Dystrophic lakes, also known ashumic lakes, are lakes that contain high amounts ofhumic substances and organic acids.[1] The presence of these substances causes the water to be brown in colour and have a generally lowpH of around 4.0-6.0.[2] The presence of humic substances are mainly due to certain plants in the watersheds of the lakes, such as peat mosses and conifers. Due to these acidic conditions, few taxa are able to survive, consisting mostly ofaquatic plants,algae,phytoplankton,picoplankton, andbacteria.[3][4] Dystrophic lakes can be found in many areas of the world, especially in the northern boreal regions.[5][6]
Dystrophia can be categorized as a condition affecting trophic state rather than a trophic state in itself.[7] Lakes typically are categorized according to the increasing productivity asoligotrophic, mesotrophic,eutrophic, and hypereutrophic.[8] Dystrophic lakes used to be classified as oligotrophic due to their lowproductivity. However, more recent research shows dystrophia can be associated with any of the trophic types. This is due to a wider possible pH range (acidic 4.0 to more neutral 8.0 on occasion) and other fluctuating properties like nutrient availability and chemical composition. Hydrochemical Dystrophy Index is a scale used to evaluate the dystrophy level of lakes. In 2017, Gorniak proposed a new set of rules for evaluating this index, using properties such as the surface water pH,electric conductivity, and concentrations of dissolved inorganic carbon, and dissolved organic carbon.[9]
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Dystrophic lakes have a high level of dissolved organic carbon. This consists of organiccarboxylic andphenolic acids, which keep water pH levels relatively stable, possibly by acting as a naturalbuffer.[10] Therefore, the lake's naturally acidic pH is largely unaffected by industrial emissions. Dissolved organic carbon also reduces the amount of ultraviolet radiation that enters the lake and can reduce thebioavailability of heavy metals by binding them.[11] There is a significantly lower calcium content in the water andsediment of a dystrophic lake when compared with a non-dystrophic lake.[3]Essential fatty acids, likeeicosapentaenoic acid (EPA) anddocosahexaenoic acid (DHA), are still present in the organisms in humic lakes, but are downgraded in nutritional quality by this acidic environment, resulting low nutritional quality of dystrophic lake'sproducers, such as phytoplankton.[12] Because of differing trophic status, some dystrophic lakes may differ strongly in their chemical composition from other dystrophic lakes.[7] Studies of the chemical composition of different types of dystrophic lakes have shown differing levels of dissolved inorganic nitrogen,lipase andglucosidase depending on water color.
Thecatchment area of a dystrophic lake is usually aconiferous forest rich or an area withpeat mosses.[13][14][3] Despite the presence of ample nutrients, dystrophic lakes can be considered nutrient-poor, because their nutrients are trapped in organic matter, and therefore are unavailable to primary producers.[15][16] A considerable amount of the organic matter in dystrophic lakes is allochthonous, meaning it is produced externally to the lake. Due to high amounts of organic matter and lack of light, it is bacterioplankton that control the rate of nutrient flux between the aquatic and terrestrial environments.[17] The bacteria are found in high numbers. These bacteria drive the food web of humic lakes by providing energy and supplying usable forms of organic and inorganic carbon to other organisms, primarily tophagotrophic andmixotrophicflagellates.[18] Decomposition of organic matter by bacteria also converts organic nitrogen and phosphorus into their inorganic forms, which are then available for uptake by primary producers including both large and small phytoplankton (algae and cyanobacteria).[4][3] The biological activity of humic lakes is, however, dominated by bacterialmetabolism. The chemistry of humic lakes makes it difficult for higher trophic levels such asplanktivorous fish to establish themselves, leaving a simplified food web consisting mostly of plants, plankton, and bacteria.[17] The dominance of the bacteria means that dystrophic lakes generally have a higherrespiration rate thanprimary production rate.[3]
The formation of a humic lakes via organic runoff has a dramatic effect on the lakeecosystem. Increases in the lake's acidity make it difficult for fish and other organisms toproliferate. The quality of the lake for use as drinking water also decreases as the carbon concentration and acidity increase. The fish that do adapt to the increased acidity may also not be fit for human consumption, due to theorganic pollutants.
Lakes are commonly known to be important sinks in thecarbon cycle. Dystrophic lakes are typically net heterotrophic due to the large amount of bacterial respiration outweighing phytoplankton photosynthesis, meaning that dystrophic lakes are larger carbon sources than clear lakes, emitting carbon into the atmosphere.[19] The elevated levels of allochthonous carbon in humic lakes are due to vegetation in the lake and catchment area, the runoff from which is the main source of organic material. However, changes in these levels can also be attributed to shifts in precipitation, changing forestry practices, reducedsulphatedeposition, and changes in temperature.[20] Contemporaryclimate change is increasing temperature and precipitation in some parts of the world, thus increasing the supply of humic substances to lakes, making them more dystrophic; this process is referred to as "brownification."[20][21]
Examples of dystrophic lakes that have been studied by scientists include Lake Suchar II in Poland, lakes Allgjuttern, Fiolen, and Brunnsjön inSweden, andLake Matheson in New Zealand.[3][9][22]
