Zooplankton in the Southern Ocean from the continuous plankton recorder: Distributions and long-term change

We provide an analysis of zooplankton distributions in the circumpolar Southern Ocean based on samples collected by the international SCAR Southern Ocean Continuous Plankton Recorder Survey (SO-CPR). We analysed SO-CPR measurements in relation to satellite and oceanographic model hind-cast data over the period 1997-2018. These environmental data were chlorophyll-a concentration, net primary productivity (VGPM model), sea-surface temperature (SST), mixed layer depth, sea ice and the spatial gradient of SST (as an indicator of ocean fronts). Boosted Regression Tree (BRT) models were used to extrapolate in space and time measurements of the abundances of six key taxonomic groups of zooplankton: Copepoda (Calanoida), Euphausiidae (numerically dominated in SO-CPR data by Thysanoessa macrura), Foraminifera, Fritillaria spp., Oithona similis, and pteropods. Based on the BRT models, we present predictions of the spatial and seasonal (October-March) environmental suitability for these groups in the Southern Ocean. Trend analysis suggests that between 1997 and 2018 the environmental suitability for copepods (both cyclopoid and calanoid), Foraminifera, and Fritillaria spp. has increased by 0.72% per year average, and at higher rates in frontal regions especially in the Indian sector of the Southern Ocean. In contrast, for pteropods in some areas (particularly over the Ross Sea shelf) the environmental suitability has significantly worsened over the last 20 years.