Seasonal distribution of photosynthetically active phytoplankton using pulse amplitude modulated fluorometry in the large monomictic Lake Biwa, Japan

The detailed seasonal distribution of photosynthetically active phytoplankton was measured using the pulse amplitude modulated fluorescence technique in the large monomictic Lake Biwa, Japan, throughout the year. At the same time, the biological and physicochemical factors (water temperature, irradiance, nutrients and phytoplankton species) affecting the primary production of phytoplankton were also measured in order to estimate the relationship between the distribution of the active phytoplankton and environmental factors. The potential maximum quantum yield (F v /F m ) of photosystem II was used as an index of the degree of potential photosynthetic competence of phytoplankton. As a result, the simultaneous measurement of F v /F m and algal biomass showed clearly the seasonal and spatial distribution patterns of the photosynthetically active phytoplankton community, and the distributions were strongly dependent on the seasonal variation in the thermally stratified structure. In addition, the value of F v /F m in the euphotic zone was found to be influenced mainly by the irradiance and to be affected indirectly by nutrient concentration and the species composition of the phytoplankton community. Greater understanding of the detailed distribution of photosynthetically active phytoplankton coupled with their biomass will encourage new developments in studies of aquatic ecosystems.