Decadal nutrient dynamics in a tropical bay: spatiotemporal variations and drivers in Haikou Bay and adjacent coastal...

Coastal eutrophication driven by anthropogenic nutrient inputs and climate change is a growing threat to marine ecosystems. This study examined the nutrient dynamics in Haikou Bay and its adjacent waters based on observational data collected from 24 cruises conducted from 2010 to 2021. The dissolved inorganic nitrogen (DIN) concentrations ranged from 3.00 μmol L^-1 to 19.3 μmol L^-1, with an average value of 10.8 ± 0.2 μmol L^-1 in Haikou Bay. Three distinct spatial patterns of DIN concentrations were identified: river mouth enrichment, a west-to-east decreasing gradient, and a homogeneous distribution. A significant negative correlation between DIN concentrations and salinity highlighted the influence of freshwater on DIN distributions. Seasonally, DIN concentrations showed a declining trend from May to September, with values decreasing from 12.4 ± 0.6 to 8.89 ± 0.64 μmol L^-1, then increased slightly thereafter. The decadal variations of DIN concentrations displayed a two-phase pattern: they increased from 2010 to 2013, then declined from 2013 to 2021. This pattern correlated with variations of freshwater input and agricultural fertilizer application from the landward side. In contrast, dissolved inorganic phosphorus (DIP) showed less pronounced spatial and decadal variabilities. The mean DIN/DIP ratio was 98.5 ± 9.0, substantially higher than the canonical Redfield ratio of 16, suggesting potential phosphorus limitation. These findings provide important insights into the decadal variations of nutrient dynamics under the combined influences of climate change and anthropogenic pressures in tropical bays.