ABSTRACT
Mesocosm studies can have many different objectives, and the study objectives will determine the appropriate sampling and analysis strategy for the test chemical. The investigator may wish to measure the test chemical for one or more of the following reasons: (a) to learn about the chemical’s fate in the aquatic environment; (b) to quantify the chemical exposure associated with observed ecological responses; or (c) to confirm the experimental treatment.
To understand the fate of the test chemical, at least water and sediment must be analyzed, and possibly also fish and vegetation. Major degradation products should be quantified. A mass balance may be attempted with radiolabeled test material. Replication of mesocosms is not essential but may be useful, because chemical dynamics will vary among test systems.
To quantify exposure, analysis of water is most important because water concentrations are the driving factor in ecological risk assessment. Sediment analyses may be useful if the test chemical binds to soil particles or is expected to be highly toxic to benthic organisms. Degradation products are quantified only if they are suspected of being toxic. For test substances with short half-lives or in studies with repeated test substance application, the exposure regime can be described in many ways (maximum concentration, average concentration, frequency or duration, etc.) and it is not always apparent which representation of exposure is the most relevant to the observed ecological effects.
To confirm treatment, water or sediment or both may be analyzed depending on the solubility and sorptive properties of the test material and on the mode of treatment. Sampling occurs immediately after application. Deposition cards may be used to quantify application rates of sprayed test materials. Treatment can also be confirmed by verifying the loading rate through analysis of stock solutions, tank mixes, and tank rinses.
Standard techniques for sampling water and sediment are briefly described, and the question of compositing is discussed. Analytical approaches depend on the test chemical; issues that must be addressed in designing a study include detection limits, method validation, storage stability, and quality control.
