Nutritional Quality of Phytoplankton Shapes Estuarine Food Webs Under Salinity and Nutrient Stress

Estuarine ecosystems, vital for biodiversity and nutrient cycling, face increasing threats from human-driven salinity changes and nutrient pollution. However, the impacts of these environmental stressors on estuarine food web dynamics remain poorly understood despite their critical role in ecosystem resilience. This study investigates how elevated salinity and nutrient levels influence the nutritional quality of phytoplankton, focusing on changes in long-chain polyunsaturated fatty acids (LC-PUFA) and their cascading effects on higher trophic consumers. Our results revealed that high salinity combined with nutrient enrichment significantly reduced phytoplankton nutritional quality. In high-salinity areas, elevated nutrient levels promoted low-quality phytoplankton, reducing phytoplankton LC-PUFA, while, in low-salinity areas, nutrient enrichment further decreased LC-PUFA synthesis. While zooplankton fatty acid profiles did not directly correlate with phytoplankton changes, suggesting potential adaptive or compensatory mechanisms of these consumers, fish fatty acid composition was closely tied to phytoplankton LC-PUFA. Declines in phytoplankton quality were associated with simplified trophic structures and weakened trophic links, particularly under high nutrient conditions. Our findings underscore the pivotal role of phytoplankton nutritional quality in maintaining the food web complexity and estuarine ecosystem functionality. Future research should prioritize the synergistic effects of multiple environmental stressors on phytoplankton LC-PUFA to inform ecosystem management and restoration strategies aimed at sustaining estuarine resilience.