Evaluating sediment denitrification and water column nitrification along an estuary to offshore gradient
Heiss, Elise Michelle
MetadataShow full item record
Humans have dramatically increased the amount of reactive nitrogen cycling through the biosphere. In coastal systems, excess nitrogen can lead to negative impacts. Thus, it is crucial to understand how nitrogen is cycled within, and eventually removed from, marine systems and the variables that regulate these processes. Sediment denitrification (the microbial conversion of nitrate (NO3^-) to dinitrogen (N2) gas) and water column nitrification (the two step oxidation of ammonium (NH4^+) to nitrite (NO2^-) and then nitrate (NO3^-)) rates were quantified along an in situ gradient of environmental conditions from an estuary to the continental shelf off Rhode Island, USA. Sediment net denitrification rates were directly measured over multiple seasonal cycles using the N2/Ar technique. Denitrification rates ranged from 20-75 μmol m^-2 hr^-1 (mean 44±4), indicating that this process removes ~5% of total reactive nitrogen entering the North Atlantic shelf region per year. Based on model results, these rates also represented a three-fold decrease in sediment nitrogen removal in New England continental shelf sediments over the past century. A literature review of marine water column nitrification observations were compiled to evaluate how ammonium, nitrite, and total oxidation rates vary worldwide. Rates of ammonium, nitrite, and total oxidation differed among estuary, continental shelf, and open ocean environments (p<0.05). This review highlights that as we continue to study marine "nitrification," it is necessary to consider both individual oxidation processes and environment type. Water column ammonium and nitrite oxidation rates were measured using stable isotope tracers off Rhode Island. At all study sites, nitrite oxidation rates (0-99 nM d^-1) outpaced ammonium oxidation rates (0-20 nM d^-1). These oxidation processes responded in dissimilar ways to in situ water column conditions (depth, salinity, dissolved oxygen, and pH), and these relationships varied with location. Nitrous oxide (N2O) production rates up to 10 times higher than ammonium oxidation indicated that ammonium oxidation may be underestimated if this byproduct is not measured. For the first time, the link between sediment metabolism and water column nitrification was also examined, and the results highlight the importance of benthic-pelagic coupling as controlling factor of water column ammonium and nitrite oxidation.