Climate features or the composition of submerged vegetation? Which factor has a greater impact on the phytoplankton structure in temperate lakes?

This study compares the composition and biomass of phytoplankton assemblages of lakes with abundant vegetation dominated by (1) charophytes (Chara-lakes) and (2) angiosperms (Potamogeton-lakes). These two groups of submerged macrophytes effectively control the phytoplankton development in lakes, but differ in the feedback mechanisms involved. Despite increasing interest, the phytoplankton development in charophyte- and angiosperm-dominated lakes under different climatic circumstances remains poorly recognized. Each type of lakes was studied in two distant (>500 km apart) regions of Poland, characterized by distinctly different climate features (western – warmer, and north-eastern – cooler), with temperature differences corresponding to the predicted magnitude of the global warming-related temperature rise in the near future. Twelve lakes were selected for this study, three Chara- and three Potamogeton-lakes in each region. In addition to phytoplankton analysis, macrophytes and climatic conditions, water chemistry, and the use of land in the catchment area were studied. Although we expected that climatic differences would have greater impact on the structure of phytoplankton assemblage than that expected due to higher macrophyte biomass and lower nutrient availability in Chara- than in Potamogeton-lakes, multidimensional statistical analyses clearly distinguished between the two macrophyte types of lakes. Significantly lower values of the total phytoplankton biomass, and the biomass of diatoms and cyanobacteria occurred in Chara- vs Potamogeton-lakes. We therefore postulate that not only abundantly developed submerged macrophytes, but also the type of vegetation are important factors structuring phytoplankton development and by interacting with the physical and chemical characteristics of water show potential in mitigating the effects of climate change