Mesozooplankton of the Scotia Sea: present, past and future
The potential impact of global warming on plankton populations and processes in the Southern Ocean is cause for concern. Greatest attention has concentrated on Antarctic krill, a keystone macroplankton species and currently the object of a commercial fishery. This thesis focuses on the mesozooplankton component of the zooplankton, in particular the Copepoda. These crustaceans are major secondary producers and are important dietary items for mesopelagic fish as well as being major recyclers of particulate carbon and nutrients. The balance between krill and copepods fundamentally influences the nature of ecosystem function, particularly with regard to carbon export and remineralization. In this synthesis of 18 years of research, summarised in eight papers, I address the question of whether there have been shifts in the distribution and community structure of mesozooplankton in the Scotia Sea over the last 70 years. Net samples obtained between 1995 and 2009 were compared with samples collected during the 1920s-1930s by the Discovery Investigations. Taking into account differences in net design between eras, no differences in overall abundances or community composition were initially apparent. Despite a median increase of 0.74oC in sea surface temperature since the 1930s, plankton distributions from contemporary sampling were geographically similar. However, further analysis showed that although ranked abundances were comparable between eras, three of the four biomass dominant copepods had increased in absolute abundance by ~20- 55% during contemporary times. Reasons for this are considered to be linked to changes within the food web, driven by decreasing krill abundance as a result of warming-induced habitat loss and the commercial exploitation of whales. Future investigations into spatial distributions and trophic overlap between krill and copepods, as well as renewed efforts in modelling the ecosystem effects consequent upon the harvesting of whales, are recommended.