Zooplankton distribution and behaviour in the Southern Ocean from surveys with a towed Optical Plankton Counter

Spatial distributions of zooplankton with lengths between about 500 μm and 8 mm are described from surveys in the vicinity of the Antarctic Polar Front in austral summer 1995/6 using an Optical Plankton Counter mounted on a towed profiling SeaSoar. The distributions, split into several logarithmically spaced size classes, are compared and related to the physical environment south of the Polar Front in the Antarctic Zone and within the Polar Frontal Zone. They also are compared with phytoplankton distributions determined from surface chlorophyll data. Both phytoplankton and zooplankton carbon densities are low in the Antarctic Zone (2–3 g C m −2), but rise to larger values in the Polar Frontal Zone (5–7 g C m −2 for zooplankton and a maximum of 6 g C m −2 at fronts for phytoplankton). Calibration of OPC derived zooplankton biovolume to carbon was achieved by comparison with dry weights from multinet samples deployed in conjunction with CTD casts. The net data showed that over 98% of zooplankton counts were copepods. Diel behaviour also was examined. Only larger copepods (over 2 mm long) displayed significant diel migration, and then only 10–20% of the standing stock; the majority remained deeper than about 100 m and their distribution patterns suggest that they may be retained aside from the main frontal jets by ageostrophic circulations associated with the front. Copepods shorter than 2 mm rose from depth over the month-long survey to become concentrated in the surface layer (the top 70–100 m). The largest copepods that could be resolved, with lengths of about 4–8 mm (possibly Rhincalanus gigas), displayed unexpected behaviour in tending to migrate to the top 0–10 m by day, descending to 40–50 m each night.