The Biogeochemical Cycling of Manganese in San Francisco Bay: Temporal and Spatial Variations in Surface Water Concentrations

Pronounced variations in the distribution of dissolved manganese within the San Francisco Bay in 1995/96 reflect spatial and temporal variations in its biogeochemical and hydraulic regimes. In the northern reach of the system (North Bay) dissolved Mn concentrations ranged from 12–210 nM at the freshwater interface, from 35–756 nM at intermediate salinities, and from 86–435 nM at the oceanic interface. At intermediate salinities in the southern reach (South Bay), the range in dissolved Mn concentrations (22–242 nM) was comparable to that of the northern reach. Dissolved Mn concentrations in the extreme South Bay, however, were one to two orders of magnitude higher (2800 nM) during the winter and even greater (5400 nM) during the summer. These high concentrations appear to be due to the relatively intense episodic diagenetic remobilization of Mn from benthic sediments. This proposed diagenetic remobilization of dissolved Mn followed a period of low dissolved Mn concentrations that occurred immediately after the spring bloom, which was attributed to the sorption of Mn onto phytoplankton, bloom-derived flocculent organic detritus, and associated bacteria