Role of water temperature variability in structuring aquatic macroinvertebrate communities : case study on the Keurbooms and Kowie Rivers, South Africa.

Abstract

Water temperature is a critical factor affecting the abundance and richness of freshwater stream aquatic macroinvertebrate communities. Variable seasonal river temperature patterns are a critical factor in maintaining temporal segregation in aquatic invertebrate communities, allowing for resource partitioning and preventing competitive exclusions, while spatial differences in water temperatures permit zonation of species. This research investigated whether the degree of predictability in a stream’s water temperature profile may provide some indication of the degree of structure and functional predictability of macroinvertebrate communities. Quarterly aquatic macroinvertebrate sampling over a single year along the longitudinal axes of two river systems, Keurbooms River in the southern Cape, and the Kowie River in the Eastern Cape, were undertaken as the core component of this research. The two river systems shared similar ecoregions and profile zones, however were expected to differ in their thermal variability, based on the hydrological index and flow regimes for their respective quaternary catchments. Hourly water temperature data were collected at each sampling site from data loggers installed at five paired sites on each stream system. The aquatic biotopes sampled were in close proximity to the loggers. Multivariate analysis techniques were performed on the macroinvertebrate and water temperature data. Macroinvertebrate taxon richness was greater on the perennial Keurbooms than the non-perennial Kowie River where, on a seasonal basis, taxon richness increased from winter to autumn on both systems. Macroinvertebrate species turnover throughout the seasons was higher for sites having lower water temperature predictability values than sites with higher predictability values. This trend was more apparent on the Keurbooms with a less variable flow regime. Temporal species turnover differed between sites and streams, where reduced seasonal flows transformed the more dominant aquatic biotopes from stones-in-current into standing pools. Findings included aquatic macroinvertebrates responding typically in a predictable manner to changing conditions in their environment, where water temperature and flow varied. The findings of this research demonstrate that macroinvertebrate taxa do respond in a predictable manner to changes in their environment. This was particularly evident in relation to variability in water temperature and flow.