Effect of environmental and spatial factors on small-sized fish assemblages in a tropical river

Izaias Médice Fernandes,Hugmar Pains Da Silva,Jhony Vendruscolo,Luzia Da Silva Lourenço,Katiele De Jesus Sacoman,José Paulo De Farias-Neto

doi:10.1590/1809-4392202002303

Abstract

ABSTRACT The river continuum concept predicts that gradual changes in river geomorphology and hydrology can drive longitudinal changes in aquatic community structure. Accordingly, we evaluated how environmental variables (electrical conductivity, pH, water transparency and water velocity), spatial factors (distance from headwaters and Moran’s eigen vector maps - MEMs), and the presence of dams affect small-sized fish assemblages along a 105-km stretch of the upper Branco River, a tributary in the Madeira River, Amazonas Basin, Brazil. Seine-net based collections were carried out at 15 sites up- and downriver from dams during the 2019 dry season. We captured a total of 4,330 individual fish belonging to three orders, nine families and 26 species (and a hybrid individual). Electrical conductivity and pH were affected by the presence of dams and the distance from headwaters. Species richness and abundance did not vary in response to environmental variables. While species richness showed no significant variation along the sampled river section, abundance showed a negative relationship with distance from headwaters. Species composition varied significantly in response to pH, linear spatial factors and the presence of dams. Our results suggest that change in species composition of small-sized fish assemblages in the upper Branco River occurs due to variation in water characteristics, inherent dispersal limitation, and in response to the presence of dams.

Highlights

The river continuum concept (Vannote et al 1980) predicts changes in community structure and ecosystem processes accompanying the gradual changes in geomorphology and hydrology that occur from a river’s headwaters to its mouth (Foubert et al 2018)
This study aimed to evaluate how the structure of small-sized fish assemblages in the Branco River, a tributary in the Madeira River basin in the southwestern Brazilian Amazon, are influenced by: i) environmental factors; ii) linear spatial factors; and iii) the presence of barriers
A sole hybrid individual resulting from a cross between Leiarius marmoratus and Pseudoplatystoma punctifer was captured in the small hydroelectric power plants (SHPs) Ângelo Cassol reservoir (P10 in Table 2 and Figure 1)

Summary

Introduction

The river continuum concept (Vannote et al 1980) predicts changes in community structure and ecosystem processes accompanying the gradual changes in geomorphology and hydrology that occur from a river’s headwaters to its mouth (Foubert et al 2018) This vision of a longitudinal gradient has dominated riverine ecological studies in recent years (Benda et al 2004). Drainage basins are dendritic structures, where rivers function as corridors through which energy, matter and living organisms are transported (Rodriguez-Iturbe et al 2009) In this structure, headwater sites, being less productive and more isolated, tend to have high endemism, lower species richness and species assemblages that are more structured by environmental variables than assamblages located downstream, which have higher connectivity and are structured by dispersal (Henriques-Silva et al 2019). Longitudinal connectivity is considered an important aspect as it allows fish and other organisms to disperse through a water course and migrate between upstream and downstream habitats (Fullerton et al 2010; Branco et al 2012), especially during the reproductive period (Lucas and Batley 1996)

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