Spatio-temporal dynamics of mesozooplankton in the subantarctic Beagle Channel: The case of Ushuaia Bay (Argentina)

The Beagle Channel is the southernmost South American fjord. Although it has been the focus of many biological studies, more in-depth knowledge of plankton and their linkage with environmental variables is lacking. The main objective of this study was to analyse the spatio-temporal variations in mesozooplankton assemblages of Ushuaia Bay, an ecologically and economically relevant area in the middle Beagle Channel, on the basis of samples collected during summer (March), autumn (June), winter (September) and spring (December) 2012 from environmentally different sites. Mesozooplankton composition and abundance exhibited strong seasonal variations, closely associated with the late winter–springphytoplankton bloom. The appendicularian Fritillaria borealis and the cyclopoid copepod Oithona similis were the dominant species during summer and autumn, whereas meroplanktonic larvae and the cladocerans Evadne nordmanni and Podon leuckarti dominated during late winter and spring, respectively. Mesozooplankton abundance was lower at coastal sites receiving freshwater and sewage discharges. Contrary to data reported a decade earlier, small-sized omnivorous species were dominant in Ushuaia Bay in 2012. The increased abundance of these species and decrease in the abundance of calanoids (e.g. Drepanopus forcipatus and Eurytemora americana) may reflect the natural year-to-year variability in phytoplankton structure and water physico-chemical properties. However, since this bay suffers from chronic pollution, other explanations, such as the influence of eutrophication on plankton structure, should also be considered. This study provides a baseline for future research on the effects of natural and anthropogenic environmental change on the Beagle Channel, thereby expanding our knowledge of plankton in subantarctic fjords.