Stable isotopic detection of ammonium and nitrate assimilation by phytoplankton in the Waquoit Bay estuarine system
Limnol. Oceanogr., 52(1), 2007, 144-155 | DOI: 10.4319/lo.2007.52.1.0144
ABSTRACT: We measured concentration and δ15N of chlorophyll a (Chl a), NO3-, and NH4+ along a salinity gradient in Childs River, Massachusetts, in winter, spring, and summer. We used the δ15N of Chl a as a proxy for the phytoplankton δ15N to minimize potential ambiguities from other material in seston. NO3- concentration ranged from 0 to 50 µmol L21 and NH4+ from 0 to 8 µmol 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 winterspring and had a summer midestuarine peak of 95 mg m-3. The δ15N of NO3- and NH4+ ranged from -10‰ to +7‰ and -3‰ to +13‰, respectively, and decreased approximately linearly with increasing salinity. The δ15N of NO3- reflected the predominance of groundwater as the source of NO3- to the estuary, whereas the δ15N of NH4+ indicated that regeneration was the main NH4+ source. Throughout the estuary, NO3- was isotopically lighter than NH4+. Phytoplankton δ15N increased from winter to summer and was relatively invariant with salinity, in contrast to the δ15N of dissolved inorganic nitrogen. A comparison of the δ15N 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‰ ± 0.6‰, which was in the lower range of other estimates, indicating that phytoplankton might have been N limited.