Standard Practice for Standardized Aquatic Microcosms: Fresh Water

Importancia y uso:

5.1 A microcosm test is conducted to obtain information concerning toxicity or other effects of a test material on the interactions among three trophic levels (primary, secondary, and detrital) and the competitive interactions within each trophic level. As with most natural aquatic ecosystems, the microcosms depend upon algal production (primary production) to support the grazer trophic level (secondary production), which along with the microbial community are primarily responsible for the nutrient recycling necessary to sustain primary production. Microcosm initial condition includes some detritus (chitin and cellulose) and additional detritus is produced by the system. The microcosms include ecologically important processes and organisms representative of ponds and lakes, but are non-site specific. To the extent possible, all solutions are mixtures of distilled water and reagent grade chemicals (see Section 8) and all organisms are available in commercial culture collections.

5.2 The species used are easy to culture in the laboratory and some are routinely used for single species toxicity tests (Guide E729; Practice D3978, Guides E1192 and E1193). Presumably acute toxicity test results with some of these species would be available prior to the decision to undertake the microcosm test. If available, single species toxicity results would aid in distinguishing between indirect and direct effects.

5.3 These procedures are based mostly on published methods (4-6), interlaboratory testing (7-10, 11), intermediate studies (12-23, 24), statistical studies (25-27) and mathematical simulation results (28). Newer studies on jet fuels have been reported (29)(See 15.1 for multivariate statistical analyses) and on the implications of multispecies testing for pesticide registration (30). Environmental Protection Agency, (EPA) and Food and Drug Administration, (FDA) published similar microcosm tests (31). The methods described here were used to determine the criteria for Acceptable Tests (Section 16). Additional papers have been published using this method for measuring chemical stress on organisms (32).

5.4 Concurrent to measuring the ecological effects, it is advisable to measure the concentration of the parent test chemical, and if possible, the transformation products ((33) see Section 12). The concentrations can be measured on either the same microcosms or on concurrent replicates. Information on the chemical concentrations of parent material and transformation products would aid in the assessment of chemical persistence, exposure, accumulation, and in interpreting, if recovery is associated with chemical degradation or biological adaptation. This protocol deals only with ecological effects, because the techniques for fate studies are in general usage.

5.5 In the microcosm, as in natural ecosystems, a population must be able to obtain its requirements from the products of other trophic levels, to maintain a birth rate equal to or greater than its death rate, and to support populations of organisms that will remove its waste products. As in natural ecosystems, several organisms might be capable of fulfilling the same function, and shifts in species dominance can occur without disruption of an ecological process. However, species that are “ecological equivalents” in one function might not be “equivalent” in other functions; for example, a filamentous alga and a single cell alga might equally produce O₂, remove NO₃, NH₃, and PO₄, but differ in the type of grazer populations they can sustain, for example, filamentous alga might support amphipods whereas unicellular algae might support Daphnia.

5.6 Results of these microcosm tests might be more likely to be indicative of natural ecosystem responses to chemicals than single species toxicity tests because microcosm tests can indicate the explosive population increases that might occur in a community when more sensitive competitors or predators are eliminated or the food supply is increased through competitive interactions. Also, microcosm tests are more likely to display the effects of chemical transformation or increased exposure to certain organisms by means of concentration of parent or degradation products in their food source or habitat.

5.7 A list of potential ecological effects is provided to serve as a summary (see Annex A1).

5.8 The microcosm test can also be used to obtain information on the toxicity or other effects of species or strains, not included in the control inocula (13). Additional modifications might be required.

5.9 Explicit Limitations of the Aquatic Microcosm Protocol: 

5.9.1 The scope of the test is limited in the following respects:

5.9.1.1 No fish or other vertebrates are included,

5.9.1.2 Predation on Daphnia is extremely limited or absent,

5.9.1.3 The ecosystem becomes nutrient limited,

5.9.1.4 The inocula are not gnotobiotic and aseptic technique is not used (except in maintaining stock cultures of microorganisms). Contaminating microorganisms are likely to be introduced with the larger organisms and during sampling.

5.9.1.5 Most detrital processing is carried out by the sediment microbial community, but this community is not clearly described or measured by this protocol.

5.9.2 Extrapolation to natural ecosystems should consider differences in community structure, limiting factors, and water chemistry (see Section 17).

Subcomité:

E50.47

Volúmen:

11.09

Número ICS:

13.060.30 (Sewage water)