Extreme Temperature Change: Water Temperatures

As average air temperatures rise, so do the average temperatures of our oceans, lakes and rivers. Open water — which is directly exposed to sunlight — is most dramatically affected. Already, water temperatures have risen enough to affect aquatic habitats and cause some species to migrate into cooler water. By some estimates, coldwater fish in North America — salmon and trout, for example — could lose as much as half their suitable habitat if average global temperatures increase by 8 degrees Fahrenheit.

How Are Water Temperatures Changing?

The 2014 National Climate Assessment reports that increasing air and water temperatures, more intense precipitation and runoff, and intensifying droughts can decrease river and lake water quality in many ways, including increases in sediment, nitrogen, and other pollutant loads. EPA reports that most lakes in the northern United States are freezing later and thawing earlier compared with the 1800s and early 1900s. Freeze dates have shifted later at a rate of roughly half a day to one day per decade, while thaw dates for most of the lakes studied have shifted earlier at a rate of half a day to two days per decade. These changing freeze and melt patterns have significant impacts on local ecosystems and water quality, and in the case of large water bodies, weather patterns.

In addition to direct impacts to water quality of freshwater resources on the mainland, changing sea surface temperatures can have profound effects on weather patterns. Sea surface temperatures have been higher during the past three decades than at any other time since reliable observations began in the late 1800s, and there is a direct relationship between the surface temperature of the ocean and the amount of moisture in the air. This means that stronger storms and more extreme weather, including more extreme cold temperatures and greater snowfall will become increasingly more likely in some regions.

Implications for Land Trusts

Warmer water affects many ecological systems that may relate to land trust management objectives, including:

• Water quality – Cold water holds more oxygen than warm water. As temperatures rise, the oxygen concentration of water declines. Algal blooms may begin earlier in the season, and last longer into the fall.
• Coral reefs – Corals are sensitive to temperature increases. Warmer-than-average waters increase the likelihood that coral reefs will suffer bleaching events.
• Increased impacts from coastal storms – Hurricanes rely on warm water for fuel. A warmer Atlantic Ocean may lead to stronger or more frequent hurricanes.
• Greater disturbance from invasive species, disease, and fire – Invasive species and diseases are able to extend their ranges into new territories. As air and water temperatures increase, evaporation also increases, which may alter ecosystems, making native species more susceptible to die-off from competition with invasive species and in some cases more at risk of disease. Drier systems also creates greater potential for more extensive fire disturbance.

Rising water temperatures are also a trigger for many other climate change impacts, such as:

• warmer air temperatures
• greater flood risk
• rising sea levels
• shifting seasons
• altered precipitation
• stronger storms
• migrating species
• ocean acidification
• coral bleaching
• declining water levels

Temperature exerts a major influence on biological activity and growth. Temperature governs the kinds of organisms that can live in rivers and lakes. Fish, insects, zooplankton, phytoplankton, and other aquatic species all have a preferred temperature range. As temperatures get too far above or below this preferred range, the number of individuals of the species decreases until finally there are none. Temperature is also important because of its influence on water chemistry.

Temperature is also important in lakes and reservoirs. It is related to the dissolved-oxygen concentration in water, which is very important to all aquatic life. Many lakes experience a “turning” of its water layers when the seasons change. In summer, the top of the lake becomes warmer than the lower layers. You’ve probably noticed this when swimming in a lake in summer – your shoulders feel like they’re in a warm bath while your feet are chilled. Since warm water is less dense that colder water, it stays on top of the lake surface. But, in winter some lake surfaces can get very cold. When this happens, the surface water becomes more dense than the deeper water with a more constant year-round temperature (which is now warmer than the surface), and the lake “turns”, when the colder surface water sinks to the lake bottom.  In Lake Superior, increasing water temperatures have led to an earlier stratification onset by about two weeks in the past 30 years, a change that the NFW predicts may mean this large water body may have larger dead zones due to lack of oxygen, and may mean the lake will be ice-free in upcoming decades. These physical and chemical changes can mean more harmful algae blooms, better conditions for invasive species, less habitat for native fish populations, and significant water quality impacts. The way that temperatures vary in lakes over seasons depends on where they are located. In warm climates the surface may never get so cold as to cause the lake “to turn.” But, in climates that have a cold winter, temperature stratifications and turning do occur.  As water temperatures changes, natural processes can be significantly altered, which may negatively affect the flora and fauna that live in and around aquatic ecosystems.

Tips for Planning: What Land Trusts Are Doing

Cooperatively Working Towards Conservation Goals.

Land trusts are responding to on-the-ground impacts of changing water temperatures in various ways. Some groups are working to restore riparian areas to establish better canopy cover to shade vulnerable cold-water habitats. Others are changing language in conservation easements to accommodate long-term variations in species and habitats. In addition to employing strategic conservation planning to reduce risks and enhance resilience, some land trusts are also supporting efforts to mitigate greenhouse gas emissions to reduce the extent of future climate change.

Land trusts must determine the right planning approach for their organization, however, more and more, conservation organizations are working with their communities to identify opportunities to reduce vulnerabilities and prepare for changing temperatures. Agencies are making similar strides to implement projects that reduce risks and plan for resilience by incorporating adaptation, mitigation, and engagement into their strategic goals and objectives. For example, the U.S. Fish and Wildlife Service’s Strategic Plan for Responding to Accelerating Climate Change identifies seven planning and development goals to support sustainable landscapes. Conservation groups are increasingly partnering with agencies as well as other nonprofits and for-profit organizations to respond to this global challenge at local levels.

Building resilience for multiple management objectives.

Resilience describes the ability of a system to persist through extreme change. By working to identify and reduce potential threats land managers can build resilience and achieve multiple management objectives. Assessing vulnerabilities is a critical step in the strategic conservation planning process that helps land trusts identify key threats to resources. Adaptive management planning enables conservation practitioners to implement interventions to reduce vulnerabilities and to monitor and revise strategies as new information becomes available.

Despite uncertainties regarding the extent of impacts due to warming temperatures, many land trusts have already engaged in planning to assess and reduce risks. Strategic Conservation Planning and Adaptive Management Planning are tools the conservation community has been increasingly employing to identify management goals. These planning processes often involve vulnerability assessments, which aim to identify risks and opportunities to enhance resilience by reducing vulnerabilities. Land trusts that have engaged in these processes often incorporate resiliency planning into their management missions. To achieve this end, land trusts across the country are prioritizing conservation of vulnerable ecosystems to protect biodiversity and, encouraging connectivity of conserved spaces to create species migration corridors. These strategies reflect increased understanding about the importance of “conserving the stage” to build system resilience throughout ecozones and across species. To learn more about what land trusts are doing to promote resilience and “conserve the stage”, visit the case studies of resilience page. There is no one-size-fits-all solution to addressing management challenges associated with warming average temperatures, however, planning that acknowledges vulnerabilities can help land trusts better achieve their conservation objectives.

Learn More

• Are We Putting Our Fish in Hot Water? Global Warming and the World’s Fisheries
• Global Climate Change Indicators, NOAA
• Heating Up the Ocean, NOAA
• Protecting Freshwater Ecosystems in the Face of Global Climate Change
• Water Impacts and Climate Change, US EPA
• Water Properties: Temperature, USGS