Nitrogen dynamics at the sediment-water interface in shallow, sub-tropical Florida Bay: why denitrification efficiency may decrease with increased eutrophication

Nitrogen (N) dynamics at the sediment-water interface were examined in four regions of Florida Bay to provide mechanistic information on the fate and effects of increased N inputs to shallow, subtropical, coastal environments. Dissimilatory nitrate (NO3 (-)) reduction to ammonium (DNRA) was hypothesized to be a significant mechanism retaining bioreactive N in this warm, saline coastal ecosystem. Nitrogen dynamics, phosphorus (P) fluxes, and sediment oxygen demand (SOD) were measured in north-central (Rankin Key; eutrophic), north-eastern (Duck Key; high N to P seston ratios), north-western (Murray Key; low N to P ratios), and central (Rabbit Key; typical central site) Florida Bay in August 2004, January 2005, and November 2006. Site water was passed over intact sediment cores, and changes in oxygen (O-2), phosphate (o-PO4 (3-)), ammonium (NH4 (+)), NO3 (-), nitrite (NO2 (-)), and N-2 concentrations were measured, without and with addition of excess (NO3)-N-15 (-) or (NH4)-N-15 (+) to inflow water. These incubations provided estimates of SOD, nutrient fluxes, N-2 production, and potential DNRA rates. Denitrification rates were lowest in summer, when SOD was highest. DNRA rates and NH4 (+) fluxes were high in summer at the eutrophic Rankin site, when denitrification rates were low and almost no N-2 came from added (NO3)-N-15 (-). Highest (NH4)-N-15 (+) accumulation, resulting from DNRA, occurred at Rabbit Key during a picocyanobacteria bloom in November. (NH4)-N-15 (+) accumulation rates among the stations correlated with SOD in August and January, but not in November during the algal bloom. These mechanistic results help explain why bioreactive N supply rates are sometimes high in Florida Bay and why denitrification efficiency may decrease with increased NO3 (-) inputs in sub-tropical coastal environments.