Suttle, C.A., J.A. Fuhrman and D.G. Capone. 1990. Rapid ammonium cycling and concentration-dependent partitioning of ammonium and phosphate: implications for carbon transfer in planktonic communities. Limnology and Oceanography 35:424-433.

A short-lived radioactive isotope of nitrogen ¹³N, half-life = 10 min) of very high specific activity (> 100 mCi/ µmol) was used to study ammonium uptake at near-ambient concentrations in natural planktonic communities. The turnover times of the dissolved ammonium 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. Up to 50% of the ambient ammonium flux into particulate material from Long Island Sound was attributable to organisms passing 1.0- µm polycarbonate filters. Partitioning of ammonium uptake among size classes was, however, very concentration-dependent. A 0.5 µM addition resulted in an increase in ammonium uptake by the > 3- µm size class of from 33% to > 80% of the total. Because slight increases in ammonium and phosphate 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.