Abstract
We analysed changes in the fish assemblage structure along a longitudinal gradient of the Paraíba do Sul River and Funil Reservoir. We tested the hypothesis that shifts from lotic to lentic environment affect the richness and structure of the assemblage which are modulated by seasonal rainfall changes. Standardised monthly samplings were carried out from October 2006 to September 2007 in four zones: 1) river upstream from the reservoir; 2) upper part of the reservoir; 3) lower part of the reservoir, and 4) river downstream from the reservoir. Fishes were caught using gillnets deployed for 15 hours. We collected a total of 4550 specimens, representing 35 species and 5 orders. The highest richness and diversity were recorded in zone 2, the transitional zone between river and reservoir. In this ecotone, lotic and lentic species overlap. Greater abundance and biomass was recorded in the river upstream from the reservoir (zone 1); however, there are no differences between the zones in the structure of assemblages during the wet season. During the dry season, the assemblage structure is more differentiated between zones, although no differences in abundance and biomass occur. The seasonal flow of the river is the major driving factor to influence the fish assemblage structure along the longitudinal gradient from the river to the reservoir.
Highlights
Reservoirs have been considered hybrid systems due to their pronounced environmental gradient; that is, they have limnological properties intermediate between those of rivers and lakes (Thornton, 1990)
Local fish assemblages can be organised across space, since each species has different tolerance limits that vary across environmental gradients (Holmgren & Appelberg, 2000)
Questions addressed here include (i) How does the fish assemblage change along the longitudinal and temporal gradient from the river upstream to the reservoir to the river downstream? (ii) What physical and chemical variables are associated with longitudinal and temporal changes in assemblage structure? In answering these questions, we test the hypothesis that shifts from lotic to lentic environment affect the richness and structure of the fish assemblage due to physical, chemical and biological changes in the system, which are modulated by seasonal rainfall changes
Summary
Reservoirs have been considered hybrid systems due to their pronounced environmental gradient; that is, they have limnological properties intermediate between those of rivers and lakes (Thornton, 1990). The upper part of a reservoir tends to be similar to a river, while the lower part has characteristics more similar to a lake. Because of this gradient, local fish assemblages can be organised across space, since each species has different tolerance limits that vary across environmental gradients (Holmgren & Appelberg, 2000). Longitudinal and seasonal gradients along reservoirs influence environmental conditions that determine patterns in fish assemblages (O’Brien, 1990; Agostinho et al, 2004a), they are rarely considered in management plans
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