Net nitrogen uptake and DON release in surface waters: Importance of trophic interactions implied from size fractionation experiments

Factors that influence the release of DON by planktonic assemblages were investigated using size fractionation experiments in the Southern California Bight and in Monterey Bay, California, USA. Incubation experiments, with either ¹⁵NH₄⁺ or ¹⁵NO₃^- as a tracer, were used to measure rates of net uptake (incorporation of DIN into particulate nitrogen [PN]) and DON release (production of DON during the incubation, through both active and passive mechanisms). DON release varied greatly among experiments and was higher when ¹⁵NO₃^- was the substrate; it accounted for 3 to nearly 100% of the gross uptake (net uptake plus DON release). Compared with incubations with the <10 μm fraction alone, addition of the <210 μm fraction resulted in distinct patterns of change in net uptake and DON release. Whether the large fraction caused an increase or a decrease in net uptake or DON release rates probably depended on species composition as well as size distribution of trophic groups. DON release rates were positively correlated with NH₄⁺ regeneration rates (p<0.00001) and the magnitude of DON release was about 40 % that of NH₄⁺ regeneration. The results imply that zooplankton and protozoan grazing - on primary producers, small heterotrophic plankton, and possibly bacteria - is a major mechanism of DON release. DON release was observed in every experiment and is clearly an important nitrogen flux in planktonic communities. If grazing is responsible for DON release, then standard incubation experiments probably underestimate the growth of phytoplankton by not accounting for grazing during incubations. The observed rates of DON release imply a rapid turnover of at least a portion of the total DON pool.