Benthic exchange along a tropical estuarine salinity gradient during dry season: Biogeochemical and ecological implications

Abstract

Benthic biogeochemistry and its coupling with pelagic ecology are less understood in tropical than in temperate estuaries, which have significant implications for global carbon and nutrient cycling. In dry season, the Mandovi Estuary of Western India develops a prominent salinity gradient (0-35 psu), and remains nutrient-limited but most productive, which stands in contrast to wet season. To understand the role of benthic exchange in the sustenance of estuarine biogeochemistry and ecosystem, we carried out biogeochemical measurements in the water column and a series of intact-core incubations along the estuarine salinity gradient, during the dry season of 2014. We observed that the lower estuary was nitrogen (N)-limited whereas the middle and upper estuary were phosphorus (P)-limited. The benthic respiration (38.6-82.87 mmol O2 m-2 d-1), organic carbon (Corg) mineralization (34.3-79.8 mmol C m-2 d-1) and nutrient exchange rates considerably varied along the salinity gradient. Variability in benthic respiration was apparently controlled by labile Corg content and faunal abundance in the sediments. Benthic exchange of NH4+ and NOx- were mainly controlled by salinity and NOx- of the water column, respectively. Benthic PO43- exchange was predominantly controlled by sedimentary Fe content and bottom water O2, whereas benthic SiO44- exchange was primarily controlled by sediment-water SiO44- gradient. Abiotic processes accounted for 37-79% of sediment O2 consumption (SOC). Benthic metabolism accounted for 34-60% of total community respiration and mineralized 43-145% equivalent of C fixed through primary production (PP). Of dissolved inorganic nitrogen (DIN) efflux (0.67-2.59 mmol m-2 d-1), NH4+ comprised 91% in the lower estuary but NOx- (NO3- + NO2-) comprised 97% in the upper estuary, and benthic nitrification apparently determined the dominant N species released from the sediments. Coupled nitrification-denitrification apparently caused the loss of 30-69% of NH4+ diffusing from the deeper sediments. The estuarine sediments were a net source of DIN and SiO44-, potentially meeting up to 31% of N and 40% of Si demand of the phytoplankton, but acted as a net PO43- sink owing to high sedimentary Fe and normoxia, and tended to buffer the estuarine water PO43- to 0.05-0.19 M.