Ammonium distribution in southern California coastal waters and its role in the growth of phytoplankton

The average concentration of ammonium in the euphotic zone of the Southern California Bight was about$0.35 \mug-atom \cdot liter^-1$in 1974-1977. Concentrations up to 5-20 μ g-atoms· liter-1 were found in Santa Monica Bay surface and bottom waters. Elsewhere maximum concentrations were 2-3 μ g-atoms · liter-1 at the bottom of the euphotic zone of inshore stations. Ammonium provided an average of 35% of the nitrogen assimilated by the phytoplankton and a much higher proportion in Santa Monica Bay. The rate of ammonium assimilation was modified primarily by the phytoplankton standing stock, and also by ambient concentration of ammonium, temperature, irradiance, and the carbon:chlorophyll a ratio of the phytoplankton. These variables accounted for 52% of the explained variance in three "winter" cruises and 83% in a summer cruise. Remineralization processes account for most of the ammonium in the bight, while human inputs account for about 10%.