Seasonal dynamics of mangrove outwelling and its impact on estuarine productivity: insights from multi-parameter analysis across tidal variations

Tropical mangroves are subjected to extreme environmental conditions yet remain highly productive ecosystems. The Mandovi and Zuari estuaries of Goa on the western coast of India are lined by mangroves and are a rich source of fisheries that supports the coastal livelihood. Despite their ecological and economic significance, the nutrient and organic matter dynamics in tropical mangrove ecosystems remain poorly studied. The hydrographic conditions in the mangrove and adjacent estuarine waters are dictated by monsoonal precipitation, freshwater input, tides, and influx of seawater, resulting in subsequent changes in their physicochemical parameters and organic matter concentrations. The impact of these changes on the phytoplankton biomass and primary productivity in estuarine waters was investigated during the pre-monsoon, monsoon and post-monsoon seasons. Dissolved inorganic nutrients, such as nitrite, nitrate, ammonia, phosphate and silicate, displayed wide seasonal variability and were higher during the low tide than those in high tide. Isotopic and elemental ratios of carbon and nitrogen revealed that primary productivity in the estuaries was enhanced by the outwelling of mangrove-derived organic matter. The highest chlorophyll a and primary productivity were observed during the pre-monsoon season, with Zuari exhibiting higher levels compared with Mandovi. Multivariate statistical analyses confirmed that primary productivity was influenced mainly by temperature, salinity, pH, dissolved oxygen and organic matter. Results from our study emphasizes the role of outwelling of organic matter from mangroves and their influence on primary productivity in adjacent estuarine waters.