Replacement of the medium for a natural phytoplankton community by tangential-flow filtration, with special emphasis on toxicity tests.

Short-term photosynthesis toxicity tests with natural phytoplankton communities have received increased attention in recent years, both as an alternative to standardized toxicity tests (Kusk and Nyholm 1991) and as an integrated part of the assessment of the health of aquatic ecosystems (Munawar et al. 1989). The results of a toxicity test with natural phytoplankton are inherently dependent on the actual sensitivity of the algae and on the bioavailability of the contaminants. Both of which can vary significantly with the structure of the phytoplankton community and the chemical composition of the medium (Hongve et al. 1980; Steemann Nielsen and Laursen 1976). If the algae or the medium are replaced before the short-term photosynthesis toxicity test it might be possible to discriminate between the influence of the algae and the medium. This technical note describes and evaluates one approach for carrying out such a replacement.