Abstract
The effect of dissolved organic carbon (DOC) on the environmental conditions of macrophytes has been studied in 35 lakes divided into soft- and hardwater: oligohumic (&lt;4.0 mg C dm<sup>-3</sup>), α-mesohumic (4.0-8.0 mg C dm<sup>-3</sup>), β-mesohumic (8.1-16.0 mg C dm<sup>-3</sup>) and polihumic (&gt;16.0 mg C dm<sup>-3</sup>). The optimum environmental conditions for macrophytes have been found in oligohumic lakes, characterised by low water colour and its good transparency. In soft- and hardwater lakes increasing concentration of DOC is accompanied with an increase in the colour (r=0.95), while the visibility decreases. With increasing DOC in the near-sediment layer the pH values decrease while the concentration of nitrogen increases and the concentration of phosphorus slightly increases. In hardwater lakes with increasing DOC concentration, the redox potential, conductivity, total hardness and calcium concentration in the near-sediment water decrease, whereas the content of CO<sup>2</sup> remains at a very low level.
Highlights
Many lakes are connected into systems of drainage ditches and channels filled with waters rich in humus substances from the drained peat-bogs
In oligohumic lakes the bulk water is colourless or weakly coloured (0-17 mg Pt dm-3; Me=4.5) and transparent. The latter feature follows from a low concentration of dissolved organic carbon (2.9±1.2 mg CDOC dm-3; Me=2.5)
The environmental conditions in the lakes have been determined by the presence of allochtonous DOC coming from humic substances
Summary
Many lakes are connected into systems of drainage ditches and channels filled with waters rich in humus substances from the drained peat-bogs. With increasing content of allochtonous carbon compounds, especially humic acids, in the lakes, the pH values of water and bottom sediments decrease, the rate of organic matter decomposition decreases, the redox potential of the sediment increases and anaerobiosis becomes more enhanced (Lampert and Sommer 1996). These changes disturb the circulation of biogenic elements (Grahn 1985), causing the plants exchange according to their demand on inorganic carbon (DIC) including the species CO2 and HCO3(Sand-Jensen 1989), and the structure and species composition of phytocenoses change (Szmeja 1992). The acidic bottom sediments restrict the appearance of many aquatic plants, leading to changes in their population structure (Roelofs 1983; Grahn 1985; Arts 1987; Szmeja 1994a, b, c) and initiate the exchange of the basiphilous and neutrophilous species by acidophilous ones, mainly bryophytes (Szmeja 2000; Boci1g 2000)
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