Trophic cascading in the Polar Frontal Zone of the Southern Ocean during austral autumn 2002

摘要

Trophic cascading resulting from coupling among phytoplankton, protozooplankton (2―200 μm) and the mesozooplankton fractions < 1, 000 μm and < 2,000 μm was investigated at three stations in the Polar Frontal Zone of the Southern Ocean during austral autumn 2002. In the absence of any grazers, net growth rate of the phytoplankton was positive and ranged between 0.209 and 0.402 day~(-1). Among the heterotrophic components of the zooplankton, protozooplankton were identified as the most important consumers of the phytoplankton biomass. The low impact of the mesozooplankton on the phytoplankton may have been related to the inability of the larger grazers to feed efficiently on the small phytoplankton cells, which dominated the total chlorophyll concentration at two of the three sampling stations. It appears that the mesozooplankton < 1,000 μm consumed protozooplankton, which resulted in a decrease in the impact of the latter organisms on the phytoplankton. The presence of predatory zooplankton (mainly chaetognaths and am-phipods) > 1,000 μm led to a decrease in the carnivory of the < 1,000-μm mesozooplankton fraction on the protozooplankton. In this situation, protozooplankton were again able to exert a considerable impact on the phytoplankton.