Taxa Composition and Biomass of the Surface-dwelling Crustaceans During Spring Pycnocline Formation in Auke Bay, Southeastern Alaska

The crustacean-dominated zooplankton community that lives in the upper 5 m of water in one of southeastern Alaska's bays was monitored before and after spring pycnocline formation. Zooplankton abundance and wet weight biomass in the upper 5 m of the surface layer reached peak values of 50,000 m–3 and 2.5 g m–1, respectively. Surface populations were dominated by barnacle nauplii, with substantial contributions by the copepods Pseudocalanus spp., Acartia spp., and Centropages abdominalis. Barnacle nauplii were only a minor component of the zooplankton in whole water-column samples. Surface zooplankton abundance and biomass were higher at night than during the day in April and early May, prior to stabilization of the water column. The day versus night differences were primarily due to diurnal vertical migration by barnacle nauplii. Obvious diurnal differences in abundance and biomass of zooplankton at the sea surface were not detected after pycnocline formation. After pycnocline formation, copepod biomass in the upper 5 m was about 5 times higher than that of the integrated water column. This may explain why many young fishes congregate in the surface layer to feed.