Environmental flow and habitat requirements of aquatic macroinvertebrates from selected rivers in the Limpopo River Basin

Abstract

Managing rivers sustainably requires a thorough understanding of the response of aquatic biota to riverine microhabitat variability. The Limpopo River Catchment (LRC) is under extreme pressure from an increase in anthropogenic activities and demands. These pressures have a significant impact on the aquatic biodiversity within the riverine ecosystems within the LRC and is therefore very important to understand the responses of aquatic organisms for conservational and management purposes. Aquatic macroinvertebrates are often used in biomonitoring of rivers and was assessed during this study to evaluate the responses towards environmental flows of main stream rivers and tributaries in the LRC. The main aims were to assess and determine the drivers of macroinvertebrate community structures and preferences towards velocity and depth. Different approaches, taxonomical and functional traits, were used to provide results which will reflect the aims of this study. Sixteen sites in the LRC were sampled during 2021, that included both main stream rivers and tributaries, and two sites in the Olifants River were sampled during 2020 and 2021. Habitats at each site were measured and described in terms of depth, velocity, hydraulic biotope, and substrates. Water quality measurements were taken in-situ and samples taken in triplicate for nutrient analysis in the laboratory. Aquatic macroinvertebrates were sampled via a surber-sampler and in duplicate within each biotope present for quantitative data collection purposes. Data analysis consisted of diversity indices, multivariate analysis (RCA and RDA) and habitat suitability curves (HSCs). Taxonomical and functional traits revealed differences in spatial variations towards water quality and habitat preferences although slight variances were shown of the responses towards velocity and depth alterations. The substratum type was uncovered to be the main driver of the macroinvertebrate community structures within the LRC. The Olifants River study revealed that overall macroinvertebrates preferred shallow, slow flowing waters and the stones in current (SIC) biotope. Tricorythus sp.(mayfly) was identified as an indicator taxon for faster flowing waters. Further studies should be conducted on the LRC in consecutive years to provide accurate indicator taxa for the main stream rivers and tributaries. Taxonomic and functional approaches complement one another and is used together to provide more detail.