Nitrogen isotope fractionation by oceanic zooplankton

Abstract

The ratio of 15N: 14N for particulate matter suspended in oceanic, surface waters is high after recent nitrate depletion and low in the stable, oligotrophic ocean. We hypothesize that zooplankters and other pelagic heterotrophs produce 15N-depleted ammonium and 15N-enriched particulate matter that are, respectively, recycled in and exported from the euphotic zone and thus cause the low values of 15N: 14N in oligotrophic seas. Heretofore, this pattern was attributed to nitrogen-fixation by the phytoplankton. We measured the ratio of 15N: 14N in the bodies and excreted ammonium of zooplankters from the northwest Pacific Ocean and compared these values to the ratio of 15N: 14N for subeuphotic, dissolved nitrate. We report that oceanic zooplankton excrete ammonium that is isotopically light relative to their bodies and subeuphotic nitrate. These results are consistent with our hypothesis and the view that the phytoplankton of oligotrophic seas is nourished primarily by nitrogen recycled within the euphotic zone. Nitrate injected into the euphotic zone may be manifest and hence detected by an increase of the ratio 15N: 14N for the particulate matter suspended therein.