The future of Finland's inland waterways

Water bodies are closely affected by changes in the air and on land. They are nevertheless isolated habitats for many species, which makes it challenging for species to disperse to new areas. As species adapt to climate change, the choice of biotope also matters; conditions in a large water body such as Lake Saimaa are different from those found in a river up in the fells or in a spring deep inside an old-growth forest.

From the land of thousands of lakes

Water covers a tenth of Finland's area. The volume of water is nevertheless small relative to the total area, because the ground surface was worn flat by the ice age and water bodies are therefore shallow. [1] More than half of Finnish rivers and lakes have a good ecological status [2], but the small volume of water makes shallow water bodies sensitive to man-made changes in the environment. [1]

The biodiversity of inland waterways leaves room for improvement. A total of 68% of the biotopes found in inland waterways in Southern Finland are classified as threatened. The situation is better in Northern Finland, and only 3% of biotopes are threatened. The situation is especially worrying as regards fluvial waters – that is rivers and streams – and all fluvial biotopes in Southern Finland are either threatened or near threatened. Lakes fare better, but the majority of biotopes found in lakes have nevertheless been classified as near threatened due to eutrophication and the accumulation of silt deposits, for example. Measures relating to agriculture and forestry, drainage and fertilisation, peat extraction, dredging, and hydraulic engineering and coastal development are among the most common reasons for the endangerment of biotopes found in inland waterways. [3]

Water bodies provide humans with nutrition, energy, transport routes, and various recreational opportunities. The wellness of aquatic ecosystems is important for the survival of these ecosystem services. Some ecosystem services are dependent on high biodiversity.

Effect of climatic conditions on water bodies

Rainfall affects the volume and quality of water in water bodies, the flow rates of rivers, the likelihood of flooding, the leaching of nutrients and solids into water bodies, and oxygen levels in water, for example. Rainfall determines the hydrological status of water bodies, which is why changes in the volume and temporal distribution of rainfall have a significant impact on the ecological status of water bodies and on biological communities [4]. Temperature affects stratification, oxygen levels, and the prevalence of different species, as each species has its own thermal optimum. [5][6]

Rainfall and increasing temperature increase eutrophication

Air temperature in Finland is likely to increase by between three and six degrees on average by the end of the current century. Together with the longer growing season, it will increase primary production in aquatic ecosystems. In the future, Finnish waterways will also be affected by rainfall, which is likely to increase by between 12 and 20 percent depending on the climate change scenario. Increased rainfall and heavy rains will increase the leaching of nutrients into water bodies especially during mild winters when there is no layer of vegetation to absorb nutrients and the ground is not frozen. High levels of nutrients accelerate plant growth in aquatic ecosystems, and climate change is expected to increase eutrophication on the whole. [7]

järviruokoa rantavedessä © Auri Sarvilinna

Reeds in shallow water. Climate change may increase vegetation in water bodies.

Due to climate change and the resulting lengthening of the growing season [8] and increased eutrophication, the volume of vegetation will increase especially along the shores of water bodies. Large emergent aquatic plants will also benefit from higher levels of carbon dioxide in the air and may therefore replace some species that are sensitive to eutrophication. Mass phytoplankton blooms may become more common and appear earlier in the year. [7] Blue-green algae are believed to benefit from global warming, because their thermal optimum is a little higher than that of other species. They are also capable of fixing atmospheric nitrogen and are therefore not as dependent on the nutrient levels found in water as many other species. [9]

levää rantavedessä © Pirjo Ferin-Westerholm

Blue-green algal blooms may increase with climate change.

Increasing productivity increases the volume of organic matter on the lake bed or riverbed as well as the numbers of organisms and their oxygen intake. This affects the composition of biological communities; typically, eutrophication reduces the diversity of species. [10] Fish that benefit from eutrophication include the zander (Sander lucioperca) and several species of the carp family (Cyprinidae) [11]. Eutrophication also affects the ability of aquatic ecosystems to act as carbon sinks [12].

On overturning, flooding, and acidification

As annual mean temperatures rise, winters and the period of ice cover become shorter, the spring overturn moves forward, and the autumn overturn moves back. As the thermal stratification of lakes continues for longer in the summer and primary production increases in water bodies, oxygen levels near the lake bed may deplete faster and anaerobic conditions prevail for longer periods of time. [7] This may harm several species and especially decomposers found in the benthic zone in lakes. Prolonged anaerobic conditions favour species that have adapted to low levels of oxygen [13]. The common bream (Abramis brama) and the crucian carp (Carassius carassius), for example, can tolerate very low levels of oxygen [11][14]. The lack of oxygen also causes nutrients stored in the lake bed or riverbed to be released and increases internal loading and eutrophication in water bodies. Thermal stratification may nevertheless decrease in some lakes in Southern Finland, and strong winds may mix water even in the summer, therefore preventing the development of anaerobic conditions. [7]

Water levels in all water bodies are expected to vary more in the future, and periodical and local flooding is likely to increase across the country. Floods are expected to become increasingly irregular, and spring floods may decrease as winter flooding becomes more common. The risk of flooding is especially high in lakes with large drainage basins, such as Lake Saimaa and Lake Päijänne. [15] During floods, surface runoff from terrestrial ecosystems increases and the capacity of water supply systems may be exceeded, which may result in wastewater being released directly into water bodies. Nutrients, solids, and harmful substances damage water quality and increase eutrophication and oxygen demand in water bodies. Flooding is associated with fish death due to the lack of oxygen. [10]

The scale of the effects of climate change varies from one type of water body to the next. Its impact on small water bodies, such as brooks and ponds, is likely to be substantial, because changes in air temperature, for example, have a strong effect on them and because they are more sensitive to thermal stress than larger water bodies. Small brooks are sensitive to changes in surface runoff. Small ponds are susceptible to the effects of summer droughts. [6]

Increasing levels of carbon dioxide in the air and nitrogen introduced by means of surface runoff cause acidification in water bodies [15], which affects all aquatic organisms and especially invertebrate animals with calcareous shells, such as snails.

Global warming brings changes to the prevalence of species

The diversity of species in freshwater ecosystems is partially dependent on the climate, and the diversity of many groups of species decreases towards the north in the northern boreal ecoregion [6]. Examples of this include dragonflies, fish, diatoms, and large aquatic plants [16][17] [18]. Increasing mean temperatures may therefore have an effect on the regional diversity of species and biological communities.

Studies indicate that the increase of spring temperatures, which began in the 1840s, has affected the composition of the biological communities found in lakes across Lapland. With global warming, the prevalence of diatom species (Bacillariophyta), in particular, has changed and diversity either decreased or remained relatively unchanged. The change is believed to be attributable to increased thermal stratification in the studied lakes, which has favoured planktic species instead of benthic ones. [19] Lake Saanajärvi also demonstrated changes in the composition of species of golden algae (Chrysophyta) and zooplankton [20]. These results can be used to project the effects of anthropogenic climate change.

Coldwater species are likely to decrease in the future and species more typically associated with warm and cool water increase and spread towards the north [6]. In terms of fish, the Arctic char (Salvelinus alpinus), the lavaret (Coregonus lavaretus), and the brown trout (Salmo trutta), in particular, rely on cold water and have a low tolerance to thermal stress [11] [21]. The metabolic rate of fish increases in warm water, and their ontogenesis and reproduction may accelerate. Competition between species as well as diseases and parasites may also increase. [22] Increasing water temperatures will be problematic especially for coldwater species in small ponds, which lack the colder deeps typical of larger water bodies where fish can escape in hot summer weather [6].

Adaptation to changing conditions is especially important for the survival of coldwater species. This may be possible at least for species with a short generation time but may be more difficult for long-living species. Top predators in the food web tend to be long-living, and a decrease in the numbers of predator species could increase the prevalence of species on the lower levels of the food web. [22]

The prevalence of different species in inland waterways may also change as a result of indirect effects. The decreasing prevalence of coniferous trees and the increase of broadleaf trees, for example, is likely to affect invertebrate aquatic species, because leaves and needles falling from trees are an important source of nutrition for them. Because leaves have a higher nutritional value than needles, invertebrates would have more nutrition available to them, which could lead to changes in the prevalence of species. [6]

Ecological connectivity between habitats and the dispersal ability of species regulate changes in geographic ranges

Little research has gone into changes in inland water species so far, but, in the case of many species, changes in geographic ranges via inland waterways may be slower than those in terrestrial ecosystems. This is due to dispersal barriers between waterways. Poor connections from one habitat to another make it more difficult for populations to survive and for new species to disperse to an area. Species with the best dispersal ability have the best chances of survival. There are considerable differences between the dispersal ability of different species, and it is possible that species found in lakes and ponds have developed better dispersal strategies than species found in fluvial waters. This is why the effects of climate change on the geographic ranges of species may show more slowly in fluvial waters. [23] This theory does not necessarily apply to all species, and in some lakes dispersal barriers may affect the ability of coldwater species, for example, to disperse to new areas. [6] Competition between species also affects the changes in the prevalence of species in each habitat.

sudenkorento järviruoolla © Jouko Lehmuskallio

Species of dragonflies may become more numerous in Finland with climate change.

In addition to certain microscopic algae, dragonflies (Odonata) are also potentially good indicators of climate change, because the lack of ecological connectivity and terrestrial ecosystems are likely to restrict the dispersal of flying insects less than that of fish, for example [6]. New species of dragonflies have been spotted in Finland every year during the 2000s, and with climate change the number of species is expected to increase further. [24] Although the arrival of new species is assumed to be attributable to warm summers, no actual longitudinal studies have been carried out in Finland on the effects of climate change on the prevalence of different species of dragonflies.

Eutrophication as well as increasing rainfall and surface runoff may increase the amount of carbon released from inland waterways

Lake sediments are the third largest natural carbon store in Finland. Approximately 0.62 Pg of carbon has been sequestered in them since the last ice age. The carbon dioxide emissions of lakes have been estimated to amount to approximately 1,400 Gg per year with approximately 65 Gg of carbon sequestered in lake bed sediments annually. Studies indicate that there is a strong correlation between annual rainfall and the annual carbon dioxide emissions of lakes. This may be due to the fact that more organic matter is leached into lakes from drainage basins during years with heavy rainfall. Eutrophication in lakes also increases the volume of readily biodegradable organic matter and increases the rate of decomposition. This increases the natural carbon dioxide and methane emissions of lakes. If rainfall increases with climate change and eutrophication accelerates, carbon dioxide emissions from natural sources may also increase. [12]

Other changes in biodiversity

The combined effect of climate change and acidification, eutrophication, changes in land use, and the impact of alien species may have unpredictable consequences on aquatic species [6] [17]. With climate change, the number of alien species introduced to inland waters by man is likely to increase further [25], as conditions become more favourable to the survival of species originating from more southern areas. Alien species affect ecosystems and their native species through competition, predation, illnesses, and parasites, for example [26].

References

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