Temporal variability in nitrification rates and related biogeochemical factors in Monterey Bay, California, USA

The scales of temporal variability in the rate of ammonium oxidation and a suite of related biogeochemical parameters were investigated in the surface layer of the water column of Monterey Bay, California, on the west coast of the USA. Samples were collected on bimonthly (approx. every 2 mo) cruises during the course of 2 yr. The signal of El Nio was evident in the water column temperature and nitrate distributions during the initial cruise in February 1998. Throughout the 2 yr period, variation in temperature and salinity was small and consistent with seasonal upwelling. The biological parameters, however, varied much more than might have been predicted by the minimal variability in hydrographic signals. Ammonium assimilation rates, chlorophyll a concentrations and bacterial abundance varied by 41-, 65-and 147-fold, respectively. Ammonium oxidation rates often exhibited a subsurface maximum near the bottom of the euphotic zone, and varied by a factor of 4.5 over the 2 yr study. The presence of the rate maximum was not obviously related to the dominant seasonal signals of hydrography or to other biological variables. Nitrification rates were much less variable in the bay than in Elkhorn Slough, a small estuary that opens onto the bay. The small variability in ammonium oxidation rates may be related to more dynamic responses in the community composition of the ammonia-oxidizing bacteria. Different groups of ammonia-oxidizing bacteria were detected as dominants in clone libraries from the Monterey Bay and Elkhorn Slough. The time scale of bimonthly sampling was evidently inadequate to resolve the response of nitrification rates to direct physical forcing in the bay.