Isotopic Approach to Determining the Fate of Ammonium Regenerated from Sediments in a Eutrophic Sub-estuary of Waquoit Bay, MA
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We measured fluxes of NH4+ and NO3- and delta N-15 of NH4+, sediment, and porewater NH4+ from incubated sediment cores along a nitrate gradient and in different seasons from Childs River, MA. NH4+ flux was low at the downstream site with the lowest concentration of organic matter (high salinity) but otherwise did not differ along the estuary. The delta N-15 of regenerated NH4+ ranged from + 6.1 parts per thousand to +15.3 parts per thousand but did not vary significantly with season or salinity; the mean for the entire estuary was +10.4 +/- 0.5 parts per thousand. Based on differences between the delta N-15 of regenerated NH4+ and sediment, and expected isotopic fractionation due to remineralization, we concluded that nitrification occurred after remineralization of NH4+. Differences between the delta N-15 of regenerated NH4+ and the delta N-15 of porewater NH4+ provided further evidence of nitrification. We estimated that 11% to 48% of remineralized NH4+ underwent coupled nitrification-denitrification before release into the water column. In spite of losses to denitrification, NH4+ flux released 1.4 mol N m(-2)year(-1) to the water column and could provide 42% of phytoplankton nitrogen requirements.