The Effects of Salinity on Nitrogen Losses from an Oligohaline Estuarine Sediment

Benthic respiration, sediment–water nutrient
fluxes, denitrification and dissimilatory nitrate reduction to
ammonium (DNRA) were measured in the upper section
of the Parker River Estuary from 1993 to 2006. This site
experiences large changes in salinity over both short and
long time scales. Sediment respiration ranged from 6
to 52 mmol m−2 day−1 and was largely controlled by
temperature. Nutrient fluxes were dominated by ammonium
fluxes, which ranged from a small uptake of −0.3 to an
efflux of over 8.2 mmol N m−2 day−1. Ammonium fluxes
were most highly correlated with salinity and laboratory
experiments demonstrated that ammonium fluxes increased
when salinity increased. The seasonal pattern of DNRA
closely followed salinity. DNRA rates were extremely low
in March, less than 0.1 mmol m−2 day−1, but increased to
2.0 mmol m−2 day−1 in August. In contrast, denitrification
rates were inversely related to salinity, ranging from 1 mmol
m−2 day−1 during the spring and fall to less than 0.2 mmol
m−2 day−1 in late summer. Salinity appears to exert a major
control on the nitrogen cycle at this site, and partially
decouples sediment ammonium fluxes from organic matter
decomposition.