Modelling flow and water temperature in the Luvuvhu catchment and their impact on macroinvertebrate assemblages

Abstract

Stream flow and water temperature regimes are master variables driving river ecosystems, which determine the spatial and temporal dynamics of plant, benthic and other taxonomic groups in streams. Stream flow and water temperature trends are changing drastically worldwide with far-reaching effects on aquatic organisms. These changes are predicted to be disproportionately higher in southern Africa compared to most other regions of the world, due to declining and unreliable rainfall, increasing human population and changing land and water uses. In this study, these issues were investigated in the Luvuvhu River catchment (LRC), a strategic water source area in an arid region of southern Africa. Stream flow in the LRC is currently subjected to major forms of flow alteration, including large dams and abstraction. Studies on long-term changes in stream flow and water temperature regime are limited in this catchment. Given the fact that these rivers are home to many flow and temperature sensitive aquatic organisms such as mayflies and blackflies, the current situation in the LRC raises important concerns around the hydraulic and thermal stress of these biota. Making things worse, very few studies have assessed the constraints towards the development of conservation and management programs for rivers and their related ecosystem services in the LRC.

Assessment of long-term changes in stream flow patterns, correlation between sub-daily and daily flow metrics, the influence of stream flow, water temperature and other environmental variables on both mayfly and blackfly assemblage structure and also the identification the indicator species along important environmental gradients were the main objectives of this study. This study also reviewed institutional challenges affecting the sustainable management of stream flow and river management frameworks to provide environmental flows. Flow gauging stations were selected across the catchment based on the availability of long-term (> 20 years) stream flow data, while mayfly and blackfly organisms were collected from twenty-three sites for not less than twelve months together with the measurements of flow, water temperature and other environmental variables.

All stream flow trend analyses pointed to declining stream flow volumes that are most pronounced during the wet seasons. This declining stream flow volumes are occurring in both tributaries and main river channels. Stream flow monitoring is poor in the LRC with flow at most of the stations not natural, where only one gauging station had natural flow records. However, most of the natural subdaily flow metrics (for example, daily minimum, daily maximum, daily delta, standard deviation, maximum hourly ramp rate, daily path length, and daily standardized delta) were related to daily flow metrics. Water temperature is slowly but significantly rising, while climatic data showed that in rainfall and air temperature in the catchment had no significant change over the last 35 years. The absence of catchment specific management institutions and strategies to address issues such as water allocation and conservation is one of the major problem in the LRC, and these has implication for aquatic biota as the habitat continues to deteriorate. Water temperature is the major driver of mayfly structure and richness. Stream flow explained some considerable amount of variation, while a large proportion of variation in blackfly community structure remained unexplained. Majority of mayflies are cold water species and are threaten by changing water temperature (> 19 °C), while blackfly species respond positively to increasing flow. The development and implementation of catchmentwide flow management strategies and abstraction thresholds would make a major contribution in minimizing the declining stream flow volumes in rivers of the LRC. To save aquatic biota in the LRC, a key focus for impounded rivers should be the provision of environmental flows for restoration of habitat for aquatic species, while for non-regulated rivers, the most effective option is to conserve and maintain their free-flowing condition. Stream flow monitoring and reporting should be prioritized to give regular accounts of the health of our rivers.