A Method for Estimating Uptake and Production Rates for Urea in Seawater using [14C] Urea and [15N] Urea

Abstract

Improved estimates of the rates of urea production and uptake by natural populations of phytoplankton were made after determining the change in 15N-atom% enrichment of urea during incubations. A [14C] urea method is described by which the change in enrichment is measured. Estimates of uptake rates are increased (relative to uptake rates determined without correction for isotope dilution) by up to 83% using a 15N accumulation model and by >210% using a 15N disappearance model. A discrepancy exists between [15N] urea removed from the aqueous phase and 15N accumulated in the particulate phase at stations occupied in the northeastern Bering Sea. The ability to find in the particulate fraction the 15N removed from solution as [15N] urea was improved by 72% following removal of the >20-μm particulate fraction. This corresponded to only a 4% reduction in the concentration of chlorophyll a and a 37% reduction in the concentration of particulate N. Removal of microzooplankton may have improved the efficiency of urea-N retention by phytoplankton.