Patterns of colonization in neotropical reservoirs, and prognoses on aging

Abstract

We review patterns of reservoir colonization and aging in neotropical reservoirs, with emphasis on the Upper Paraná River Basin, Brazil. The filling stage was a period of rapid and intense transformation, an abrupt transition from a lotic to a lentic environment. This stage lasted 8-80 d, which was relatively short in comparison with reservoirs in temperate zones or other reservoirs in the Amazon basin. Release of nutrients, increases in water transparency, decreases in turbulence, and development of thermal and oxygen stratification were key factors characterizing this transition. Anoxic events were critical during this stage. During the first few days individuals of various fish species occupied the entire reservoir and water column, independent of the habitat they had inhabited in the river, or the type they would eventually inhabit in the new lake. As anoxia progressed, fish moved to the embayments and tributaries including the main river. Species that remained in the reservoir concentrated mainly in shallow, littoral areas and near the mouth of tributaries. The season of the filling stage was extremely relevant to the colonization process. The ecological transformations observed during the filling stage of neotropical reservoirs are likely more accentuated than in temperate reservoirs, because water temperature remains warm around the year. After the original upsurge, no severe changes in abiotic environment were identified, but a gradual colonization by submersed aquatic macrophytes was evident in some reservoirs, and changes in the fish communities were identified in all reservoirs. In one reservoir, diversity of macrophyte communities became higher than in natural enviromments within its basin. There was a general absence of fish species pre-adapted to colonize the pelagic zones, and species diversity was higher in the littoral zone. Many species that colonized neotropical reservoirs reproduced in lateral tributaries or upstream stretches. In at least one reservoir, reproductive effort by the fish assemblage in the first two years was low and increased thereafter. Migrators and large-bodied fish reached peak abundance shortly after impoundment, but assemblages shifted towards a dominance by sedentary, medium-sized species. As stability was approached after the upsurge period, diets shifted towards more stable, autochthonous resources. In general, detritivores-illiophages showed sharp reductions, whereas herbivores and zooplanktivores increased. Piscivores increased in some reservoirs but decreased in others; often they became abundant where pre-adapted lacustrine species existed within the basin. Anticipated environmental changes such as reservoir age, including nutrient accumulation, siltation, and habitat deterioration, particularly in the littoral zone are expected to cause faster changes in the fish assemblages. The most notable ones include a reduction in the number of top predators, mean size of species, and species richness. Pelagic species are expected to become dominant as benthic organisms decline and littoral habitat deteriorate. Short-lived species with fast growth or reproductive compensation are expected to have survival advantages. In advanced stages of aging, fish communities are anticipated to consist of species typical of murky, shallow water, with low oxygen. These aging effects are long-term, but will become noticeable in littoral areas sooner.