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

We measured concentration and δ 15 N of chlorophyll a (Chl a), NO 3 ‐ , and NH 4 + along a salinity gradient in Childs River, Massachusetts, in winter, spring, and summer. We used the δ 15 N of Chl a as a proxy for the phytoplankton δ 15 N to minimize potential ambiguities from other material in seston. NO 3 ‐ concentration ranged from 0 to 50 µmol L21 and NH 4 + from 0 to 8 µmol L ‐1 ; both forms decreased with increasing salinity. NO 3 ‐ concentration was generally higher than NH 4 + . 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 δ 15 N of NO 3 ‐ and NH 4 + ranged from ‐10‰ to +7‰ and ‐3‰ to +13‰, respectively, and decreased approximately linearly with increasing salinity. The δ 15 N of NO 3 ‐ reflected the predominance of groundwater as the source of NO 3 ‐ to the estuary, whereas the δ 15 N of NH 4 + indicated that regeneration was the main NH 4 + source. Throughout the estuary, NO 3 ‐ was isotopically lighter than NH 4 + . Phytoplankton δ 15 N increased from winter to summer and was relatively invariant with salinity, in contrast to the δ 15 N of dissolved inorganic nitrogen. A comparison of the δ 15 N of phytoplankton, NO 3 ‐ , and NH 4 + indicated that phytoplankton in Childs River derived 53% to 97% of their N from NH 4 + . Phytoplankton acquired their stable nitrogen isotopic ratio upstream, then maintained that ratio during downstream transport. The fractionation factor for phytoplankton NH 4 + uptake was +4.0‰ ± 0.6‰, which was in the lower range of other estimates, indicating that phytoplankton might have been N limited.