Nitrogen transformations in the Southern California Bight

Oxidation and assimilation of ammonium and nitrate were measured in profiles from the surface to 1000 m at four stations in the Southern California Bight. Assimilative processes dominated the turnover of these nitrogen compounds in the nutrient depleted photic zone; however, at the bottom of the photic zone, nitrification and assimilation were equally important in the turnover of both compounds. Although ammonium oxidation was strongly inhibited by light in the surface samples, nitrification probably contributed to rapid nitrogen cycling within the photic zone. Assimilation of ammonium and nitrite, integrated over the photic zone, was about 3-fold greater than calculated regenerated production, estimated from ¹⁴CO₂ uptake measurements. Integrated nitrification rates below the photic zone usually exceeded new primary production estimates. Most ammonium oxidation occured in a discrete maximum around the depth of 0.2% surface light intensity; rates within the maximum exceeded rates in shallower and deeper samples by up to 80-fold. Nitrite oxidation rates exceeded ammonium oxidation rates, and did not display the prominent subsurface maximum found in the former. Abundance of both ammonium and nitrite oxidizing bacteria varied less with depth than did oxidation rates. Autotrophic activity of the nitrifiers, measured by ¹⁴C-autoradiography combined with immunofluorescent counts, was minimal in the surface samples, and increased with depth, but was not directly correlated with nitrification rates, measured by ¹⁵N techniques.