Nitrogen dynamics at the VERTEX time-series site

Abstract

Euphotic zone concentrations and fluxes of nitrate, ammonium, particulate and dissolved organic nitrogen were measured over an 18 month period at the VERTEX time-series site in the oligotrophic northeast Pacific (33°N, 139°W). Variations in all N-forms were significant but not clearly linked to the temporal hydrographic cycle. Inorganic-N uptake (nitrate + ammonium) from ¹⁵N tracer experiments generally paralleled primary productivity variations, peaking in summer; ammonium accounted for most of the uptake (∼90%) and temporal variability. Comparisons of ¹⁵N results with estimates of autotropic N-uptake from ¹⁴C incorporation into protein suggest that as much as 40% of the annual inorganic-N uptake was due to microheterotrophs; peak heterotrophic N-uptake occurred in summer when heterotrophic biomass was at its maximum. Nitrate uptake (new production) was less variable than ammonium uptake and annually equivalent to particulate nitrogen export from sediment traps. Dissolved organic-N (DON) represented the largest and most variable N-pool, accounting for 80–90% of the total nitrogen in the euphotic zone. Vertical DON gradients, however, were small and temporally invariant, implying little contribution to the biogenic nitrogen export from the euphotic zone.