Long-term change in zooplankton communities of the Southern Ocean between 1997 and 2018: implications for fisheries and ecosystems

We provide an analysis of zooplankton distributions in the circumpolar Southern Ocean based on samples collected by the international Southern Ocean Continuous Plankton Recorder Survey (SO-CPR) between 1991 and 2018. We analysed SO-CPR measurements in relation to satellite and oceanographic observations over the period 1997–2018. These environmental data were: phytoplankton biomass, primary productivity, sea-surface temperature (SST), mixed layer depth, sea ice and the spatial gradient of SST (as an indicator of ocean fronts). Boosted Regression Tree models were used to investigate relationships between the abundances of key groups of zooplankton, occurrence of characteristic zooplankton communities, and environmental conditions. Analysis is presented on the environmental suitability for six broad zooplankton groups: Copepoda, Euphausiidae (numerically dominated in SO-CPR data by Thysanoessa macrura), Foraminifera, Fritillaria spp., Oithona similis and pteropods. Trend analysis suggests that the environmental suitability for copepods, Foraminifera, and Fritillaria spp. may lead to increasing abundances at between 0.59–0.83% per year averaged across the Southern Ocean, and with much higher rates of increase in some regions. In contrast, the environmental suitability for pteropods is predicted to be decreasing in the Ross Sea region. Eight characteristic zooplankton communities were identified based on multivariate cluster analysis. The analysis suggests that the areal extent of the northern (Subantarctic) communities have not changed or decreased in area between 1998 and 2018, whereas the main Polar Frontal communities have expanded, and the area occupied by the southernmost (sea-ice) zooplankton community has decreased in area. These analyses are relevant to evaluating the conservation value of the Ross Sea region Marine Protected Area against its specific objectives.