Abstract
The copepod community structure, with special emphasis on small-sized species, was studied over the southern Patagonian shelf in late summer 2004, applying the first plankton sampling in the region with a fine-mesh (66 μm) net. The key role of the copepods Drepanopus forcipatus and Calanus australis was confirmed, but also the high abundance and frequency of occurrence of the microcopepods Oithona helgolandica and Microsetella norvegica and of the medium-sized copepod Ctenocalanus vanus were revealed. Copepod community structure was nearly homogenous over the entire study area. Drepanopus forcipatus, O. helgolandica and M. norvegica were identified as the typical species of the region, although secondarily C. australis and Oithona atlantica also contributed significantly to community similarity across the area. The study of interspecific relationships of dominant copepods indicated that D. forcipatus and C. australis were associated positively with O. helgolandica , while C. vanus , and M. norvegica constituted a separate assemblage with Clausocalanus brevipes and O. atlantica . The importance of fine-mesh-size nets for collecting the smaller size fractions of mesozooplankton and for accurately portraying the mesozooplankton assemblage structure in the area is stressed by this study.
Highlights
Quantitative assessment of planktonic organisms is a key issue in determining food web structure and material cycling in pelagic ecosystems
Ctenocalanus vanus, Calanus australis and Microsetella norvegica contributed much less to total numbers, accounting for 1 to 2% each
Drepanopus forcipatus and O. helgolandica occurred at all stations (FO=100%)
Summary
Quantitative assessment of planktonic organisms is a key issue in determining food web structure and material cycling in pelagic ecosystems. Reliable data on the mesozooplankton (0.2-20 mm), an important size class of the zooplankton community that links primary producers with higher trophic levels, are essential in order to accurately interpret the structure of marine pelagic ecosystems. The widespread use of coarse plankton nets with standard mesh sizes of 200330 μm to collect mesozooplankton has led researchers to focus primarily on large copepod species, and to substantially underestimate the abundance of smaller organisms. The structure and function of the mesozooplankton community have been analysed in the region primarily for the larger, most abundant and most frequently occurring species captured by those standard nets. Despite its importance, the overall structure of the zooplankton community of the Patagonian shelf has not been studied in detail, and species diversity patterns are poorly described
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