Benthic Metabolism and Nutrient Cycling along an Estuarine Salinity Gradient
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Benthic metabolism and nutrient exchange across the sediment-water interface were examined over an annual cycle at four sites along a freshwater to marine transect in the Parker River-Plum Island Sound estuary in northeastern Massachusetts, U.S. Sediment organic carbon content was highest at the freshwater site (10.3%) and decreased along the salinity gradient to 0.2% in the sandy sediments at the marine end of the estuary. C:N ratios were highest in the mid estuary (23:1) and lowest near the sea (11:1). Chlorophyll a in the surface sediments was high along the entire length of the estuary (39-57 mg chlorophyll a m(-2)) but especially so in the sandy marine sediments (172 mg chlorophyll a m(-2)). Chlorophyll a to phaeophytin ratios suggested most chlorophyll is detrital, except at the sandy marine site. Porewater sulfide values varied seasonally and between sites, reflecting both changes in sulfate availability as overlying water salinity changed and sediment metabolism. Patterns of sediment redox potential followed those of sulfide. Porewater profiles of inorganic N and P reflected strong seasonal patterns in remineralization, accumulation, and release. Highest porewater NH4+ values were found in upper and mid estuarine sediments, occasionally exceeding 1 mM N. Porewater nitrate was frequently absent, except in the sandy marine sediments where concentrations of 8 mu M were often observed. Annual average respiration was lowest at the marine site (13 mmol O-2 m(-2) d(-1) and 21 mmol TCO2 m(-2) d(-1)) and highest in the mid estuary (130 mmol O-2 m(-2) d(-1) and 170 mmol TCO2 m(-2) d(-1)) where clam densities were also high. N2O and CH4 fluxes were low at all stations throughout the year. Over the course of a year, sediments varied from being sources to sinks of dissolved organic C and N, with the overall spatial pattern related closely to sediment organic content. There was little correlation between PO43- flux and metabolism, which we attribute to geochemical processes. At the two sites having the lowest salinities, PO43- flux was directed into the sediments. On average, between 22% and 32% of total system metabolism was attributable to the benthos. The mid estuary site was an exception as benthic metabolism accounted for 95% of the total, which is attributable to high densities of filter-feeding clams. Benthic remineralization supplied from less than 1% to over 190% of the N requirements and 0% to 21% of the P requirements of primary producers in this system. Estimates of denitrification calculated from stoichiometry of C and N fluxes ranged from 0% for the upper and mid estuary site to 35% for the freshwater site to 100% of sediment organic N remineralization at the marine site. We hypothesize that low values in the upper and mid estuary are attributable to enhanced NH4+ fluxes during summer due to desorption of exchangeable ammonium from rising porewater salinity. NH4+ desorption during summer may be a mechanism that maintains high rates of pelagic primary production at a time of low inorganic N inputs from the watershed.