Effects of toxic cyanobacteria on a plankton assemblage: Community development during decay of Nodularia spumigena

Abstract

We studied the development of the plankton community in an artificially created toxic Nodularia spumigena bloom during a 2 wk enclosure study at the SW coast of Finland in the Baltic Sea. We measured bacterial abundance, dominant phytoplankton groups and ciliates, as well as concentrations of phytoplankton pigments, fatty acids, nodularin, protein and nutrients. A high POC:chl a (<10 μm) ratio (427 ± 185), a decrease in the polyunsaturated:total fatty acid ratio (from 0.4 to 0.2), and a reduction in cyanobacteria filament length indicated decay of N. spumigena during the course of the experiment. Along with cyanobacterial decay, high concentrations of ammonium (last day: 2.7 ± 2.0 μmol l-1), nitrate (0.1 ± 0.01 μmol l-1), and orgamc nutrients were released into the water, whereas chl a and the cyanobacterial pigments, echinenone and zeaxanthin, decreased. Nodularin was found in the mesocosms during the whole experiment. A strong increase in filamentous bacteria was detected by the middle of the experiment, most likely indicating a response to grazing pressure. Two ciliate species, Mesodinium rubrum and Urotricha sp., decreased dramatically during the experiment, probably due to predation by the increasing mesozooplankton community. The ciliate Euplotes sp. flourished in the bags and was best suited to escape predation due to its protecting lorica and its surface affinity. No direct harmful effects of the cyano-bacteria on the microorganisms could be documented. We conclude that these blooms provide a potential food source for the heterotrophic food chain, from bacteria, flagellates and ciliates to crustacean zooplankton, and possibly fish.