Midsummer heat wave effects on lacustrine plankton: Variation of assemblage structure and fatty acid composition

The effects of the 2003 European heat wave on a freshwater plankton assemblage and its fatty acid (FA) composition were investigated. Composition and FA profiles of four size categories of planktonic organisms collected in 2003 were compared to those of the colder year 2002. Temperature affected plankton population selecting thermophilic ones and had direct and indirect effects on its nutritional quality given by its FA patterns. Among algae, Ceratium hirundinella dominated and occurred at higher temperatures than described in the literature. Cyanobacteria showed a limited development. Diaphanosoma brachyurum was the dominant zooplanktonic species. Thermal environment appeared to favour the dominance of cladocerans and a decline in copepods abundance with cyclopoids (Acanthocyclops robustus) favoured over calanoids. Variation of FA composition with temperature was reflected in the relative proportion of saturated FA (SAFA), monounsaturated FA (MUFA) and polyunsaturated FA (PUFA). Organism exposure to overheated water resulted in increased SAFA and MUFA content at the expense of PUFA. This effect can be promoted in phytoplankton with high light intensity and sunshine duration.