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.
Next - Section III. Chemical Sinks
Chemical Oceanography Table of Contents