Macroinvertebrate functional responses to human disturbance and flow cessation in Afromontane-savannah rivers

Abstract

Wildlife, people, and livestock rely on riverine ecosystems in arid and semi-arid areas as primary water sources. Studies on the influence of human activities and livestock on the ecological health of these seasonal systems are thus necessary. This is more relevant given the increasing demand for water as human populations grow, which leads to over-abstractions and, sometimes, cessation of flows in streams and rivers during the dry periods. Although the structural composition of macroinvertebrate communities has been utilized to indicate the ecological integrity of streams and rivers, macroinvertebrate functional feeding groups (FFGs) are less studied, especially in intermittent Afrotropical streams. We used macroinvertebrate FFGs as indicators of water quality and ecological integrity of streams influenced by different levels of human disturbance and flow variability in the Afromontane-savanna Bura and Wundanyi rivers in Taita Taveta County, Kenya. A total of 18 sampling sites were identified for sampling and grouped into three (3) disturbance categories (low– n = 7, moderate - n = 4, and disturbed - n = 7) and two categories of flow permanence (permanent – n = 9, and seasonal – n = 9). At each site, sampling of physicochemical water quality parameters and macroinvertebrates was done twice during the wet and dry seasons. Ratios of five FFGs (collector-gatherers, collector-filterers, scrapers/grazers, predators, and shredders) were used to derive five metrics that are surrogates of ecosystem attributes in the rivers. There was a significant difference (p < 0.05) in the concentrations of dissolved organic carbon, total phosphorus, pH, and electrical conductivity between the three site categories. Seasonal sites recorded higher electrical conductivity and total dissolved solids compared to permanent streams. Total suspended solids and particulate organic matter were higher during the dry season. FFGs responded to the disturbance gradient, seasonality, and flow variability in the study area with high numbers of predators and scrapers during dry season, suggesting that the human disturbance influenced the functional composition of macroinvertebrates in the rivers. The findings also show that flow variability (seasonal vs flow permanence) played a important role in structuring communities and determining ecosystem functioning. Therefore, in addition to general human disturbance, there is also a need to study the impact of excessive water withdrawals or changes in natural flow regimes of streams and rivers on aquatic communities and the development of indices to assess their effects.