Stable isotopic detection of ammonium and nitrate assimilation by phytoplankton in the Waquoit Bay estuarine system
York, Joanna K., Gabrielle Tomasky, Ivan Valiela, and Daniel J. Repeta
Limnol. Oceanogr., 52(1), 2007, 144–155

We measured concentration and d¹⁵N of chlorophyll a (Chl a), NO₃^-, and NH₄⁺ along a salinity gradient in Childs River, Massachusetts, in winter, spring, and summer. We used the d¹⁵N of Chl a as a proxy for the phytoplankton d¹⁵N 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 d¹⁵N of NO₃^- and NH₄⁺ ranged from –10‰ to +7‰ and –3‰ to +13‰, respectively, and decreased approximately linearly with increasing salinity. The d¹⁵N of NO₃^- reflected the predominance of groundwater as the source of NO₃^- to the estuary, whereas the d¹⁵N of NH₄⁺ indicated that regeneration was the main NH₄⁺ source. Throughout the estuary, NO₃^- was isotopically lighter than NH₄⁺. Phytoplankton d¹⁵N increased from winter to summer and was relatively invariant with salinity, in contrast to the d¹⁵N of dissolved inorganic nitrogen. A comparison of the d¹⁵N 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.