United States

Environmental Protection 

Agency

Office of Chemical Safety

and Pollution Prevention

(7101)

EPA 712-C-006

January 2012

Ecological Effects

Test Guidelines

OCSPP 850.4500:

Algal Toxicity
i

NOTICE

This guideline is one of a series of test guidelines established by the United States 

Environmental Protection Agency’s Office of Chemical Safety and Pollution Prevention 

(OCSPP) for use in testing pesticides and chemical substances to develop data for 

submission to the Agency under the Toxic Substances Control Act (TSCA) (15 U.S.C. 2601, 

et seq.), the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) (7 U.S.C. 136, et 

seq.), and section 408 of the Federal Food, Drug and Cosmetic (FFDCA) (21 U.S.C. 346a). 

Prior to April 22, 2010, OCSPP was known as the Office of Prevention, Pesticides and Toxic 

Substances (OPPTS). To distinguish these guidelines from guidelines issued by other 

organizations, the numbering convention adopted in 1994 specifically included OPPTS as 

part of the guideline’s number. Any test guidelines developed after April 22, 2010 will use 

the new acronym (OCSPP) in their title.

The OCSPP harmonized test guidelines serve as a compendium of accepted scientific 

methodologies and protocols that are intended to provide data to inform regulatory decisions 

under TSCA, FIFRA, and/or FFDCA. This document provides guidance for conducting the 

test, and is also used by EPA, the public, and the companies that are subject to data 

submission requirements under TSCA, FIFRA, and/or the FFDCA. As a guidance 

document, these guidelines are not binding on either EPA or any outside parties, and the 

EPA may depart from the guidelines where circumstances warrant and without prior notice. 

At places in this guidance, the Agency uses the word “should.” In this guidance, the use of 

“should” with regard to an action means that the action is recommended rather than 

mandatory. The procedures contained in this guideline are strongly recommended for 

generating the data that are the subject of the guideline, but EPA recognizes that departures 

may be appropriate in specific situations. You may propose alternatives to the 

recommendations described in these guidelines, and the Agency will assess them for 

appropriateness on a case-by-case basis.

For additional information about these test guidelines and to access these guidelines 

electronically, please go to http://www.epa.gov/ocspp and select “Test Methods & 

Guidelines” on the left side navigation menu. You may also access the guidelines in 

http://www.regulations.gov grouped by Series under Docket ID #s: EPA-HQ-OPPT-2009-

0150 through EPA-HQ-OPPT-2009-0159, and EPA-HQ-OPPT-2009-0576.
Page 1 of 26

OCSPP 850.4500: Algal Toxicity.

(a) Scope―

(1) Applicability. This guideline is intended to be used to help develop data to submit to 

EPA under the Toxic Substances Control Act (TSCA) (15 U.S.C. 2601, et seq.), the 

Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) (7 U.S.C. 136, et seq.), and 

the Federal Food, Drug, and Cosmetic Act (FFDCA) (21 U.S.C. 346a).

(2) Background. The source material used in developing this harmonized OCSPP test 

guideline includes the OPPT guideline under 40 CFR 797.1050 Algal Acute Toxicity 

Test; OPP 122-2 Growth and Reproduction of Aquatic Plants (Tier 1) and OPP 123-2 

Growth and Reproduction of Aquatic Plants (Tier 2) (Pesticide Assessment Guidelines 

Subdivision J); Non-target Plants: Growth and Reproduction of Aquatic Plants Tiers 1 

and 2 Standard Evaluation Procedure; OECD 201 Algal Growth Inhibition Test; ASTM E 

1218-04, Standard Guide for Conducting Static 96-h Toxicity Tests with Microalgae; and 

OPP Pesticides Reregistration Rejection Rate Analysis: Ecological Effects. This 

guideline was formerly Public Draft OCSPP 850.5400 (April, 1996). 

(b) Purpose. This guideline is intended for use in developing data on the toxicity of chemical 

substances and mixtures (“test chemicals” or “test substances”) subject to environmental effects 

test regulations. This guideline prescribes test procedures and conditions designed to determine 

the inhibition of growth and the pattern of growth in algae exposed to a test substance. This 

guideline was written specifically for the freshwater green alga Pseudokirchneriella subcapitata 

(formerly known as Selenastrum capricornutum) and the marine diatom Skeletonema costatum

(see paragraph (e)(3)(i) of this guideline). Use of Navicula pelliculosa or other test species may

require some specific modifications in test procedures. Methods for toxicity testing with the 

cyanobacterium species Anabaena flos-aquae (formerly classified as a blue-green alga) can be 

found in OCSPP 850.4550. The Environmental Protection Agency will use data from these tests 

in assessing the hazard and risks a test substance may present in the aquatic environment. This 

guideline should be used in conjunction with OCSPP 850.4000 (Background and special 

considerations for conducting ecological effects tests with terrestrial and aquatic plants, 

cyanobacteria, and terrestrial soil core microcosms), which provides general information and 

overall guidance for the plant test guidelines and OCSPP 850.1000 (Background and special 

considerations for conducting ecological effects tests with aquatic and sediment-dwelling fauna 

and aquatic microcosms), which provides general information for conducting toxicity tests in an 

aqueous matrix.

(c) Definitions. The definitions in OCSPP 850.1000 and OCSPP 850.4000 are applicable to this 

guideline. In addition, the following more specific definitions also apply:

Algicidal refers to having the property of killing algae.

Algicidal concentration is the lowest concentration tested which allows no net growth of 

the population of test organisms during either exposure to the test substance or during the 

recovery period in the absence of test substance.

Algistatic refers to having the property of inhibiting algal growth.
Page 2 of 26

Algistatic concentration is the highest concentration tested which allows no net growth of 

the population of test organisms during exposure to the test substance but permits 

regrowth when the organisms are placed in test substance-free medium.

Biomass is the dry weight of living matter present in a population and expressed in terms 

of a given volume, e.g. mg algae per liter. Because dry weight is difficult to measure 

accurately, surrogate measure of biomass, such as cell counts, are typically used in this 

test.

Growth refers to an increase in algal biomass in a specified period of time.

Static system for this test refers to a system in which old nutrient medium (test solution) 

is not renewed or replaced during the period of the test.

(d) General considerations―

(1) Summary of the test. Organisms of a particular species of microalgae are 

maintained under static conditions in test vessels containing nutrient medium alone and 

nutrient medium to which the test substance has been added. Over an exposure period of 

96 hours, data on population growth (cell density) are obtained on a daily basis. The test 

is designed to determine the quantity of test substance that results in a 50 percent (50%) 

inhibition (IC50) in biomass yield, growth rate, and the area under the growth curve. The 

results of the test are expressed as 96-hour median inhibition concentration (IC50) values. 

In addition, the no observed effect concentration (NOEC) for these response measures are 

determined. The results are used to establish toxicity levels, evaluate hazards or risks to 

aquatic plants, and to indicate if further testing at a higher tier is necessary. Note 

historically in OCSPP pesticide and industrial chemical guidelines the term ECx was used 

to cover both the current OCSPP 850.4000 definition of ECx (concentration where x% of 

the population exhibit the effect (e.g., mortality)) and ICx (concentration resulting in a x% 

decrease or inhibition effect on an attribute of the population (e.g., growth rate)).

(2) General test guidance. The general guidance in OCSPP 850.4000 and OCSPP

850.1000 applies to this guideline except as specifically noted herein.

(3) Range-finding test. A range-finding test is usually conducted to establish the 

appropriate test substance solution concentrations for the definitive test. In the rangefinding test, the test organisms are exposed to a series of widely-spaced concentrations of 

the test substance (e.g., 0.1, 1.0, 10, 100 milligrams per liter (mg/L), etc.). The details of 

the range-finding test do not have to be the same as the definitive testing in that there are 

no replicates, and the number of test organisms used, and duration of exposure may be 

less than in definitive testing. In addition, the types and frequency of observations made 

on test organisms are not as detailed or as frequently observed as that of a definitive test 

and results are analyzed using nominal concentrations. However, the range-finding test 

will be more useful the greater the similarity between the range-finding test and the 

definitive test.

(4) Definitive test. The goal of the definitive test is to determine the concentrationresponse curves, 96-hour IC50 values (with 95% confidence intervals and standard error), 
Page 3 of 26

and NOEC and lowest observed effect concentration (LOEC) values for algal population 

growth based on yield, growth rate, and area under the growth curve for each species 

tested. The slopes of the concentration-response curves, and associated standard errors, 

and 95% confidence intervals of the slopes should be determined for all of the population 

growth measures, if possible, for each species. However, at a minimum, the full 

concentration-response curve (curve between IC05 to IC90) is determined for the most 

sensitive measure of effect for a given species using a minimum of five concentrations of 

the test chemical, plus appropriate controls. Recommend adding one or two additional 

test concentrations in the lower tail of the concentration-response curve for the most 

sensitive endpoint to ensure bracketing of the most sensitive NOEC or IC05 value. For a 

satisfactory test for a given species, the lowest treatment concentration is below the yield, 

average specific growth rate, and area under the growth curve for all IC50 values and is at 

or below the NOEC (or IC05) for all growth measures. Analytical confirmation of test 

concentrations is performed as described in OCSPP 850.1000. A summary of test 

conditions is provided in Table 3 of this guideline and validity elements for an acceptable 

definitive test are listed in Table 4 of this guideline.

(5) Limit test. In some situations, it is only necessary to ascertain that both the 96-hour 

IC50 values for growth measures are above a certain limit concentration, and that at this 

limit concentration there is no observable adverse effect on growth. For pesticides, a 

limit test has also been referred to as a Tier I test or Maximum Challenge Concentration 

test. In an algal limit test, at least four replicate test vessels are exposed to a single “limit 

concentration,” with the same number of test vessels containing the appropriate control 

solution(s). The multiple-concentration definitive test may be waived for a given test 

species if the following two conditions are met for yield, average specific growth rate and 

area under the growth curve. First, the “limit” treatment response is statistically less than 

a 50% decrease from the control response (i.e., IC50 value > limit concentration), and 

second, the limit treatment responses are not significantly reduced (or inhibited) as 

compared tithe control response (i.e., NOEC > limit concentration). For most industrial 

chemicals, 1,000 mg/L or the limits of water solubility or dispersion are considered 

appropriate as the limit concentration. For pesticides, the limit concentration is 

equivalent to the maximum label rate (pounds of active ingredient per acre (lbs a.i./A)) 

directly applied to a one acre pool that is 6 inches deep (21,280 cubic feet (ft3) or 602,581 

liters). For example, a 1 lb a.i./A (or 453,592 milligrams (mg) a.i. per acre) application 

rate and assuming a water density of 1 gram per milliliter would have a limit 

concentration of 0.75 mg a.i./L. Except for the number of treatment groups, an 

acceptable limit test follows the same test procedures, is the same duration, and has the 

same number of controls as the multi-concentration definitive test (Table 3). Acceptable 

limit tests like definitive tests include analytical confirmation of the exposure 

concentration.

(e) Test standards―

(1) Test substance. For industrial chemicals, the substance to be tested should be 

technical grade unless the test is designed to test a specific formulation, mixture, or enduse product. For pesticides, the use of the typical end-use product (TEP) instead of the 

technical grade active ingredient (TGAI) is preferred for all aquatic plant phytotoxicity
Page 4 of 26

tests. If there is more than one TEP with the same inert substances, the one with the 

highest percent a.i. and/or the one most commonly used should be tested. If there is more 

than one TEP with different inert substances, a TEP representative of each different inert 

substance should be tested in the range-finding test and at a minimum the most sensitive 

one tested in the definitive or limit test. Adjuvants are not used with TEP or TGAI 

testing of algae. OCSPP 850.1000 and OCSPP 850.4000 list the type of information that 

should be known about the test substance before testing, and discusses methods for 

preparation of test substances.

(2) Test duration. The test duration is 96 hours.

(3) Test organism―

(i) Species. 

(A) This test guideline prescribes testing conditions and standards for 

three algal species: the freshwater green alga Pseudokirchneriella 

subcapitata (formerly known as Selenastrum capricornutum and also as 

Raphidocelis subcapitata); the marine diatom, Skeletonema costatum; and 

the freshwater diatom, Navicula pelliculosa. For pesticides all three of 

these species are tested at a minimum. These species have been 

historically used for this type of testing. For testing industrial chemicals, 

the specific algal species tested are selected on a case-by-case basis.

(B) Some species that may be tested in addition include Desmodesmus 

subspicatus (formerly Scenedesmus subspicatus), Microcystis aeruginosa, 

Thalassiosira pseudonana, Dunaliella tertiolecta and Phaeodactylum 

tricornutum. Appropriate references (as given in paragraph (j)(3), (j)(7), 

(j)(8), (j)(9), (j)(10), and (j)(15) of this guideline) should be consulted 

regarding the correct culturing and testing conditions for these species.

(C) Procedures for testing the cyanobacterium, Anabaena flos-aquae, 

formerly classified as a blue-green alga, are found in OCSPP 850.4550.

(ii) Source, age and condition. 

(A) All algae used for a particular test should be from the same source and 

the same stock culture. Also, the clone of all species should be specified. 

Algae to be used in toxicity tests may be initially obtained from 

commercial sources and subsequently cultured using sterile technique. 

Upon receipt of a culture of a species not previously maintained in the 

testing facility, a period of six weeks culturing is recommended to 

establish the ability to successfully maintain a healthy, reproduciblegrowing culture. The algal inoculum to begin the toxicity test should be 

from logarithmically-growing stock cultures (typically 3- to 7-days old). 
Page 5 of 26

(B) A culture should not be used for starting a test under the following 

conditions:

(1) If it is not in the log growth phase;

(2) If visual examination at 400 power shows it is contaminated by 

fungi or other algae, or if the health of the culture is doubtful in 

any respect;

(3) If test algae were used in a previous test, either in a treatment 

or a control.

(iii) Culturing procedures. 

(A) Algal cultures should be maintained in freshwater or saltwater 

medium as described in the references listed in paragraphs (j)(1), (j)(3), 

(j)(6), and (j)(9) of this guideline. The cultures should be maintained 

under the same conditions as used for testing.

(B) Aseptic stock transfer should be performed on a regular schedule (e.g., 

once or twice weekly) to maintain a supply of cells in or near the 

logarithmic growth phase. Long-term maintenance of cultures on a solid 

medium containing 1% agar in sterile Petri plates or test tubes may be 

desirable. However for a satisfactory test, the algal inoculum used to 

initiate toxicity testing is from a liquid culture shown to be actively 

growing (i.e. capable of logarithmic growth within the test period) in at 

least two subcultures lasting 7 days each prior to the start of the definitive 

test.

(C) Stock algal cultures of P. subcapitata and N. pelliculosa should be 

shaken on a rotary shaking apparatus. Culture vessels containing S. 

costatum should be shaken by hand once or twice daily. If clumping of 

cells is not experienced, S. costatum may be continuously shaken at 

approximately 60 cycles per minute.

(4) Administration of test substance―

(i) Preparation of test solutions―

(A) Stock solutions or direct addition. Test solutions are prepared by

adding the test substance to the nutrient medium either as direct addition 

or by addition of a stock solution. Typically, a stock solution of the test 

substance is prepared and aliquots of the stock solution added to medium 

to obtain the desired test concentrations. Guidance for preparation of test 

solutions, especially for difficult or low solubility test substances, is 

provided in OCSPP 850.1000.
Page 6 of 26

(B) Solvents. The recommended solvent for algal toxicity tests is N,Ndimethyl-formamide, as solvents such as acetone can cause stimulation of 

bacterial growth. The concentration of solvent should preferably be the 

same in all test treatments and should not exceed 0.1 milliliters per liter 

(mL/L).

(C) Stock solution pH adjustment. The pH may be adjusted in stock 

solutions to match the pH of the medium if pH change does not affect the 

stability of the test substance in the stock solution or test solution. 

Hydrochloric acid (HCl) and sodium hydroxide (NaOH) may be used for 

this adjustment if warranted. The pH should not be adjusted after the 

addition of the test substance or stock solution into the test medium except 

as discussed in paragraph (e)(9)(iv)(C)(2) of this guideline.

(D) Exposure technique. 

(1) This test is conducted using a static exposure technique. 

Although semi-continuous algal culturing techniques are available, 

they have not been commonly employed in algal toxicity testing 

and their use is not required.

(2) When a substance is known to have a tendency to vaporize, 

closed test flasks with increased head-space may be used. 

Attempts should be made to determine the amount of the substance 

which remains in solution, and extreme caution is advised when 

interpreting results of test with volatile chemicals using closed 

systems.

(ii) Treatment levels. For a given plant species a minimum of five test solution 

concentrations are tested for multi-concentration definitive testing, plus the 

appropriate control(s). A range-finding test can be used to establish the 

appropriate test solution concentrations for the definitive test (see paragraph 

(d)(3) of this guideline). For scientifically sound estimates of a given point 

estimate (e.g., IC50, IC05) test substance concentrations should immediately 

bracket the point estimate(s) of concern. The concentrations should be a 

geometric progression of twofold at a minimum (e.g., 0.1, 0.2, 0.4, 0.8, and 1.6 

milligrams per liter (mg/L)). While a twofold progression is preferred, threefold 

and fourfold progressions are acceptable. If a fourfold or higher series 

progression is used, the rationale for using this large an interval between 

concentrations and the effect on the accuracy and reproducibility of the point 

estimate(s) and NOEC should be provided. For an acceptable study, the lowest 

test treatment level should be lower than the IC50 values for yield and average 

specific growth rate based on cell density. The lack of a NOEC for an effect 

measure is not critical as long as the response-curve for the effect measure is 

acceptable for calculation of the 5% inhibition concentration (IC05). It is 

recommend that one or two additional test concentrations in the lower tail of the 

concentration-response curve of the most sensitive endpoint for a given species be 
Page 7 of 26

added to ensure bracketing of both the most sensitive IC50 value and the most 

sensitive NOEC (or IC05) value. For a limit test, there is single treatment 

concentration, plus the appropriate control(s). Guidance on the limit 

concentration is provided in paragraph (d)(5) of this guideline.

(iii) Introduction of test organisms. 

(A) In preparation for the test, appropriate volumes of nutrient medium 

and/or test solution are placed in the test vessels. Within 30 minutes algae 

are then introduced into the test vessels, which are subsequently placed in 

a growth chamber or a laboratory testing area. Inoculum should be 

impartially or randomly distributed among the test vessels in such a 

manner that test results show no significant bias from the distributions.

(B) Test vessels within the testing area are positioned in a random manner 

or in a way in which appropriate statistical analyses can be used to 

determine the variation due to placement. Because illumination may vary 

at different positions within the growth chamber, and since this parameter 

has an important influence on algal growth, it is recommended that the test 

vessels be randomly re-positioned each day.

(5) Controls. 

(i) Every test includes a negative control consisting of the same nutrient medium, 

conditions, procedures, and algae from the same culture, except that none of the 

test substance is added. In addition, vehicle (solvent) controls are also included if 

a solvent is used to dissolve or suspend the test substance. 

(ii) For a satisfactory test, cell counts in the controls should increase by a factor 

of at least 100 times for P. subcapitata and a factor of at least 30 times for S. 

costatum by test termination (i.e., logarithmic growth in the controls).

(iii) At test termination the coefficient of variation for mean control yield should 

be less than 15% and it should be less than 15% for average specific growth rate, 

which is a logarithmically-transformed variable.

(6) Inoculum concentration and replicates. The minimum number of replicates per 

treatment and control is four to provide acceptable confidence in the results and the 

ability to conduct the statistical hypothesis tests. Each test vessel should be inoculated at 

an initial population density to allow sufficient growth under the test conditions to 

provide accurate quantification without resulting in nutrient or carbon dioxide limitation. 

Recommended initial population density is 10,000 cells/mL for Pseudokirchneriella 

subcapitata (formerly Selenastrum capricornutum) and Skeletonema costatum. These 

inoculum concentrations are known to result in acceptable population densities at the end 

of 96 hours under the specified test parameters for these species. The use of other test 

species such as Navicula pelliculosa may require higher initial inoculum concentrations, 

and the initial population density necessary to result in sufficient growth over the test 

period should be determined on a case-by-case basis. In no case should a test be started 
Page 8 of 26

with less than 10,000 cells/mL. Each test vessel should contain an equal volume of test 

solution and the same initial inoculum volume and concentration. The volume of 

inoculum used should not dilute the test solution (i.e., should typically be less than 2 

mL). The volume of inoculum to be added to each vessel is calculated based upon the 

cell concentration in the stock solution, the volume in the test vessel, and the desired 

initial cell concentration. 

(7) Facilities, apparatus and supplies. Normal laboratory equipment and especially the 

following are necessary:

(i) Containers for culturing and testing algae. Erlenmeyer flasks should be 

used for test vessels. The flasks may be of any volume between 125 and 500 

milliliters (mL) as long as the same size is used throughout a test and the test 

solution volume does not exceed 50% of the flask volume. To permit gas 

exchange but prevent contamination, the flasks should be covered with foam 

plugs, stainless steel caps, Shimadzu enclosures, glass caps or screw caps. 

Containers and covers that may contact the test solution should be chosen to 

minimize sorption of test substances, and not contain substances that can be 

leached or dissolved into aqueous solutions in quantities that can affect the test 

results. The acceptability of foam plugs should be investigated prior to use 

because some brands have been found to be toxic. For a satisfactory test for a 

given test species, all test vessels and covers in the test are identical.

(ii) Growth chamber or laboratory environment. 

(A) A growth chamber or a laboratory environment that can hold the test 

vessels and will maintain the air temperature, lighting intensity, and 

photoperiod specified in this test guideline. If necessary for the given test 

species, a mechanism for continuously shaking the test vessels should be 

incorporated into the growth chamber or controlled environment room.

(B) Facilities should be well ventilated and free of fumes that may affect 

the test organisms. Construction materials and equipment that may 

contact the stock solution, test solution, or nutrient medium should be 

chosen to minimize sorption of test substances and not contain substances 

that can be leached or dissolved into aqueous solutions in quantities that 

can affect the test results. Refer to OCSPP 850.1000 for additional 

information on appropriate construction materials.

(iii) Environmental monitoring equipment. Equipment for determination of 

test conditions (e.g., pH meter, photosynthetically active radiation (PAR) light 

sensor, etc.)

(iv) Cleaning and sterilization. Apparatus for preparing sterile nutrient media. 

Apparatus for sterilizing glassware and maintaining aseptic technique during 

culturing and testing. New test vessels may contain substances that inhibit growth 

of algae. They are therefore to be cleaned thoroughly and used several times to 
Page 9 of 26

culture algae before being used in toxicity testing. All glassware used in algal 

culturing or testing is to be cleaned and sterilized prior to use.

(v) Microscope. Microscope capable of 100 to 400 power magnification.

(vi) Algal cell counting equipment. An apparatus for enumerating algae, e.g., 

hemacytometer, plankton counting chamber, or electronic particle counter. An 

alternative method to performing cell counts is to determine the chlorophyll a

concentration through spectrophotometric or fluorometric methods, as described 

in the reference in paragraph (j)(1) and (j)(6) of this guideline.

(vii) Nutrient media. 

(A) Water used for preparation of nutrient medium should be of reagent 

quality (e.g., ASTM Type I water). Freshwater algal nutrient medium is 

prepared by adding specified amounts of reagent-grade chemicals to 

reagent water. Marine algal nutrient medium is prepared by adding 

reagent grade chemicals to synthetic salt water or filtered natural salt 

water, or by preparing a complete saltwater medium. Salinity for 

saltwater medium should be 30 parts per thousand (ppt) ± 5 ppt. 

(B) Formulation and sterilization of nutrient medium used for algal culture 

and preparation of test solutions should conform to those currently 

recommended by ASTM for freshwater and marine algal toxicity tests 

described in the reference in paragraph (j)(1) of this guideline and Tables 

1 and 2. Chelating agents (e.g. ethylenediaminetetraacetic acid (EDTA)) 

are included in the nutrient medium for optimum cell growth. Lower 

concentrations of chelating agents, down to one-third of the normal 

concentration recommended for AAP medium, may be used in the nutrient 

medium used for test solution preparation if it is suspected that the 

chelater will interact with the test substance. Nutrient medium should be 

freshly prepared for algal testing or may be stored under refrigeration in 

the dark at 4 degrees Celsius (

oC). Nutrient medium should be sterilized 

by autoclaving or filtering (0.22 micrometer (μm) membrane filter). At 

the start of the test, the pH of the nutrient medium should be 7.5 ± 0.1 for 

freshwater algal medium and 8.0 ± 0.1 for marine algal medium. The pH 

may be adjusted prior to addition of the test substance with 0.1 normal (N) 

or 1N sodium hydroxide (NaOH) or hydrochloric acid (HCl).

(8) Environmental conditions. Environmental conditions during the test should be 

maintained as specified below:

(i) Temperature. The test temperature is 24 oC for P. subcapitata and N. 

pelliculosa, and 20 oC for S. costatum. Excursions from the test temperature 

should be no greater than ± 2 oC. Temperature monitoring is described in 

paragraph (e)(9)(iv)(A) of this guideline.
Page 10 of 26

(ii) Lighting and photoperiod. Test vessels containing P. subcapitata and N. 

pelliculosa are illuminated continuously; those containing S. costatum are 

provided a 14-hour light/10-hour dark photoperiod. Cool-white fluorescent lights 

providing 60 micromoles per square meter per second (μmo/m2/s), for cool-white 

fluorescent lighting this is approximately equivalent to 4300 lux, are used. 

Additional information on the use of lighting in plant toxicity tests can be found 

in the references given in OCSPP 850.4000.

(iii) Shaking. Test vessels containing these species should be placed on a rotary 

shaking apparatus and oscillated at approximately 100 cycles/min during the test. 

The rate of oscillation should be determined at the beginning of the test or at least 

once daily during testing if the shaking rate is changed or changes. Test vessels 

containing S. costatum should be shaken by hand once or twice daily. If clumping 

of cells is not experienced, S. costatum may be continuously shaken at 

approximately 60 cycles per minute.

Table 1.—Preparation of Medium for Freshwater Algae

This medium (referred to as AAP medium) is prepared by adding 1 milliliter (mL) of each of the 

macronutrient stock solutions and 1 mL of the micronutrient stock solution listed below to approximately 

900 mL reagent grade water and then diluting to 1 liter (L).

Macronutrient stock solutions are made by dissolving the following into 500 mL of reagent grade water: 

 Sodium nitrate (NaNO3) - 12.750 grams (g)

 Magnesium chloride hexahydrate (MgCl2 • 6H2O) - 6.082 g

 Calcium chloride dihydrate (CaCl2 • 2H2O) - 2.205 g

 Magnesium sulfate heptahydrate (MgSO4 • 7H2O) - 7.350 g

 Potassium phosphate (K2HPO4) - 0.522 g

 Sodium bicarbonate (NaHCO3) - 7.500 g

The micronutrient stock solution is made by dissolving the following into 500 mL of reagent grade water:

 Boric acid (H3BO3) - 92.760 milligrams (mg)

 Manganese chloride tetrathydrate (MnCl2 • 4H2O) - 207.690 mg

 Zinc chloride (ZnCl2) - 1.635 mg

 Ferric chloride hexahydrate (FeCl3 • 6H2O) - 79.880 mg

 Cobalt chloride hexahydrate (CoCl2 • 6H2O) - 0.714 mg

 Sodium molybdate dihydrate (Na2MoO4 • 2H2O) - 3.630 mg

 Copper chloride dihydrate (CuCl2 • 2H2O) - 0.006 mg [Typically must be prepared by serial dilution].

 Ethylenediaminetetraacetic acid disodium salt dihydrate (Na2EDTA • 2H2O) - 150.000 mg 

 Sodium selenite pentahydrate (Na2SeO3 • 5H2O) - 0.005 mg [Used only in medium for stock cultures of 

diatom species]

For freshwater diatom species only, add sodium metasilicate nonahydrate (Na2SiO3 • 9H2O) as another 

macronutrient. May be added directly (202.4 mg) or by way of stock solution to give 20 mg/L silicon (Si) 

final concentration in medium (see reference in paragraph (j)(1) of this guideline).

Adjust pH to 7.5 ± 0.1 with 0.1 N or 1.0 N sodium hydroxide (NaOH) or hydrochloric acid (HCl).

Filter the media into a sterile container through a 0.22 micrometer (μm) membrane filter. Store medium in 

the dark at approximately 4 degrees Celsius (oC) until use.
Page 11 of 26

Table 2.—Preparation of Medium for Saltwater Algae

This medium (referred to as MAA medium) is prepared by adding the aliquots of Metal Mix, Minor Salt Mix 

and Vitamin Mix listed below into a sterile container containing approximately 900 milliliters of natural or 

artificial salt water (salinity 30 ± 5 ppt) then diluting to 1 L. Either natural salt water that has been filtered 

through a 0.22 μm membrane filter or reconstituted salt water is used.

Add the amounts given below to prepare medium used for toxicity testing. Add twice the amounts given 

to prepare medium for use in maintenance of stock cultures.

Add 15 mL of Metal Mix

Add 10 mL of Minor Salt Mix stock solution

Add 0.5 mL of Vitamin Mix stock solution (Add 1 mL of vitamin mix if Thalassiosira is used).

The Metal Mix is prepared by adding the specified amount of chemicals in the order listed below to 900 

milliliters (mL) reagent water and diluting to 1 liter (L).

Metal Mix:

Ferric chloride hydrate (FeCl3 • H2O) - 0.048 g

Manganese chloride tetrathydrate (MnCl2 • 4H2O) - 0.144 g

Zinc sulfate heptahydrate (ZnSO4 • 7H2O) - 0.045 g

Copper sulfate pentahydrate (CuSO4 • 5H2O) - 0.157 mg

Cobalt chloride hexahydrate (CoCl2 • 6H2O) - 0.404 mg

Boric acid (H3BO3)- 1.140 g

Ethylenediaminetetraacetic acid disodium salt dihydrate (Na2EDTA • 2H2O) - 1.0 g

The Minor Salt Mix is prepared by adding the specified amounts of the chemicals listed below to 900 mL 

reagent water and diluting to 1 L.

Minor Salt Mix:

Potassium phosphate (K3PO4) - 0.3 g

Sodium nitrate (NaNO3) - 5.0 g

Sodium metasilicate nonahydrate (NaSiO3 • 9H2O) - 2.0 g

The Vitamin Mix is prepared by adding the specified amount of chemicals in the order listed below to 900 

mL reagent water and diluting to 1 L.

Vitamin Mix:

Thiamine hydrochloride - 500 mg

Biotin - 1 mg

cyanocobalamin (vitamin B12) - 1.0 mg

Adjust MAA medium pH to 8.0 ± 0.1 with 0.1 N or 1.0 N NaOH or HCl. Store medium in the dark at 

approximately 4 oC until use.

(9) Observations―

(i) Measurement of test substance. Analytical confirmation of dissolved test 

concentrations is performed at a minimum at test initiation and at test termination 

for static tests, as described in OCSPP 850.1000. The analytical methods used to 

measure the amount of test substance in a sample are validated before beginning 

the test, as described in OCSPP 850.1000. Samples for analysis should be 

collected as described in OCSPP 850.1000, with the following exception: at the 

end of the test and after aliquots have been removed for algal growth-response 

determinations, microscopic examination, mortal staining, or subculturing, the 

replicate test containers for each chemical concentration may be pooled into one 

sample. An aliquot of the pooled sample may then be taken and the dissolved 

concentration of test substance in solution determined after all algal cells have 

been removed, either by centrifugation or filtration. The effect of centrifugation 

or filtration upon recovery of the test substance should be determined during 

method validation. As an additional procedure, the mass of test substance sorbed 
Page 12 of 26

to the algae may be determined. To measure sorbed test substance, separate and 

concentrate the algal cells from the remaining pooled sample by centrifuging or 

filtering and measure the test substance concentration in the algal-cell 

concentrate.

(ii) Test solution appearance. Observations are made daily on test substance

solubility (e.g., surface slicks, clarity, precipitates, or material adhering to the 

sides of the test vessels) and recorded.

(iii) Dilution water quality. The dilution water source used to prepare media 

should be periodically analyzed to document and characterize the hardness, 

alkalinity, pH, conductivity, total organic carbon (TOC) or chemical oxygen 

demand magnitude and variability, and to ensure that pesticides, PCBs and toxic 

metals are not present at concentrations that are considered toxic. See OCSPP

850.1000 for guidance on dilution water.

(iv) Environmental conditions―

(A) Temperature. It is impractical to measure the temperature of the 

solutions in the test vessels while maintaining axenic conditions. 

Therefore, one or two extra test vessels may be prepared for the purpose of 

measuring the solution temperature during the test. Alternatively, hourly 

measurements of the air temperature (or daily measurements of the 

maximum and minimum) are acceptable. Because the test vessels are 

placed in an environmental chamber or incubator, the air temperature is 

more likely to fluctuate than the water temperature.

(B) Light intensity. Light intensity should be monitored at test initiation 

at the approximate level of the test solution at each test vessel position in 

the growth chamber or environmental control room. If it is suspected 

during the test that light intensity has changed by 15% or more, 

monitoring of light intensity should be conducted daily. A

photosynthetically active radiation (PAR) sensor is used to measure light 

quality. The light intensity should not vary more than ± 15% from the 

selected light intensity at any test vessel position in the incubator or 

growth chamber. Because illumination may vary at different positions, 

and since this environmental parameter has an important influence on 

growth, it is recommended that the test vessels be randomly re-positioned 

on a regular schedule (e.g., daily) to minimize spatial differences.

(C) pH. 

(1) The pH of all test solutions is measured at test initiation and 

termination. This measurement may be made on the bulk test 

solutions at test initiation and on samples of pooled replicates of 

each test treatment at test termination provided none of the 
Page 13 of 26

replicates appear to be “outliers” with respect to growth, in which 

case individual pH measurements should be made.

(2) Before addition of the inoculum, if the test substance is highly 

acidic and reduces the pH of the test solution below 5.0 at the first 

measurement, or is highly basic and increases the pH of the test 

solution similarly, appropriate adjustments to pH should be 

considered and the test solution measured for pH on each day of 

the test.

(v) Measures of effect―

(A) Cell count. At the end of 24, 48, 72, and 96 hours, at a minimum the 

algal cell density (cells per milliliter) in all test vessels are enumerated by

performing cell counts using direct microscopic observation or using an 

electronic particle counter. Additional confirmation measurements 

include chlorophyll a (measured spectrophotometrically or 

fluorometrically).

(1) Microscopic counting. Microscopic counting of cells can be 

performed using a hemacytometer or an inverted microscope with 

settling chambers. Precision is proportional to the square root of 

the number of cells counted. For microscopic counting, two 

samples should be taken from each test vessel and two counts 

made of each sample. Whenever feasible, at least 400 cells per test 

vessel should be counted in order to obtain ± 10% accuracy at the 

95% confidence level. 

(2) Electronic particle counter. An alternative method to 

enumerate large numbers of cells very rapidly is to use an 

electronic particle counter. To use this method, an electronic 

particle count for a sample is converted to a cell count using data 

developed by the laboratory demonstrating the correlation between 

electronic particle counts and microscopic counts for each algal 

test species. Automated particle counting, although the most rapid 

and sensitive method, has limitations, some related to particle 

interferences. If the test solution does not have a low background 

in the particle size range of the test species, masking errors will 

result. An additional test vessel at each concentration containing 

test substance and growth medium but no algae can allow 

measurement of, and if needed, correction for, potential particle 

interference.

(B) Appearance and condition. Note daily any unusual cell shapes, 

color differences, differences in chloroplast morphology, flocculations, 

adherence of algae to test vessels, or aggregation of algal cells. These 

observations are qualitative and descriptive, and are not used in endpoint 
Page 14 of 26

calculations. They can be useful in determining additional effects of test 

substances. In addition, microscopic observations at test termination are 

performed to determine whether the altered growth response between 

controls and test algae (at the concentrations of test substance

demonstrating an effect) was due to a change in relative cell numbers, cell 

sizes, or both. 

(C) Algistatic and algicidal determination. At the end of the definitive 

test, determination of algistatic and algicidal effects may be performed. 

When performed algistatic effects may be differentiated from algicidal 

effects by either Method 1 or Method 2 described in paragraphs 

(e)(9)(v)(C)(1) and (e)(9)(v)(C)(2) of this guideline, respectively.

(1) Method 1. Add 0.5 mL of a 0.1% solution (weight/volume) of 

Evans blue stain to a 1-mL aliquot of algal suspension from a 

control vessel and to a 1-mL aliquot of algae from the test vessel 

having the lowest concentration of test substance which completely 

inhibited algal growth. If algal growth was not completely 

inhibited, select an aliquot of algae for staining from the test vessel 

having the highest concentration of test substance that inhibited 

algal growth. Wait 10 to 30 minutes, examine microscopically, 

and determine the percent of the cells that stain blue (indicating 

cell mortality). A staining control is also performed concurrently 

using heat-killed or formaldehyde-preserved algal cells; 100% of 

these cells should stain blue. This method will work for S. 

costatum and possibly Navicula spp., but probably will not work 

with P. subcapitata.

(2) Method 2. Remove 0.5 mL aliquots of test solution containing 

growth-inhibited algae from each replicate test vessel having the 

lowest concentration of test substance that completely inhibited 

algal growth. If algal growth was not completely inhibited, select 

aliquots from the highest concentration of test substance that 

inhibited algal growth. Combine these aliquots into a new test 

vessel and add a sufficient volume of fresh nutrient medium to 

dilute the test substance to a concentration that does not affect 

growth (using the original test vessel size and solution volume is 

generally appropriate). Aliquots from the control test vessels are 

also transferred to clean medium. Incubate these subcultures under 

the environmental conditions used in the definitive test for a period 

of up to 9 days, and observe periodically (e.g., every other day) for 

algal growth (direct or indirect methods) to determine if the 

algistatic effect noted after the 96-hour definitive exposure test is 

reversible. This subculture test may be discontinued as soon as 

growth occurs.
Page 15 of 26

(f) Treatment of results―

(1) Response variable calculation. Algal population density is the biomass 

measurement normally used in this test guideline to evaluate the inhibitory and 

stimulatory effects of the test substance. Three response variables are calculated from 

algal density: final population density, also referred to as yield, as described in paragraph 

(f)(1)(i) of this guideline; average specific growth rate, as described in paragraph 

(f)(1)(ii) as described in this guideline; and area under the growth curve, as described in

paragraph (f)(1)(iii) in this guideline.

(i) Final population density or yield. To correctly represent yield, the initial 

biomass values should be subtracted from the final biomass values for each test 

vessel. Since the initial values are extremely small relative to the final values, this 

correction has a small impact upon the test results but is nonetheless 

recommended. 

Y  b1  b0 Equation 1

where:

Y = final yield of observed biomass (cell density)

b0 = biomass (cell density) at test initiation

b1 = biomass (cell density) at test termination

(ii) Average specific growth rate. Average specific growth rate (sometimes 

called relative growth rate) is the rate of growth over a given time interval. The 

growth rate cell density for each test vessel (replicate) over a given time interval 

is calculated as given in Equation 2. At a minimum, the average specific growth 

rate is calculated for the time interval between test initiation and termination. The 

average specific growth rate during the course of the test (days 0 to 1, 1 to 2, 2 to 

3, etc.), also called the section-by-section growth rate, is calculated also for each 

test vessel to assess effects on the pattern of growth of the test substance

occurring during the exposure period, such as an increased lag phase.

   

t

b b

r

j i

i j

ln  ln

  Equation 2

where:

ri j = average specific growth rate per day (day-1

) of observed biomass 

(cell density) from time i to j.

bi = observed biomass (cell density) at beginning of the observation 

interval, time i
Page 16 of 26

bj = observed biomass (cell density) at end of the observation interval, 

time j

t = time interval from i to j in days

(iii) Area under the growth curve. The area under the growth curve for each 

test vessel (replicate) based on cell density is calculated using Equation 3. At a 

minimum the area under the growth curve from test initiation through termination 

is calculated.

      1

1 0

2 1

1 2 0

1

1 0

2

2

2

2

2





 









  

  









  

 









 

 n n

n n

t t

b b b

t t

b b b

t

b b

A Equation 3

where:

A = area under the growth curve for biomass (cell density)

b0 = observed biomass (cell density) at test initiation

b1 = observed biomass (cell density) at time t1

b2 = observed biomass (cell density) at time t2

bn = observed biomass (cell density) at time tn

t1 = time (units day) of the first measurement after test initiation

tn = time (day) of the nth measurement after test initiation

(2) Summary statistics―

(i) Environmental conditions. Calculate descriptive statistics (mean, standard 

deviation, coefficient of variation, minimum, maximum) by treatment level for 

temperature and pH. Calculate descriptive statistics (mean, standard deviation, 

coefficient of variation, minimum, maximum) by test vessel position for light 

intensity.

(ii) Test substance concentration. Calculate descriptive statistics (mean, 

standard deviation, minimum, maximum, coefficient of variation) by test vessel 

and treatment level of the test substance soluble concentration. For each 

treatment level, compare the initial test substance concentration with test 

substance concentration at the end of the test in each treatment. If the test 

substance was not stable calculate a rate of decline of the test substance; a timeweighted mean concentration should be calculated under these circumstances 

(e.g., exponential decay calculate the area under the exponential decay 

concentration curve divided by the total exposure days). For pesticides under 

unstable test substance conditions, the measured test substance concentration at 

test initiation is considered appropriate rather than the time-weighted average 
Page 17 of 26

because of the exposure estimate currently used by OPP for calculating risk 

estimates.

(iii) Cell density. For each treatment level and observation time calculate the 

mean, standard deviation, and coefficient of variation for algal cell density. 

Calculate the mean treatment yield, average specific growth rate, and area under 

the growth curve based on changes in algal cell density from test initiation to test 

termination. Calculate the mean treatment section-to-section growth rate for cell 

density.

(iv) Appearance and condition. Morphological symptoms of plant injury should 

be summarized in tabular form by time of observation, treatment, and replicate. 

Definition of any index values used for morphological symptoms, indicating the 

severity of the symptom(s), should be provided.

(3) Percent inhibition―

(i) Treatment level. For yield, average specific growth rate, and area under the 

growth curve calculate the percent inhibition (%I) at each treatment level at 96 

hours using Equation 4.

  

C

C X

I

100 %

 Equation 4

where:

C = the control mean response value (yield, average specific growth rate, 

and area under the growth curve); and

X = the treatment mean response value (yield, average specific growth 

rate, and area under the growth curve, respectively). Stimulation is 

reported as negative %I.

(ii) Growth pattern. If there are substantial differences between the section-bysection growth rates and the average growth rates for a test vessel this indicates 

deviation from theoretical exponential growth. In this instance, compare specific 

growth rates from exposed cultures during the time period of maximum inhibition 

to those for controls during the same period. The same time interval should be 

used for each test vessel in all treatments.
Page 18 of 26

(4) Doubling time of controls. The doubling time (Td) of cell density in controls at test 

termination is calculated using Equation 5.

Control

d

r

T

ln(2)

 Equation 5

where:

rControl = mean of the control average specific growth rate test vessel values from 

paragraph (f)(1)(ii) of this guideline.

(5) Limit test―

(i) IC50 values. To ascertain that the yield, average specific growth rate, and area 

under the growth curve 96-hour IC50 values based on algal cell density occur 

above the “limit” concentration, a one-sided test which compares the difference 

between two sample groups to a fixed value (or difference) is performed for each 

of these response measures. For a comparison of sample means, the difference 

defining the IC50 compared to controls is operationally defined as a 50% reduction 

or inhibition from the control sample mean (Equation 6). The null hypothesis 

(H0) stated in terms of true population parameters is that the difference of the 

“limit” treatment mean response (μlimit) from the control mean response (μcontrol) is 

greater than or equal to a 50% reduction, compared to the control (i.e., H0: μcontrol

- μlimit > δ0). The alternative hypothesis (HA) is that this difference is less than a 

50% reduction, compared to the control (HA: μcontrol - μlimit < δ0). An example of a 

parametric two-sample comparison test is the Student’s t-test. If the null 

hypothesis is rejected, the inhibition level for the given response measure (i.e. 

yield or the average specific growth rate based on cell density) in the limit 

treatment as compared to the control is declared to be less than 50% (i.e., IC50 > 

limit concentration). If the null hypothesis is not rejected, the limit treatment as 

compared to the control response is declared to be 50% or greater (i.e., IC50 < 

limit concentration).

   100  0  xcontrol  p Equation 6

where:

δ0 = difference between two parameters, defined in this case as a p% 

reduction from the control sample mean;

control x = control sample mean response (e.g., yield or the average specific 

growth rate based on cell density); and

p = percent reduction from the control sample mean, which is 50 in the 

case of the IC50.
Page 19 of 26

(ii) NOEC. To ascertain that there is no observable effect at the limit treatment 

(i.e., NOEC > limit concentration) for a given response measure (yield or the 

average specific growth rate based on cell density), the limit treatment response is 

compared to the control treatment response using a one-sided two-sample 

parametric or nonparametric test, as appropriate (see OCSPP 850.4000). The 

minimum significant difference detectable by the test or a similar estimate of the 

sensitivity of the test should be determined and reported.

(iii) Multiple-dose definitive testing. 

(A) A multiple-dose definitive test is performed for a given test species if 

either the effect or inhibition level for one or more response measures (i.e., 

yield or the average specific growth rate based on cell density) in the limit 

treatment as compared to the control response at test termination are 

declared to be 50% or greater effect (i.e., the null hypothesis is not

rejected) or the NOEC is less than the limit concentration.

(B) Multiple-dose definitive testing may be waived for a given test species 

if at test termination the “limit” treatment response is both statistically less 

than a 50% decrease from the control response and there is no observable 

adverse effect from the control response for all measures of effect (yield or 

the average specific growth rate based on cell density).

(6) Multiple-dose definitive test―

(i) Concentration-response curve, slope and IC50. For concentration-response 

tests statistical procedures are employed to calculate the 96-hour IC50 value 

(standard error and 95% confidence interval) for yield, average specific growth 

rate, and area under the growth curve based on cell density. If a concentrationresponse curve model was fit to the data to determine an IC50 value, the model 

parameters (e.g., slope) and their uncertainty estimates (e.g., standard error) 

should be recorded. The values for each test vessel, not the mean for each 

treatment, should be used as the response variable in fitting the model.

(ii) NOEC. The 96-hour NOEC values for yield, average specific growth rate, 

and area under the growth curve based on cell density are determined. For 

pesticides if a 96-hour NOEC value can not be determined the 96-hour IC05 value 

(standard error and 95% confidence interval) is estimated and used in place of the 

NOEC. For industrial chemicals, the specific ICx used in place of a NOEC that 

can not be determined will vary, consult with the Agency. Methods, assumptions, 

and results of the statistical approaches used should be recorded.

(iii) Statistical methods. Statistical procedures for modeling continuous toxicity

data are available and should be used (see references in paragraphs (j)(2), (j)(4) 

and (j)(14) of this guideline and OCSPP 850.1000). Additional discussion about 

endpoints and statistical procedures is found in OCSPP 850.1000 and OCSPP

850.4000.
Page 20 of 26

(iv) Algicidal and Algistatic concentrations. If applicable, results from the 

staining procedure or the recovery phase are used to determine the algistatic 

concentration (refer to paragraph (e)(9)(v)(C) of this guideline).

(g) Tabular summary of test conditions. Table 3 lists important conditions that should prevail 

during the definitive test. Except for the number of test concentrations, Table 3 also lists the 

important conditions that should prevail during a limit test. Meeting these test conditions will 

greatly increase the likelihood that the completed test will be acceptable or valid.

Table 3.—Summary of Test Conditions for the Algal Toxicity Test

Test type Static

Test duration 96 hours 

Test matrix Synthetic growth medium appropriate for the test species

Temperature 24 oC for P. subcapitata and N. pelliculosa; 20 oC for S. costatum. 

Constant during test within ± 2 oC.

Light quality Cool-white fluorescent

Light intensity 60 μmol/m2/s

Photoperiod Continuous light for P. subcapitata and N. pelliculosa; 14 hour 

light:10 hour dark for S. costatum

Shaking Continuous at 100 oscillations per minute for P. subcapitata and N. 

pelliculosa; manual, once or twice daily, for S. costatum

Salinity 30 ± 5 ppt for saltwater species (S. costatum)

Test vessel size 125 - 500 mL Erlenmeyer flasks 

Test solution volume Less than or equal to 50% of the volume of the test vessel

Age of inoculum From logarithmically-growing stock cultures (typically 3 - 7 days old)

Inoculum concentration 10,000 cells per milliliter (cells/mL) for P. subcapitata and S. 

costatum. At least 10,000 cells/mL for other species. Inoculum 

volume less than 2 mL.

Number of replicate test vessels 

per concentration

Four (minimum)

Test concentrations Unless performing limit test, minimum of 5 test concentrations plus 

appropriate controls

Test concentration preparation Aqueous solutions prepared by adding test substance to synthetic 

nutrient medium, directly or via vehicle

Measures of effect (Measurement 

endpoints)

96-hour IC50 and NOEC (or IC05) values for yield, average specific 

growth rate, and area under the growth curve based on algal cell 

density

(h) Test validity elements. This test would be considered to be unacceptable or invalid if one or 

more of the conditions in Table 4 occurred or one or more performance objectives in Table 4 

were not met. This list should not be misconstrued as limiting the reason(s) that a test could be 

found unacceptable or invalid. However, except for the conditions listed in Table 4 and in 

OCSPP 850.4000, it is unlikely a study will be rejected when there are slight variations from 

guideline environmental conditions and study design unless the control organisms are 

significantly affected, the precision of the test is reduced, the power of a test to detect differences 

is reduced, and/or significant biases are introduced in defining the magnitude of effect on 

measurement endpoints as compared to guideline conditions. Before departing significantly 
Page 21 of 26

from this guideline, the investigator should contact the Agency to discuss the reason for the 

departure and the effect the change(s) will have on test acceptability. In the test report, all 

departures from the guideline should be identified, reasons for these changes given, and any 

resulting effects on test endpoints noted and discussed.

Table 4.—Test Validity Elements for the Algal Toxicity Test

1. All test vessels and closures were not identical.

2. Treatments were not randomly assigned to test vessels, and test vessels were not randomly assigned 

to positions in the growth chamber.

3. A medium (untreated) control [and solvent (vehicle) control, when a solvent was used] was not 

included in the test.

4. The concentration of solvent in the range used affected growth of the test species.

5. During the 96 hour test period, cell counts in the controls did not increase by a factor of at least 100 

times for P. subcapitata and a factor of at least 30 times for S. costatum (i.e., logarithmic growth in the 

controls was not reached during the test).

6. A minimum of five test concentrations were not used in the definitive test.

7. Controls were contaminated with the test substance. 

8. The lowest test concentration level was not less than the 96-hour yield, average specific growth rate, 

and area under the growth curve IC50 values based on cell density.

9. For testing with industrial chemicals a surfactant or dispersant was used in the preparation of a stock 

or test solution.

10. Temperature and light intensity were not measured as specified during the test.

(i) Reporting―

(1) Background information. Background information to be supplied in the report 

consists at a minimum of those background information items listed in paragraph (j)(1) of 

OCSPP 850.4000.

(2) Guideline deviations. Provide a statement of the guideline or protocol followed. 

Include a description of any deviations from the test guideline or any occurrences which 

may have influenced the results of the test, the reasons for these changes, and any 

resulting effects on test endpoints noted and discussed.

(3) Test substance. 

(i) Identification of the test substance: common name, IUPAC and CAS names, 

CAS number, structural formula, source, lot or batch number, chemical state or 

form of the test substance, and its purity (i.e. for pesticides, the identity and 

concentration of active ingredient(s)).
Page 22 of 26

(ii) Storage conditions of the test chemical or test substance and stability of the 

test chemical or test substance under storage conditions if stored prior to use.

(iii) Methods of preparation of the test substance, stock solutions, and the 

treatment concentrations used in the range-finding and definitive test, or limit test.

(iv) If a vehicle (solvent) is used to prepare stock or test substance provide: the 

name and source of the vehicle, the nominal concentration(s) of the test substance

in the vehicle in stock solutions or mixtures, and the vehicle concentration(s) used 

in the treatments and solvent control.

(4) Plant test species. 

(i) Scientific and common name, plant family, and strain.

(ii) Source and method of species and strain verification.

(iii) Culture practices, including culturing media used, and conditions.

(iv) Acclimation period, if applicable.

(v) Age (stage) of inoculum at test initiation.

(5) Test system and conditions. Description of the test system and conditions used in 

the definitive or limit test, and any preliminary range-finding tests.

(i) Description of the growth chamber, or laboratory location, type of lights, and 

oscillation rates and type of apparatus.

(ii) Description of the test vessels: size, type, material, fill volume.

(iii) Volume of test solution in the test vessels.

(iv) Description of preparation of inoculum used to begin test.

(v) Inoculum volume and density added to each test vessel.

(vi) Number of test vessels (replicates) per treatment level and control(s).

(vii) Description of the preparation of the synthetic growth media used including 

the preparation date, concentration of all constituents, the initial pH, and storage 

conditions and duration prior to use in test.

(viii) Description of the dilution water and any water pretreatment: source/type; 

pH; total organic carbon content; particulate matter content; metals, pesticides, 

and chlorine concentration. Describe the frequency and sample date(s) for 

documenting dilution water quality.
Page 23 of 26

(ix) Methods used for treatment randomization and assignment of inoculum to test 

vessels.

(x) Date of introduction of test organisms to test solutions and test duration.

(xi) Exposure technique.

(xii) The photoperiod and light source.

(xiii) Methods and frequency of environmental monitoring performed during the 

definitive or limit study for temperature, light intensity, pH.

(xiv) Methods and frequency of measuring test substance to confirm exposure 

concentrations.

(xv) Methods and frequency of measuring cell density, and any other 

measurements or observations of phytotoxic symptoms.

(xvi) For the definitive and limit test, all analytical procedures should be 

described. The accuracy of the method, method detection limit, and limit of 

quantification should be given.

(6) Results. 

(i) Tabulation of test substance analytical results by test vessel and treatment 

(provide raw data) and descriptive statistics (mean, standard deviation, minimum, 

maximum, coefficient of variation).

(ii) Environmental monitoring data results (test solution or air temperature, light 

intensity, and pH) in tabular form (provide raw data for measurements not made 

on a continuous basis), and descriptive statistics (mean, standard deviation, 

minimum, maximum).

(iii) For preliminary range-finding tests, if conducted, the cell density at each 

treatment level and in the control(s). A description and count of morphological 

phytotoxic effects, if recorded, at each treatment level and in the control(s).

(iv) For a limit test, tabulate for the limit concentration and the control(s) by

replicate, the cell density at test initiation, at 24, 48, and 72 hours, and at test 

termination, the treatment means and standard deviations (provide the raw data).

(v) For the definitive test, tabulation by test vessel and treatment of cell density at 

test initiation, at 24, 48, and 72 hours, and at test termination and treatment means 

and standard deviations (provide the raw data).

(vi) For the limit and definitive tests, tabulation by test vessel and treatment of 

yield, average specific growth rate, and area under the growth curve for cell 

density.
Page 24 of 26

(vii) For the limit and definitive tests, tabulation of the mean treatment yield, 

average specific growth rate, and area under the growth curve values, treatment 

standard deviations for these variables, and the treatment %I (or stimulation) in 

yield, average specific growth rate, and area under the growth curve relative to the 

control values.

(viii) For the limit and definitive test, tabulation of observed morphologic signs of 

toxicity (size, abnormal shape or color changes, and any other observed effect) by 

observation time, treatment level and replicate.

(ix) Graphs of the 96-hour concentration-response data for yield, average specific 

growth rate, and area under the growth curve based on cell density.

(x) For a limit test, provide the results of hypothesis tests.

(xi) For the limit test, provide a description of the statistical methods used 

including software package, and the basis for the choice of method.

(xii) For the definitive study and for those effect measures (yield, average specific 

growth rate, and area under the growth curve for cell density) with data sufficient 

to fit a concentration-response relationship, tabulation of the slope of the 

concentration-response curve and its standard error and 95% confidence limits 

and any goodness of fit results.

(xiii) For the definitive test, tabulation of 96-hour IC50 values for yield, average 

specific growth rate, and area under the growth curve for cell density.

(xiv) For the definitive test, a tabulation of the 96-hour NOEC and LOEC values 

for each response variable (yield, average specific growth rate, and area under the 

growth curve based on cell density). For pesticides, the IC05 and 95% confidence 

interval should be reported for response data where an NOEC could not be 

determined.

(xv) Description of statistical method(s) used for point estimates, including 

software package, for determining IC50 values, fitting the dose-response model, 

and the basis for the choice of method. Provide results of any goodness-of-fit 

tests.

(xvi) Description of statistical method(s) used for NOEC and LOEC 

determination, including software package, and the basis for the choice of 

method. If an IC05 value is used in place of a NOEC provide a description of 

statistical method(s) used for point estimates, including software package, for 

determining IC05 values, fitting the dose-response model, and the basis for the 

choice of method. Provide results of any goodness-of-fit tests.

(xvii) If determined, report the algistatic and algicidal concentrations.
Page 25 of 26

(j) References. The following references should be consulted for additional background 

material on this test guideline.

(1) American Society for Testing and Materials. ASTM E 1218-04. Standard guide for 

conducting 96-h toxicity tests with microalgae. In: Annual Book of ASTM Standards, 

Vol. 11.06, West Conshohocken, PA. Current edition approved 2004.

(2) Bruce, R.D. and D.J. Versteeg, 1992. A statistical procedure for modeling continuous 

toxicity data. Environmental Toxicology and Chemistry 11:1485-1494.

(3) Miller, W.E., J.C. Greene and T. Shiroyama, 1978. The Selenastrum capricornutum

Printz Algal Assay Bottle Test: Experimental Design, Application, and Data 

Interpretation Protocol. EPA-600/9-78-018. Corvallis, OR.

(4) Nyholm, N., P.S. Sorenson, K.O. Kusk, and E.R. Christensen, 1992. Statistical 

treatment of data from microbial toxicity tests. Environmental Toxicology and Chemistry

11:157-167.

(5) Organization for Economic Co-operation and Development, 2006. OECD Guidelines 

for Testing of Chemicals, TG 201, Freshwater Alga and Cyanobacteria, Growth 

Inhibition Test. Updated guideline, adopted 23 March, 2006. 

(6) Payne, A.G. and R.H. Hall, 1979. A method for measuring algal toxicity and its 

application to the safety assessment of new chemicals. pp 171-180 in L.L. Marking and 

R.A. Kimerle (eds.). Aquatic Toxicology, ASTM STM 667, American Society for 

Testing and Materials, Philadelphia, PA.

(7) Provasoli, L., 1963. Growing marine seaweed, Proceedings of the 4

th International 

Seaweed Symposium, Pergamon Press, Vol. 4, pp. 9-17. 

(8) Stein, J., Handbook of Phycological Methods, Cambridge University Press.

(9) U.S. Environmental Protection Agency, 1974. Marine Algal Assay Procedure: Bottle 

Test, Eutrophication and Lake Restoration Branch, Pacific Northwest Environmental 

Research Center, Corvallis, OR. 

(10) U.S. Environmental Protection Agency, 1978. Bioassay Procedures for the Ocean 

Disposal Permit Program, EPA-600/9-78-010, Environmental Research Laboratory, Gulf 

Breeze, FL. 

(11) U.S. Environmental Protection Agency, 1982. Pesticide Assessment Guidelines 

Subdivision J Hazard Evaluation: Nontarget Plants. Office of Pesticides and Toxic 

Substances, Washington, D.C. EPA-540/9-82-020, October 1982.

(12) U.S. Environmental Protection Agency, 1986. Hazard Evaluation Division Standard 

Evaluation Procedure, Non-target Plants: Growth and Reproduction of Aquatic Plants -
Page 26 of 26

Tiers 1 and 2. Office of Pesticides Programs, Washington, D.C. EPA 540/9-86-134, 

June 1986.

(13) U.S. Environmental Protection Agency, 1994. Pesticides Reregistration Rejection 

Rate Analysis: Ecological Effects, Office of Prevention, Pesticides and Toxic Substances, 

Washington, D.C. EPA 738-R-94-035

(14) VanEwijk, P.H. and J.A. Hoekstra, 1993. Calculation of the EC50 and its confidence 

interval when subtoxic stimulus is present. Ecotoxicology and Environmental Safety

25:25-32. 

(15) Walsh, G.E. and S.V. Alexander, 1980. A marine algal bioassay method: results 

with pesticides and industrial wastes. Water, Air and Soil Pollution 13:45-55.