The effects of changes in diversity on phytoplankton community metabolism

ABSTRACT Changes in composition of phytoplankton communities cause drastic changes in functioning of aquatic ecosystems. In this study, we hypothesized that the shifts in composition of the phytoplankton community of Lake Kinneret induced notable shifts in the relationship between primary production and decomposition of organic matter in the lake ecosystem. To test this, we created an approach based on a statistical analysis of long-term dynamics of diversity indices and their relationships with potential perturbations: changes in lake water level, nutrient loads, and annual average epilimnetic temperature. Biodiversity of the algal community was estimated using Simpson’s Diversity Index and community-weighted mean trait value (functional diversity index). Specific community respiration and photosynthesis assimilation number were considered as the functional traits characterizing the role of phytoplankton in cycling organic matter. The observed temporal trends in the diversity indices provided estimates of the shift in the biotic community from large, relatively faster aerobic decomposing and slower photosynthesizing producers towards slower aerobic decomposing and faster photosynthesizing producers. These trends were confirmed by the results of direct determination of community respiration and (indirectly) by the reduced microbial activity in the epilimnetic waters of the lake.