The role of total phosphorus in eutrophication of freshwater and brackish-water parts of the Neva River estuary (Baltic Sea).

Estuarine ecosystems worldwide are increasingly affected by eutrophication, which degrades water quality, reduces biodiversity, and disrupts ecosystem functioning. In the Baltic region, mitigation efforts have focused primarily on reducing external nutrient inputs - especially total phosphorus (TP) - based on the assumption that phosphorus is the principal driver of algal blooms in freshwater-dominated estuarine zones. The Neva River estuary, the largest freshwater inflow into the Baltic Sea, offers a relevant case study. Between 2003 and 2016, municipal phosphorus discharges from St. Petersburg were reduced nearly tenfold, from 2307 to 290 tons per year, due to targeted improvements in wastewater treatment. Despite these efforts, chlorophyll a concentrations and plankton primary production have continued to rise. In this study, we analyzed a 20-year dataset (2003-2023) of TP, chlorophyll a, and plankton primary production across the freshwater and brackish parts of the Neva River estuary. Using Multivariate Linear Mixed Models and Stepwise PERMANOVA, we quantified the contribution of TP to eutrophication indicators and assessed spatiotemporal variability. Results show that TP accounted for only 5 % and 4 % of the variance in chlorophyll a and primary production in freshwater and brackish zones, respectively. In contrast, spatial and temporal factors explained a much larger proportion of variability: Station - 14.9 %, Year - 13.0 % in the freshwater zone, and Station - 4.0 %, Year - 40.0 % in the brackish zone. These findings highlight the limited predictive power of phosphorus as a standalone indicator of eutrophication. Our analysis suggests that phosphorus-only monitoring and management strategies, such as those implemented in the Neva River estuary, may be insufficient to achieve water quality improvements. Cost-saving decisions that prioritize narrow interventions can result in higher long-term environmental and economic costs, especially when they fail to address the complexity of nutrient dynamics in estuarine systems. To improve eutrophication prediction and management, we recommend the integration of multiple nutrient parameters (e.g., nitrogen, silicon, iron), along with biogeochemical, hydrodynamic, and climate modeling approaches. Our findings also illustrate that different estuarine zones may require tailored modeling strategies: freshwater zones benefit from biogeochemical-hydrodynamic-climatic models, while in brackish zones, biogeochemical and climate models may suffice. This integrated approach can enhance monitoring accuracy, reduce operational costs by optimizing sampling efforts, and support more effective estuarine management worldwide.