Thermokarst lake to terrestrial ecosystem
Thermokarst lake dominated landscapes are transforming into terrestrial ecosystems (e.g.: tundra). There is a natural fluctuation between these two ecosystems. However, the rate and scale at which those fluctuations are occurring are increasing due to permafrost melting caused by the increasing atmospheric temperatures associated with climate change. Warmer air temperature increases soil temperature, which melts permafrost (permanently frozen soils found in Arctic regions). The shift in ecosystems occurs when permafrost degradation becomes severe enough for the lakes to get permanently drained, creating the necessary conditions for vegetation to establish. The increased rate and scales of these land cover changes has extensive impacts on food and freshwater provisioning, but its greatest impact is on carbon sequestration. The melting of permafrost releases greenhouse gases, i.e. carbon dioxide (CO2) and methane (CH4), which further increase climate change, creating a powerful reinforcing feedback.
Key direct drivers
- Global climate change
- Rock and Ice
Key Ecosystem Processes
- Soil formation
- Nutrient cycling
- Water cycling
- Fuel and fiber crops
- Climate regulation
- Regulation of soil erosion
- Natural hazard regulation
- Aesthetic values
- Security of housing & infrastructure
Typical spatial scale
Typical time scale
- Irreversible (on 100 year time scale)
- Contemporary observations
Confidence: Existence of RS
- Contested – Reasonable evidence both for and against the existence of RS
Confidence: Mechanism underlying RS
- Well established – Wide agreement on the underlying mechanism
Links to other regime shifts
Due to thermokarst processes such as permafrost melting, numerous areas of the Arctic landscape are extremely lake-rich. The water balance of those lakes may either be maintained due to impermeable permafrost that limits groundwater flow (Marsh 2009) or lost due to seasonal drainage, a natural characteristic of the Arctic system. However, a widespread, unnatural decline in lake abundance is being observed within the Arctic due to decades of rising atmospheric temperatures in the region that has been deteriorating the state of the permafrost (Smith et al. 2005). The lake-rich landscape of the Arctic is therefore slowly transforming into a drained, dry landscape increasingly dominated by vegetation such as shrubs and graminoids (Hinzman 2005).
Thermokarst lake ecosystem
The thermokarst lake-dominated landscape is formed as a response to some imposed disturbance, such as road construction, wildfire or climatic warming, which causes permafrost to thaw, creating irregular surface topography depressions, called thermokarsts. Those may appear as near-surface massive ice melts, allowing the surface to subside (Hinzman et al. 2005). As permafrost melts over time, those depressions become filled with melted water forming the lakes characteristic of the Arctic region.
Driven by external and internal factors, Arctic lakes are drying up at an unnatural speed within the region. This shift is accompanied by changes in vegetation, for example, increased shrub- and graminord abundance and a decrease in the cover of mosses and lichens through a broad expanse of the Arctic (Hinzman et al. 2005). The changes in vegetation may then further transform the landscape through sequential effects on foraging mammals and birds, as well as aquatic fauna and insects (Hinzman et al 2005).
Drivers and causes of the regime shift
Shift from thermokarst lakes to terrestrial ecosystem
The main external driver of this regime shift is climate change, which is caused by elevated greenhouse gas emissions (IPCC 2007). Increased temperatures warm the soil and warms the permafrost. As the permafrost thaws, greenhouse gasses such as CO2 and CH4 that are trapped in the ice are released into the atmosphere, further increasing the effects of climate change (Hinzman 2005; Karlsson et al. 2011). At a local scale, the effects of climate change are incremental. However, as this interaction is occurs over large areas, the effects are aggregated and self-reinforcing, which is significant since permafrost and seasonally frozen grounds store about 25% of the total global soil carbon stock (IPCC 2007).
At a local scale, initial thawing of the permafrost from increased atmospheric temperatures creates a dynamic landscape of depressions (thermokarst) in which lakes are formed (IPCC 2007). As permafrost become more degraded, drainage through outlet channels occurs. Since the temperature of melted water is higher than that of ice, it will exacerbate the melting of permafrost, causing another reinforcing feedback. Once the layer of frozen ground is completely penetrated the lakes can become permanently drained allowing for vegetation to be established (Marsh et al. 2008).
Shift from terrestrial ecosystem to thermokarst lake ecosystem
Heightened air- and soil temperatures melt the ice-rich soils and permafrost on which forests in the Arctic region develop, changing the physical conditions for the forest growing on top of it. When the roots of the trees get flooded, the trees die and ponds and lakes eventually replace the forest (Hinzman 2005). However, it is important to note that the shift from a terrestrial ecosystem to a lake dominated one is not as frequent as the shift from lakes to terrestrial ecosystems. The reason for this are the reinforcing feedbacks as mentioned above.
How the regime shift works
The warming of soil temperatures leads to the melting of continuous permafrost and the subsequent increase in abundance of discontinuous or sporadic permafrost. Melting of permafrost causes depressions in the soil called thermokarsts. Increased precipitation in the form of snowfall may have either an insulating or cooling effect on soil temperatures depending on several variables such a timing, duration, accumulation, and melting processes of seasonal snow cover, density, structure, and thickness of seasonal snow cover etc. (Zhang 2005). Depending upon the local surface energy balance, the thawed ground may refreeze or the permafrost can continue to degrade (Hinzman et al. 2005). On sites where continuous permafrost is thawing the active layer of soil in which melted water can move deepens allowing for the formation of thermokarst lakes (Hinzman et al. 2005). In freezing seasons, if the temperature is low enough and the hydraulic parameters on site suitable, the formation of these lakes can contribute to the re-formation of permafrost soils. Ice-rich permafrost can also prevent surface water from infiltrating to deeper groundwater zones, causing surface soils to be very wet. Decreased albedo due to changes in vegetation, the extension of snow-free and ice-free periods on terrestrial and lake surfaces and abundance, and reduction in the area occupied by glaciers and continental ice sheets in high latitudes may act as a positive feedback into atmospheric warming (Hinzman et al. 2005).
On sites where there is an abundance of discontinuous permafrost the active layer also deepens allowing for increased surface area for sub-ground water movement that increases the rate of drainage. Thermokarst lakes and ponds may begin to fill or drain depending upon the direction of the hydraulic gradient beneath the lake (Hinzman et al. 2005). Therefore it is not the depth of the active layer alone that determines the rate of drainage. However, increased drainage of lakes may prevent the reformation of permafrost from lack of available water for freezing in winter months and it can be inferred through advection of heat from decreased albedo. Once these drained lake beds become permanently drained they are likely to be re-vegetated which will change the albedo and may cause the extension of snow-free and ice-free periods and may act as feedback into increasing atmospheric temperature (Hinzman et al. 2005).
The affects of the shifts from lake to terrestrial ecosystems are aggregated at a local, regional and global scale due to the large scale and frequency at which those dynamics occur. The whole regime as described in the causal loop diagram below is a re-enforcing feedback that at its current state is leading to an overall decrease in lake dominated ecosystems and a increase in terrestrial ecosystems (Smith et al. 2005).
Impacts on ecosystem services and human well-being
Even though this regime shift occurs at the local scale, its consequences go beyond that scale because it reshapes regional hydrology (through changes in water balance and surface water- connection and fragmentation) and alters regional supplies of the ecosystem (Karlsson et al. 2011). This regime shift therefore affects provisioning ecosystem services, as it will decrease the availability of freshwater. The above-ground stored freshwater, which is of vital importance to migratory birds, fish, and other wildlife used by indigenous people, will shift to below-ground stored freshwater (Artic Science Journeys Radio Stories 2005). Globally, however, freshwater will become a more abundant ES due to reductions permafrost and subsequently ice sheets and glaciers resulting from the regime’s feedbacks to climate change. Nevertheless, this global increase in the freshwater ES is not a positive one: “Increased freshwater delivery to the Arctic Ocean from reductions in ice sheets and glaciers result in rising sea level (...)” (Hinzman et al. 2005). At a local scale traditional hunting and fishing practices will be impaired (Vincent et al. 2013). Arctic freshwater systems provide important migratory routes for fish stock. Due to this regime shit, those routes would be greatly altered in connectivity among lakes and river channels, as well as in terms of their physical coupling to the coastal marine ecosystem (Vincent et al. 2012). Globally, fishing practices would suffer as well. Increased discharge of water into the Arctic Ocean due to draining results in increased nutrient and external organic matter inputs to the Arctic Ocean that affect primary processes at the base of the marine food web (Vincent et al. 2012). Climate regulation is heavily affected on a global scale through the processes of this regime shift: as it sets up a positive feedback cycle in which the release of methane and carbon dioxide through the newly exposed soil and melted permafrost feeds into global warming. The shift from lakes to terrestrial ecosystems heightens gully erosion, a type of erosion that occurs when water is channeled across unprotected land and washes away the soil along the drainage lines. As permafrost degrades further, it causes erosion and slumping of lake edges and stream channels
The Arctic lakes are of vital importance to indigenous people as they are a source of fresh drinking water and also provides important winter transport routes. These lakes not only allows indigenous people access to their traditional hunting and fishing areas, but also to transport goods to remote communities and industries such as mining centers (Vincent et al. 2013). The regime shift would also mean an increase in products extracted from woods such as timber commodities and fuel: “increased areas of tree growth in the Arctic could serve to (…) take supply more wood products and related employment, providing local and global economic benefits” (Hassol et al. 2004).
There are few management options available to maintain the thermokarst lake ecosystems due to the long time scale at which permafrost melts and the natural dynamics within this system e.g. hydraulic fluctuations or site-specific soil conditions. One management option is to continue to reduce the emissions of greenhouse gases while simultaneously investigating further opportunities for carbon offsetting schemes.
For hydroelectric reservoirs, shifting ice conditions will have both positive and negative effects, and may require adaptive changes in operating procedures, with attention to minimize negative impacts associated with ice jams and ice breakup downstream of the spillway (Vincent et al. 2013). Fisheries management plans will also need to be adapted to the changes in migration and productivity of northern fish populations with ongoing climate change (Vincent et al. 2013). These essential resources require the development of integrated freshwater management plans, which include consideration of alternate water sources as traditional supplies change in quantity or quality (Vincent et al. 2011). The Arctic will require increasing vigilance and appropriate water management strategies to avoid and minimize the impacts of changing water impacts in the future (Vincent et al. 2013).
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