The role of the hydrological cycle on the distribution patterns of fish assemblages in an Andean stream

The seasonal and interannual changes of the fish assemblage in the main channel and littoral zone of the Guarinó River, a torrential system located in the Colombian Andean foothills, were examined in relation to the physical and chemical environmental changes associated with the hydrological cycle and the El Niño-La Niña-Southern Oscillation (ENSO) between 2007 and 2010. Four samplings per year (in dry season and high water) were performed. Environmental variables (temperature, pH, conductivity, turbidity, oxygen, total nitrogen, orthophosphate, depth and flow rate) were contrasted with ANOVAs and a discriminant analysis to establish temporal patterns. Biological descriptors (richness, density and biomass) were contrasted with ANCOVAs. The temporal patterns of the taxonomic and functional composition and the density of the fish assemblages were examined with respect to environmental variables through canonical discriminant analysis per habitat. Interannual differences were significant with regard to density and richness for the main channel habitat; while in the littoral zone, the differences were significant between both the year and seasons. Discriminant analysis showed variations in species composition and relative abundance between the main channel and the littoral zone under contrasting hydrological conditions of El Niño-La Niña. High flows from 2008 to 2009 (La Niña) favoured resident species (e.g. Creagrutus magdalenae) and small benthic Siluriformes (e.g. Chaetostoma spp.), but was limited to migratory species (e.g. Prochilodus magdalenae). From 2009 to 2010 (El Niño), the most common species were reduced and rare species increased. River flow, temperature and oxygen were the variables that had the largest influences on the seasonal and interannual differences in the fish assemblage structure of the Guarinó River. The results suggested that the presence and abundance of species and functional groups in different habitats were regulated by stochastic conditions, such as droughts, floods and drag events associated with the Southern Oscillation phenomenon (ENSO), acting as a hydrological disturbance on the habitats and generating different responses according to the adaptive characteristics of the species.