Unusual nitrification patterns in a eutrophic coastal basin analyzed by geochemical, isotopic (δ15N) and microbiological data from a highly-resolved in situ time series.

Sebastian Haas1, Brent M. Robicheau2, Julie LaRoche2, Tim Kalvelage3, Subhadeep Rakshit3 and Douglas Wallace3, (1)University of Washington, Earth and Space Sciences, Seattle, WA, United States, (2)Dalhousie University, Department of Biology, Halifax, NS, Canada, (3)Dalhousie University, Department of Oceanography, Halifax, NS, Canada
Nitrification, the microbially catalyzed oxidation of ammonium to nitrate via nitrite, is central for the remineralization of the macronutrient nitrogen. Novel aspects of this process (e.g. “comammox”, complete nitrification catalyzed by a single microorganism) are still being discovered. Importantly, the environmental conditions that favor individual steps of nitrification (ammonium oxidation versus nitrite oxidation), or nitrification compared to competing processes (e.g. ammonium assimilation), remain insufficiently understood.

Here we present analyses of a multi-variable dataset from a multi-year, weekly-resolved time series in the coastal Bedford Basin, NS, Canada. The eutrophic, fjord-like basin experiences annual Winter convection (~January-April), during which the entire water column (71 m) overturns, followed by stable Summer stratification that can last until the end of a given year. In some years, nitrate accumulation in the bottom water (60 m) starts immediately after the end of Winter convection and continues throughout the stratified period. Interestingly, in other years there are week- to month-long delays with little or no nitrate accumulation during the stratified period, despite an abundance of ammonium (>1 μM). In years with delayed nitrification, the stratified period can be separated into an ‘ammonification period’ of net ammonium accumulation and a subsequent ‘nitrification period’ of net ammonium removal and accumulations of nitrate and, in some years, nitrite. A combination of time-series generated geochemical (nutrient concentrations, isotopic composition (δ15N) of nitrate, nitrite, ammonium), physical (e.g. temperature, salinity), and molecular (16S rRNA gene sequencing and quantitative PCRs of amoA, a marker gene for ammonium oxidation) data were used to analyze our observations. Correlations with potentially linked forcing parameters have been found and will be presented.