Effects of cyanobacterial allelochemicals on a natural plankton community

Abstract

We investigated the effects of cell-free filtrates of the cyanobacteria Nodularia spumigena, Aphanizomenon sp. and Anabaena sp. on a brackish-water plankton community (< 150 mu m). In a laboratory experiment, we monitored the concentration of chlorophyll a and cell numbers of bacteria, phytoplankton and ciliates in the experimental units treated with cyanobacterial filtrates and the control medium. Cyanobacterial filtrates altered the cell numbers of many phytoplankton groups; they stimulated both colonial (Snowella spp.) and filamentous cyanobdcteria (Pseudanabaena spp., Anabaena spp., Aphanizomenon sp., N, spumigena), a chlorophyte (Oocystis sp.), a dinoflagellate (Amphidinium sp.) and nanoflagellates, but inhibited cryptophytes. The filtrates also increased the numbers of bacteria at the beginning of the experiment, whereas they had no effect on ciliate abundances. Our results suggest that cyanobacteria may enhance the effects of eutrophication by stimulating various phytoplankton groups. The specific compounds causing this stimulation remain to be studied. On the other hand, cyanobacteria also seem able to reduce the biomass of certain phytoplankton species, via an allelopathic mechanism.