Key findings, science gaps, and recommendations for freshwater ecosystems in the ACIA
This is Section 8.8 of the Arctic Climate Impact Assessment Lead Authors: Frederick J.Wrona,Terry D. Prowse, James D. Reist; Contributing Authors: Richard Beamish, John J. Gibson, John Hobbie, Erik Jeppesen, Jackie King, Guenter Koeck, Atte Korhola, Lucie Lévesque, Robie Macdonald, Michael Power,Vladimir Skvortsov,Warwick Vincent; Consulting Authors: Robert Clark, Brian Dempson, David Lean, Hannu Lehtonen, Sofia Perin, Richard Pienitz, Milla Rautio, John Smol, Ross Tallman, Alexander Zhulidov
In general, changes in climate and UV radiation levels in the Arctic are very likely to have far-reaching impacts, affecting aquatic species at various trophic levels, the physical and chemical environment that makes up their habitat, and the processes that act on and within freshwater ecosystems. Interactions of climatic variables such as temperature and precipitation with freshwater ecosystems are highly complex and can be propagated through the ecosystem in ways that are not readily predictable. This reduces the ability to accurately project specific effects of climate and UV radiation change on freshwater systems. This is particularly the case when dealing with threshold responses (i.e., those that produce stepwise and/or nonlinear effects). Forecasting ability is further hampered by the poor understanding of arctic freshwater systems and their basic interrelationships with climatic and other environmental variables, as well as by a paucity of long-term freshwater monitoring sites and integrated hydro-ecological research programs in the Arctic.
A significant amount of the understanding of potential impacts is based on historical analogues (i.e., historical evidence from past periods of climate change), as well as from a limited number of more recent studies of ecosystem response to environmental variability. Paleo-reconstructions indicate that during the most recent period of climatic warming, which followed the Little Ice Age, the Arctic reached its highest average annual temperatures observed in the past 400 years, resulting in glacier retreat, permafrost thaw, and major shifts in freshwater ecosystems. Examples of ecosystem effects included altered water chemistry, changes in species assemblages, altered productivity, and an extended growing-season length. Importantly, however, past natural change in the Arctic occurred at a rate much slower than that projected for anthropogenic climate change over the next 100 years. In the past, organisms had considerable time to adapt; their responses may therefore not provide good historical analogues for what will result under much more rapid climate change. In many cases, the adaptability (i.e., adaptation, acclimation, or migration) of organisms under rapidly changing climate conditions is largely unknown. Unfortunately, no large-scale attempts have been made to study the effects of rapid climate change on aquatic ecosystems using controlled experiments, as have been attempted for terrestrial systems (e.g., see International Tundra Experiment studies of tundra plant response in section 22.214.171.124). However, field studies in areas that have recently experienced rapid changes in climate have provided important knowledge. Information about ongoing climate change impacts is provided by results from long-term environmental monitoring and research sites in the Arctic.
Key findings (8.8.1)
This section lists a number of broad-scale findings for major components of arctic freshwater ecosystems. Although it was possible in this assessment to evaluate mesoscale regional differences in, for example, the timing and severity of the freshet and/or breakup on large rivers, difficulties in ecological downscaling of most climatic and related hydrological changes precluded regional discrimination of variations in impacts. Hence, most of the following statements are broad-scale and not regionally specific. To indicate the probability of each impact occurring, the ACIA lexicon regarding the range of likelihood of outcome (section 8.1.2) has been applied to the findings (i.e., the bold-text statement(s) in each bullet). Each assigned likelihood is the product of a multi-author scientific judgment based on knowledge synthesized from the scientific literature, including the previously noted case studies, and the interpretation of effects deduced from the ACIA-designated model projections. The same level of likelihood of occurrence is applied to the subsequent, more detailed descriptions following each major finding.
Ecological impacts of changes in runoff, water levels, and river-ice regimes
A probable shift to a more pluvial system with smaller and less intense freshet and ice breakup is very likely to decrease the frequency and magnitude of natural disturbances, and reduce the ability of flow systems to replenish riparian ecosystems, particularly river deltas. As rainfall becomes a more prominent component of high-latitude river flow regimes, and as temperatures rise, freshets will be less intense and ice breakup less dramatic. Furthermore, differential changes in climatic gradients along the length of large arctic rivers will produce varying responses in freshet timing and breakup severity. Decreased frequency and intensity of physical disturbances will result in decreased species richness and biodiversity in riverine, deltaic, and riparian habitats.
Reduced climatic gradients along large northern rivers are likely to alter ice-flooding regimes and related ecological processes. Projected differential rates of temperature increases across the major latitudinal ranges of some large northern rivers and the corresponding reduction in the latitudinal spring temperature gradient are likely to reduce the frequency and magnitude of dynamic river-ice breakups and lead to more placid thermal events. Because such disturbances play a major role in maintaining habitat complexity and associated species richness and diversity, this will have significant implications for ecosystem structure and function.
A very probable increase in winter flows and reduced ice-cover growth is very likely to increase the availability of under-ice habitat. High-latitude rivers that typically freeze to the bottom during the winter will experience increased flow in response to increasing precipitation and winter temperatures, increasing base flow, and declining ice thickness. The subsequent presence of year-round flowing water in these river channels will increase habitat availability, ensuring survival of species previously restricted by the limitation of under-ice habitat. Migration and the geographic distribution of aquatic species (e.g., fish) may also be affected.
A probable decrease in summer water levels of lakes and rivers is very likely to affect quality and quantity of, and access to, aquatic habitats. In areas where combinations of precipitation and evaporation lead to reductions in lake and/or river water levels, pathways for fish movement and migration will be impaired, including access to critical habitat. In addition, declining water levels will affect physical and chemical processes such as stratification, nutrient cycling, and oxygen dynamics.
Changes in biogeochemical inputs from altered terrestrial landscapes
Enhanced permafrost thawing is very likely to increase nutrient, sediment, and carbon loadings to aquatic systems. This is very likely to have a mixture of positive and negative effects on freshwater chemistry. As permafrost and peat warm and active layers deepen with rising temperatures, nutrients, sediment, and organic carbon will be flushed from soil reserves and transported into aquatic systems. Increased nutrient and organic carbon loading will enhance productivity in high-latitude lakes, as well as decrease exposure of biota to UV radiation. Conversely, heavily nutrient-enriched waters (i.e., systems with enhanced sediment and organic matter loads), may result in increased light limitation and reduced productivity.
An enhanced and earlier supply of sediment is likely to be detrimental to benthic fauna. As soils warm in high-latitude permafrost landscapes and become more susceptible to erosion, surface runoff will transport larger sediment loads to lakes and rivers. Aerobes in lake and river bottom sediments will initially be threatened by oxygen deprivation due to higher biological oxygen demand associated with sedimentation. Larger suspended sediment loads will also negatively affect light penetration and consequently primary production levels. Similarly, negative effects such as infilling of fish spawning beds associated with increased sediment loads are also likely in many areas.
Increases in DOC loading resulting from thawing permafrost and increased vegetation are very likely to have both positive and negative effects. The balance will be ecosystem- or site-specific. For example, as DOC increases, there will be a positive effect associated with reduced penetration of damaging UV radiation, but also a negative effect because of the decline in photochemical processing of organic material. An additional negative effect would be a decrease in primary production due to lower light availability (quantity and quality).
Alterations in ponds and wetlands
Freshwater biogeochemistry is very likely to be altered by changes in water budgets. As permafrost soils in pond and wetland catchments warm, nutrient and carbon loading to these freshwater systems will rise. Nutrient and carbon enrichment will enhance nutrient cycling and productivity, and alter the generation and consumption of carbon-based trace gases.
The status of ponds and wetlands as carbon sinks or sources is very likely to change. High-latitude aquatic ecosystems function as sinks or sources of carbon, depending on temperature, nutrient status, and moisture levels. Initially, arctic wetlands (e.g., peatlands) will become sources of carbon as permafrost thaws, soils warm, and accumulated organic matter decomposes. Decomposition rates in aquatic ecosystems will also increase with rising temperatures and increases in rates of microbial activity. Increases in wetland water levels could enhance anaerobic decay and the production and release of methane.
Permafrost thaw in ice-rich environments is very likely to lead to catastrophic lake drainage; increased groundwater flux is likely to drain others. As permafrost soils warm, freshwater bodies will become increasingly coupled to groundwater systems and experience drawdown. Lake drawdown will result in a change in the limnology and the availability and suitability of habitat for aquatic biota. Over the long term, terrestrial habitat will replace aquatic habitat.
New wetlands, ponds, and drainage networks are very likely to develop in thermokarst areas. Thawing permafrost and melting ground ice in thermokarst areas will result in the formation of depressions where wetlands and ponds may form, interconnected by new drainage networks. These new freshwater systems and habitats will allow for the establishment of aquatic species of plants and animals in areas formerly dominated by terrestrial species.
Peatlands are likely to dry due to increased evapotranspiration. As temperatures increase at high latitudes, rates of evapotranspiration in peatlands will rise. Drying of peat soils will promote the establishment of woody vegetation species, and increase rates of peat decomposition and carbon loss.
Effects of changing lake-ice cover
Reduced ice thickness and duration, and changes in timing and composition, are very likely to alter thermal and radiative regimes. Rising temperatures will reduce the maximum ice thickness on lakes and increase the length of the ice-free season. Reduced lake-ice thickness will increase the availability of under-ice habitat, winter productivity, and associated dissolved oxygen concentrations. Extension of the ice-free season will increase water temperatures and lengthen the overall period of productivity.
A longer open-water season is very likely to affect lake stratification and circulation patterns. Earlier breakup will lead to rapid stratification and a reduction in spring circulation. In certain types of lakes this will cause a transfer of under-ice oxygen-depleted water to the deep water of stratified lakes in the summer (i.e., lakes will not get a chance to aerate in the spring). A longer open-water season will result in an increase in primary production over the summer that will lead to increased oxygen consumption in deeper waters as algae decompose. Correspondingly, fish habitat will be substantially reduced by the combination of upper-water warming and the low-oxygen conditions in deeper water. As a result, certain fish species (e.g., lake trout) may become severely stressed.
Reduced ice cover is likely to have a much greater effect on underwater UV radiation exposure than the projected levels of stratospheric ozone depletion. A major increase in UV radiation levels will cause enhanced damage to organisms (biomolecular, cellular, and physiological damage, and alterations in species composition). Allocations of energy and resources by aquatic biota to UV radiation protection will increase, probably decreasing trophic-level productivity. Elemental fluxes will increase via photochemical pathways.
Aquatic biota, habitat, ecosystem properties, and biodiversity
Climate change is very likely to affect the biodiversity of freshwater ecosystems across most of the Arctic. The magnitude, extent, and duration of the impacts and responses will be system- and location-dependent, and will produce varying outcomes, including local and/or regional extinctions or species loss; genetic adaptations to new environments; and alterations in species ranges and distributions, including invasion by southern species.
Microbial decomposition rates are likely to increase. Rates of microbial decomposition will rise in response to increasing temperatures and soil drying and aeration. Enhanced decomposition of organic materials will increase the availability of [[dissolved organic carbon]] and emissions of carbon dioxide, with implications for the carbon balance of high-latitude lakes and rivers and, in particular, wetlands, which are significant carbon reserves.
Increased production is very likely to result from a greater supply of organic matter and nutrients. Organic matter and nutrient loading of rivers, lakes, and wetlands will increase as temperatures and precipitation rise. Thawing permafrost and warming of frozen soils with rising temperature will result in the release of organic matter and nutrients from catchments. Rising temperatures will increase the rates and occurrence of weathering and nutrient release. Organic matter contributions may also increase with the establishment of woody species. Primary productivity will rise, the effects of which may translate through the food chains of aquatic systems, increasing freshwater biomass and abundance. In some instances, high loading of organic matter and sediment is very likely to limit light levels and result in a decline in productivity in some lakes and ponds.
Shifts in ranges and community composition of invertebrate species are likely to occur. Temperature-limited species from more southerly latitudes will extend their geographic ranges northward. This will result in new invertebrate species assemblages in arctic freshwater ecosystems.
Fish and fisheries
Shifts in species ranges, composition, and trophic relations are very likely to occur. Southern species will shift northward with warming of river waters, and are likely to compete with northern species for resources. The ranges of anadromous species may shift as oceanic patterns shift. The geographic ranges of northern or arctic species will contract in response to habitat impacts as well as competition. Changes in species composition at northern latitudes are likely to have a topdown effect on the composition and abundance of species at lower trophic levels. The broad whitefish, Arctic char complex, and the Arctic cisco are particularly vulnerable to displacement as they are wholly or mostly northern in their distribution. Other species of fish, such as the Arctic grayling of northern Alaska, thrive under cool and wet summer conditions, and may have less reproductive success in warmer waters, potentially causing elimination of populations.
Spawning grounds for cold-water species are likely to diminish. As water temperatures rise, the geographic distribution of spawning grounds for northern species will shift northward, and is likely to contract. Details will be ecosystem-, species-, and site-specific.
An increased incidence of mortality and decreased growth and productivity from disease and/or parasites is likely to occur. As southern species of fish migrate northward with warming river waters, they could introduce new parasites and/or diseases to which arctic fish species are not adapted, leading to a higher risk of earlier mortality and decreased growth.
Subsistence, sport, and commercial fisheries will possibly be negatively affected. Changes in the range and distribution of fish species in northern lakes and rivers in response to changing habitat and the colonization of southerly species have implications for the operation of commercial fisheries and will possibly have potentially devastating effects on subsistence fishing. Changes in northern species (e.g., range, abundance, health) will diminish opportunities for fisheries on such species, calling for regulatory and managerial changes that promote sustainable populations. Subsistence fisheries may be at risk in far northern areas where vulnerable species, such as the broad whitefish, the Arctic char complex, and Arctic cisco, are often the only fish species present. Fisheries will have to change to secure access, and to ensure that fishery function and duration of operation are effective, given a change in fish species and habitat. Alternatively, new opportunities to develop fisheries may occur.
Aquatic mammals and waterfowl
Probable changes in habitat are likely to result in altered migration routes and timing. Migration routes of aquatic mammals and waterfowl are likely to extend northward in geographic extent as more southerly ecosystems and habitats develop at higher latitudes with increasing temperature. Migration may occur earlier in the spring with the onset of high temperatures, and later in the autumn if high temperatures persist. Breeding-ground suitability and access to food resources will be the primary drivers of changes in migration patterns. For example, wetlands are important feeding and breeding grounds for waterfowl, such as geese and ducks, in the spring. As permafrost landscapes degrade at high latitudes, the abundance of thermokarst wetlands may increase, promoting the northward migration of southerly wetland species, or increasing the abundance and diversity of current high-latitude species.
An increased incidence of mortality and decreased growth and productivity from disease and/or parasites will possibly occur. As temperatures rise, more southerly species of mammals and waterfowl will shift northward. These species may carry with them new diseases and/or parasites to which northerly species are not adapted, which is likely to result in both an increased susceptibility to disease and parasites, and an increase in mortality.
Probable changes in habitat suitability and timing of availability are very likely to alter reproductive success. Aquatic mammals and waterfowl are highly dependent on the availability and quality of aquatic habitats for successful breeding, and in the case of waterfowl, nesting. Northern species may have diminished reproductive success as suitable habitat either shifts northward or declines in availability and access. Northward migration of southern species may result in competitive exclusion of northern species from habitat and resources.
Increases in temperature and precipitation are very likely to increase contaminant capture in the Arctic. Projected increases in temperature and changes in the timing and magnitude of precipitation will affect the deposition of contaminants at high latitudes. Climate change will accelerate rates of contaminant transfer. Climate scenarios currently project a “wetter” Arctic, increasing the probability of wet deposition of contaminants such as heavy metals and persistent organic pollutants.
Episodic releases of high contaminant loadings from perennial snow and ice are very likely to increase. As temperatures rise at high latitudes, snow and ice accumulated over periods of years to decades will melt, releasing associated stored contaminants in the meltwater. This will increase episodes of high contaminant loadings into water, which may have toxic effects on aquatic organisms. Permafrost degradation may also mobilize contaminants. Lower water levels will amplify the impacts of contamination on high-latitude freshwater bodies.
Arctic lakes are very likely to become more prominent contaminant sinks. Spring melt waters and associated contaminants typically pass through thermally stratified arctic lakes without transferring their contaminant burden. Contaminant capture in lakes will increase with reduced lake-ice cover (decreased stratification), increased mixing and primary production, and greater organic carbon and sediment loading. Contaminants in bottom sediments may dissociate from the solid phase with a rise in the rate of organic carbon metabolism and, along with contaminants originating from cryogenic concentration, may reach increasing levels of toxicity in lake bottom waters.
The nature and magnitude of contaminant transfer in the food web are likely to change. Changes in aquatic trophic structure and zoogeographic distributions will alter biomagnification of contaminants, including persistent organic pollutants and mercury, and potentially affect freshwater food webs, especially top-level predatory fish (e.g., lake trout) that are sought by all types of fisheries.
Cumulative, synergistic, and overarching interactions
Decoupling of environmental cues used by biota is likely to occur, but the significance of this to biological populations is uncertain. Photoperiod, an ultimate biological cue, will not change, whereas water temperature, a proximate biological cue, will change. For arctic species, decoupling of environmental cues will probably have significant impacts on population processes (e.g., the reproductive success of fish, hatching and feeding success of birds, and the migratory timing and success of birds and anadromous fish may be compromised).
The rate and magnitude at which climate change takes place and affects aquatic systems are likely to outstrip the capacity of many aquatic biota to adapt or acclimate. Evolutionary change in long-lived organisms such as fish cannot occur at the same rate as the projected change in climate. The ability to acclimate or emigrate to more suitable habitats will be limited, thus effects on some native arctic biota will be significant and detrimental. Shorter-lived organisms (e.g., freshwater invertebrates) may have a greater genetic and/or phenotypic capacity to adapt, acclimate, or emigrate.
Climate change is likely to act as a multiple stressor, leading to synergistic impacts on aquatic systems. For example, projected increases in temperature will enhance contaminant influxes to aquatic systems, and independently increase the susceptibility of aquatic organisms to contaminant exposure and effects. The consequences for the biota will in most cases be additive (cumulative) and multiplicative (synergistic). The overall result will be higher contaminant loads and biomagnification in ecosystems.
Climate change is very likely to act cumulatively and/or synergistically with other stressors to affect physical, chemical, and biological aspects of aquatic ecosystems. For example, resource exploitation (e.g., fish or bird harvesting) and climate change impacts will both negatively affect population size and structure.
Key science gaps arising from the assessment (8.8.2)
In conducting this assessment, a number of key gaps in scientific understanding became evident. These are noted throughout the chapter, and include:
- the limited records of long-term changes in physical, chemical, and biological attributes throughout the Arctic;
- differences in the circumpolar availability of biophysical and ecological data (e.g., extremely limited information about habitat requirements of arctic species);
- a lack of circumpolar integration of existing data from various countries and disparate programs;
- a general lack of integrated, comprehensive monitoring and research programs, at regional, national, and especially circumpolar scales;
- a lack of standardized and networked international approaches for monitoring and research;
- the paucity of representative sites for comparative analyses, either by freshwater ecosystem type (e.g., small rivers, wetlands, lakes) or by regional geography (ecozone, latitude, elevation);
- the unknown synergistic impacts of contaminants and climate change on aquatic organisms;
- a limited understanding of the cumulative impacts of multiple environmental stressors on freshwater ecosystems (e.g., land use, fisheries, forestry, flow regulation and impoundment, urbanization, mining, agriculture, and poleward transport of contaminants by invasive/replacement species);
- the unknown effects of extra-arctic large-river transport on freshwater systems induced by southern climate change;
- a limited knowledge of the effects of UV radiation–temperature interactions on aquatic biota;
- a deficient knowledge of the linkages between structure (i.e., biodiversity) and function of arctic aquatic biota;
- a poor knowledge of coupling among physical/chemical and biotic processes; and
- a lack of coupled cold-regions hydrological and ecological theories and related projective models.
Filling these gaps, the most outstanding of which include inter-regional differences in the availability of, and access to, circumpolar research (hence the North American and European bias in this assessment), would greatly improve understanding of the effects of climate and UV radiation change on arctic freshwater ecosystems. Furthermore, comprehensive monitoring programs to quantify the nature, regionality, and progress of climate change and related impacts require development and rapid implementation at representative sites across a broad range of the type and size of aquatic ecosystems found within the various regions of the Arctic. Coupling such programs with ongoing and new research will greatly facilitate meeting the challenges sure to result from climate change and increased UV radiation levels in the Arctic.
Science and policy implications and recommendations (8.8.3)
A number of the above gaps in scientific understanding could be addressed by the following policy and/or program-related adjustments:
- Establish funding and mechanisms for the creation of a coordinated network of key long-term, representative freshwater sites for comparative monitoring and assessment studies among arctic regions (e.g., creation of a Circumpolar Arctic Aquatic Research and Monitoring Program).
- Based on the results of this assessment, establish a science advisory board (preferably at the international level) for targeted funding of arctic freshwater research.
- Secure long-term funding sources, preferably for an international cooperative program, for integrated arctic freshwater research.
- Adjust current northern fisheries management policies and coordinate with First Nations resource use and consumption.
- Establish post-secondary education programs focused on freshwater arctic climate change issues at both intra- and extra-arctic educational institutions, preferably involving a circumpolar educational consortium.
Chapter 8: Freshwater Ecosystems and Fisheries
8.2. Freshwater ecosystems in the Arctic
8.3. Historical changes in freshwater ecosystems
8.4. Climate change effects
8.4.1. Broad-scale effects on freshwater systems
8.4.2. Effects on hydro-ecology of contributing basins
8.4.3. Effects on general hydro-ecology
8.4.4. Changes in aquatic biota and ecosystem structure and function
8.5. Climate change effects on arctic fish, fisheries, and aquatic wildlife
8.5.1. Information required to project responses of arctic fish
8.5.2. Approaches to projecting climate change effects on arctic fish populations
8.5.3. Climate change effects on arctic freshwater fish populations
8.5.4. Effects of climate change on arctic anadromous fish
8.5.5. Impacts on arctic freshwater and anadromous fisheries
8.5.6. Impacts on aquatic birds and mammals
8.6. Ultraviolet radiation effects on freshwater ecosystems
8.7. Global change and contaminants
8.8. Key findings, science gaps, and recommendations