Essential fatty acid concentrations of different seston sizes and zooplankton: a field study of monomictic coastal lakes

Plankton particle size is a central issue for aquatic ecologists. In this field study, we examined essential fatty acids (EFAs) in different plankton particle sizes and obtained initial estimates of how EFA concentrations, and thus high dietary lipid quality for consumers, vary with seston size. We measured EFA concentrations in three seston size classes (i.e. micro = 0.7-35 mu m; meso = 35-64 mu m and macroseston = 64-100 mu m) and in cladocerans and calanoid copepods of monomictic, coastal lakes. Algal pigment analysis identified a mixture of Chlorophyta, Cryptophyta, diatoms and Cyanophyta in all seston size classes. Total EFA concentrations did not vary with increasing seston size. There was no difference in linoleic (LIN), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acid concentrations among the three seston size classes; however, alpha-linolenic (ALA) and arachidonic (ARA) acid concentrations were significantly higher in microseston. For herbivorous zooplankton, concentrations of LIN, ALA and EPA did not differ significantly between cladocerans and calanoid copepods; however, ARA concentrations were significantly higher in cladocerans and DHA concentrations were significantly higher in calanoid copepods. Our results indicate that microseston represents the most nutritious dietary component, per unit biomass, with respect to ALA and ARA and that seston size alone does not predict EFA concentrations, i.e. dietary access to larger seston particle sizes is not necessarily associated with access to particles with higher EFA concentrations.