem that the titration to a 

pH above 10 reversed the trend and the flocculent, 

whether organic or inorganic, started to dissipate. At 

the end of the experiment, the remaining flocculent in 

column D was resuspended and TP was measured. The 

final TP concentration was 43.1 μg/L, about 94% of the 

initial concentration of 46.1 μg/L, after allowing for 

early contamination of the column. Thus, the flocculent 

accounted for nearly all of the lost TP. If it is assumed 

that the same rate of precipitation and dissolution of 

solids occurred in the control column and treatment 

columns, and the TP release rates are corrected 

accordingly, then the average flux rates of TP in the 

treatment columns are -0.8, 1.9, and 1.3 mg/m2/day 

during phases I, II, and III, respectively (table 2). 

Resuspended Sediment Study 

The suspended sediment did not show a potential 

to repartition rapidly into the aqueous phase in any of 

the treatment columns regardless of the conditions of 

pH, background aqueous concentration, or alkalinity 

tested (fig. 7). In order to facilitate a comparison with 

the Stable Interface Study, release rates of phosphorus 

from the sediments were calculated for Phases I and II 

of this experiment and are summarized in table 3. The 

Phase I release rates of SRP were zero or negative in 

every treatment column (fig. 7), reflecting the fact that 

the tendency of the sediments for the first 10–15 days 

after being suspended in the columns was to take on 

SRP, not release it (table 3). This was true of the 

column in which the pH had been raised to 9.1 (column 

A), the column that had been preconditioned to create 

low aqueous phase concentrations (column B), the 

column in which the pH was kept at the ambient value 

(column C), and the column in which the pH was raised 

to 10.2 while greatly increasing the alkalinity (column 

D). The affect of the higher pH in columns A and D as 

compared to column C during this phase seems to be to 

moderate somewhat the ability of the sediment to take 

on SRP from the water column. This is likely because 

at higher pH the substitution of hydroxide ions for 

phosphate ions on iron hydroxides introduces a geochemical gradient that opposes the process that is 

driving the uptake of SRP by the sediments, but the 

gradient is not strong enough to override the opposing 

process. 

During Phase II of this experiment, the sediment 

in treatment columns A, B, and C released SRP to the 

water column (fig. 7). The release rates during this 

phase in column A at pH 10 and column C at pH 7.9 

(table 3) were comparable at 0.9 and 0.5 μg/L/day, 

11 
Figure 7. Soluble reactive phosphorus (SRP) concentrations in each 

column during the Resuspended Sediment Study. Column B was 

preconditioned, as described in the text, to remove SRP from the 

water column. Columns A, B, and E were sparged with CO2-reduced 

air (CRA) for 6 days prior to adding sediment, which raised the pH in 

these columns to approximately 9.1. Column C was sparged with 

ambient air during this time. Phase I began with the addition of 

sediment to columns A, B, C, and D and lasted 18 days. Columns A, 

B, and E continued to be sparged with CRA during phase I, column C 

continued to be sparged with ambient air, and column D was titrated 

to a pH of 10.2. At the beginning of phase II, columns A, B, and E 

were titrated to a pH of 10 or greater, while column C continued to 

be sparged with ambient air. 

respectively, indicating very little influence of pH 

during days 20 to 40 of the experiment. The lowest 

release rate of 0.4 μg/L/day was calculated for the 

column that had been pre-conditioned to have very low 

aqueous phase concentrations at the start of the 

experiment, so this evidence also suggests that 

phosphorus transfer between the sediments and the 

water is not controlled by a simple partitioning process 

that one would expect to respond dramatically to a 

change in the geochemical gradient. 

The control column E to which no sediment was 

added showed a decline in SRP over time (fig. 7), 

indicating that SRP was slowly converted into other P 

forms. Therefore the Phase II release rates of SRP from 

the sediments calculated for the other treatment 

columns may be underestimated, but the rates 

calculated for column E (-0.3 and -0.1 μg/L/day for 

Phases I and II, respectively) do not indicate that the 

correction would be large (table 3). 

Phosphorus Fractionation Study 

Results of the phosphorus fractionation study are 

summarized in table 4. SRP was the largest dissolved 

fraction, except in column C where SRP had been 

stripped from the water by preconditioning. The results 

do not indicate that a large fraction of either DOP or 

DAHP was liberated from resuspended sediments 

throughout the course of the experiment, although the 

total dissolved unreactive phosphorus (DUP=DOP+ 

DAHP) was a nontrivial fraction of the TDP, and was 

the largest fraction of the TDP in column C, where SRP 

had been stripped from the water by preconditioning. 

Notably in column B, to which no sediment was 

added, a significant fraction of TSP developed between 

the time that the column was filled with filtered water 

and the time that the fractionation was done, about 27 

days. This seems to be evidence that over time colloidal 

material and particles larger than 0.45 μm are formed, 

incorporating phosphorus. This is consistent with the 

flocculation noted in the Stable Interface Study. The 

beginning of the experiment showed that SRP was 68% 

of TP, leaving DUP with 32%, assuming that TSP 

immediately after filtering was 0%. Therefore, 14% of 

the TP that originally was SRP, and 9% of the TP that 

originally was DUP, were converted into the TSP pool 

over a period of 27 days. The same type of conversion 

could be occurring in the other columns to which 

sediment was added, which would suggest that roughly 

40% of the TSP in column A and 30% of the TSP in 

columns C and D was generated within the column. 

12 
[μ

Column Phase μ a b 

I 9.1 ± 0.1 -0.5 -0.2 

A 

II ± 0.4 0.9 0.3 

I 9.1 ± 0.1 0.0 0.0 

B 

II ± 0.2 0.4 0.1 

I 7.9 ± 0.1 -1.0 -0.3 

C 

II 7.9 ± 0.1 0.5 0.1 

D I ± 0.2 -0.4 -0.1 

I 9.2 ± 0.1 -0.3 

E 

II ± 0.5 -0.1 

a 

. 

b 

μ

SRP % % % % 

pH μg/L SRP TSP 

A 10.0 33.6 11 1 31 57 

B 9.0 29.9 0 23 56 21 

B 10.4 30.0 12 11 54 23 

C 10.4 2.1 17 2 4 77 

D 7.9 26.1 4 3 21 72 

Table 3. Column pH and release rates by phase for the Resuspended Sediment Study 

g/L, micrograms per liter; mg/kg, milligrams per kilogrgam] 

Median pH Soluble reactive phosphorus (SRP) release rate 

pH units g/L/day mg/kg/day 

10.0 

10.2 

10.2 

no sediment 

10.1 no sediment 

Release rates were derived from the difference between the beginning and the ending phosphorus concentration divided 

by the number of days

 Release rate in milligrams of SRP per kilogram of suspended sediment (dry) per day. 

Table 4. Resuspended Sediment Study column pH, SRP concentration, and percent of the total 

phosphorus as dissolved organic phosphorus (DOP), dissolved acid-hydrolyzable phosphorus (DAHP), 

soluble reactive phosphorus (SRP), and total suspended phosphorus (TSP) 

[Column B was sampled prior to and after titration to pH 10.4; g/L, micrograms per liter; %, percent] 

Column DOP DAHP 

DISCUSSION 

The maximum rate of phosphorus release from 

the sediments to the water column observed during the 

Stable Interface Study was approximately 1 and 

2 mg/m2/day of SRP and TP, respectively. The effect 

of raising the pH from 8.1 to first 9.1 and then to 10.2 

on the release rate was not pronounced and did not 

indicate a strong dependence of the dominant processes 

on pH. The treatment columns were consistent in 

showing that initially upon being put into contact with 

the lake water the sediments removed SRP from the 

water column rather than releasing SRP to the water 

column. After about 5 days the water in the column 

was largely depleted of SRP and the trend reversed; the 

sediments slowly released SRP to the water column 

over the next 50 days until the experiments were 

terminated. 

The results of the Resuspended Sediment Study 

showed that even when diffusion limitations on the 

processes responsible for the transfer of phosphorus 

across the sediment-water interface were eliminated, 

the phosphorus in the sediments did not rapidly 

repartition into the aqueous phase. This was true under 

all the conditions tested: ambient and photosynthetically elevated pH, very low SRP concentration in the 

aqueous phase, and elevated pH accompanied by a 

large increase in alkalinity. The treatment columns in 

the Resuspended Sediment Study were notable for their 

similarities: all showed that initially upon suspension in 

the water, the sediments took up phosphorus from 

solution. After a period of 10 to 15 days the columns 

still being monitored showed that the exchange of 

13 
phosphorus reversed and that SRP was slowly released 

to the water column through the remaining 10 to 20 

days of the experiments. It also was shown in the 

Resuspended Sediment Study that the initial uptake 

of phosphorus by the sediments was reduced at a 

photosynthetically elevated pH of 9.1 or 10.2 compared 

to uptake at an ambient pH of 8, probably because of 

the substitution of hydroxide ions for phosphate ions 

on iron hydroxides. This substitution is the geochemical process that, it was hypothesized, would 

cause rapid desorption of phosphorus from the sediments at high pH, i.e., internal loading. The results of 

the column experiments have indicated that this 

geochemical process has a moderating influence on the 

exchange of phosphorus between the sediments and the 

water, but that it is not the most important process 

governing that exchange. 

The rapid increase in chlorophyll a (fig. 2) in late 

May and early June for Shoalwater Bay in 2001 

corresponds to an increase in pH of 2.5 units and TP 

concentrations that increase 10 μg/L/day. The data 

shown are from one site in 1 year, but the trend in TP is 

typical of the increase observed in most years at 

multiple sites in Upper Klamath Lake monitored by the 

Klamath Tribes. Because the release rates from bottom 

sediments measured in these experiments are only 

about 10 percent of the actual phosphorus release rate 

calculated during times of maximum internal loading 

in Upper Klamath Lake, it is difficult to conclude that 

pH is the primary factor controlling the timing and 

magnitude of P release during these internal loading 

periods. 

It should be noted that sediment collected from a 

single site, Shoalwater Bay, may have responded 

differently than would sediments from other parts of 

the lake. Additionally, the timing of the sample 

collection may have been less than optimal. While it 

could be assumed that the sediment had already 

undergone phosphorus release due to high lake pH 

prior to sample collection, during preliminary 

laboratory studies using lake sediment collected on 

May 25, 2001, phosphorus release/uptake rates similar 

to those in tables 2 and 3 were observed. Additionally, 

data from these preliminary experiments indicated a 

sequence of SRP uptake and release similar to the data 

presented in figures 5, 6, and 7. 

The classic theory for redox-driven phosphate 

release from iron hydroxides at the sediment/water 

interface was developed by Einsele (1938) and 

Mortimer (1941, 1942). Later, the theory for a 

strictly geochemical mechanism for the release of 

phosphate from iron hydroxides under oxic conditions 

that was greatly enhanced at high pH was developed 

(MacPherson et al., 1958; Anderson, 1975; Lijklema, 

1976). Based on the literature, either geochemical or 

microbiological processes might control phosphorus 

release, but it is not clear what set of environmental 

conditions (redox, pH, temperature, etc.) is likely to 

result in control by one or the other type of process. 

However, because sediment bacteria mineralize organic 

detritus that falls to the bottom, these bacteria must be 

involved at some point in recycling the phosphorus that 

is incorporated annually into algal biomass. Whether 

these bacteria represent a transient phosphorus pool 

that can be ignored or a persistent reservoir of 

phosphorus in the sediments that must be considered 

is a matter of debate (Gachter and Meyer, 1993), but 

it has been demonstrated that benthic bacteria can 

operate as a short-term sink for bioavailable 

phosphorus (Montigny and Prairie, 1993; Tornblom 

and Rydin, 1998; Clavero et al., 1999; Khoshmanesh 

et al., 1999). How and under what conditions the 

phosphorus stored in sediment bacteria is returned 

to the water column is also a matter of debate, but it 

probably occurs under both aerobic and anaerobic 

conditions. Polyphosphate-metabolizing bacteria 

store polyphosphates under aerobic conditions when 

phosphate is available in excess of immediate requirements for growth. These bacteria use the stored 

polyphosphate as an energy source when conditions 

turn anaerobic (Gachter et al., 1988; Davelaar, 1993; 

Khoshmanesh et al., 2002), which releases phosphate 

to the surrounding medium. Alternatively, the release 

of phosphorus incorporated into bacterial cells under 

aerobic conditions has been attributed to cell lysis 

(Montigny and Prairie, 1993; Tornblom and Rydin, 

1998; Clavero et al., 1999). 

For comparison, a summary of the results of 

several laboratory studies that measured phosphorus 

release from a stable sediment-water interface is 

provided in table 5. The release rates found in the 

literature vary widely, and the results depend not 

only on the character of the sediments involved but 

also on the experimental conditions. Several studies 

of phosphorus release from sediments under oxic 

conditions found release rates similar to those presented here (Drake and Heaney, 1987; Boers and 

van Hese, 1988; Boers, 1991; Appan and Ting, 1996; 

Anderson and Ring, 1999; James et al., 2000). In 

cases where the same sediments were tested under oxic 

and anoxic conditions, the release rate under anoxic 

conditions generally was found to be greater (Boers 

14 
et al., 1994; Appan and Ting, 1996; Moore et al., 1998; 

Anderson and Ring, 1999; James et al., 2000). Anderson (1975) found a large phosphorus release rate of 

20-80 mg/m2/day under oxic conditions, but the system 

was dominated by calcium rather than iron chemistry. 

Some studies have demonstrated greatly enhanced 

release of phosphorus from sediments at high pH, 

which was not found in this study, but usually the pH 

was increased with a strong base in these studies 

(Jacoby et al., 1982; Drake and Heaney, 1987; Appan 

and Ting, 1996). Boers (1991) concluded that raising 

the pH with strong base enhanced phosphorus release 

rates relative to raising the pH by removing CO2. 

A few studies have demonstrated the importance of 

temperature (Kelderman, 1984; Anderson and Ring, 

1999), which is usually taken as an indication of a 

biologically mediated reaction, and one study (Boers 

and van Hese, 1988) concluded that temperature 

affected the outcome of the experiments more than pH. 

In the context of table 5, the results presented here are 

not atypical. 

The results of this study indicate that the current 

understanding of internal loading in Upper Klamath 

Lake should be revised to include a more important 

role for microbiology. Our observations in the columns 

would not have detected any change in the way the 

phosphorus was partitioned within the sediments, yet it 

appears likely that some transfer to and from bacterial 

biomass within the sediments occurred. The switch 

between sediment uptake of phosphorus to sediment 

release of phosphorus at several weeks into the 

experiments may indicate either (a) biological uptake 

under initially favorable conditions followed by cell 

lysis when conditions became unfavorable, or (b) the 

superposition of opposing biological and geochemical 

reactions operating at different time scales. Tornblom 

and Rydin (1998) suggested the former interpretation 

to explain their results of measuring phosphorus release 

from sediment cores collected from Lake Erken, 

Sweden. Their results had many similarities to those 

presented here—sediment cores were observed to take 

up SRP for the first 12 days of the experiment, then a 

reversal occurred and the cores released SRP between 

days 12 and 20. These investigators also tracked 

bacterial biomass through the course of the experiment 

and were able to show that biomass decreased coincident with the switch between SRP uptake and release. 

This paper also was one of several recent works that 

have demonstrated that NaOH extraction, a standard 

step in sequential extractions for sediment phosphorus, 

likely extracts bacterial polyphosphates that go 

undetected because they do not react to the colorimetric 

techniques used to analyze the NaOH extract (Hupfer 

et al., 1995; Goedkoop and Pettersson, 2000; 

Khoshmanesh et al., 2002). Goedkoop and Pettersson 

(2000) found the nonreactive fraction of NaOH 

extractable P (NaOH-nrP) to be 13–20% of total P in 

Lake Erken sediments; Tornblom and Rydin (1998) 

found the NaOH-nrP fraction to be greater 

than the reactive fraction (NaOH-rP) in Lake Erken 

sediments, and NaOH-nrP amounted to 23–40% of 

the total P in their cores. 

The results of previous work with Upper 

Klamath Lake sediments are inconsistent with the idea 

that the phosphorus source in the sediments could be 

bacterial or organic in nature. Wildung et al. (1974, 

1977) showed that the spring 1969 increase in 

biological activity in the lake coincided with a 

reduction in the inorganic P sediment fraction, in 

particular the fraction extractable in NaOH and 

associated with iron hydroxides. There are at least two 

reasons why the conclusions of Wildung et al. (1974, 

1977), that the source of phosphorus in the sediments 

is primarily inorganic, could be flawed. First, these 

conclusions were strongly supported by data from 

only one site, Howard Bay, and the phosphorus in 

these sediments was shown to be more labile in 

laboratory experiments than phosphorus in sediments from other sites (Wildung and Schmidt, 1973). 

Thus, the sediments from Howard Bay may not be 

representative of sediments over the rest of the lake. 

Second, it is possible that these investigators did not 

detect changes in a significant fraction of bacterial 

phosphorus in the sediments because it was extracted 

by NaOH but was not reactive to colorimetric techniques. Yet another possibility is that the primary 

source of phosphorus really is the inorganic fraction, 

and that bacteria play an important role not only in the 

mineralization of organic matter, but also in the 

solubilization of inorganic phosphate. Bacteria that can 

solubilize phosphate salts under aerobic conditions 

have been isolated from Upper Klamath Lake 

sediments (Harrison, 1970; Harrison et al., 1972). 

15 
°

Rate Oxidation 

Reference 2

/day) pH Notes 

20 –80 10 

° °C, 

0.2, 0.6 

0.3, 5.3 

6.5 

1.2, 2.3 9 

(1988) 

1.0, 7.6 

1.7, 3.6 

2.1, 10.3 

3.3, 4.9 

3.0, 14.8 

6.5 

9 

8.3 

9.5 

oxic 

Boers, et al. (1994) 19 

28 2 

Boers (1991) 1.8 

3.8 

9.5 

2 (estimate) 7.6 

75 10.6 

1.0, 1.9 

0.7, 1.3 10.3 

5.6 10 

4.9 

8.3 

where 

oxic °C, 

Moore, et al. (1998) 

18 

Table 5. Experimentally determined areal release rates of phosphorus exchange between sediments and water 

[Nonitalicized text indicates exchange rates of orthophosphate as P. Italicized text indicates exchange rates of total phosphorus as P. The pH was elevated by 

the addition of strong base unless indicated by an *. The term “amb” is used for ambient. C, degrees Celsius. The term “DO” is used for dissolved oxygen] 

(mg/m conditions 

Anderson (1975) oxic lakewater dominated by calcium chemistry, not iron 

chemistry 

Andersen and Ring (1999) 1.27, 2.48 

7.19, 9.78 

amb 

amb 

oxic lakewater 

anoxic lakewater 

the two rates are at 12 C and 19 

respectively; these rates are for littoral 

sediments, generally lower rates were 

observed for profundal sediments 

Appan and Ting (1996) oxic lakewater the two rates are results for sediments 

from two different reservoirs 

Boers and van Hese 0.2 – 4 

0.2 – 5 

anoxic lakewater 

pH effects were found to be minor; temperature effects were more important 

amb oxic “synthetic” lakewater maximum values as extracted from figure 

amb anoxic “synthetic” lakewater 

8.3* oxic “synthetic” lakewater 

9.3* 

15.6 

Drake and Heaney (1987) oxic lakewater 

Fisher and Wood (this study) 9.2* oxic lakewater Stable sediment interface—no strong 

dependence on pH, sediments adsorb 

phosphorus for first 4 days. 

Jacoby, et al. (1982) oxic lakewater first 30 days at amb pH the sediment 

appears to adsorb phosphorus 

James, et al. (2000) amb oxic lakewater average of values determined from sediment in a sediment accumulation zone; 

amb anoxic lakewater values much less for sediment elseKelderman (1984) 0.4, 2.6, 7, 11 amb rates obtained at 5, 10, 15, and 20 

respectively 

Kleeberg and Kohl (1999) ~28 amb ~40% DO saturated lakewater 

50.82 amb ~90% DO saturated lakewater 

5.79 amb oxic lakewater maximum values observed over several 

12.3 amb anoxic lakewater sites and sampling dates 

Mozeto, et al. (2001) 219 – 493 amb anoxic calculated from porewater gradients 

Ruban and Demare (1998) 6.4 – 6.7 anoxic lake water 

Sondergaard, et al. (1992) 4 – 12 amb oxic lakewater lower range for a stable sediment inter60 – 70 face; higher range for resuspended sediments 

The sediments used in this study were rich in 

phosphorus (1,027 mg/kg of dry sediment), yet the 

phosphorus exhibited relatively little mobility, at least 

when compared to the expectations for internal loading 

that have developed based on monitoring in the lake 

over the last decade. Neither high pH (9.2 to 10.2) nor 

resuspension of the sediment in a high pH environment 

had a marked effect on the ability of the sediments to 

release phosphorus to the water column. Either these 

experiments did not duplicate the environmental 

conditions in Upper Klamath Lake that mobilize the 

phosphorus in the sediments, or the manner in which 

the experiments monitored changes was flawed, or 

both. 

These experiments relied on SRP concentration 

to indicate the transfer of biologically available 

16 
 
phosphorus from the sediments to the water column. 

This reliance on SRP may be misguided, for two 

reasons. First, in the water column, SRP is likely to 

be highly transient in the presence of consumers— 

planktonic bacteria or algae—and, in any event, SRP 

may overestimate free orthophosphate by 1–2 orders of 

magnitude (Eisenreich and Armstrong, 1980). Second, 

both bacteria and algae can utilize DOP, although the 

degree of preference for phosphate or DOP, as well as 

the competitive advantage of bacteria or algae in 

utilizing DOP is a matter of debate (Currie and Klaff, 

1984; Cotner and Wetzel, 1992). The results of the 

fractionation study did not indicate that a conversion of 

sediment-bound phosphorus to the dissolved phase as 

either DOP or DAHP took place; if anything, part of 

the dissolved phosphorus appeared to be incorporated 

into particles larger than 0.45 μm over time. In this 

case, therefore, it does not appear that the monitoring 

of SRP alone caused us to miss a large release of 

organic phosphorus from the sediments to the water in 

the column. It seems likely, then, that our experiments 

did not duplicate some important environmental 

conditions that mobilize the sediment phosphorus in 

the lake. 

Observations of internal loading in the lake are 

not based on SRP, but rather on TP; in fact, SRP 

concentrations often remain low while the most rapid 

internal loading occurs, as happened in 2001 (fig. 2). 

The observed increase in TP is assumed to reflect an 

increase in algal biomass. This assumption is supported 

by the fact that TP and chlorophyll a are correlated 

(Kann, 1998), but certainly A. flos-aquae is not the 

only compartment containing the TP—there must be 

phosphorus incorporated into zooplankton, rotifers, 

and bacteria (Vadstein et al., 1993) as well as dissolved 

organic matter. It is not just the cycling of phosphorus 

between the sediments and the water column that 

supports the algal bloom, but the cycling among all of 

the water column compartments. For example, Kann 

(1998) discussed the occurrence of Daphnia pulicaria 

in Upper Klamath Lake, and possible links between the 

dominance of Daphnia and A. flos-aquae. This points 

to additional speculation as to why these experiments 

were unable to demonstrate phosphorus transfer from 

the sediments to the water column of the magnitude 

observed in the lake—perhaps the consumers must be 

in place to drive (or at least accelerate) the transfer. For 

example, the production of alkaline phosphatase would 

have been inhibited in these experiments because the 

water was filtered to remove most bacteria and algal 

growth was prevented. This alkaline phosphatase 

would allow consumers to use an organic P source 

from the sediments if they were brought into contact 

with it either at the sediment-water interface or during a 

re-suspension event. The source of phosphorus would 

not have to be organic; however, sediment bacteria can 

solubilize inorganic phosphates when an adequate 

energy source is available (Harrison, et al., 1972). 

Harrison (1970) concluded that this solubilization 

mechanism also was stimulated by the geochemical 

disequilibrium caused by biological uptake of 

phosphates in the water column. 

SUMMARY 

Upper Klamath Lake sediments collected from 

the lake bottom and then placed in contact with lake 

water at a stable sediment/water interface exhibited an 

initial capacity to take up SRP from the water column 

rather than release phosphorus to the water column. 

After about 4 days, and after the sediments had taken 

up most of the SRP in the water, the transfer of 

phosphorus reversed and the sediments began to 

release phosphorus back into the water column. This 

trend continued for about 50 days until the experiments 

were terminated, during which the pH was increased 

twice, first from 8.1 to 9.2 and then from 9.2 to 10.3. In 

both cases the effect of raising pH on the phosphorus 

release rate was insignificant. The release rates of SRP 

were 1.4, 1.0, and 0.7 mg/m2/day for the first 2 weeks 

at pH 8.1, the next 4 weeks at pH 9.2, and the next 2 

weeks at pH 10.3, respectively. 

Sediments suspended in the water column also 

exhibited an initial capacity to take up SRP from the 

water rather than release it. Immediately after 

suspension in the water column and for at least the next 

10 days, the sediments pulled SRP from the water 

column at all the pH values tested. The effect of 

increasing the pH was to slow the uptake, as can be 

seen by comparing columns A and C in the Resuspended Sediment Study. At pH 8 in column C, the rate 

of phosphorus uptake was 1.0 μg/L/day, and at pH 9 in 

column A it was 0.5 μg/L/day (table 3). The transfer of 

SRP reversed between 10 and 15 days into the 

experiments, and the sediment continued to release 

SRP slowly for the next 30 days until the experiments 

were terminated. The effect of increasing the pH 

during this time was to accelerate the release rate of 

phosphorus to the water column, as can be seen by 

comparing columns A and C again. The rate of 

phosphorus release was 0.5 μg/L/day at pH 8 in column 

17 
C, and 0.9 μg/L/day at pH 10 in column A (table 3). 

These experiments demonstrated that even when the 

diffusive limitations on phosphorus transfer that might 

have been present at the stable sediment-water interface were removed, the phosphorus in the sediments 

did not rapidly repartition into the aqueous phase. 

While some effect due to increased pH was apparent, 

these experiments were not able to demonstrate that 

either pH-driven desorption from iron hydroxides or 

wind-driven sediment resuspension are critical in 

explaining the internal loading that occurs annually in 

Upper Klamath Lake. 

The results of the column experiments suggest 

that sediment bacteria may play an important role in 

the cycling of phosphorus between lake sediments and 

the water column. This study was designed to measure 

phosphorus release as a geochemical phenomenon, in 

particular the release of phosphorus adsorbed to iron 

hydroxides and released under conditions of elevated 

pH or increased surface area contact with the water 

(resuspension), and therefore cannot confirm a large 

role for sediment bacteria. For this reason, further work 

on defining the mechanism of internal loading to Upper 

Klamath Lake should consider evaluating the following 

processes: 

1. An important fraction of the phosphorus stored in the 

sediment may be incorporated into benthic 

bacteria, and this fraction may grow in response 

to environmental conditions, such as warming 

water and sediment temperatures. 

2. The release of phosphorus to the water column may 

be microbially driven and triggered by conditions 

that lead to massive lysis of bacterial cells and a 

rapid release of the phosphorus stored in benthic 

bacteria to the water column. 

3. Consumers may need to be present to initiate or 

accelerate the transfer of phosphorus from the 

sediments to the water column. For example, A. 

flos-aquae may be producing alkaline 

phosphatase in Upper Klamath Lake, which 

might promote P release from the sediments. 

4. Planktonic bacteria may play an important role in the 

phosphorus cycle, and internal loading may be 

manifested first as an increase in this phosphorus 

compartment, which then works its way through 

the food chain, but is rarely manifested as a 

significant increase in SRP. 

18 
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20 
___________________________________________________________________________________________ 

__________________________________________________________________________________________ 

APPENDIX
 
 

21
 
22
 
APPENDIX A: SUMMARY OF DATA QUALITY 

Preliminary Column Tests 

A preliminary test was conducted on both the 

acrylic sediment cup and the acrylic laboratory column 

to determine if the acrylic material sorbs phosphorus. 

For this test, a column was filled with 23.2 L of 

deionized water spiked with KH2PO4 to make a 

40 μg/L SRP solution. The column was sampled for 

SRP over a period of 18 days. During this time the 

average SRP concentration was 41.9 μg/L based on 10 

samples, indicating no loss of phosphorus to the 

column. 

As an additional preliminary study, phosphorus 

uptake by bacteria colonizing the interior walls of the 

column was investigated. A column involved in a 

preliminary stable interface experiment was drained 

and sampled for TP at the end of the test to determine if 

phosphorus was lost over the course of the test. A 

section of the interior wall of the column 50.8 cm tall, 

15.2 cm from the bottom of the column, and half of the 

interior circumference was sampled for TP by scraping 

with a plastic spatula. The spatula was then rinsed with 

deionized water into a collection bottle. Additional 

deionized water was analyzed as a TP blank. From 

these analyses it was concluded that as much as 5.3 μg 

of TP could have been sorbed to the interior wall. This 

would have amounted to a 0.23 μg/L decrease in the 

water column TP concentration when the column was 

full. This decrease is below the method detection limit 

and considered negligible. 

Method Detection Limit 

A method detection limit (MDL) was calculated 

for the ascorbic acid method used for both SRP and TP 

analysis as outlined in 1030, C2 in Standard Methods 

for The Examination of Water and Wastewater, 20th 

edition (Clesceri et al., 1998). The MDL is expected 

to fall within a range that is between one-fourth and 

one-fifth of the standard reference sample (SRS) 

concentration in order to be statistically relevant. 

Three SRP reference samples were prepared 

(2.0 μg/L, 1.7 μg/L, and 0.9 μg/L) and analyzed from 

9/4/01 to 1/7/02. The 2.0 μg/L and 0.9 μg/L SRSs both 

resulted in MDLs of 0.26 μg/L. The MDL for the 2.0 

μg/L SRS was less than the expected range, while the 

MDL for the 0.9 μg/L SRS was greater than the 

expected range. The 1.7 μg/L SRS resulted in an MDL 

of 0.49 μg/L, which is also greater than one-fifth of the 

reference concentration. With two MDL determinations resulting in overestimates of the MDL and one 

resulting in an underestimate of the MDL, the average 

of all three (0.3 μg/L) rounded to the nearest tenth was 

taken as the SRP MDL for the study. All SRP concentrations in this study were above this MDL. 

A 4.0 μg/L TP reference sample was prepared 

and analyzed from 9/6/01 to 11/29/01 to determine an 

MDL for TP. The resulting MDL (0.72 μg/L) was a 

small underestimate of the expected MDL based on the 

SRS concentration; however, the total phosphorus 

concentrations determined throughout this work were 

more than 20 times this level. 

Quality Assurance/Quality Control 

Internal Standard Reference Samples 

An internal QA/QC program provided a measure 

of precision and accuracy for the phosphorus analysis 

from this work. SRP analysis was conducted on a 0.5 

μg/L internal SRS from 6/5/01 through 7/3/01, and 

on a 42 μg/L SRS between 6/1/01 and 7/26/01. TP 

determinations were conducted on a 30 μg/L 

internal SRS from 8/9/01 through 9/27/01, and on 

a 42 μg/L SRS from 6/29/01 through 8/27/01. Figure 

A.1 shows accuracy and precision for SRP and TP, 

divided into two concentration ranges, as calculated by 

subtracting the SRS concentration from each analytical 

result. SRP concentrations show a nearly normal 

Gaussian distribution with no apparent bias in the low 

range (≤ 30 μg/L), while the higher range (> 30 μg/L) 

shows a positive bias that is less than 0.2 μg/L. This 

suggests that SRP concentrations above 30 μg/L could 

be slightly overestimated. TP shows no apparent bias in 

either the low or high range (fig. A.1). 

External Reference Samples 

The USGS Oregon District laboratory supplies 

monthly synthetic standard reference samples, river 

samples, and spiked samples to Clean Water Services 

of Washington County, Oregon, and the USGS 

National Water Quality Laboratory, as part of a 

quality-control program. Additionally, the Oregon 

District Laboratory conducts a yearly sample split 

among laboratories performing analytical work for the 

23 
Designated Management Agencies (DMA) of the 

Tualatin River Basin. Eight low range (≤ 30 μg/L) and 

11 high range (> 30 μg/L) standard reference samples 

were analyzed for SRP in the Oregon District lab as 

part of the QA/QC program for this work. Fifteen of 

these samples were also analyzed for TP (high range). 

The percent differences from each reference sample’s 

most probable concentration is given in table A.1. A 

Tualatin River sample and a spiked duplicate of this 

sample was obtained during the yearly DMA split. This 

set of samples was analyzed to determine the phosphorus recovery as part of the QA/QC program for this 

work. The percent recovery obtained by the Oregon 

District laboratory was 103.1% for SRP and 97.2% for 

TP. 

As an additional measure of accuracy and 

precision, the USGS Branch of Quality Assurance 

(BQA) supplied an SRS at a concentration of 86 μg/L. 

Between 7/12/01 and 8/29/01, 15 SRP analyses were 

conducted on a 1:10 dilution of this reference sample 

resulting in a mean measured concentration of 

8.4 μg/L, with a standard deviation of 0.13 μg/L. 

Between 7/12/01 and 7/23/01, the diluted reference 

sample was analyzed for TP. This resulted in a mean 

measured concentration of 8.5 μg/L, with a standard 

deviation of 0.37 μg/L. The percent differences from 

the BQA sample concentration are given in table A.1. 

Figure A.1. Accuracy of analytical results for internal soluble reactive phosphorus and total phosphorus standard reference samples when 

compared to the reference sample mean. 

24 
Sample Sample MPCc Observed valued Percent a Range typeb mg/L mg/L differenced 

5/7/01 OR Dist. Synth. 

6/4/01 OR Dist. Synth. 

7/2/01 OR Dist. Synth. 

0.00, 3.45 

4.76, 4.76 

0.026 4.00 

> 60 μg/L 30 μg/L

7/24/02 DMA 0.00, 0.00 

8/6/01 OR Dist. Synth. 0.016 -5.88 

10/1/01 OR Dist. Synth. 0.024 9.09 

11/5/01 OR Dist. Synth. -4.00, -4.00 

12/3/01 OR Dist. Synth. 7.41, 3.70 

5/7/01 OR Dist. Synth. 2.56, 2.56 

6/4/01 OR Dist. Synth. 8.11, 10.81 

7/2/01 OR Dist. Synth. 0.038 8.57 

7/2/01 OR Dist. 15.22, 15.22 

7/2/01 OR Dist. Spike 12.90, 11.29 

7/12/01 Synth. ± -2.65 ± 1.69, n = 15 

7/24/01 DMA 4.21, 3.16 

8/6/01 OR Dist. Synth. 0.033 0.00 

10/1/01 OR Dist. Synth. 0.042 7.69 

11/5/01 OR Dist. Synth. -3.03, -3.03 

11/5/01 OR Dist. 1.52, 0.00 

12/3/01 OR Dist. Synth. 5.71, 5.71 

Sample Sample MPCc d 

Range a typeb mg/L mg/L d 

7/2/01 OR Dist. 0.059 -3.28 

7/2/01 OR Dist. 0.091 -16.51 

7/2/01 OR Dist. 14.68, 15.60 

7/2/01 OR Dist. 9.26, 9.26 

7/24/01 DMA ± ±

7/24/01 DMA ± ±

8/6/01 OR Dist. 0.066 -7.04 

8/6/01 OR Dist. μ

0.106 0.00 

10/1/01 OR Dist. 0.00, 0.00

> 60 

10/1/01 OR Dist. 0.00, 0.00 

11/5/01 OR Dist. 0.068 -2.86 

11/5/01 OR Dist. 0.092 -5.15 

11/5/01 OR Dist. ± ±

12/3/01 OR Dist. 3.13, 1.56 

12/3/01 OR Dist. 1.06, 1.06 

Table A.1. External reference sample date, source, type, most probable concentration, and observed concentration and 

percent difference from the MPC grouped by concentration range for soluble reactive phosphorus and total phosphorus 

Soluble reactive phosphorus external reference samples 

Date source 

0.029 0.029, 0.030 

0.021 0.022, 0.022 

0.025 

Stream 0.011 0.011, 0.011 

0.017 

0.022 

0.025 0.024, 0.024 

0.027 0.029, 0.028 

0.039 0.040, 0.040 

0.037 0.040, 0.041 

0.035 

Stream 0.046 0.053, 0.053 

0.062 0.070, 0.069 

BQA 0.086 0.084 0.001, n = 15 

Stream 0.095 0.099, 0.098 

0.033 

0.039 

0.033 0.032, 0.032 

Stream 0.066 0.067, 0.066 

0.035 0.037, 0.037 

Total phosphorus external reference samples 

Observed value Percent 

Date source difference 

Synth. 0.061 

Synth. 0.109 

Stream 0.109 0.125, 0.126 

Spike 0.162 0.177, 0.177 

Stream 0.077 0.090 0.001, n = 3 16.88 1.30, n = 3 

Stream 0.175 0.185 0.006, n = 3 5.52 3.49, n = 3 

Synth. 0.071 

g/L 

Synth. 0.106 

Synth. 0.077 0.077, 0.077 

Synth. 0.122 0.122, 0.122 

Synth. 0.070 

Synth. 0.097 

Spike 0.185 0.174 0.002, n = 3 -5.77 0.83, n = 3 

Synth. 0.064 0.066, 0.065 

Synth. 0.094 0.095, 0.095 

a OR Dist.—Oregon District USGS supplied reference samples obtained as part of a monthly QC program with Clean Water Services of Washington County, Oregon; DMA—USGS supplied reference samples as part of 

the Tualatin Basin Designated Management Agencies Interlaboratory Quality Control Sample Split (July 24, 2001); BQA—USGS Branch of Quality 

Assurance reference sample split and analyzed by various labs. b Synth.—Synthetic reference sample; Stream—samples of stream water obtained from the Tualatin Basin by USGS for use in the monthly QC program with Clean Water Services of Washington County, Oregon; Spike— 

spiked stream water samples supplied by USGS. c

 Most probable concentration. The MPC for Oregon District synthetic 

samples is the target concentration at which the reference was prepared. The 

MPC for Oregon District stream samples is the value reported by the USGS 

National Water Quality Laboratory. The MPC for the Oregon District spike 

references is the value reported by the USGS National Water Quality Laboratory plus the spike concentration. The MPC for DMA and BQA reference 

samples is the consensus of all labs participating in the sample split. 

d Reference samples with a single analysis are shown as a single value. 

Reference samples analyzed in duplicate are shown as two values separated 

by a comma. Reference samples with three or more analysis values are reported as the mean ± a standard deviation followed by the number of samples 

analyzed. 

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