IV. Water Column Characteristics and Processes
Nutrients are subject to dynamic temporal variability (Figure 1; Broenkow and McKain 1972; Shea and Broenkow 1982). The main cause of the seasonal differences in nutrients is variation in wind-driven upwelling. When the wind blows south along the coast, an offshore transport of surface waters results. This water is replaced with cold nutrient-rich water from a depth of 25-300 m (Breaker and Broenkow 1989; and see Physical Oceanography section). Surface NO3 is between 0-2 µM during non-upwelling periods, and up to 30 µM during strong upwelling. Processes which affect upwelling therefore affect surface water nutrient concentrations. For example, at a deep water station located in the middle of the Monterey Bay (CALCOFI station 3, 36°46.7'N 122° 01.3'W), Chinburg and Lasley (1977) found a surface temperature of 14.52°C with a NO3 concentration of 2.4 µM during a non-upwelling period. During an upwelling period they found a surface temperature of 10.12°C with a NO3 concentration of 24.1 µM. In shallower water close to shore and over shelves, the effect of upwelling is not as pronounced. In these regions, the upwelled water comes from above the nutricline with a corresponding lower concentration of nutrients. The upwelling periods in Figure 1 (Smethie 1973) have surface NO3 concentrations between 1-5 µM due to the shallow bottom depth.
During non-upwelling periods, internal tides can have a major effect on nutrient concentrations nearshore (Broenkow and McKain 1972; Shea and Broenkow 1982; and see Physical Oceanography section). Within 10 km of the canyon head, in water depths of 100-400 m, bottom currents are aligned predominantly along the canyon axis of which the strongest component is tidal (Broenkow and McKain 1972). The fluctuation of the tides has been observed to cause internal tide heights of 50-120 m (Shea and Broenkow 1982). At high internal tide, nutrient-rich water from the canyon is forced over the sill into shallower parts of the Bay. At one station north of the canyon, Shea and Broenkow (1982) observed a 68% increase in the concentration of PO4 between the periods of high and low internal tide. In northern Monterey Bay, Shea and Broenkow (1982) estimated that the transport of nutrient-rich water could result in an increased euphotic zone productivity of 0.6 g C/m2/day.
As can be seen in the examples above, nutrient dynamics in the MBNMS are influenced by a number of factors which can strongly and quickly affect nutrient concentrations and distributions, especially in Monterey Bay. These spatial and temporal changes in nutrients have profound effects on the productivity of the region. Unfortunately, these complex patterns cannot readily be resolved using conventional discrete methods of shipboard sampling and analysis. Recently, researchers at Moss Landing Marine Laboratories and the Monterey Bay Aquarium Research Institute have pioneered the development of long term in situ chemical sensors based on the principle of osmotic pumps and colorimetric analysis (Jannasch et al. 1994). These osmotically pumped submersible chemical analyzers, "Osmoscanners," have been deployed in the Monterey Bay for months at a time and reveal high frequency, dynamic trends in the temporal distribution of nitrate at the Oasis mooring site (Johnson and Jannasch 1994). Future deployments of such analyzers will improve our understanding of nutrient dynamics in the MBNMS.
Recent advances in the development in analytical procedures and instrumentation, coupled with ultra-clean techniques for sampling and analysis, have given scientists the first reliable measurements of trace metal distributions from ocean waters (Bruland 1983). Since these advances have been so recent, credible historical trace metal data for Monterey Bay and environs is limited. In general the trace metal chemistry of the MBNMS is characteristic of the waters of the California Current. As such, the trace metal concentrations offshore reflect those values except where perturbed by nearshore point sources. Some of the reported surface water trace metal concentrations for stations in the MBNMS are given in Table 1. Metals, such as iron, may even be limiting phytoplankton production in some MBNMS areas (Johnson et al. 1993).
Trace metals, such as cadmium, zinc and nickel, exhibit nutrient-like profiles with low concentrations at the surface and deep water enrichments. These profiles result from biological and chemical processes and lead to strong linear correlations between trace metals and nutrients. The trace metal to nutrient ratios within the MBNMS are shown in Table 2 and are similar to ratios found in open ocean areas. These relationships are, therefore, diagnostic of allochthonous (i.e. transported to local region) metal inputs and may be useful in identifying metal pollution in MBNMS waters.
Biological processes can strongly influence the oceanic speciation of trace metals, particularly manganese, iron, cobalt, nickel, copper and zinc (Bruland et al. 1991). Of these, iron, zinc and manganese are often transferred and recycled between different classes in the plankton community (Hutchins and Bruland 1994,1995; Hutchins et al. 1993,1995). Other trace metals can be organically complexed. Organic acids can function as ligands which bind trace metals, thereby making them biologically unavailable to phytoplankton. Research indicates that phytoplankton and bacteria influence water column chemistry by producing metal-binding ligands. Without these ligands, which bind as much as 99% of some trace metals, dissolved metal ion concentrations in the MBNMS could be toxic to many of its organisms (Sunda 1990). To date only a few studies of trace metal complexation have been performed in the bay (Bruland 1989,1992a,1992b; Bruland et al. 1991; Coale 1988; Coale and Bruland 1989, 1990; Capadaglio et al. 1990). These studies indicate that the majority of the metals in the MBNMS are organically complexed, which result in phytoplankton body burdens comparable to open ocean values (Martin and Knauer 1973).
The organic constituents in the water column are largely impacted by organic matter that enters via runoff or is produced within the MBNMS by its marine life. Dissolved organic carbon (DOC) levels in Monterey Bay can reach levels of 123 µM near the surface (Hansell et al. 1993) and may significantly influence trophic interactions within the water column.
The composition of particular organic carbon (POC) in the water column has not been thoroughly charcterized. Those specific constituents that have been determined include amino acids (18-90% of organic carbon), wax esters (<1-20%), lipids (5-20%), triacylglycerols (<1-3%) and fatty acids (<1-11%), and their rate of flux in MBNMS waters is characteristic of that found in other regions of the Eastern Pacific (Lee and Cronin 1984, Wakeham et al. 1984).
Anthropogenically introduced POC and DOC compounds include synthetic organics and aromatic hydrocarbons such as are found in pesticides and petroleum products. Typical water column concentrations in the MBNMS are in the low picogram per litre range for organochlorines and the low nanogram per litre range for petroleum (de Lappe et al. 1983).
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