Consequences of Groundwater and Surface Water Exchange for Instream Habitat and Aquatic Macroinvertebrates

Abstract:

Aquatic macroinvertebrates are a fundamental component of river ecosystems, as their functional role is to process energy from plants and organic materials and transfer it to their fish predators. The composition and distribution of macroinvertebrate communities in stream systems is influenced by flow-related habitat variables, such as depth and velocity. Water-quality-related habitat variables, especially temperature, also influence the suitability of freshwater habitats for different invertebrate taxa. Because the exchange of groundwater and surface water across the streambed influences both streamflow and water quality characteristics, the spatial and temporal dynamics of groundwater and surface water exchanges can be an important control on the distribution of macroinvertebrate habitats in connected aquifer-stream systems. Most importantly, locations of groundwater discharge can provide critical flows and thermal habitats during the hot low-flow conditions that often persist in the summer months in groundwater-fed streams. While there is a general understanding of the importance of groundwater and surface water exchange for maintaining habitat characteristics, few studies have characterized macroinvertebrate community responses to changes in habitat in direct relation to groundwater and surface water exchange. In this study, groundwater and surface water exchange dynamics are characterized through a field study at Otter Park, Langley, a stream that historically dries during the summer. A hydrological network consisting of instream piezometers, hydrometric stations, and streambed temperature loggers is used to monitor streambed flux and streamflow characteristics continuously, with focus on the summer low-flow period. Macroinvertebrates are sampled throughout the stream reach and over the summer low flow period to examine macroinvertebrate community responses to spatial and temporal variability of instream habitat. The results of this study will help our understanding of how aquatic ecosystem components depend on the spatial and temporal variation of groundwater and surface water exchange. This understanding is important for river management situations, especially those concerned with determining minimum instream flow requirements for aquatic species.