Linear water column stratification and euphotic depth determine the number of phytoplankton taxa that create biomass peaks in a hypertrophic oxbow lake

Abstract

The mixing regime, the spatial distribution of nutrients and light determine the distribution of phytoplankton in lakes to a large extent. Linear stratification is a unique phenomenon among the various forms the lakes can stratify, representing a continuous and gradual water temperature decrease with depth. Here, we aimed to understand how mixing, nutrient and light affect the vertical distribution of phytoplankton in the case of linear water column stratification using the taxonomic and functional group approaches. We sampled phytoplankton and physical and chemical variables in the Malom-Tisza oxbow lake (Hungary) monthly from May to September between 2007 and 2009. Our results revealed that multiple biomass peaks of taxa belonging to distinct phytoplankton functional groups could develop in response to the strong linear stratification of the water column. Although several different species represented the functional groups, only one or two species developed the peaks. Light irradiance did not influence the vertical distribution of biomass and taxonomic richness of phytoplankton, but the depth of the euphotic zone determined the number of distinct biomass peaks. We found that diversity indices could not reflect the phytoplankton compositional differences well in the case of linear stratification, but similarity indices calculated among water column layers.