Hydrological control of ecosystem functioning in a coastal lagoon

Abstract Located at the land–ocean interface, coastal lagoons are specifically vulnerable to increasing threats from anthropogenic activities and climate change. Understanding the hydrological and ecological responses of these lagoon systems to global and local changes is essential. In this study, the spatial and seasonal (wet and dry seasons) variations of water level, salinity, nutrients, and ecosystem metabolism are investigated to understand the impacts of environmental changes on fish and invertebrates in Ambinanibe coastal lagoon (southeast Madagascar). Our results indicate that Ifaho River flow is the main driver of ecological functioning in the Ambinanibe lagoon. Seasonal variations in freshwater inflow directly affect salinity, nutrient concentrations, dissolved oxygen, and sediment dynamics. Changes in river discharge and lagoon–ocean connectivity also drive shifts in the trophic interactions and community structure of fish and invertebrates, demonstrating the strong sensitivity of lagoon ecosystems to hydrological variability. Overall, this study provides a robust scientific basis for understanding and predicting future ecological responses to changes in freshwater inflow, and for mitigating the ecological impacts of climate change and anthropogenic pressures.