Stable isotopic detection of ammonium and nitrate assimilation by phytoplankton in the Waquoit Bay estuarine system

We measured concentration and delta N-15 of chlorophyll a (Chl a), NO3-, and NH4+ along a salinity gradient in Childs River, Massachusetts, in winter, spring, and summer. We used the delta N-15 of Chl a as a proxy for the phytoplankton delta N-15 to minimize potential ambiguities from other material in seston. NO3- concentration ranged from 0 to 50 mu mol L-1 and NH4+ from 0 to 8 mu mol L-1; both forms decreased with increasing salinity. NO3- concentration was generally higher than NH4+. Chl a concentrations ranged between 1 and 15 mg m(-3) in winter-spring and had a summer midestuarine peak of 95 mg m(-3). The delta N-15 of NO3- and NH4+ ranged from -10%(0) to +7%(0) and -3%(0) to +13%(0), respectively, and decreased approximately linearly with increasing salinity. The delta N-15 of NO3- reflected the predominance of groundwater as the source of NO3- to the estuary, whereas the delta N-15 of NH4+ indicated that regeneration was the main NH4+ source. Throughout the estuary, NO3- was isotopically lighter than NH4+. Phytoplankton delta N-15 increased from winter to summer and was relatively invariant with salinity, in contrast to the delta N-15 of dissolved inorganic nitrogen. A comparison of the delta N-15 of phytoplankton, NO3-, and NH4+ indicated that phytoplankton in Childs River derived 53% to 97% of their N from NH4+. Phytoplankton acquired their stable nitrogen isotopic ratio upstream, then maintained that ratio during downstream transport. The fractionation factor for phytoplankton NH4+ uptake was +4.0%(0) +/- 0.6%(0), which was in the lower range of other estimates, indicating that phytoplankton might have been N limited.