Isotopic Approach to Determining the Fate of Ammonium Regenerated from Sediments in a Eutrophic Sub-estuary of Waquoit Bay, MA

摘要

We measured fluxes of NH_4~+ and NO_3~- and δ~(15)N of NH_4~+, sediment, and porewater NH_4~+ from incubated sediment cores along a nitrate gradient and in different seasons from Childs River, MA. NH_4~+ 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 δ~(15)N of regenerated NH_4~+ ranged from +6.1‰ to +15.3‰ but did not vary significantly with season or salinity; the mean for the entire estuary was +10.4 ± 0.5‰. Based on differences between the δ~(15)N of regenerated NH_4~+ and sediment, and expected isotopic fractionation due to remineralization, we concluded that nitrification occurred after remineralization of NH_4~+. Differences between the δ~(15)N of regenerated NH_4~+ and the δ~(15)N of porewater NH_4~+ provided further evidence of nitrification. We estimated that 11% to 48% of remineralized NH_4~+ underwent coupled nitrification-denitrification before release into the water column. In spite of losses to denitrification, NH_4~+ flux released 1.4 mol N m~(-2) year~(-1) to the water column and could provide 42% of phytoplankton nitrogen requirements.