. Elements of ecology. Ecology. Aquatic Environment 249 the seasonal circulation does not reach the bottom, with the result that the deepest layers may be completely devoid of oxygen through- out the year. In the temperate regions of the ocean the thermal stability built up during the summer is similarly broken down in the autumn. Turbu- lence produced by wind and waves then carries oxygen-rich water downward, and this deep stirring continues all winter. The down- ward extent of the winter mixing is sufficient to reach the bottom in coastal areas but its effect does not go below the permanent
. Elements of ecology. Ecology. Aquatic Environment 249 the seasonal circulation does not reach the bottom, with the result that the deepest layers may be completely devoid of oxygen through- out the year. In the temperate regions of the ocean the thermal stability built up during the summer is similarly broken down in the autumn. Turbu- lence produced by wind and waves then carries oxygen-rich water downward, and this deep stirring continues all winter. The down- ward extent of the winter mixing is sufficient to reach the bottom in coastal areas but its effect does not go below the permanent thermo- cline of the open sea (Fig. ). Seasonal changes in temperature and wind action thus do not provide for the delivery of oxygenated water to the great depths of the May June July Aug. Sept. Oct. Nov. Fig. Changes in oxygen diu-ing the warm part of the year at the indicated depths in Lake Mendota, Wisconsin. (Needham and Lloyd, 1937, after Birge and Juday.) Oxygen is supplied to the ocean abyss by deep permanent currents that originate near the surface in high latitudes, sink to intermediate levels or to the bottom, and flow for thousands of miles toward and beyond the equator (Fig. ). This deep flow of water is dynam- ically integrated with the horizontal current systems with the result that the vertical movements of water follow a complicated pattern (Sverdrup et al., 1942). As a simplified generality, however, the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Clarke, George L. (George Leonard), 1905-. New York, Wiley
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