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Dystrophic Lakes

Dystrophic lakes, also known as humic lakes, are lakes that contain high amounts of humic substances and organic acids. The presence of these substances causes the water to be brown in colour and have a generally low pH of around 4.0-6.0. Due to these acidic conditions, there is little biodiversity able to survive, consisting mostly of algae, phytoplankton, picoplankton, and bacteria Ample research has been performed on the many dystrophic lakes located in Eastern Poland, but dystrophic lakes can be found in many areas of the world.

Classification of Dystrophic Lakes

The main trophic levels used to describe the properties of lakes are oligotrophic, mesotrophic, eutrophic, and hypereutrophic. Dystrophic lakes used to be classified as oligotrophic lakes due to their low productivity. However, more recent research shows dystrophia can be attributed to any of the trophic levels. This is due to a wider possible pH range (acidic 4.0 to more netural 8.0 on occasion) and other fluxuating properties like nutrient availability and chemical composition. Therefore, dystrophia can be categorized as a condition affecting tropic state rather than its own trophic state.

Chemical Properties

Dystrophic lakes have a high level of dissolved organic carbon (DOC). DOC contains organic carboxylic and phenolic acids, which keeps water pH levels relatively stable by acting as a natural buffer. Therefore, the lake’s naturally acidic pH is largely unaffected by industrial emissions. DOC also reduces the entry of UV radiation and can reduce the bioavailability of heavy metals by binding them. There is a significantly lowered calcium content in the water and sediment of a dystrophic lake when compared with a regular lake. Essential fatty acids like EPA and DHA are still present in the organisms in humic lakes, but are downgraded in nutritional quality by this acidic system due to the high numbers of low quality producers such as phytoplankton. HDI (Hydrochemical Dystrophy Index) is a scale used to evaluate the dystrophy level of lakes. In 2016, Gorniak proposed a new set of rules for evaluating the HDI of a lake, using properties such as the surface water pH, electric conductivity, level of dissolved inorganic carbon (DIC), and level of dissolved organic carbon (DOC). As dystrophia is a descriptor attributed to other trophic levels, differences in chemical composition can be expected when comparing different types of dystrophically labelled lakes. Studies of the chemical composition of dystrophic lakes have shown heightened levels of dissolved inorganic nitrogen and higher activities of lipase and glucosidase in polyhummic lakes when compared with oligohumic lakes. In the surface microlayers of the lake waters, there are higher levels of phosphatase activity than in the subsurface microlayers when the lake is oligohumic. The opposite is true when the lake is polyhumic. Both oligohumic and polyhumic lakes show higher aminopeptidase activity in the subsurface microlayers than in the surface microlayers.

Life in Dystrophic Lakes

The catchment area of a dystrophic lake is usually a coniferous forest rich with peat mosses that spread along the water surface. Dystrophic lakes can be considered nutrient poor, despite the presence of ample nutrients, because their nutrients are trapped in organic matter and are therefore unavailable. This organic matter is derived from the catchment area, as it is allochthonous (terrestrially derived) organic matter that fills this aquatic environment. Due to this organic matter rich environment, bacterioplankton are responsible for the rate of nutrient flux between the aquatic and terrestrial environments. The bacteria most commonly found in humic lakes are phagotrophic and mixotrophic flagellates. These are found in high numbers and have great growth potentials under dystrophic conditions. These bacteria drive the food web of humic lakes by providing energy and supplying usable forms of organic and inorganic carbon. Dystrophic lakes have high biovolumes of algae present. There is also an abundance of picoplankton and phytoplankton present. The main activity of humic lakes, however, is their high bacterial metabolism which dominates the food web. The humic lake environment cannot support higher trophic levels such as planktivorous fish and therefore contains a small simplified food web consisting mostly of plants, plankton, and bacteria. The presence of these few organisms and lack of other biodiversity causes these lakes to have a higher respiration rate than primary production rate.

Dystrophic Lakes and Climate Change

Lakes are commonly known to be important sinks in the carbon cycle. Due to their high levels of dissolved organic carbon, dystrophic lakes are significantly larger carbon sinks than clear lakes. The heightened levels of carbon concentrations in humic lakes can partially be explained by vegetation patterns in the catchment area, whose runoff greatly impacts the composition of the lakes because it is the main source of organic material. However, changes in these levels can also be attributed to shifts in precipitation, modifications of soil mineralization rates, reduced sulphate deposition, and changes in temperature. Changes in these factors can all be attributed to climate change. The creation of a humic lake via organic runoff has dire consequences for the ecosystem as a whole. Chemical composition changes that increase the lake’s acidity make it difficult for fish and other organisms to proliferate. 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, as the organic pollutants and mediation of heavy metal concentrations is also impacted by the changes in chemical composition of a humic lake.

Examples of Dystrophic Lakes

Examples of dystrophic lakes that have been studied by scientists include Lake Suchar II in Poland, and lakes Allgjuttern, Fiolen, and Brunnsjön in Sweden.