Benthic-pelagic coupling of carbon and nitrogen along a tropical estuarine gradient (Gulf of Nicoya, Costa Rica)

Abstract Tropical estuaries are very little studied compared to temperate ones, despite their generally high primary production rate and hence potential contribution to global carbon and nitrogen budgets. The Gulf of Nicoya (northwest Costa Rica Pacific coast) is one of the most productive estuaries worldwide. However, how changes in benthopelagic coupling affect biogeochemical cycling of carbon and nitrogen over seasons and along the estuarine gradient is not known for this and, in general, other tropical estuaries. For this purpose, we performed two sampling campaigns during the rainy and dry seasons to investigate the contribution of the pelagic and benthic compartments to the carbon and nutrient pools and dynamics of organic matter remineralization in the estuary. Areal concentrations of chlorophyll a (Chla), organic carbon (C) and total nitrogen (N) integrated along the water column photic layer and in the surface sediment generally reached maximum concentrations in the middle of the estuary. Although, integrated Chla in the water column and sediment were in the same range, integrated C and N in the surface sediment (5 cm) were 2–10 times higher than in the water column. The C:Chla and N:Chla ratios in the suspended particulate matter were much higher than those typical of phytoplankton indicating a strong contribution of allochthonous organic matter. However, the C:N ratio in the water column was frequently lower than the Redfield ratio suggesting an important contribution of particulate inorganic N, particularly in the rainy season. Diffusive O2 sediment uptake rate and the O2 penetration depth within the sediment correlated with the organic carbon content in the sediment. In contrast, total oxygen utilization and total CO2 fluxes showed higher variability along the estuary but no significant statistical correlations with any other sediment biogeochemical variables. Organic carbon flux (JC) to the sediment was highly variable along the estuary and between seasons. Despite differences in absolute terms, the fraction of JC accumulated within the sediment for the whole inner part of the Gulf of Nicoya was about 60% in both seasons. Total C burial in the inner part of the gulf was 390 and 813 Tn C d−1 in the rainy and dry seasons, respectively, representing 1.8 and 3.8% of the daily Costa Rica CO2 emissions.