Nitrate-based primary production in nutrient-depleted surface waters off California

Biological models of oceanic, subtropical and tropical surface waters conceive of a euphotic zone bisected by the nutricline forming two layers, a nutrienf-depleted but light-rich Upper layer and a nutrient-rich but light-poor lower layer. New production, driven by nitrate, and the generation of the sinking flux of biogenic particles take place primarily in the lower layer where nitrate is present. This study used a chemiluminescent analyzsr (GARSIDE, 1982) to show that nitrate is present ut nanomolar concentrations above the nitracline in southern California coasfal waters. The nitrate concentrations often decreased over time on incubation of water samples implying its utilization by phytoplankton. This utilization appears to drive a low but significant rate of nitrate-based production in fhe Upper layer. Depending upon thc source of the nitrate this production may be new or regenerated production.