Chemical Oceanography
II. Chemical Sources

The MBNMS receives its source waters from the California Current during most of the year and the Davidson Current in late summer and fall. Along with these currents, the surface waters receive nutrient input from wind-induced upwelling that usually occurs in spring and summer. These oceanographic features are described in the Physical Oceanography section and in Breaker and Broenkow (1989).

Several freshwater inputs and two major estuaries also contribute to the nearshore chemical characteristics of the Sanctuary. The major freshwater sources are from the Sacramento and San Joaquin rivers which enter the Sanctuary through the San Francisco Bay estuary. These source waters are substantially impacted by the diversion of water to Southern California and agricultural activities in the Sacramento and Central Valleys. The chemistry is modified substantially during mixing processes occurring in the extensive estuary of the anthropogenically influenced San Francisco Bay (Flegal et al. 1991). Input from the Salinas and coastal range rivers, however, are not substantially chemically modified by an estuary prior to entering the MBNMS. These freshwater inputs comprise minor sources of chemical constituents to the Sanctuary (NOAA 1992).

In addition to the surface water sources of dissolved and particulate chemical species, several benthic sources have also been identified. Among these are the flux of metals from sediments. Johnson and co-workers (Johnson et al. 1992) have found that metal fluxes from the sediments are elevated in regions of high rates of organic carbon remineralization. These high organic carbon flux/remineralization regions are found in the upper shelf waters which are typical throughout the MBNMS. Contrary to previous belief, the flux of metals (manganese, cobalt, iron and copper) is lower in the deeper waters normally associated with the oxygen minimum zone.

Cold seep venting has also been recently discovered within Monterey Bay. This venting is thought to result from a variety of geological processes including sediment compression (as in accretionary wedges) and submarine aquifer outcropping (see Geology and Cold Seep Communities sections). The extent of these venting sites is not yet known, but they may contribute significant levels of iron, manganese, sulfide and silica to the immediate vicinity where they have been shown to support local chemosynthetic communities (Ferioli et al. 1994; Cold Seep Communities section). The isotopic composition of nitrogen in clam tissue off the Monterey Peninsula is being examined as a possible indicator of uptake of dietary nitrogen from cold seeps (Rau et al. 1990). Although cold seep fluids do not propagate far from their origins (Ferioli et al. 1994) and the abundance of cold seeps within the Sanctuary has not yet been determined, extensive cold seep venting could have some influence on the chemical composition of near bottom waters.

Atmospheric sources of chemical constituents to the Sanctuary are minimal due to the prevailing onshore winds but occasional high terrigenous inputs accompany offshore winds during the fall and winter (Breaker and Broenkow 1989). Pollution sources, identified and described by California Department of Fish and Game's Marine Pollution ('Mussel Watch') program, include sewage outfalls and runoff associated with agricultural drainage. These sources are described in the Water Quality section and publications by the Regional Water Quality Control Board and the MBNMS Water Quality Protection Program.