Limiting nutrient and its use efficiency of phytoplankton in a shallow eutrophic lake, Lake Kasumigaura

In the management of lakes for a healthy environment, it is crucial to assess the limiting nutrient(s) and understand its use efficiency of the phytoplankton production. We hypothesized that dissolved inorganic phosphorus (DIP) and/or nitrogen concentrations (DIN) describe the state of nutrient limitations on phytoplankton production. Using the 38 years of monthly limnological data at eight stations in Lake Kasumigaura, we analyzed the relationships between the available nutrients for building phytoplankton biomass and chlorophyll a (Chla). Better correlations were obtained by subtracting the nutrients in tripton (non-living particulate matter) from the total P and N (available nutrients: TP′ and TN′). We determined the thresholds for P and N limitation (DIP: 0.01 mg l−1; DIN: 0.15 mg l−1) using the slopes of regression, and we examined this assessment in one of four limitation conditions (only P, only N, both, or none) for the respective times and stations. Close relationships between TP′ and Chla for only the P limitation data and between TN′ and Chla for only the N limitation data were observed. Multiple regression models confirmed that TP′ and TN′ predominantly determined Chla in only the P and N limitation conditions, respectively. Factors affecting nutrient use efficiency were also discussed.