New insights on biomass production in lakes: Integration of Carlson trophic state index and vertically generalized production model

• Integrated TSI and vertically generalized production model to manage eutrophication. • We calculated phytoplankton primary production and the TSI in Qeshlagh dam reservoir. • Mesotrophic conditions support higher primary production compared to eutrophic states. • Excessive eutrophication limits light and hinders phytoplankton primary productivity. Carlson trophic state index (CTSI) is widely adopted for eutrophication studies because of its simplicity and ease of access to basic data, making it particularly popular in data scare regions. However, it provides only a generalized view of eutrophication and lacks a quantitative description of biomass and the accumulation of algal blooms. In contrast, the empirical models for estimating biomass production, such as the vertically generalized production model (VGPM), require extensive and complex data, which limits its practical application in resource-limited regions. In this study, we introduced a new model that integrates the CTSI and VGPM to estimate primary production using easily measurable environmental factors. We assessed the developed model performance using the data sampled from the Qeshlagh dam reservoir, Iran. CTSI results revealed a dominantly eutrophic state in the QDR. VGPM model showed the mesotrophic conditions support higher biomass primary production (mean: 4,753.89 mg C m −2 d −1 ) compared to eutrophic states (mean: 2,373.63 mg C m −2 d −1 ). Contrary to the VGPM, which requires rarely measured parameters, our developed model acceptably estimated the primary production in the QDR, with a correlation coefficient of 0.87. The introduced model identified the primary production is positively impacted by the maximum rate of carbon fixation, the depth of euphotic zone, and chlorophyll- a . Further investigations using multivariate statistical methods showed that primary production values estimated by the model were inversely correlated with CTSI, suggesting that excessive eutrophication (i.e., higher CTSI values) limits light penetration and hinders primary production. These findings provide important insights for estimating the biomass primary production in data-scarce regions, managing eutrophication, and improving water quality in affected eutrophic reservoirs.