Chemical Oceanography
III. Sinks

Chemical removal from MBNMS waters takes place by a variety of processes, one of the most important of which is nearshore precipitation during estuarine mixing (Sholkovitz 1976). This can dramatically affect the fluxes of chemicals, such as iron, humic acids and fulvic acids which may be rapidly oxidized or precipitated very near shore. Chemicals which are mixed further offshore are incorporated into the biogeochemical cycle of elements which is driven by biological uptake, scavenging onto particles, transport to depth via sinking of marine snow or fecal pellets, and incorporation into the sediments. In addition, several water column processes can influence this cycling, including photochemical reactions in the surface waters (Johnson et al. 1994) and remineralization of detrital constituents on marine snow particles (Steinberg et al. 1994). Studies of chemical scavenging in Monterey Bay indicate that scavenging rates are directly correlated to primary production in the Bay. This leads to short dissolved residence times on the order of a few days and the rapid removal of particle reactive species such as thorium (Coale and Bruland 1985) and iron (Gordon et al. 1982), when production rates are high.

Large primary production and high rates of grazing result in a high particulate organic carbon flux which sweeps the water column of particle-bound chemicals. In particular, it has been shown that the occasional swarms of salps common in the Monterey Bay, can remove essentially all particles of 0.2 microns or larger within three days (Bruland and Silver 1981; Coale and Bruland 1985; Silver and Bruland 1981; and see Pelagic Zone section). More commonly, organisms such as copepods, larvaceans, euphausids and siphonophores are responsible for grazing of phytoplankton and producing fecal pellets with sinking rates of 100 m/day or more. The flux of this material has been measured as part of the Vertex program (Martin et al. 1983). These carbon fluxes measured in the MBNMS range from 440 mg C/m2/day in the upper 50 m to 30 mg C/m2/day at depth of 2000 m. In a more recent study, Pilskaln et al. (1995) report the results of a time series study of carbon flux in Monterey Bay. These values range from 500 mgC/m2/day to 2600 mgC/m2/day and are seasonally coherent showing high particle fluxes at times of high primary production.

The rain of organic material from the surface ocean is directly linked to rates of decomposition in the sediments. However, since the majority of organic carbon reaching the ocean floor is respired at the sediment-seawater interface, carbon burial is not equivalent to carbon flux in the water column. Sedimentation rates are difficult to ascertain within the MBNMS due to its geographical variability, highly episodic environmental perturbations, and the dynamic nature of oceanographic processes. Berelson et al. (1996) reported sediment mass accumulation rates of 1 - 10 mg/cm2/yr in the open ocean parts of the MBNMS whereas they can be up to an order of magnitude higher in the nearshore sections (van Geen 1992). Approximately 60-75% of sediments enter the MBNMS via coastal streams, and the majority of these fluvial sediments are mobilized and deposited within a few kilometers of shore in less than 40 m water depth (Arnal et al. 1973, Best and Griggs 1994). Depositional processes for organic carbon in the offshore regions are less well known. As organic matter sinks from the photic zone, about 75% of the material is transformed by biological processes in the upper water column so that inputs to the deep sea consist mainly of dissolved organic and inorganic materials (Pace et al. 1987). It is difficult to determine the fate of this refractory pool, thus confounding efforts at determining the rate at which organic carbon is incorporated into offshore sediments. In general, it is felt that less than 1% of the autochthonously generated (i.e. of local origin) organic material reaches the pelagic sea floor.