Spatial and temporal changes in the riverine macroinvertebrate community composition in the Berg River, and the expected response following the development of the Berg River Dam

Abstract

The main objective of the Berg River Baseline Monitoring Programme (BRBMP) was to describe the historical and current state of the chemical, physical and biological characteristics of the river and its estuary, floodplains and groundwater links that are most likely to be affected by the construction of the Berg River Dam. The dam will have a particularly significant impact on the flow regime of the river, and so the emphasis of the BRBMP was on determining the relationship between flow and the chemical, physical and biological characteristics of the river. The riverine macroinvertebrates component of the BRBMP comprised the collection of data from five sites along the length of the BergRiver—two in the foothill zone and three in the lowland river zone. The macroinvertebrates were sampled in each season for three years, from 2003 to 2005, using a rapid bio-assessment method, the South African Scoring System (Version 5). The macro-invertebrates were identified to family level in situ, and in the laboratory to species level, where possible. Distinctive communities were found to inhabit the various biotopes sampled, and these communities were significantly different between the foothill and lowland river sites. The upstream site was far less impacted than any of the other sites. Significant differences were found between the communities sampled in the various years at most of the sites, with the exception of the upstream site. At most sites, there was a decrease in diversity, a loss of the more sensitive taxa and decreased frequency of occurrence for many taxa, from 2003 to 2005, which was a markedly wetter year, with more frequent and higher discharge floods. However, the macroinvertebrate communities inhabiting the most upstream site appeared to be fairly resilient to the physical perturbations associated with the increased discharges and flood frequencies of 2005. Seasonal fluctuations in community composition were more marked at the foothill sites than at the lowland river sites. The paper concludes with some expected macroinvertebrate responses to the modifications in flow, geomorphology and water quality that are expected in the river reaches below the Berg River Dam. The impacts of the Dam are expected to be greatest in the river reaches between the dam and the confluence with the Franschhoek River, where the river will rely solely on the environmental flow releases from the dam.