Measuring gross uptake of 15N‐labeled nitrogen by marine phytoplankton without particulate matter collection: Evidence of low 15N losses to the dissolved organic nitrogen pool

Present 15 N tracer techniques allowing indirect estimates of gross uptake of dissolved inorganic nitrogen (DIN) from measurements of DIN net uptake and DIN loss to the dissolved organic nitrogen (DON) pool (DIN gross uptake = DIN net uptake + DIN loss) require collection by filtration of particulate organic nitrogen (PON). We introduce a simple and reliable method that avoids this filtration step and permits direct determination of DIN gross uptake. The method involves (1) collection after 15 N tracer addition and incubation of a 300-ml sample, (2) removal of initial DIN, (3) conversion of total organic nitrogen (PON + DON) to DIN, (4) recovery of formed DIN, and (5) quantification of mass and 15 N enrichment of initial and formed DIN. Gross uptake rates obtained in parallel from experiments without particulate matter collection (WOPMC) and with particulate matter collection (WIPMC) were in good agreement (R 2 = 0.99) with values ranging from about 0.2 nM h-' (open ocean) to 200 nM h -1 (culture). Variation in gross uptake rate in a set of five replicate samples, processed using both the WOPMC and WIPMC methods, was less than 7%. Losses of DIN to the DON pool were found to be negligible in laboratory cultures and represented on average only 15% of gross DIN uptake in the open ocean. Experimental considerations suggest that high DIN losses (≥50% of gross DIN uptake) to the DON pool are unlikely for healthy and actively growing cells and probably are limited to dying cells loosing their organic nitrogen content during postbloom situations (cell lysis) and/or during sloppy feeding by zooplankton (cell rupture).