East Branch Pecatonica River
Stream Temperature / Groundwater Discharge
Remotely-sensed thermographic profiles and in-stream temperature histories validated a one-dimensional stream temperature model that highlights the threat of global climate change to ecologically valuable and economically significant trout fisheries in southwest Wisconsin (study site - Figure 1). This work demonstrates the value of remotely-sensed thermal data for stream temperature model validation and analyses of thermal heterogeneity. Without thermal infrared data, the significant depression in stream temperature from ~1.5-3 km would not be easily identified (Figure 2). The profile developed with thermal infrared data increases our understanding of spatially-variable groundwater inflow to the stream. This finding will benefit fisheries managers seeking to locate and protect thermal refugia, where maximum daily stream temperatures are depressed due to the inflow of cool groundwater.
Figure 1. Study site location. (1) Location of the East Branch Pecatonica River (2) Sample thermal image collected over the East Branch Pecatonica River (3) Modeled stream reach (10.47 km) and digital elevation model
Figure 2. Comparison of remotely-sensed (RS), in-stream (In) and simulated (Sim) temperatures on July 24, 2008 for the East Branch Pecatonica River. The profile moves downstream left to right (0 km is upstream, 10.47 km is downstream). Remotely-sensed temperatures were sampled using a thermal infrared camera. In-stream records at 2.75 km, 7.3 km and 7.75 km were recorded using HOBO loggers. Simulated temperatures were modeled using Heat Source V.8.0.4 software.
Twelve stream temperature model simulations were used to evaluate the potential threats to brook trout and brown trout at the East Branch Pecatonica. Both increases in air temperature and decreases in baseflow drive maximum daily stream temperature high and in combination threaten the stream's fishery (Figure 3). In the most extreme climate change scenario, stream temperature may cross thermal tolerance thresholds developed for similar fisheries in Wisconsin and Michigan (Wehrly et al. 2007) (Figure 4). This result has implications beyond this study site. Stream temperature in higher flow streams or those with smaller groundwater input may be more susceptible to change due to changes in stream flow. At this study site, the absolute difference between the base case and the recharge simulations is greater at the downstream location than the upstream location. On a larger stream, cases with stream flow alteration may reach thermal tolerance thresholds. Wider, shallower streams warm faster and may reach critical thermal maxima for trout.
Figure 3. Maximum daily temperature for model simulations of the base case (no change in air/groundwater temperature or recharge), the base case plus 1°C, the base case plus 3°C and the base case plus 5°C at 7.75 km [top]. Maximum daily temperature for model simulations of the base case (no change in air/groundwater temperature or recharge), the base case -30% recharge and the base case +30% recharge at 7.75 km [bottom]. A decrease in recharge decreases the volume of stream flow allowing the stream to warm more readily.
Figure 4. Comparison of simulated cases at an upstream (7.75 km) and downstream (2.75 km) location to the maximum daily maximum curve (solid line) developed by Wehrly et al. (2007). The absolute difference between the base case and the change in recharge cases is greater at the downstream location for all simulations. This is a consequence of greater stream flow downstream; reductions in stream flow are amplified in the simulated stream temperature at locations further downstream. Only the +5°C, -30% recharge case crosses the empirical threshold developed by Wehrly et al. (2007), however this is a relatively upstream segment of the East Branch Pecatonica and ecologically and economically valuable trout fisheries may exist downstream that may be threatened in less-extreme climate change scenarios. Additionally, this has implications for higher order streams and fisheries (wider, higher volume) which may be at risk in less extreme scenarios.
This study demonstrates that climate change poses a risk to trout fisheries in southwest Wisconsin. Site specific conditions, such as stream flow and stream width to depth ratios, are critical controls on stream temperature. Thermal infrared and in-stream data may be used in combination to validate a freely-available one-dimensional stream temperature model to predict the impacts of climate change on stream temperature. Thermal tolerance thresholds, such as the maximum daily maximum temperature developed by Wehrly et al. (2007), may be used to assess species' risk.
Wisconsin Groundwater Coordinating Council/Water Resources Institute
Wehrly, K.E., Wang, L.Z. and Mitro, M., 2007. Field-based estimates of thermal tolerance limits for trout: Incorporating exposure time and temperature fluctuation. Transactions of the American Fisheries Society, 136(2): 365-374.