Effective allocation of limited water resources between human and ecological needs is critical for sedentary species whose low mobility makes them sensitive to changes in flow. One such group, North American freshwater mussels, are a highly imperiled taxa with over a third of species listed as threatened or endangered. Where human alternation of river networks has reduced water availability, mussel communities are impacted by low flows leading to heat stress. Climate change is likely to further exacerbate these impacts by increasing the frequency and intensity of drought events.
We present a case study on the Kiamichi River in southeastern Oklahoma, USA to explore how upstream reservoir management decisions may be used to mitigate impacts on mussels from future drought intensification. We fit generalized linear models (GLM) with historic data to establish relationships between climatic variables (precipitation, air temperature), in-stream discharge, upstream reservoir releases, and in-stream temperature. We estimate temperature thresholds for mussel survival from the literature and in combination with daily precipitation and temperature projections from a set of downscaled climate models for this region, we calculate the frequency with which mortality events will increase.
Results/Conclusions
In the past decade, reservoir regulation caused more dry days across the downstream mussel beds than was typical over recent recorded history. We illustrate how in 2011 a no-release decision and drought combined to incur heat stress and mussel mortality. Under dry future climate change scenarios, similar drought events in this watershed will increase in length (days) by up to 26% and frequency (per year) by up to 127%.
Our results suggest that these droughts may be mitigated by more timely and larger releases by reservoir operators. By combining our GLM estimate of the relationship between flow and temperature during summer months (thermal sensitivity: 0.73) with historic conditions and reservoir releases (1995-2014), we demonstrate how additional releases of water may keep mussel beds below a 30°C threshold. Annual totals for additional releases varied widely between 0.6 and 35,000 acre-feet; however, the distribution was strongly skewed and half of these years could be successfully managed with under 3300 acre-feet (just 10% of current downstream water rights held by local communities). Thus, maintenance of environmental flows for mussel communities may have only modest impacts on reservoir storage for societal needs and we recommend that conservation of low-mobility organisms be added to water management objectives of reservoirs in this region.