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

York, Joanna K., Gabrielle Tomasky, Ivan Valiela, Daniel J. Repeta

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.

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