IMPACTS OF GROUNDWATER AND SURFACE WATER EXCHANGE ON INSTREAM HABITAT AND AQUATIC MACROINVERTEBRATES: CONSEQUENCES FOR ENVIRONMENTAL FLOW REQUIREMENTS IN GROUNDWATER-FED STREAMS

In environmental flow assessments, the responses of aquatic organisms to changes in flow-related habitat characteristics (e.g. depth and velocity) and water-quality-related habitat characteristics (e.g. temperature and dissolved oxygen) with changing discharge are central for developing recommendations for maximum limits to hydrological alterations by proposed activities. Traditional flow assessments, however, have ignored the role of groundwater and surface water exchange in controlling the flow regime and the distribution of thermal habitats, which are important to aquatic species. Furthermore, flow assessments have historically focussed on fish habitat availability, while discounting other important factors, such as the abundance of prey. The purpose of this study is to examine the ecological consequences of changes to the thermal and hydrological regime of a stream, specifically in relation to groundwater-surface water dynamics, through a field study at Otter Park in Langley, British Columbia. A hydrological network consisting of instream piezometers, hydrometric stations, and streambed temperature loggers is used to monitor streambed flux and streamflow characteristics from the spring to the fall to examine how changes in groundwater and surface water exchange dynamics impact the distribution of instream habitat. Benthic macroinvertebrates, a fundamental group of stream taxa whose function role is to process and transfer energy to their fish predators, are collected over this range of flow conditions to assess the responses of invertebrate communities to changes in instream habitats. Ultimately, this research will help our understanding of ecological flow requirements in groundwater-dependent ecosystems, which can be leveraged to make more robust environmental flow recommendations, especially in regions where non-stationarity in hydrological conditions due to climate change and human activities threaten the stability of groundwater and surface water exchange dynamics.