Interactive influences of climate change and agriculture on aquatic habitat in a Pacific Northwestern watershed

Climate change and agricultural intensification are two potential stressors that may pose significant threats to aquatic habitats in the inland Pacific Northwest over the next century. Climate change may impact running water through numerous pathways, including effects on water temperature and stream flow. In certain regions of the Pacific Northwest, agricultural activities, such as crop production, may become more profitable if water projects result in more irrigation water. If so, riparian buffers in these areas may be converted into cropland, which may in turn affect aquatic habitats through increases in sediment and agrochemical runoff into streams. We used currently available downscaled temperature and hydrology data in combination with a habitat quality framework developed for Pacific salmon and trout (Oncorhynchus spp.) to predict how different levels of each stressor, alone and in combination, may impact aquatic habitats in an inland Pacific Northwest watershed dominated by high-value agriculture-the Umatilla Subbasin. We developed spatially explicit predictions for how changes in stream flow and water temperature associated with three climate change scenarios and loss of riparian buffers in two agricultural intensification scenarios may impact aquatic habitats. We also examined the cumulative effects of the interaction of extreme climate change and agricultural intensification scenarios. Our results show that all three climate change scenarios are expected to primarily impact aquatic habitat in the upper Subbasin. In contrast, agricultural intensification scenarios did not have large impacts on temperature, but are predicted to affect other water quality variables in the lower Subbasin. A moderate scenario of agricultural intensification had relatively little effect on aquatic habitat, whereas the removal of all riparian buffers in agriculturally viable areas had a substantially negative effect on sediment, embeddedness, and large woody debris in the lower Subbasin. Interactions between the most extreme climate change and agricultural intensification scenarios reflected a complementarity of effects, with climate change primarily affecting the upper Subbasin and agricultural intensification primarily impacting the lower Subbasin. This work suggests that the Umatilla Subbasin and similar watersheds will present a challenging habitat for warm water-and pollution-intolerant species in the coming century.