Metabolic balance in the shallow eutrophic Lake Inbanuma, Japan

Metabolism is a pivotal process that governs the biogeochemical carbon cycle by both producing and consuming organic matter. Gross primary production (GPP), ecosystem respiration (ER), net ecosystem production ( NEP = GPP − ER ), and the metabolic balance of GPP and ER were analyzed in Lake Inbanuma from July to December 2023 and May to December 2024. Water temperature, nutrient (nitrate + nitrite, ammonium, phosphate) and chlorophyll a (Chl a ) concentrations were also measured. GPP was generally higher in summer and fall than in other seasons, exceeding 10 mgO 2 L −1 d −1 . GPP and NEP exhibited a significant positive linear relationship. The threshold GPP (when GPP = ER ) was 0.75 mgO 2 L −1 d −1 , markedly lower than values from other eutrophic lakes. GPP/Chl a was significantly positively correlated with phosphate and ammonium concentrations. Furthermore, GPP/Chl a was more affected by phosphate than by ammonium. These results suggest that phosphate concentration is a key factor controlling phytoplankton growth in Lake Inbanuma. NEP remained above zero throughout the study period, with an average GPP/ER ratio of 4.9, suggesting that Lake Inbanuma was autotrophic during the study period. The variations in GPP and NEP were closely related to changes in Chl a concentration, which explained approximately 40% of the total variation.