Mesozooplankton interactions with the shelf around the sub-Antarctic Prince Edward Islands archipelago

Mesozooplankton surveys were conducted in April/May for four consecutive years (1996-1999) in the vicinity of the Prince Edward Islands (PEIs), Southern Ocean. The PEIs are located in the Polar Frontal Zone, directly in the path of the east-flowing Antarctic Circumpolar Current. Zooplankton were collected by oblique tows using a Bongo net fitted with 300 μm mesh. The abundance, biomass and average side of the mesozooplankton in the upstream (USR), inter-island (IIR) and downstream (DSR) regions indicated that some groups and species were significantly affected by their interaction with the shallow shelf waters of the PEIs. Total mesozooplankton abundance and biomass were typically highest in the DSR, but no consistent pattern was evident in the USR and IIR, Copepods, euphausiids and fish were generally of a low average size in the IIR. This small size was largely attributed to the reduced abundance, or complete absence, of mesopelagic species from the shelf region. Of total biomass, the mesopelagic species Euphausia longirostris, Euphausia similis, Pleuromamma abdominalis, Paraeuchaeta biloba and Oncaea antarctica together contributed an average of 16% to the USR, 2% to the HR and 15% to the DSR. Conversely, epipelagic species showed no consistent pattern of abundance and biomass distribution between regions. The low incidence of mesopelagic species over the island shelf was attributed mainly to reduced advection of deep water into the shelf region (average depth = 200 m), rather than predation, particularly during the through-flow mode between the islands. This resulted in substantial regional differences in euphausiid community structure. The epipelagic species Euphausia vallentini and Thysanoessa vicina completely dominated the IIR, comprising on average 89% of total euphausiid biomass in this region. However, predation may be important, during the water-trapping mode between the islands. Advection of zooplankton into the IIR appeared to be affected by the proximity of the Subantarctic Front (SAF). In 1996, when the SAF was far north of the PEIs, reduced current velocities resulted in some degree of water retention over the shelf and an increased predation impact. Conversely, when the SAF was close to the PEIs in 1999, more large plankton were transported over the island shelf. High current velocities and productivity associated with the SAF appear to increase the biomass and size of allochthonous zoopilankton/nekton advected info the IIR. and consequently may have increased the availability of prey to land-based predators. The long-term southward movement of the SAF recently observed in the vicinity of the PEIs may therefore have important implications for the ecosystem of these islands.