No. 31. Marine ShelfDavid H. Johnson
Physical Setting. Along the coasts of Oregon and Washington, the Marine Shelf (also called the Continental Shelf) habitat exists as a relatively shallow, flat, submerged area, which varies from about 9 to 40 miles (14 to 64 km) in width. At about the 656-ft (200-m) isobath, roughly the edge of the marine shelf, the bottom drops off more steeply to the continental slope, which is indented by several major submarine canyons. Beyond the shelf and slope are deep abyssal oceanic waters. Landscape Setting. This habitat is located between the Nearshore Marine and Oceanic habitats; at about one third of the way into the Strait of Juan de Fuca, this habitat adjoins the Inland Marine Deeper Water habitat. Structure. The marine shelf extends seaward from the 60-ft (20 m) to the 656-ft (200 m) isobath. It is occasionally divided into the inner (0-328 ft [100 m] deep) and outer (328-656 ft [100-200 m] deep) shelf areas, reflective of differing oceanographic influences. The bottom substrate of the shelf is mostly sand, giving way to silt on the outer edges, and is described as smooth as a result of sediment accumulation. Currents over the shelf tend to follow the seasonal pattern of the oceanic currents (i.e., northward during winter [Davidson current]). The mean surface temperatures in summer were >5°C lower, and mean salinities were 0.1-0.3% higher in upwelling areas than farther offshore. These conditions reflect an active upwelling process 83. The shelf areas off Oregon and Washington are known for their heavy surface waves; extremes of wave heights ranging from 49 to 95 ft (15 to 29 m) have been recorded. More typical are waves of 20 to 33 ft (6 to 10 m) during storm events. Composition. The Marine Shelf, as with other deep-water regions, does not support rooted plant life. Strickland and Chasan 194 offered a synthesis of phytoplankton; the Washington shelf has been ranked in the highest productivity category of U.S. continental shelves. Other Classifications and Key References. Strickland and Chasan 194 offered a synthesis of information on Coastal Washington, as related to understanding impacts of offshore oil and gas exploration. The National Oceanic and Atmospheric Administration 163 reflects the Final Environmental Impact Statement/Management Plan for the Olympic Coast National Marine Sanctuary. Bottom et al. 33, Dodimead et al. 73, Favorite et al. 83, Thomson 203, and Ware and McFarlane 213 offered classification schemes and terminology for understanding the oceanic systems in the Northeastern Pacific. Natural Disturbance Regimes. Currents over the shelf tend to follow the seasonal pattern of the oceanic currents, but also are strongly influenced by local winds, bottom and shoreline configuration, and freshwater input. On average, water flows southward in the upper 328 ft (100 m) during summer, and northward below that. Water over the shelf flows generally northward at all depths during the winter 194. In addition to ocean currents, this habitat is heavily influenced by freshwater infusion from the Columbia River, the largest river on the Pacific west coast. The Columbia River effluent amounts to approximately 60% of the freshwater entering the Pacific Ocean between San Francisco and the Strait of Juan de Fuca in winter, and >90% the rest of the year 21. Furthermore, the 12- to 16-mile (20 to 25 km)-wide Strait of Juan de Fuca, separating Vancouver Island from mainland Washington, is a glacially excavated channel that is the primary avenue for vigorous estuarine exchange between the shelf and the inland marine waters of Washington and British Columbia. Effects of Management and Anthropogenic Impacts. The chief human influence on the ecology of Oregon and Washington marine waters is fishing, especially bottom and mid-water trawl fishing in shelf waters for flatfish and Pacific hake. Other significant fisheries include salmon (inner shelf), shrimp (outer shelf), and albacore (slope). The effects of overfishing have been documented. Other risks to the shelf environment include pollution, contaminants, and oil spills. Status and Trends. Until the early 1990's, the Minerals Management Service of the U.S. Department of the Interior had planned to conduct Lease Sales for offshore oil and gas exploration in federal waters on the outer marine shelf of Oregon and Washington 194. A moratorium on these leases is now in place. Designated in 1994, the Olympic Coast National Marine Sanctuary covers 2.11 million acres (854,251 ha) and is managed to protect its natural resources while encouraging compatible commercial and recreational uses. [ Top ] [ Literature Citations ] [ Wildlife-Habitat Types - Table 1 ] |
