3.1 General Description of the Marine Environment 3.1.1 Weather, Oceanography, and Geology Weather Conditions Along the Coast Of

Chapter 3. Environmental Settings 3.1 General Description of the Marine Environment 3.1.1 Weather, Oceanography, and Geology Weather conditions along the coast of California are influenced by oceanographic conditions of the eastern Pacific Ocean boundary current region. Ocean currents can be thought of as a simple combination of the geostrophic current field and the field of Ekman drift (Bakun and Parrish 1980). Large water masses, principally wind-driven, rotate in a general clockwise direction in the North Pacific due to the Coriolis effect, which results from the earth’s rotation. The dominant oceanographic feature of the eastern Pacific boundary is the California current, a broad, slow-moving current that originates about 500 km off the Oregon, Washington and southern British Columbia coasts between 45° and 50° N latitude (Hickey 1979, Williams et al. 1980). Driven primarily by wind stress patterns over the North Pacific, it flows southward in a band 500-1,000 km wide and 100-500 m deep at a mean speed of 10-30 cm/sec (Hickey 1979, Williams et al. 1980). Water in the California Current is characterized by low temperatures and low salinity; near the coast and north of Cape Mendocino it originates primarily from the west wind drift and is primarily subarctic in type (Hickey 1979, Williams et al. 1980). The percentage of subtropical water increases towards the south and west (Hickey 1979). The California Current is characterized by large flow variability, and the mean southward flow is only in a large-scale sense (Bernstein et al. 1977, Owen 1980, Parrish et al. 1981). In contrast to the cold current moving southward along the coast, there is an intermittent northerly moving inshore counter-current, called the Davidson Current consisting of semitropical warm waters moving in a general northwesterly direction. The colder current is predominant along the California coast, especially north of Point Conception, because prevailing winds and currents in that area come from the north and northwest. During fall and winter months, however, these winds weaken and the warm Davidson Current becomes more predominant (Department of Navigation and Ocean Development, 1971). Inshore of the dominant currents, a general upwelling or rising of subsurface waters occurs seasonally around islands and headlands along the coast. Upwelling along the west coast results from the interaction of the California Current and the winds generated by the North Pacific High (Hickey 1979). Due to the Coriolis effect, these northwesterly along-shore winds entrain surface waters to the west, or away from the coast, a process known as Ekman transport (Thurman 1975, Beer 1983). The transported water is replaced by cold, nutrient-rich, subsurface water. Water is upwelled from depths greater than 60 m south of Cape Mendocino (Huyer 1983). The region of maximum southward wind stress shifts northward from around 25° N in January to about 39° N in July; the strongest winds are observed in July off northern California, where the offshore pressure gradient is steepest. South of San Francisco, upwelling may occur year-round, with a peak in April (Huyer 1983). The offshore extent of the primary upwelling zone appears to be 10-20 km along the entire coast (Parrish et al. 1981, Huyer 1983). Upwelling is extremely important for the productivity of our coastal waters because the rising water brings nutrient salts into the lighted layers, which results in the proliferation of phytoplankton, the basis of the marine food chain. The two most conspicuous centers of upwelling along the California coast occur at Point Conception (35°N) and Cape Mendocino (41°N) (Sverdrup, Johnson & Fleming, 1942; Jones and Stokes Associates, Inc., 1981). 3-1 The coastal climate is primarily controlled by moisture-laden, prevailing northwesterly winds sweeping on shore from the semipermanent Pacific Anticyclone, a high pressure area known as the North Pacific High. The winds resulting from the pressure gradient between the North Pacific High and a low pressure area over the desert Southwest begin in April and continue until the fall. At this time the North Pacific High moves southward reaching about 28° N latitude in February (Huyer 1983). The air temperature variations between day and night are normally small, summers are cool, winters are moderately warm, and there is considerable fog. From south to north, air temperature variations increase and there is a greater contrast between summer and winter. Fog is more frequent and lasts longer in the north (De Santis, 1985). Rainfall patterns vary with latitude and altitude all along the California coast, with mountainous areas receiving much more rain than lower altitudes. Northcoast areas average about 30-40 inches per year. In contrast, the annual rainfall along the southern California coast averages about 12 inches per year. San Francisco, located approximately in the middle of these extremes, receives about 18 inches of rain a year. The rainy season normally begins by late September, with the greatest precipitation occurring in December and January. The dry season starts about June. Snowfall is uncommon along the coast, except in the northern counties, where it does occur occasionally (Department of Parks and Recreation, 1970). Ocean surface water temperatures and salinities also vary from an average of 54.5°F and 32.5°/oo salinity in the north, to an average of 67° F and 33.45°/oo salinity in the south. California has, in fact, two very distinct oceans. Point Conception, situated approximately two-thirds of the way down the California coast, marks an abrupt change in the character of our ocean waters. North of Point Conception, the waters are uniformly 10°F colder, and have 1°/oo lower salinity, than the waters south of the Point. North of the Point, the northwesterly winds are much stronger, which tend to make the ocean waters more turbulent (Sverdrup, Johnson & Fleming, 1942; Bureau of Land Management, 1974). Point Conception is, in addition to being an oceanographic boundary, a biological boundary as well. Southwesterly from Point Conception, a large eddy is formed where the Davidson Current, deflected westward by the Point, is turned southward by the California Current. The abrupt temperature and salinity change at the north edge of the eddy acts as an effective barrier to the mixing of fish and shellfish species acclimated to environmental conditions found to the north and south. Much of the California coastline is mountainous and characterized by a mix of rocky headlands, cliffs, and beaches, with few major estuaries and embayments. From Crescent City, south to the Los Angeles Basin, only a few alluvial plains are found at the mouths of broad valleys. The-shoreline is relatively straight, lacking in barrier beaches and lagoons for protection (Department of Parks and Recreation, 1970; United States Army Corps of Engineers, 1971; National Marine Fisheries Service, 1990). The morphology of the coastal region and the nearshore subtidal region of California has been shaped essentially by three processes: 1) the convergence of tectonic plates, 2) seismic activity along the San Andreas Fault zone, and 3) sea level fluctuations. California straddles the Pacific and North American plates in such a way that San Francisco is on the North American plate while Los Angeles is on the Pacific plate. The two plates are separated by the San Andreas Fault, which is a relatively straight, northwest-tending fault that extends 992 miles from the Salton Sea through the Coast Ranges to the Mendocino Escarpment (Jones and Stokes Associates, Inc, 1981b; Anderson et al., 1990). Both plates generally move the opposite directions with the American plate moving in a southeast direction while the Pacific plate tends to move northwest. When these plates collided, the North American plate was lifted up and over the other plate producing the Coastal and Transverse mountain ranges. 3-2 Anderson et al. (1990) attributed the current geomorphology of the north coast (southern Oregon to Cape Mendocino) to tectonic convergence. This area has undergone significant uplifting as a result of the collision of three plates: the Gorda, Pacific, and the North American plates. While the geomorphology of the central coast (Cape Mendocino to Morro Bay) is attributed to the right-lateral, strike-slip motion of the San Andreas fault more than to the convergence of the two plates. The faulting action has created folded, sheared, and metamorphosed jumble blocks with a north-south orientation (Anderson et al., 1990). Coincidental to the geologic processes were sea level fluctuations. The fluctuations, which were caused by changes in the worldwide climatic conditions, lead to periods of shoreline advance and retreat (Jones and Stokes Associate, Inc, 1981b). During glacial periods, the shoreline retreated as far as the edge of the present day continental shelf. During interglacial times, the shoreline advanced to near modern levels. Each of these oscillations in sea level lead to varying degrees of erosion of the shelf and coastal mountains. Evidence of this can be found in the presence of broad, gently sloping, wave-cut terraces on land (Anderson et al., 1990). These terraces have been lifted to their present day levels through a combination of seismic activity and the retreat of glaciers. The weight of the ice masses pushed the underlying land down much like the effect of putting weight on a dock does. Much of the land surface of the Oceanside-San Diego area represents a series of marine terraces that were cut into the coastal plain that parallels the Peninsular ranges (Hertlein and Grant, 1954). The ocean bottom seaward of the California coastline is called the Continental Shelf. This shelf, part of the continent that is presently submerged beneath the ocean waters, is an area where a considerable portion of our marine resources occur. The Continental Shelf off California varies considerably in width.

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