Rapid ammonium cycling and concentration‐dependent partitioning of ammonium and phosphate: Implications for carbon transfer in planktonic communities

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Abstract
A short‐lived radioactive isotope of nitrogen (13N, half‐life = 10 min) of very high specific activity (>100 mCi µmol−1) was used to study NH4+ uptake at near‐ambient concentrations in natural planktonic communities. The turnover times of the dissolved NH4+ pool in Long Island Sound ranged from tens of hours between April and early June to a fraction of an hour between mid‐June and late July. The NH4+ turnover time was also rapid in a nearby eutrophic lake. Up to 50% of the ambient NH4+ flux into particulate material from Long Island Sound was attributable to organisms passing 1.0‐µm polycarbonate filters. Partitioning of NH4+ uptake among size classes was, however, very concentration‐dependent. A 0.5 µM addition resulted in an increase in NH4+ uptake by the >3‐µm size class of from 33% to >80% of the total. Similar results were obtained for PO43− uptake into particulate matter from the Sargasso Sea. Although 1 µm, a 100 nM pulse resulted in about 50% of the uptake entering the >1‐µm fraction. Because slight increases in NH4+ and PO43– concentration result in greater uptake by larger organisms with higher C : N and C : P ratios, distributing these nutrients in “patches” of elevated concentration results in more C being transferred to higher level consumers.