Intermediate-size cell dominance in the phytoplankton community of an eutrophic, estuarine ecosystem (Guadalhorce River, Southern Spain)

The size–abundance spectrum (SAS) of phytoplankton is controlled by the interplay of physical and biological factors whose particular relevance varies between ecosystems. Here we report the results of a study of phytoplankton SAS in a system of eight estuarine shallow, eutrophic lagoons (Guadalhorce river, South Spain). SAS were obtained through a combination of flow cytometry and image analysis microscopy techniques covering six orders of magnitude from picoplankton to microplankton. Cell numbers were classified into a log2 scale of cell volume to model the log–log relation between cell abundance (cells/mL) and cell volume (μm 3 ). The resulting averaged phytoplankton SAS can be described by a log–log transformed, power model with a slope of – 0.62 (that is, there is an allometric relation between the size and abundance of cells). The distribution of biovolume (μm 3 /l) in broader size categories is characterized by the dominance of nanoplankton (67.4%), followed by microplankton (30.1%) and picoplankton (2.5%). The minor relative contribution of picoplankton to total biovolume can be explained by a combination of high and variable rates of nutrient inputs, light stress and grazing. The biomass dominance of intermediate-size cells (nanoplankton) is coherent with experimental findings describing the unimodal size scaling of growth rate, with maximum values centered in this size category.