The spatio-temporal distribution of zooplankton communities in the Southern Ocean south of Australia : high resolution sampling by the Continuous Plankton Recorder and its implications for long-term monitoring

The Polar regions are considered to be particularly susceptible to the effects of climate change. Although there is evidence for warming in the Antarctic during the last century, few long-term ecological monitoring programmes have been initiated in this region. Zooplankton have been demonstrated to be excellent environmental indicators and in 1991 a Continuous Plankton Recorder (CPR) survey was established in the Southern Ocean south of Australia for monitoring purposes. Using data collected between October 2001 and March 2002 this thesis aimed to provide baseline data for the Southern Ocean CPR survey, and the first high resolution (— 5 nm), ocean-basin scale sampling of zooplankton communities south of Australia. As a primary step the sampling characteristics of the CPR were investigated by direct comparison of depth integrated NORPAC net samples with CPR samples (average depth — 10.5 m). Data were collected on a repeat transect along 140°E, between November and December 2001. Species richness and Shannon-Weaver diversity increased, while dominance decreased with depth. The CPR under-represented components of the zooplankton community relative to NORPAC samples from the equivalent depth zone (0-20 m), particularly fast moving and soft bodied taxa, however, both of these surface sample groups were characterised by high densities of Oithona similis and appendicularia. Multivariate analysis demonstrated that the CPR effectively identified the location of the biogeographic boundary represented by the Southern Sub-Antarctic Front (SAF-S). After calibration of the CPR, two major components of natural ecosystem variability were investigated: spatial and temporal. The fine-scale horizontal structure of zooplankton communities was quantified from an 1170 nm transect along 140°E, conducted between 25 February and 3 March 2002. Six distinct zooplankton communities were identified, characterised by sharp boundaries which were strongly correlated with oceanographic fronts, demonstrating both the biogeographic importance of fronts and the sensitivity of zooplankton to their physical environment. The high degree of community complexity south of Australia reflected the regions unique oceanographic structure, characterised by multiple branches of the Sub-Antarctic Front, Polar Front, and Southern Antarctic Circumpolar Current Front. Zooplankton communities were also influenced by phytoplankton community structure and mesoscale processes (e.g. eddy shedding, cross-frontal advection). The seasonal component of temporal variability was investigated for two distinct regions, the SIZ and the Sub-Antarctic / Polar Frontal Zone (SAZ / PFZ). Six transects were completed in the SIZ between November and March along 140°E, where the SIZ is characteristically narrow. Zooplankton densities were low before ice melt (22-28 November transect). Peak densities (1963 ind.m-3) were recorded on the 10-15 January transect, dominated by small copepods, large calanoid copepodites (C1-3), Rhincalanus gigas, foraminiferans, pteropods and appendicularians. The densities of these "Peak Community" taxa decreased through February and March. Conversely, the densities of > C4 copepodite Calanoides acutus, Calanus propinquus and C. simillimus, Salpa thompsoni (salps) and Euphausia superba (krill) increased through the season, and these taxa had become important community components by March. Six transects were completed in the SAZ / PFZ at monthly intervals between October and March. Community structure was dominated by a small group of taxa throughout the season, including foraminiferans, Oithona spp. , appendicularians, Calanus simillimus, Rhincalanus gigas, Ctenocalanus citer, Clausocalanus spp. , pteropods and chaetognaths. Salps were an important component north of the SF-N throughout the season. Unlike the SIZ no seasonal shift in community structure was evident in the SAZ / PFZ. Peak densities to the north and south of the northern Sub-Antarctic Front were recorded in February and March respectively indicating a poleward lag in seasonal development. The CPR was demonstrated to be an extremely effective sampling apparatus in the Southern Ocean. The high resolution spatial and seasonal data collected during this study gave unique insights into Southern Ocean zooplankton ecology, and provided a base against which future variability, both inter-annual and long-term, can be measured.