| Zooplankton were trawled during the U.S. Antarctic Marine Living Resources (AMLR) 1991 and 1992 cruises in the Elephant Island region using a 1 m{dollar}sp2{dollar} Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS). Samples were collected over the shelf break and slope in 1991, and over the shelf as well as over the shelf break and slope, in 1992. Two regional water masses were present: the Weddell-Scotia Confluence water (WSC) and Transition Type II (TTII) water. Analysis of variance multiple regression tests, Spearman's nonparametric r correlations, and hierarchical cluster analyses showed varying degrees of associations between abundances of different kinds of zooplankton and to the different water masses, diel periods, areas, and bottom topography. Results demonstrate that at spatial scales of 10{dollar}sp3{dollar} to 10{dollar}sp4{dollar} meters, water masses were the most important physical factor influencing the abundance of most types of zooplankton, and that bottom depth was locally important within these two water masses. Diel changes in abundance described by previous investigators were documented, with Euphausia superba and Euphausia frigida more abundant at twilight, and the other common species, in general, more abundant at night. Interannual variations in abundance were observed for most of the zooplankton in the WSC water, although further studies are needed to fully understand the physical and biological forcing for interannual variations. Euphausia superba swarms were not correlated with the abundance of the other zooplankton, suggesting that krill swarms do not exclude the other zooplankton from their aggregations via food competition as previously thought. Patchiness was also detected on a fine scale (tens to hundreds of meters) for the copepods Calanoides acutus, Calanus propinquus, Rhincalanus gigas, Themisto gaudichaudii and the chaetognaths, showing net-to-net variability within the same water parcel. This study demonstrates the need to sample physical environmental variables concurrently with biological factors, on all scales of variability, when analyzing zooplankton communities. |
