Abstract
The effect of hydrological connectivity of the fish assemblages was assessed on a floodplain in the SE Gulf of California, Mexico using a Before–After Control-Impact (BACI) design. Community structure attributes of species abundance, biomass, richness, diversity, and differences in the structure of fish assemblages were compared between two periods (January to June in 2011 and 2015) and two flood plains, one designated as the control zone which was divided by a road, and another one designated as the treatment zone which from January – June 2011 was divided by a road, and then sampled from January - June 2015, as the site was rehabilitated by removing the road in the year 2012. Fish were sampled at monthly intervals using a seine net at different stations in both floodplains. A total of 7024 organisms, comprising of 14 species belonging to 11 families, were analysed. In the control zone during both periods and in the treatment zone before removal of the road Poecilia butleri was the most abundant species and two exotic species (Tilapia) accounted for more than 95% of the total biomass. After the removal of the road in the treatment zone Poeciliopsis latidens was the most abundant species and the relative biomass of both species of tilapia decreased to 85%, but was still the most important in terms of biomass. However, richness and diversity were higher and the structure of the fish assemblages was different in the treatment zone after the road was removed, due to the presence of species with estuarine and marine affinity in this zone and period. Our results indicate that hydrological connectivity proved to be a key factor influencing the fish composition and abundance in the floodplain system in the region. Hence, a better hydrological connectivity implied higher abundance and diversity, likely related to an increase in the habitat complexity. The priority in the management of the ecological functioning of the floodplain system should be set on minimising the modifications of the natural flow and thus avoiding the changes on the fish biota.
Highlights
Seasonal coastal floodplains along tropical latitudes are amongst the most biologically productive and diverse ecosystems on earth (Tockner and Stanford, 2002; Junk et al, 2006) and are an important component of rivers and wetlands, acting as a nursery habitat and refuge for fish (Junk and Wantzen, 2004)
This same pattern was observed in the present study, as abiotic factors seem to meaningfully define changes in fish composition through time
The floodplains analyzed were brackish at the beginning of the year, and in general had good water quality, but as time passed they became hypersaline and hypoxic, until the control zone in both studied periods, and the rehabilitated zone during 2011, dried up by June. In this area the dry season runs from mid-November to mid-June (Amezcua et al, 2019), so all the sampled months occurred during the dry season, and as time passed, the temperature increased, causing evaporation, and a reduction in the depth of the system, an increase in the salinity and temperature of the water, and a decrease in the dissolved oxygen
Summary
Seasonal coastal floodplains along tropical latitudes are amongst the most biologically productive and diverse ecosystems on earth (Tockner and Stanford, 2002; Junk et al, 2006) and are an important component of rivers and wetlands, acting as a nursery habitat and refuge for fish (Junk and Wantzen, 2004). These dynamic systems are seasonally flooded during flow and ebb currents resulting on water level variations. Contrary, during the dry season the floodplains are prone to an alteration of the hydrological connectivity (Jardine et al, 2012), which limits the fish movement and certainly provokes the mortality of many organisms trapped in dried habitats
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