Isotope-dilution and its effects on measurements of nitrogen and phosphorus uptake by oceanic microplankton

Abstract

Nitrogen (NH,+) and phosphorus (pod3-) uptake and recycling were measured in time-series exper~ments in waters off the coast of Hawaii and in the Sargasso Sea. Patterns of uptake and recycling were similar but regionally distinct. In H a w a ~ i , uptake rates were initially rapid but essentially ceased after only a few hours in lnshore experiments; recycling rates continued for 24 h. Offshore, recycling rates were variable but decreased significantly with incubation time; uptake, on the other hand, continued for 24 h and paralleled photosynthetic carbon uptake. Uptake and recycling fluxes balanced after 24 h. In the Sargasso Sea, both soluble reactive phosphorus (SRP) uptake and recycling rates decreased wlth incubation time but were also in balance Isotope-dilution, resulting from the in vitro production (recycling) of unlabelled substrate was significant and if ignored, could account for 1.5 to 3-fold underestimates in computed uptake rates.