Mesoscale zooplankton distribution patterns and euphausiid population ecology in the south-west Atlantic

Abstract

Two mesoscale net sampling surveys were conducted in the south-west Atlantic between 34° and 55 °S. The first survey was in the austral spring of 1990 and used both an RMT8 net which was trawled obliquely down to 200 or 300 m and caught mainly macrozooplankton and a Bongo net which was deployed at the surface and sampled mesozooplankton. The second survey was in the austral spring of 1991 and used a Bongo net which was deployed obliquely down to 50 m and sampled mesozooplankton. This thesis considers the species composition and abundance of these samples and represents one of the first insights into the mesoscale biogeography of zooplankton communities in the south-west Atlantic.

155 species from 9 taxonomic groups were considered including euphausiids, hyperiid amphipods, chaetognaths, salps, siphonophores, and nektonic/planktonic fish. Multivariate analyses were used to highlight species assemblage distribution patterns and determine strongly correlated environmental variables. In the 1990 RMT8 samples, species assemblages showed a distribution pattern related to the location of water masses, which was reflected in a combination of water mass and latitude being the most strongly correlated environmental variables. In the 1990 Bongo samples, a combination of seasurface temperature and latitude were most strongly correlated environmental variables and different species assemblages showed a pattern of being located in exclusive temperature ranges. The two sample sets did exhibit some common distribution patterns especially in the warm, sub-tropical waters to the north and the Falkland Shelf to the south. However, there were fundamental differences in the mid-latitudes regions, possibly reflecting the reduced ability of larvae to counteract expatriating forces when compared with adults. Further comparisons made between the 1990 and 1991 Bongo sample sets highlighted some of the causal factors behind distribution patterns. For instance, the precise definition of the boundary between sub-tropical and sub-Antarctic assemblages by the 17.3°C isotherm despite the multitude of expatriating phenomena suggested that many organisms were at the edge of their physiological limits in this region. In polar waters, distribution patterns were consistent but temperatures variable suggesting that advection rather than temperature tolerance was more influential. Further data from Montu (1977) and the Discovery Investigations was examined to add a seasonal dimension to the above patterns as well as providing an insight into the importance of population ecology on community distribution. Studies were concentrated on euphausiid species from which it was apparent that size structure and species dominance changed considerably with season. Estimates of the productivity of these species showed that weight-specific rates were comparable with more sub-tropical regions despite biomass levels being proportionally low.

The use of satellite thermal images for predicting faunal distribution patterns was assessed with respect to future biogeographic analysis of this region. Images were a good predictor at the sub-tropical boundary but a poor predictor in other regions highlighting the fact that in situ net sampling methods still appear to be the most effective and reliable investigative tools for biogeographic analysis.

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