Climate change impacts on ecosystems reduce their ability to improve water quality and regulate water flows.

This finding is from chapter 8 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: The key messages and supporting chapter text summarize extensive evidence documented in the Ecosystems Technical Input Report, Impacts of Climate Change on Biodiversity, Ecosystems, and Ecosystem Services: Technical Input to the 2013 National Climate Assessment.7406884d-2302-4644-aa50-12ed8baf4fd7 This foundational report evolved from a technical workshop held at the Gordon and Betty Moore Foundation in Palo Alto, CA, in January 2012 and attended by approximately 65 scientists. Technical inputs (127) on a wide range of topics related to ecosystems were also received and reviewed as part of the Federal Register Notice solicitation for public input.

Description of evidence base: The author team digested the contents of more than 125 technical input reports on a wide array of topics to arrive at this key message. The foundational Technical Input Report7406884d-2302-4644-aa50-12ed8baf4fd7 was the primary source used. Studies have shown that increasing precipitation is already resulting in declining water quality in many regions of the country, particularly by increasing nitrogen loading.2513bdf8-409d-4564-a569-490457fad85b 5cb1fd05-b566-4048-9989-c49553b77755 c0431825-7915-41e6-adbe-09285acf9168 2def4038-abbc-43aa-b816-c8b195e2cf5b f68f6208-6991-4325-8854-881c76072096 This is because the increases in flow can pick up and carry greater loads of nutrients like nitrogen to rivers.5cb1fd05-b566-4048-9989-c49553b77755 c0431825-7915-41e6-adbe-09285acf9168 2def4038-abbc-43aa-b816-c8b195e2cf5b f68f6208-6991-4325-8854-881c76072096 One model for the Mississippi River Basin, based on a doubling of CO2, projects that increasing discharge and nitrogen loading will lead to larger algal blooms in the Gulf of Mexico and a larger dead zone.0c7283dc-9cfb-4669-b0ec-0e81c7285bd5 The Gulf of Mexico is the recipient system for the Mississippi Basin, receiving all of the nitrogen that is carried downriver but not removed by river processes, wetlands, or other ecosystems. Several models project that declining streamflow, due to the combined effects of climate change and water withdrawals, will cause local extinctions of fish and other aquatic organisms,9dc780be-c325-40c4-a7ea-4846b7c53b59 particularly trout in the interior western U.S. (composite of 10 models, A1B scenario).38a94887-f469-4fce-8feb-75fc8e55568e The trout study38a94887-f469-4fce-8feb-75fc8e55568e is one of the few studies of impacts on fish that uses an emissions scenario and a combination of climate models. The researchers studied four different trout species. Although there were variations among species, their overall conclusion was robust across species for the composite model. Water quality can also be negatively affected by increasing temperatures. There is widespread evidence that warmer lakes can promote the growth of harmful algal blooms, which produce toxins.11b14f32-9d24-45c2-b953-d8a57cbab116

New information and remaining uncertainties: (for example, fertilization) on nitrogen losses from watersheds,2513bdf8-409d-4564-a569-490457fad85b c0431825-7915-41e6-adbe-09285acf9168 and how the interactions between climate and human actions (for example, water withdrawals) will affect fish populations in the west.9dc780be-c325-40c4-a7ea-4846b7c53b59 38a94887-f469-4fce-8feb-75fc8e55568e However, few studies have projected the impacts of future climate change on water quality. Given the tight link between river discharge and pollutants, only areas of the U.S. that are projected to see increases in precipitation will see increases in pollutant transport to rivers. It is also important to note that pollutant loading – for example, nitrogen fertilizer use – is often more important as a driver of water pollution than climate. 2513bdf8-409d-4564-a569-490457fad85b c0431825-7915-41e6-adbe-09285acf9168

Assessment of confidence based on evidence: Given the evidence base and uncertainties, there is high confidence that climate change impacts on ecosystems reduce their ability to improve water quality and regulate water flows. It is well established that precipitation and associated river discharge are major drivers of water pollution in the form of excess nutrients, sediment, and dissolved organic carbon (DOC) transport into rivers. Increases in precipitation in many regions of the country are therefore contributing to declines in water quality in those areas. However, those areas of the country that will see reduced precipitation may experience water-quality improvement; thus, any lack of agreement on future water-quality impacts of climate change may be due to locational differences.

References :

• Relating Net Nitrogen Input in the Mississippi River Basin to Nitrate Flux in the Lower Mississippi River (f68f6208)
• The influence of climate on average nitrogen export from large watersheds in the Northeastern United States (5cb1fd05)
• CLIMATE: Blooms Like It Hot (11b14f32)
• Effects of climate change on hypoxia in coastal waters: A doubled CO2 scenario for the northern Gulf of Mexico (0c7283dc)
• Flow regime, temperature, and biotic interactions drive differential declines of trout species under climate change (38a94887)
• Coupling between climate variability and coastal eutrophication: Evidence and outlook for the northern Gulf of Mexico (2def4038)
• Coextirpation of host-affiliate relationships in rivers: the role of climate change, water withdrawal, and host-specificity (9dc780be)
• Nitrogen fluxes from the landscape are controlled by net anthropogenic nitrogen inputs and by climate (2513bdf8)
• Influences of climate, hydrology, and land use on input and export of nitrogen in California watersheds (c0431825)
• Impacts of Climate Change on Biodiversity, Ecosystems, and Ecosystem Services. Technical Input to the 2013 National Climate Assessment (7406884d)