Climate Change Impact on Freshwater Wetlands, Lakes & Rivers
Freshwater ecosystems provide a wide range of goods and services. Wetlands exhibit extensive biodiversity, function as filters for pollutants, and are important for carbon sequestration and emissions. Rivers transport water and nutrients from the land to the oceans and provide crucial buffering capacity during droughts especially if fed by mountain springs and glaciers. Lakes serve as sediment and carbon sinks and provide crucial repositories of information on past climate changes (USGRP, 2009).
Climate change impacts on inland aquatic ecosystems will be caused by the direct effects of rising temperatures and rising CO2 concentrations to indirect effects caused by changes in the regional or global precipitation and the melting of glaciers and ice cover (Ibid). A summary of likely impacts appears below (IPCC, 2007; Bates, et al., 2008):
- Many wetlands have world conservation status (Ramsar sites, World Heritage sites). Their loss could lead to significant extinctions, especially among amphibians and aquatic reptiles.
- The most vulnerable wetlands regions include: Arctic and sub-Arctic ombrotrophic (‘cloud-fed’) bogs, depressional wetlands with small catchments, monsoonal wetlands in India and Australia, boreal peatlands, North America’s prairie pothole wetlands and African Great Lake wetlands.
- Rising temperature will lower water quality in lakes through a fall in oxygen concentrations, release of phosphorus from sediments, increased thermal stability, and altered mixing patterns.
- Ice cover on lakes and rivers will continue to break up earlier and the ice-free periods will increase. This increases the length of time that deep lakes may become oxygen deplete.
- Higher temperatures will negatively affect micro-organisms and benthic invertebrates and the distribution of many species of fish. The thermal optima for many mid- to high-latitude cold-water species are lower than 20°C.
- Species extinctions are expected when warm summer temperatures and anoxia (oxygen depletion) eliminate deep cold-water refuges. In the southern Great Plains of the USA, water temperatures are already approaching lethal limits for many native stream fish.
- Invertebrates, waterfowl and tropical invasive species are likely to shift poleward with some potential extinctions.
- Major changes will be likely to occur in the species composition, seasonality and production of planktonic communities (e.g., increases in toxic blue-green algal blooms) and their food web interactions resulting in changes in water quality.
- Enhanced UV-B radiation and increased summer precipitation will significantly increase dissolved organic carbon concentrations, altering major biogeochemical cycles.
- Tropical lakes may respond with a decline in fish yields.
- Numerous arctic lakes will dry out with a 2-3°C temperature rise.
- The seasonal migration patterns and routes of many wetland species will need to change and some may be threatened with extinction.
- Small increases in the variability of precipitation regimes will significantly impact wetland plants and animals at different stages of their life cycles.
- In monsoonal regions, increased variability risks diminishing wetland biodiversity and prolonged dry periods promote terrestrialization of wetlands.
- In dryland wetlands, changes in precipitation regimes may cause biodiversity loss.
- Changes in climate and land use will place additional pressures on already-stressed ecosystems along many rivers in the world.
- An increase or decrease in freshwater flows will also affect coastal wetlands by altering salinity, sediment inputs and nutrient loadings.
- Expansion in range for many invasive aquatic weeds.
- Water levels are expected to increase in lakes at high latitudes, where climate models indicate increased precipitation, while water levels at mid- and low latitudes are projected to decline.
- Closed lakes are most vulnerable to a change in climate because of their sensitivity to changes in the balance of inflows and evaporation. Changes in inflows to such lakes can have very substantial effects and, under some climatic conditions, they may disappear entirely.
The figure below (Munson and Axler, 2009) shows the various zones of a lake. The littoral zone is the zone near shore where sunlight can penetrate the water to the sediment and allows aquatic plants (macrophytes) to grow. The limnetic zone is the open water region where light does not penetrate to the bottom. The bottom sediment is known as the benthic zone, and has a surface layer abundant with organisms such as insect larvae and crustaceans. The euphotic zone is the region of the lake where sunlight can penetrate enough to allow photosynthesis.
A pristine large lake is one which experiences seasonal water turnover (circulation from top to bottom). Warmer water is less dense than cooler water. Cooler water tends to sink while warmer water will rise. Seasonal turnover occurs during spring after ice melt and during fall when surface waters begin to cool. Turnover is essential to the biodiversity of large lakes because the process brings oxygen from the surface to the deeper waters and lifts nutrients previously trapped at the bottom of the lakes. During summer, the warm water layer at the top inhibits turnover. This is know as stratification. The figures below illustrate lake turnover and stratification.
According to Euro-Limpacs (2010) climate change will have the following impacts:
Rivers: A general upstream movement of river zones, particularly affecting species bound to small streams and springs, which can not move further upstream. Most fish of small rivers, especially the salmonids, are cold-adapted and will be particularly affected by rising temperatures.
Lakes: Climate change impacts will be significant, and they will mainly act directly through temperature increase, which prolongs the ice-free period and thus the duration of summer stratification. This may enhance eutrophication and lead to oxygen depletion in deep zones during summer, thus eliminating the refuge for coldwater-adapted fish species.
Wetlands: Climate change threatens the stability of mires (wet, swampy ground; bog; marsh) by increasing decomposition rates due to higher temperatures and lowering of water tables. Reduction of the carbon store, increased flux of CO2 and possibly of CH4, contribute to further amplification of greenhouse gas production. Disruption of intact mire surfaces also increases the generation of ‘coloured water’, with potential effects on aquatic ecology, the breeding cycles of important fish species, and flood control.
According to an exhaustive review of the literature, the Netherlands Institute of Ecology (2010) concludes that climate change will have the following impact on shallow lakes :
- reduce the numbers of several species of birds;
- favor toxic algae dominance in phytoplankton communities;
- cause more serious incidents of botulism among waterfowl and enhances the spreading of mosquito borne diseases;
- benefit invaders originating from the Ponto-Caspian region;
- stabilize turbid, phytoplankton-dominated systems, thus counteracting restoration measures;
- destabilize macrophyte-dominated clear-water lakes;
- increase carrying capacity for primary producers, especially phytoplankton, thus mimicking eutrophication;
- affect higher trophic levels as a result of enhanced primary production;
- have a negative impact on biodiversity which is linked to the clear water state;
- affect biodiversity by changing the disturbance regime.
Stager & Thill (2010) studied that Lake Champlain region and have determined:
Significant climatic changes, which most scientists agree are now driven primarily by human-generated greenhouse gases, are already under way in the Champlain Basin: mean annual air temperatures warmed by 2.1°F (1.2°C) between 1976 and 2005, with the most significant seasonal warming during summer and autumn. Total annual precipitation during that time period was approximately 3 inches greater than it was during the preceding 8 decades. Freeze-up on Lake Champlain is happening two weeks later than in the early 1800s—that is, in the increasingly rare winters when ice covers the main body of the lake at all. Rising surface water temperatures may increase the stability and duration of warm-season stratification in Lake Champlain, potentially making the lake more susceptible to nuisance phytoplankton blooms and low-oxygen (“hypoxic”) conditions.
Lake Champlain is one of the largest lakes in North America, and its watershed supports a high level of biodiversity. The authors conclude that ongoing climate change is likely to affect many of these organisms and their habitats in important ways. The figure below (Ibid) summarizes five key ecosystems that are likely to be impacted (click for larger version):
Hall and Stuntz (2007) considered the impact of climate change on the U.S. Great Lakes region and concluded:
- Spring and summer temperatures in the Great Lakes region may increase by as much as 9º F (5º C) and 7.2º F (4º C), respectively, by 2050;
- Lake levels in Lake Michigan and Lake Huron may drop by as much as 4.5 ft (1.38 m) due to a combination of decreased precipitation and increased air temperature/evapotranspiration;
- Groundwater will be impacted, as aquifer levels and recharge rates are expected to drop;
- Lower lake levels and rising temperatures (both in the air and water) will significantly impact fisheries, wildlife, wetlands, shoreline habitat, and water quality in the Great Lakes region;
- Tourism and shipping, which are critically important to the region, are especially vulnerable to climate change impacts;
- Water shortages in other regions will raise the threat of Great Lakes diversions.
- Water temperatures are expected to rise during the next century, and warmer water can lead to anoxia.
- Summer surface water temperatures in Lake Superior have already risen 4ºF (2.2ºC) over the past 27 years.
- The metabolic rates of sediment bacteria which consume oxygen increase as water warms.
- Biological productivity and respiration in the water column also increase, providing more decomposing bottom matter and robbing the water of oxygen. At the same time, warmer temperatures decrease dissolved oxygen saturation values, limiting the amount of oxygen in the water.
- A warming environment may also increase thermal stratification in the Great Lakes, further depleting oxygen in the waters. The Great Lakes mix vertically—or turn over—each spring and fall, when the near-surface water reaches 39ºF (4ºC), the temperature of maximum density for water. Climate models predict that the surface water temperatures of deep lakes will stay above 39ºF (4ºC) in some years. As a result, vertical mixing may occur only once a year. Not only would this deplete oxygen in the lakes, altering their deep water chemistry, but it would deprive phytoplankton and detritus-eating organisms of nutrients necessary for growth and survival. The entire food chain could be impacted.
- Lower water levels restrict the access of commercial navigation throughout the lakes. Shippers will have to reduce the amount of cargo they carry and make more frequent trips to transport the same amount of cargo. According to the U.S. Great Lakes Shipping Association, for every inch (2.5 cm) of lower lake levels, a cargo ship must reduce its load by 99 to 127 tons (90 to 115 metric tons).
- Changes in the habitats and ranges of fish, waterfowl, other birds, and mammals could have a negative impact on angling, hunting, and bird watching – major tourist attractions.
Warming of the African Great Lakes due to climate change may create conditions that increase the risk of cholera transmission in the surrounding countries (Ecosystem Assessment, Reid et al., 2005).
Johnson et al. (2009) modeled he possible effects of changing climate on a southern and a north-eastern English river (the Thames and the Yorkshire Ouse, respectively) in relation to water and ecological quality throughout the food web based on IPCC low and high CO2 emission scenarios for 2080. Their results appear in the figure below (click for larger version).
Their review suggests that lowland British rivers both in the South and North East will have less water and be warmer than before. Unless there is a drastic reduction in nutrients, for much of the year the rivers will probably be a darker shade of green than they are now. The species composition will undoubtedly change and a number of wetland environments, particularly those depending on rain water or river water, are likely to decline. Consumers, particularly in the South East, will have to pay more for their water as it becomes a scarce commodity and the costs of maintaining quality increases. The authors also suggest that there are a number of ‘wild cards’ which could have drastic effects on the river ecosystem and which need further analysis, for example:
- Alien species might bring new pathogens that infect and overwhelm native species in rivers such as the Thames and Ouse.
- A dry summer following an extremely dry winter might stop flow in the Thames and lead to catastrophic eutrophication.
- Higher carbon loadings in river bed-sediments might dangerously deplete oxygen levels in some slow-flowing rivers.
River ecosystems around the world are threatened by climate change. For example, salmon and other coldwater fish species in the United States are at particular risk from warming. Rising temperatures affect salmon in several important ways. As precipitation increasingly falls as rain rather than snow, floods could wash away salmon eggs incubating in the streambed. Warmer water leads eggs to hatch earlier in the year, so the young are smaller and more vulnerable to predators. Warmer conditions increase fish metabolism, taking energy away from growth and forcing the fish to find more food, but earlier hatching of eggs could put them out of sync with the insects they eat. Earlier melting of snow leaves rivers and streams warmer and shallower in summer and fall. Diseases and parasites tend to flourish in warmer water. Studies suggest that up to 40 percent of Northwest salmon populations may be lost by 2050 (USGRP, 2009).
Over half of the wild trout populations are likely to disappear from the southern Appalachian Mountains because of the effects of rising stream temperatures. Losses of western trout populations may exceed 60 percent in certain regions. About 90 percent of bull trout pictured below (USFWS, 1998) , which live in western rivers in some of the country’s most wild places, are projected to be lost due to warming. Pennsylvania is predicted to lose 50 percent of its trout habitat in the coming decades. Projected losses of trout habitat for some warmer states, such as North Carolina and Virginia, are up to 90 percent (Ibid).
Jon Kusler, Associate Director and co-founder of the Association of State Wetland Managers, and the 2009 winner of the Society of Wetland Scientist’s Lifetime Achievement Award, writes in Common Questions: Wetland, Climate Change, and Carbon Sequestering (2006) that wetlands are more sensitive to climate change for three major reasons:
- Flora and fauna in wetlands are more sensitive to changes in water levels than those of lakes, rivers, and streams. For example, lowering long-term water levels even a few inches can be the difference between a wetland or dry ground.
- Wetlands have been cut off from other wetlands by dams, dikes, roads, and other alterations so wetland plants and animals cannot migrate to other wetlands in response to changes in temperature or water levels.
- Mankind has already stressed wetlands which has reduced the biodiversity. A reduced biodiversity makes wetlands more vulnerable to small changes in temperature and water levels.
According to Kusler, the types of wetlands that are most vulnerable include:
- Coastal and estuarine wetlands primarily due to rising sea levels. For example, a two foot rise in the Florida Everglades will move the land/sea boundary inland by several kilometers.
- Permafrost and other open tundra wetlands due to rising temperatures. Melting permafrost may turn these wetlands into open water. Boreal forests may then encroach on the region.
- Wetland boreal forests through the loss of southern forests, invasion of northern trees into tundra, and increased fire and pest outbreaks.
- Alpine wetlands near the top of mountains because rising temperatures leave these wetlands with no place to go.
- Prairie potholes reduced due to evaporation from rising temperatures. Waterfowl may be reduced as a result.
Kusler notes the following impacts on wetland goods and services:
- Reduction in cold water fish such as trout while warm water fish such as bass increase. As coastal wetlands disappear, ocean fish that depend on them will be reduced.
- Shellfish reduction due to a decrease in the size of coastal wetlands and rising sea levels above the coastal flat regions.
- Waterfowl production reduced especially in the Prairie Pothole Region.
- Habitats for rare and endangered species reduced leading to species extinctions.
- Decrease in food chain support for all species that depend on coastal and inland wetlands.
- Water quality and pollution buffering decreased with decreasing wetlands.
- Loss of coastal wetlands which provide wave and storm buffering may lead to more coastal erosion and higher storm surges in hurricanes and winter storms.
According to Twilley (2007):
The wetlands of the U.S. Gulf Coast provide services that are significant to the quality of life in the region, help sustain the national economy, and help protect life and property from climate extremes. Fisheries, recreation, and tourism have all thrived in the Gulf Coast region alongside urban development, agriculture, shipping, and the oil and gas industries. However, some regions of the Gulf Coast, such as the Mississippi River Delta and Florida Everglades, are experiencing some of the highest wetland loss rates in the U.S., largely because of engineered modifications to regional watersheds and coastal landscapes. Such modifications increase the vulnerability of these wetlands to future climate variability and change.
Gulf Coast wetlands support economic and ecological productivity as well as quality of life in many ways. Wetlands provide food, refuge, and nurseries for fish and shellfish, and they support the region’s large commercial and recreational fishing industries. As a result, Louisiana’s commercial fisheries account for about 30 percent of the nation’s total fish catch. In addition, Gulf Coast wetlands provide stopover habitat for an estimated 75 percent of the waterfowl migrating along the Central Flyway. Wetland soils and vegetation naturally store water, filter sediment and pollutants from fresh water supplies, and help stabilize shorelines by reducing erosion and storm surges associated with rising sea levels.
Degradation of coastal wetlands through land development and water management reduces the capacity of wetlands to provide significant ecosystem services that reduce the risks of living and working in coastal landscapes. For example, extensive coastal wetland landscapes, especially forested ecosystems, can reduce storm surge and wind energy during tropical storms and cyclones, minimizing hurricane damage to life and property. In part because of recent hurricanes, local, state, and federal agencies have renewed their emphasis on coastal wetland restoration in the Gulf Coast region. However, such programs may fail without effective planning for future climate change, including accelerated sea-level rise and the potential intensification and increased frequency of hurricanes.
Inundation by rising sea levels is one of the most direct threats faced by coastal and estuarine wetlands in the Chesapeake Bay region. Large wetlands such as the Blackwater Wildlife Refuge in Maryland and the Guinea Marshes in Virginia are already showing decreased areas of vegetative cover as a result of inundation and erosion. Extensive oxbow wetlands at the headwaters of the Bay’s tidal tributaries are not keeping pace with rising sea level. Inundation of coastal wetlands by rising sea levels may stress the ecosystems in ways that enhance the potential for invasion of less desirable species, such as Phragmites australis (one of six species identified as causing, or having the potential to cause, significant degradation of the aquatic ecosystem of the Bay). Reported impacts include significant loss of plant diversity, changes in marsh hydrology with the development of Phragmites, and a reduction in insect, bird, and other animal species (Najjar, et al. (2010).
According to Galatowitsch, Frelich, & Phillips-Mao (2009), Climate change projections suggest that by 2069, average annual temperatures will increase 3 oC with a slight increase in precipitation (6%) in Minnesota. However, with a 3 oC rise in temperature, a 30% increase in precipitation is required for wetlands to maintain their current state. Decreases in water supply to Minnesota wetlands will likely cause significant shifts in plant communities and will favor several invasive species, especially reed canary grass (Phalaris arundinacea). Of critical conservation concern is the anticipated impacts to calcareous fens which are sustained by mineral-rich groundwater discharge and support a relatively large proportion of rare plant species. There are approximately 100 fens in the state, 20% of the total known for all of North America. Climate change will decrease water to these rare species.
Wetlands contain about 20 percent of all terrestrial carbon stocks so they are an important carbon sink. Covering just six percent of Earth’s land surface, wetlands – including marshes, peat bogs, swamps, river deltas, mangroves, tundra, lagoons and river floodplains – contain an estimated 771 billion tons of greenhouse gases, both CO2 and more potent methane, an amount in CO2 equivalent comparable to the carbon content of today’s entire atmosphere (Leahy, 2010).
It is estimated that climate change may result in a loss of up to 85 percent of wetlands in the future. That loss would also release enough carbon and methane to almost certainly tip the climate into an era of extreme and rapid change, experts believe (Ibid).