Distribution patterns of macrozooplankton assemblages in the southwest Atlantic

A zooplankton net sampling survey was conducted in the southwest Atlantic between 34* and 55* S during austral spring 1990. The survey grid covered shelf, slope and oceanic regions and contained 44 stations spaced at 100/200 km intervals. Sampling was carried out with an RMT8 net which was deployed obliquely from the surface to either 200 m over shelf regions or 300 m off-shelf. A total of 155 species were identified from 9 taxonomic groups including euphausiids, hyperiid amphipods, chaetognaths, salps, siphonophores and nektonic/planktonic fish. Log-transformed relative numerical abundances for each species were used in hierarchical classification to elucidate macroscale patterns of distribution. Factor analysis was also used on the most abundant and widely distributed species to reveal mesoscale patterns. Classification analysis produced 4 station groups which showed distinct geographic integrity and strong relationships to the positions of water masses. Group 1 was related to the Falkland shelf, Group 2, the Polar Frontal Zone (PFZ), Group 3, the Sub-Antarctic water mass (SAZ) and Group 4, the Sub-Tropical water mass (STZ). The dissimilarity matrix showed that the STZ group was very distant from the other 3 station groups, suggesting that its fauna is distinctly different to that found throughout the rest of the survey grid. By far the highest number of species showed strongest fidelity to the STZ group with 5 out of the 8 species clusters associated to that station group. Factor analysis gave further resolution to the station group patterns. The distribution of Factor 1 suggested that the warm Falkland Current return carried sub-tropical species into the SAZ. Factors 2 and 3 showed that there were at least 2 distinct faunal zones within the PFZ and Factor 4 highlighted the potential influence of the Falkland Current on the distribution of the shelf community. Overall it was evident that there was a strong correspondence between the distribution of faunal groups and water masses. However, the differences between SAZ, PFZ and Falkland shelf groups were mostly characterised by differences in abundance, whereas the STZ/SAZ boundary was marked by distinct differences in species composition.