Spring plankton communities in the southern Patagonian shelf: Hydrography, mesozooplankton patterns and trophic relationships
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AbstractA strong interest in the southern Patagonian shelf has emerged in recent years, along with the increasing recognition of its high biological productivity. Knowledge of the pelagic food web structure that supports the richness of this system is still developing, but there are indications that mesozooplankton occupy a pivotal position, as consumers of smaller plankton and as vital prey for fish and squid. All plankton communities in the size 2μm-20mm, total and size-fractioned chlorophyll a (Chl a), nutrients and hydrology were surveyed simultaneously in October 2005 between 47°S-55°S. Picoplankton, nanoplankton and microplankton were taxonomically and functionally (autotrophs, heterotrophs) sorted within each size fraction. Plankton data and trophic relationships were examined through multivariate statistics. At that time fairly homogeneous thermal conditions prevailed over most of the shelf but weak saline horizontal gradients were evident. N/P ratios indicated no N or P limitation for phytoplankton. Surface concentrations of total Chl a were particularly high in the Grande Bay area at ca. 51°S near shore (28.6mgm-3) and at ca. 47°S on the shelf-break (7.7mgm-3). At both locations the contribution of the Chl a>5μm fraction was remarkably high. The dinoflagellate Prorocentrum minimum (10.106cellsL-1) and the diatom Thalassiosira cf. oceanica (1.3.106cellsL-1) were respectively blooming at these sites. Otherwise <10μm plankton prevailed overall. Copepods largely dominated the >200μm fraction. Three mesozooplankton assemblages typical of the inner, middle, and outer shelf were identified. The inner and middle shelf assemblages overlapped slightly but were spatially separated from the outer shelf community. Adults and late copepodids of Drepanopus forcipatus were typical of the inner shelf assemblage. Middle-shelf species included the copepod Ctenocalanus vanus, the amphipod Themisto gaudichaudii and the chaetognath Sagitta tasmanica, while an assortment of taxa characterized the outer sector. Latitudinal patterns in mesozooplankton community composition were less noticeable than cross-shelf patterns. No clear distribution of phytoplankton and protozooplankton assemblages was apparent when the whole <200μm plankton community structure was considered. In contrast, communities in the optimal size food for copepods (>10-200μm) were slightly different across shelf. Overall, spatial patterns of mesozooplankton and food availability matched weakly, suggesting a poor coupling between consumers and their prey communities at the time. Significant correlations were found particularly with large autotrophs and heterotrophs.
JournalJournal of Marine Systems