Fish assemblages of the northern Argentine coastal system: spatial patterns and their temporal variations

The springtime spatial pattern of demersal fish assemblages in the northern Argentine coastal ecosystem (34 degrees-41 degrees S) and their relationships with environmental variables were investigated over 6 years (1981, 1983, 1994, 1995, 1998 and 1999). For each fish assemblage area for each year, we identified the most common species and those species that had the highest ability to discriminate it from adjacent assemblage areas. Within each area, species were considered as resident (common in all years) or facultative (contributing to less than 90% of similarity within any year). The analysis (detrended canonical correspondence analysis), conducted using species abundances (t nm(-2)) and environmental data, indicated that six fish assemblage areas are consistently distinguishable. The fish assemblage areas correspond to inner, central and middle regions of the Rio de la Plata estuary, and inner, central and middle regions of the coastal shelf. Spatial differences in the composition among areas were primarily due to changes in species composition rather than changes in relative abundance, and were primarily related to water mass characteristics. Salinity in the estuary and temperature on the shelf were more relevant to the spatial structuring of the fish community in each environment. The assemblage areas showed a remarkable degree of temporal persistence in species composition and geographical location. The facultative species contributed more to the species composition temporal dynamics than resident species. El Nino (La Nina) condition, associated with increase (decrease) of the Rio de la Plata freshwater discharge, produced a movement of the estuarine assemblage areas to the coastal shelf (upriver). Under normal freshwater discharge conditions, the boundaries of the fish assemblages were relatively more stable.