Mesozooplankton in the Southern Ocean: Spatial and temporal patterns from Discovery Investigations

- Plankton samples from 1930s Discovery Investigations in the Southern Ocean analysed.
- Depth was the strongest factor separating samples.
- Mean zooplankton abundance in the epipelagic was ∼25 times greater than at 1000 m.
- Seasonal signals across all data became less distinct with depth.
- Rarefaction analysis revealed that depth was a major influence on diversity.

Mesozooplankton samples taken during the Discovery Investigations in the Southern Ocean in the 1930s were analysed from a series of 5 transects along 80°W. The samples provide a unique level of depth-discrete resolution across large spatial scales, over most of the productive austral season. Stratified net hauls were taken between 0 and 1000 m within the period December 1933 to November 1934. Within the epipelagic (0-100 m), median zooplankton abundance (278 ind. m−3) was ∼22 times greater than at 1000 m. A 3-4-fold variability of abundance in the epipelagic contrasted with depths >250 m where variability was <1-fold. Depth was the strongest factor separating samples (ANOSIM, R = 0.66 p = 0.1%), with a clear biological distinction between epipelagic and upper and lower mesopelagic horizons. Results from multi-dimensional scaling indicated that, when plankton abundance was integrated over all depth horizons, 3 different groups could be identified. These 'communities' were consistent with the spatial extents of Antarctic, Polar Frontal Zone, and sub-Antarctic water-mass regimes. Such groupings became less distinct when considering only deeper horizons (500-1000 m) and excluding seasonal migrants. Seasonal signals across all data became less distinct with depth. Rarefaction analysis indicated that diversity increased with depth. Although depth alone was the most important influence on sample diversity, (r2 = 0.60), water mass regime and month improved the fit (r2 = 0.71). Overall plankton diversity was highest in the sub-Antarctic zone. Following atmospheric and ocean warming that has taken place close to the study area in the last 80 years we hypothesise that species richness may increase in the Antarctic water masses as sub-Antarctic species increasingly encroach south.