IW7 Organic matter

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Partly dissolved organic matter is carried into water bodies by run-off waters from land and especially mires. Some of the organic matter is a result of degradation of aquatic macrophytes, but its proportion is relatively small. Organic matter concentration is studied by measuring the chemical oxygen demand (COD_Mn) of the water. COD_Mn value refers to the amount of organic matter which can be chemically oxidized. This may be humus, waste water, emissions from animal husbandry or other solid organic particles. The COD_Mn value varies with season especially in rivers but somewhat also in lakes. Melting snow in spring and heavy rain in autumn increase the run-off from land, resulting in high organic matter concentration of water bodies.

The average organic matter concentration seems to have increased steeply during the early 20th century in certain rivers which empty into the Gulf of Bothnia. Sufficient data is not available for the middle of the 20th century, but from 1970s to 1990s the organic matter concentrations decreased steadily in these rivers. The winter time concentrations seem to have decreased more and also remained lower than those of other seasons. Average COD_Mn values for Finnish lakes do not show any clear long term trend. In humic lakes the variation between years has been greater than in clear water lakes.

Organic matter in water environment

Organic matter concentration is usually greatest in small and shallow headwater lakes, especially if there is a large proportion of mires in the catchment area. High organic matter concentrations are also found in the lowlands of Ostrobothnia and Southern Finland, where lakes are fewer. Water typically stays longer in large lakes, which enables organic matter to deposit and sediment in the bottom of the lake, thus lowering the COD_Mn value. Microbes decompose organic matter more effectively when it is deposited rather than dissolved in water. Some zoobenthos species also consume deposited organic matter.

The organic matter which originates from terrestrial environments has a significant impact on water bodies as habitats which is why humus is said to connect terrestrial and aquatic ecosystems to each other. Humus is not very important nutrient source for plants since it decomposes quite slowly. It may however bind nutrients and act as their carrier into water bodies. Thus it acts as a sort of nutrient storage from which for example phosphorus is released to the use of primary producers. Humus is a part of so called natural organic load. Due to this humic lakes are more sensitive to the anthropogenic nutrient load than clear water lakes. As loading of organic matter increases due to for example drainage, the relatively large microbe populations of humic lakes become even more abundant and active. This decreases the oxygen concentration of water. Then again microbes are a source of nutriment for many zooplankton and zoobenthos species.

Dark water colour in humic lakes and rivers weakens their light conditions. Rapid decrease in the amount of light from top to the deeper layers of water effects the abundance and species diversity of plankton. Dark water also binds more heat than clear water does, which may affect the occurrence of some cool water species, for example salmonids. In addition, humic waters are usually more acid than clear waters, since humus contains substances which are weak acids.