| Resumo: |
The spatial and temporal changes of the fish assemblage areas composition at the coastal ecosystem of the western South Atlantic (34 degree -41 degree S) during six springs (1981, 1983, 1994, 1995, 1998 and 1999), were examined. For each assemblage area the most common species and those species that discriminate it from adjacent assemblages areas within years, were identified. Within each area, also resident (common in all years) and facultative (contributing to less than 90% of similarity within any year) species, were studied. Also were identified what parameters control the main structuring of the fish community on the spatial and temporal patterns. In order to determine how the environmental factors influenced the fish community structure, the aggregate indicators of ecosystem status: dominance curves, ABC curves, and integrated community size spectrum, were analyzed. The analysis (DCCA), carried out on the species abundance (t x nm super(-2)), indicated that there are fish assemblage areas consistently distinguishable between them during all years, by their environmental characteristics and species combination that structuring the areas. The fish assemblage areas correspond to Inner, Central and Middle region of the Rio de la Plata estuary (Argentina-Uruguay), and Inner, Central and Middle region of the coastal shelf (Argentine Sea). The differences in the area composition were primarily due to changes on species composition more than changes on their relative abundance. The spatial changes in the assemblage areas were primarily related with water masses physical characteristics. Different factors, as the salinity in the estuary and the temperature in the shelf, were more relevant on the spatial structuring of fish community in each environment. The assemblage areas showed a remarked degree of temporal persistence of their boundaries, with some temporal variation in their extension and localization. The temporal analysis indicates the fishery as one of the factors more influent on the temporal variations of fish community of the fish assemblage areas. Significant differences on the species composition were observed between most of the analyzed years, mainly associated to changes on the relative abundances more than changes on species composition. Most of the changes on species biomass implied decreasing of biomass, this being particularly evident on chondrichthyes species. The percentage of contribution to the temporal dissimilarity of the areas by resident or facultative species indicated that facultative species contributed more to the temporal dynamics of species composition within assemblages than resident species. This relative stability of resident species composition provides a degree of predictability that may confer some utility to these areas as reasonable proxies for ecosystem management areas. The aggregate indicators of ecosystem status during the last 18 years, suggested that the community structure progressively tends to be dominated by less species and smaller individual, with higher perturbation level.
Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales.
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