It is characterised by a high content of difficult-to-decompose, high-molecular-weight humic acids formed by bacterial decomposition of organic matter. The humic acids present in the water give it a dark colour (brown, brownish-brown) and a low pH. The dark colour of water is due to the microbiological decomposition of organic matter - the remains of plants and animals. Humification is the biochemical decomposition of dead organic material with the formation of highly polymeric humic substances (humic acids, humins, fulvic acids). It occurs in the presence of small amounts of oxygen. Aquatic humus is an organic substance dissolved in water consisting of persistent humic acids.