Abstract
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.
Highlights
The zooplankton fauna of the Bering Sea is divided into 4 major faunistic groups (Vinogradov 1956).The southern Bering Sea oceanic group consists of calanoid taxa occurring in both the Bering Sea and the northern Gulf of Alaska (Brodsky 1957).Despite the similarity in zooplankton species composition in the Bering Sea and North Pacific, plankton surveys to the north and south of the western Aleutian Islands over a 15 yr period revealed differences in mean biomass in the upper 80 m of the water column: 14.3-18.0 versus 24.1-28.4 g m-3 to the south and north of the islands, respectively (Motoda & Minoda 1974)
The water mass to the south of the Aleutians consists of the Alaska Stream, a strong current which flows along the shelf break from the eastern Gulf of Alaska to the western Aleutian Islands, and forms the northern boundary of the Pacific subarctic gyre (Reed 1984)
We present 4 examples of the distribution of temperature, salinity, chlorophyll concentration and acoustically determined biomass (ADB) along transects crossing the front between Bering Sea water and Pacific water or the mixing regime in the passes
Summary
The differences in zooplankton biomass are probably related to differences in water mass types to the north and south of the Aleutians. Examination of acoustic data from the Bering Sea basin north of Kiska Island in June 1992 revealed intense sound scattering in the pycnocline separating the upper mixed layer from the cold winter layer beneath (Coyle in press). We examine the geographic distribution of sound scattering intensity and the species composition and abundance of major zooplankton taxa relative to the distribution of water masses of Pacific and Bering Sea origin. We compare data from 1992 and 1995 to address interannual variability These observations may help explain the biomass differences observed earlier (Motoda & Minoda 1974) and document the critical influence of frontal systems and pycnoclines on the mesoscale distribution of zooplankton biomass in the western Aleutian Islands
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