Nutrient regeneration and oxygen consumption by sediments along an estuarine salinity gradient

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Abstract
Nutrient fluxes and O2 consumption (SOD) across the sediment-water interface were measured in situ along with vertical profiles of dissolved and particulate-phase nutrients in sediments and overlying water at 8 locations along the salinity gradient of Chesapeake Bay [USA] during spring and summer. Strong spatial and temporal patterns were evident. Highest rates of sediment NH4+ regeneration and SOD occurred in summer at mid-salinity (12 to 17 .permill.) stations. Sediment fluxes of dissolved inorganic P (DIP) were always low, possibly due to relatively oxidized conditions in surficial sediments. NO3- fluxes generally were directed into the sediments in spring and from the sediments during summer, and in both seasons fluxes were proportional to NO3- concentrations in overlying waters. Seasonal shifts in sediment O2N flux ratios suggest that denitrification may have been important in spring but not summer. Significant relations were inferred between C:N:P composition of suspended materials and surficial sediments and the magnitude and direction of sediment-water fluxes. Although accretion of particulate N in sediments was less than 6% of NH4+ regeneration in the lower bay, it was similar to rates of NH4+ flux in the low salinity region, suggesting that burial represents a significant sink for N in some estuarine zones. SOD constituted an important term in water column O2 budgets at all stations (16-50% of total respiration), and sediment regeneration of NH4+ was capable of supplying 13 to 40% of calculated phytoplankton N requirement, being more important during the summer period of higher productivity.