THE PHYSIOLOGY OF POLAR MARINE ZOOPLANKTON

Thc polar marine environment is characterised by low stable temperatures with seasonal variations ranging from +/-3-degrees-C at lower latitudes to only +/-0.2-degrees-C at high latitudes. The Arctic basin is dominated by multi-year ice. whereas the Antarctic is subject to large seasonal changes in the cover by annual sea ice. Primary production is intensely seasonal nearshore but probably less so in offshore waters where significant production is associated with the marginal ice zone. Oxygen consumption in polar zooplankton is low compared with temperate and tropical species. Annual growth rates are generally slow and, especially in herbivores, highly seasonal. It is likely that fast growth rates are possible for polar zooplankton in areas of high food availability such as ice-edge blooms, but these growth rates are not usually achieved in the more oligotrophic open-ocean areas. Lipid stores in polar herbivorous zooplankton are generally high, although some euphausiids and gelatinous zooplankton also rely on degrowth to provide energy over winter. Ice-edge blooms are of great importance to the polar marine food web although the quantitative significance of winter feeding under ice has yet to be resolved. Comparison of data on lipid storage and oxygen consumption for polar zooplankton indicates that there arc large differences in the energy requirements of benthos and crustacean zooplankton. This is probably related to the high metabolic cost of staying in the water column. In contrast gelatinous zooplankton (salps, ctenophores, medusae and siphonophores) have a low energy throughput, related to a body composition which renders them essentially neutral in buoyancy and a slow but efficient means of locomotion. Under good feeding conditions many species can therefore grow and reproduce very rapidly. This emphasises the distinct energetic regime of gelatinous zooplankton, now known to be a group of major ecological importance in most waters of the world.