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Distribution of acoustically determined biomass and major zooplankton taxa in the upper mixed layer relative to water masses in the western Aleutian Islands


Hydrographic, acoustic and zooplankton data were collected during June 1992 and June 1995 along transects to the north and south of Kiska and Buldir Islands in the western Aleutian Islands to look for mesoscale relationships between zooplankton community composition, biomass, and associated frontal systems and pycnoclines. Hydrographic data revealed a front dividing the Bering Sea water mass from the mixed regime in the passes and Alaska Stream water south of the islands. Chlorophyll concentration and ADB (acoustically determined biomass) were consistently higher on the Bering Sea side of the front in June 1992. Elevated ADB in the Bering Sea was due to aggregates of Neocalanus cristatus copepodid V in the pycnocline separating the upper mixed layer from the underlying cold intermediate layer. N. cristatus abundance was significantly higher (p < 0.05) in Bering Sea water in June 1992 than in Pacific water in June 1992 and Bering Sea water in June 1995. N. fleminger abundance was significantly higher in Bering Sea water than in Pacific water in both 1992 and 1995. N. plumchrus and Eucalanus bungil stages III and IV were more abundant in Pacific water than in Bering Sea water. The results clearly demonstrate the utility of interfacing acoustics, net samples and hydrographic data to elucidate mesoscale associations of zooplankton to fronts, water masses and pycnoclines.

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