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

Abstract

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 (O2), phosphate (o-PO43−), ammonium (NH4+), NO3−, nitrite (NO2−), and N2 concentrations were measured, without and with addition of excess 15NO3− or 15NH4+ to inflow water. These incubations provided estimates of SOD, nutrient fluxes, N2 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 N2 came from added 15NO3−. Highest 15NH4+ accumulation, resulting from DNRA, occurred at Rabbit Key during a picocyanobacteria bloom in November. 15NH4+ 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.