Benthos of a large neotropical river: spatial patterns and species assemblages in the Lower Paraguay and its floodplains

Abstract

We studied the composition and spatial structure of benthic invertebrate assemblages and their physical and chemical environment on a 331 km long section of the Lower Paraguay River between Asuncion (Paraguay) and the river's confluence with the Parana River near Corrientes (Argentina). Applying the same methods used in former studies in the Upper Paraguay and Parana Rivers, we sampled bottom fauna, bed sediments and water at ten riverine stations, two tributaries, and six floodplain lake sites. Seventy-six taxa, mostly oligochaetes (18 species) and chironomids (17 species), were identified. There was a clear distinction between central sites, river banks, and floodplain habitats. Highly significant regressions between sediment composition and biotic parameters were obtained (clay/diversity, R = 0.60, p < 0.0001; silt/diversity, R = 0.61, p < 0.0001; organic matter/diversity, R = 0.63, p <0.0001; coarse-middle sand/density, R = 0.75, p = 0.001). Central channel sites were dominated by the psammophilous oligochaete Narapa bonettoi, which attained densities of 10-3,500 ind. m -2 . The river banks had higher diversity indices, but lower average densities than the main channel center. Some sites with solid substrates were densely colonized by an invading mollusk species, Limnoperna fortunei, which had maximum densities of 13,500 ind. m -2 . Diversity was highest in the floodplain lakes (Shannon's H = 2.4 ± 0.67) compared with banks (H = 1.79 ± 0.77) and central sites above the confluence of the Bermejo River (H = 0.89 ± 0.35). This tributary carries large amounts of fine sediments and organic matter into the Paraguay River, thereby reducing biodiversity and abundance of Narapa bonettoi. The faunal composition of the Lower Paraguay was more similar to that of the Middle Parana than to that of the Upper Paraguay. From this and former studies, two general patterns of river ecology become apparent: 1) naturally flowing sand-bed rivers can have continuous species assemblages over very long distances, characterized by few, well-adapted species that occur in high densities, but contribute relatively little biomass to the riverine foodweb; and 2) the biodiversity in the floodplain water bodies is higher than in the river channel as a result of the greater habitat diversity.