A stable isotopic assessment of sources and assimilation of nitrogen by estuarine phytoplankton

Increased delivery of land-derived nitrogen (N) to estuaries alters the ratio of N forms available to phytoplankton. While phytoplankton prefer ammonium (NH4+), eutrophication tends to increase nitrate (NO3-). I used a natural abundance stable isotopic approach to determine whether phytoplankton adjust the amounts of N taken up as NH4+ or NO3- according to the relative availability of the two N forms in the environment.; First, since benthic regeneration of organic matter provides a major source of N for phytoplankton, I incubated sediment cores from a salinity gradient in Childs River, on Cape Cod, to determine the flux and delta 15N of regenerated NH4+ and loss of NO 3- to denitrification. Benthic NH4 + flux increased with temperature and was depressed at high salinity. delta 15N of regenerated NH4+ averaged +10.1+ 0.5‰, and ranged from +6.1 to +15.3‰, but did not vary significantly with season or salinity. 34 to 65% of remineralized NH4 + underwent nitrification-denitrification, but the net benthic NH 4+ flux (1.4 mol N m-2yr-1) could provide 42% of phytoplankton N requirements.; To assess whether the relative availability of NH4 + and NO3- impacts the form of N assimilated by phytoplankton, I compared results from Childs River, where NO3 - concentrations were high, with the Tijuana River, CA, where NH 4+ was elevated.; Childs River phytoplankton assimilated 53-97% of their N as NH4 +, although NO3- was generally much more available. Dissolved inorganic nitrogen (DIN) decreased with increasing salinity and was dominated by NO3-. The delta 15N of DIN indicated that wastewater was the major source of NO 3- to the estuary, while regeneration supplied NH 4+.; Although NH4+ concentrations in the Tijuana River were elevated compared to Childs River, phytoplankton did not increase the relative amount of N assimilated as NH4+ in accordance with its increased availability. NH4+ concentrations were up to 3000 times higher than NO3- concentrations, and averaged ∼200 times higher. DIN delta 15N values indicated that raw sewage inputs were the major source of DIN to the estuary. Although high NO3- concentrations are often associated with eutrophic conditions, phytoplankton assimilate most N as NH4+. This paradox implies that the mechanism by which NO3- contributes to eutrophication may be indirect.