Marginal Seas

Marginal seas, which separate coastal zones from open oceans, often exist as large indentations into continental landmasses. Some of the major marginal seas include the Arabian Sea, Baltic Sea, Bay of Bengal, Bering Sea, Beaufort Sea, Black Sea, Gulf of California, Gulf of Mexico, Mediterranean Sea, Red Sea, Ross Sea, Weddell Sea, and all four of the Siberian Seas (Barents, Kara, Laptev, and East Siberian).

Marginal seas are similar to open oceans with respect to being created by large-scale geological processes, exhibiting biodiversity , and possessing layered water circulation patterns. The degree of water circulation between marginal seas and open oceans varies with respect to location. The primary differences between marginal seas and open oceans are associated with depth and proximity to landmasses. Marginal seas, which are generally shallower than open oceans, are more influenced by human activities, river runoff, climate, and water circulation.

Human Impacts

Humans utilize nearshore environments, including coastal waters and marginal seas, for food and fuel resources, as well as for various purposes related to scientific, industrial, and recreational activities. The greatest human impact on marginal seas is related to the fisheries industry. Ninety percent of the world's fisheries exist within coastal waters that are located less than 200 kilometers (124 miles) from the shoreline. Other human activities that have adversely affected marginal seas include industrial enterprises such as sewage disposal, dredge disposal, offshore oil drilling, and accidental releases of pollutants, including petroleum products, radioactive waste, detergents, and plastics.

Coastal waters and marginal seas are more susceptible to pollution than open ocean regions because of the high concentration of human activities near coastlines and rivers. Pollutants from the nearby landmasses are introduced into marginal seas in concentrations that are thousands of times greater than in open oceans. Pollutants enter marginal seas by way of water discharge and airborne deposition.

Biological growth of marine organisms in marginal seas can be either stimulated or inhibited depending on the nature and concentration of the particular pollutant—for example, whether it is a nutrient or a toxin. For example, the discharge of domestic sewage leads to elevated nutrient concentrations, which could result in increased primary productivity for phytoplankton, possibly including those species responsible for harmful algal blooms.

Biomass Production and Primary Productivity

Biomass is comprised of plants, herbivores (plant-eaters), carnivores (animaleaters), and omnivores. Marine biomass production originates with primary productivity, which in turn is affected by the availability of sunlight, carbon dioxide, nutrients such as nitrates and phosphates, and trace elements.

Marginal seas generally exhibit intermediate levels of primary production, with the highest rates found in coastal upwelling regions and the lowest primary production occurring in open ocean regions. Hence, the highest biomass production rates occur in coastal upwelling zones, the lowest in open oceans regions, and intermediate rates in marginal seas.

Regional variations in primary productivity primarily occur as a result of water column chemistry (e.g., nutrient and trace elements concentrations) and dominant physical processes. For nearshore regions such as coastal waters and marginal seas, the dominant physical processes influencing primary productivity are river runoff, water column mixing, and turbidity . River runoff and water column mixing introduce dissolved nutrients, trace elements, and suspended particles into the photic (light) zones of nearshore regions. Although the addition of dissolved nutrients and trace elements to coastal waters and marginal seas serves to increase primary production, the addition of suspended particles increases water turbidity, which results in reduced sunlight penetration and decreased primary productivity.

Water Circulation

Water circulation patterns in marginal seas depend largely on bathymetry , fresh-water input (e.g., river runoff and precipitation) and evaporation. If river runoff and precipitation exceed evaporation, as is the case in the Black and Baltic Seas, the excess fresh water will tend to flow seaward near the sea surface, diluting the marginal sea. If evaporation exceeds river runoff and precipitation, as in the Mediterranean Sea, the marginal sea water becomes saltier, then sinks and flows towards the less salty openocean region.

Within marginal seas, these general water circulation patterns are often modified by the local bathymetry, which in some instances serves to restrict water flow. The water circulation patterns of four major marginal seas are described below.

Black and Baltic Seas.

The Black Sea and Baltic Sea basins, which exhibit maximum depths of approximately 400 meters (1,312 feet) and 2,200 meters (7,216 feet), respectively, both possess sills that restrict subsurfacewater circulation. While the surface waters of the Black and Baltic Seas are able to flow over the sills and introduce lower salinity water into the open ocean, the flow of the saltier subsurface waters is blocked by these bathymetric features. This type of subsurface-water restriction often leads to stagnation, which may eventually result in local oxygen depletion. In the Black Sea, this oxygen depletion process has led to subsurface anaerobic conditions and significant decrease in biomass with depth. Due to the shallowness of the Baltic Sea, anaerobic conditions only exist in the deepest areas, where stagnation occurs.

Mediterranean Sea.

The Mediterranean Sea, which is divided by a 400meter (1300-foot) sill into two subbasins, is connected to the Atlantic Ocean via the Straits of Gibraltar, to the Black Sea via the Bosporus Strait, and to the Red Sea via the manmade Suez Canal. Atlantic Ocean water enters this marginal sea through the Straits of Gibraltar as a surface flow. This ocean water replaces a fraction of the water that evaporates in the eastern Mediterranean Sea.

Upon arrival to the northern coast of the eastern basin, a portion of the Atlantic Ocean water is cooled (13°C or 56°F) and made saltier by evaporation, which results in its sinking in the Adriatic Sea. During the winter as the remaining Atlantic Ocean water continues to flow towards Cyprus, it sinks to a depth ranging from 200 to 600 meters (656 to 1,968 feet), where it forms the Mediterranean Intermediate Water. This water mass then flows along the North African coast into the North Atlantic Ocean via the Straits of Gibraltar. Because of its density, upon introduction into the North Atlantic, the Mediterranean Intermediate Water sinks to approximately 1,000 meters (3,280 feet), where it mixes with Atlantic Ocean water. This mixing process results in the formation of the Mediterranean water mass.

The circulation between the Mediterranean Sea and the Atlantic Ocean is typical of closed, restricted basins in which evaporation exceeds precipitation. As such this type of circulation pattern is often referred to as Mediterranean circulation, which is opposite of estuarine circulation.

Gulf of Mexico.

Compared to the Black, Baltic and Mediterranean Seas, the Gulf of Mexico is a much less complex marginal sea. The Gulf of Mexico possesses an extensive, broad continental shelf and a maximum water depth of approximately 3,600 meters (11,808 feet). The Gulf of Mexico is connected to the Atlantic Ocean via the Straits of Florida and the Caribbean Sea via the Yucatán Strait.

Surface water in the Gulf of Mexico is as shallow as 90 meters (295 feet) in the winter and 125 meters (410 feet) during the summer. Surface salinities values for this marginal sea generally range between 36.0 and 36.3. In the northern Gulf of Mexico region, Mississippi River runoff influences surface waters as far as 150 meters away from the shore, resulting in salinities as low as 25. Gulf of Mexico surface-water temperatures range from 18°C to 21°C (64°F to 70°F) in the north and 24°C to 27°C (75°F to 81°F) in the south.

A unique feature of the Gulf of Mexico's surface circulation pattern is the Loop Current, which results from the Caribbean Current entering the Gulf of Mexico through the Yucatán Strait and upon arrival, turning in a clockwise direction and "looping" around a warm "dome" of Gulf of Mexico surface water.

see also Algal Blooms, Harmful; Algal Blooms in the Ocean; Bays, Gulfs, and Straits; Beaches; Coastal Ocean; Coastal Waters Management; Fisheries, Marine; Human Health and the Ocean; Light Transmission in the Ocean; Ocean Basins; Ocean Currents; Ocean Mixing; Pollution of the Ocean by Industrial Wastes; Pollution of the Ocean by Sewage.

Ashanti Johnson Pyrtle