Physical, chemical and biological controls of nutrient fluxes from fine-grained, organic-rich sediments in the Indian River Lagoon, Florida

Releases and biogeochemical controls of dissolved nitrogen and phosphorus from fine-grained, organic-rich sediments in the Indian River Lagoon, Florida, were determined using (1) interstitial water chemistry, (2) laboratory incubations and experiments, and (3) in situ chambers. Fluxes of nitrogen, essentially all as ammonium ions, and phosphorus, essentially all as orthophosphate ions, averaged 2000 ± 1000 and 130 ± 90 µmol/m²/day, respectively. This internal recycling of ammonium and phosphate from fine-grained, organic-rich sediments that comprise at least 10% of the sediments throughout the northern lagoon total 300 metric tons/yr and 50 metric tons/yr, respectively, and were greater than external inputs to this system. Ammonium fluxes varied spatially in response to physical and chemical differences in sediment composition. Seasonal and experimental changes in temperature resulted in a >50% differences in fluxes of ammonium and phosphate. High fluxes of dissolved sulfide supported dense mats of sulfur-oxidizing bacteria that provided stability to an otherwise unconsolidated fluff layer; this negative feedback loop reduced the net flux of nitrogen and phosphorus into the overlying water.