Characteristics of the Oceanographic Environment in the Aleutian Basin of the Bering Sea during Spring

摘要

The characteristics of the oceanographic environment in the Aleutian Basin of the Bering Sea during spring in 1996, 1997, and 1999 were clarified. An investigation of the water properties revealed five basic layers in the Bering Sea during spring: (1)a surface layer of warm and low-salinity water induced by solar heating, (2) a subsurface layer of cold and low-salinity water propagated slowly by heat from the surface layer, (3) a thermocline layer where salinity was constant but temperature sharplydecreased, (4) a temperature inversion layer, and (5) a deep layer with a gradual decrease in temperature and increase in salinity toward the bottom. The ranges of water temperature and salinity were 1.8-5.5°C and 31.81-34.08 in 1996, 1.5-7.2°C and 31.9-34.06 in 1997, and 0.5-5.6°C and 32.0-34.11 in 1999, respectively. The water temperature of the surface layer was approximately 1.6°C higher in 1997 than in 1996 and 1999. The lowest temperature at a depth of 100-150 m was about 1 °C lower in 1999 than in 1996 and 1997. Nutrient levels (nitrate, phosphate, and silicate) contributing to the control of the growth of phytoplankton were higher in the Aleutian Basin than in the eastern continental shelf and Bogoslof Island area. This was closely associated with the phytoplankton distribution. Nutrient concentrations were lowest at a depth of 25 m. The high primary production at that depth was confirmed from the vertical distribution of chlorophyll a. Chlorophyll a levels were above 4.0 mu L~(-1) in some areas in 1996 and 1999, but below 2.0 mu L~(-1) in most areas in 1997. Zooplankton density was about three times higher in 1999 than in 1997.