Conclusions

We modeled four coastal watersheds that drain to important estuaries in the Pacific Northwest as part of a larger interagency investigation into the possible effects of anthropogenic climate change on estuaries and ecosystem services they provide. Our findings show that change in the freshwater inflow regimes of these watersheds is likely. In particular, increases in autumn flow and increases in the top 5 percent of flow are probable. As in previous studies, the results also show that most of the considerable uncertainty in these projections is owing to the climate models used. Not only were there large discrepancies in the degree of the changes of many indices, but there also were differences in direction as well. In these rainwater dominated coastal watersheds, the primary driver of flow changes will be shifts in precipitation, which are difficult to simulate accurately. Nevertheless, our outputs allow coastal managers to see a range of possible changes, which may help them prepare for climate change. Our simulated flow output also can be used as input to models of estuarine ecosystems and water dynamics that can be used to better understand possible climate-driven shifts in the location, quality, and extent of critical estuarine habitat.

Although this study modeled only freshwater flow changes, these flow changes will likely lead to shifts in numerous other important estuarine variables, especially salinity. Altered salinity could affect the distribution and abundance of native species, and encourage the spread of invasive species. Freshwater flow declines in summer also could lead to reduced levels of dissolved oxygen, which will have negative implications for salmonids and could lead to hypoxia. Bacterial impairments also may increase, because drier summers may be followed by more extreme heavy rains in autumn. In the past, such weather patterns have contributed to high concentrations of fecal coliform levels in the Tillamook Bay. Therefore, the potential changes in freshwater flow highlighted by this study have many implications for water-quality conditions and distribution of aquatic organisms in Pacific Northwest estuaries, but further research is needed to determine the degree and likelihood of these possible shifts.

First posted February 28, 2013