Sediment Denitrification and Nutrient Fluxes in the San José Lagoon, a Tropical Lagoon in the Highly Urbanized San Juan Bay Estuary, Puerto Rico

Direct measurements of net N2 production, oxygen, and inorganic nutrient fluxes at the sediment–water interface were conducted in the San José Lagoon (SJL), a tropical coastal lagoon located at the San Juan Bay Estuary, Puerto Rico, that is heavily impacted by anthropogenic activities. Sediment of the SJL had high organic matter (average = 16.7 %; SD = 9.0), pore water ammonium (541 μM ± 422), and pore water soluble reactive phosphorus (SRP) (57 μM ± 48) concentrations. High fluxes of oxygen (−1999 μmol m−2 h−1 ± 2180) and N2–N (192 μmol m−2 h−1 ± 315) fluxes were observed. Similarly, high fluxes of ammonium (770 μmol m−2 h−1 ± 701) and SRP fluxes (167 μmol m−2 h−1 ± 138) were measured, with moderate nitrate fluxes (−40 μmol m−2 h−1 ± 53) mostly directed into the sediment. The dissolved inorganic nitrogen and soluble reactive phosphorus ratio (DIN/SRP) was low relative to the Redfield ratio and suggested that the sediment would support SJL as a nitrogen-limited system. An experiment with higher nitrate concentration resulted in a significant response (p < 0.05) of denitrifying microbes and ammonium fluxes to a 100-μM KNO3 addition. Net N2 production was an important sink in the nitrogen balance of the SJL, though most remineralized N was released as ammonium. Efforts to mitigate eutrophication in the SJL require consideration of the substantial role that sediments play in biogeochemical cycles.