Phytoplankton community structure in five subtropical shallow lakes with different trophic status (Uruguay): a morphology-based approach

Phytoplankton abundance and biomass can be explained as a result of spatial and temporal changes in physical and biological variables, and also by the externally imposed or self-generated spatial segregation. In the present study, we analyzed contrasting-season changes in the phytoplankton communities of five subtropical shallow lakes, covering a nutrient gradient from oligo-mesotrophy to hypereutrophy, using a morphologically based functional approach to cluster the species. Six environmental variables accounted for 46% of the total phytoplankton morphological groups variance, i.e., turbidity (Secchi disk), conductivity, total phosphorus, total nitrogen, total zooplankton abundance, and herbivorous meso:microzooplankton density ratio. The differences in resource availability and zooplankton abundance among the systems were related with important changes in phytoplankton composition and structure. Within phytoplankton assemblages, adaptations to improve both light and phosphorus/nitrogen uptake were important in nutrient-poor systems; while grazing-avoidance mechanisms, such as colonial forms or bigger individuals, seemed relatively important only in eutrophic Lake Blanca, where light was not a limiting factor. However, this was not observed in the nutrient-rich Lake Cisne, where low light availability (due to clay resuspension and dark water color) was identified as the main structuring factor. Our results suggest that the composition of phytoplankton morphologically based functional groups appear to reliably describe the trophic sate of the lakes. However, other factors, such as nonbiological turbid condition, or zooplankton composition, may interact rendering interpretations difficult, and therefore, deserve further studies and evaluation.