Abstract
Abstract. In the last decades, the mining exploitation of large areas in Lusatia (Eastern Germany) but also in other mining areas worldwide has led to the formation of hundreds of pit lakes. Pyrite oxidation in the surrounding dumps makes many such lakes extremely acidic (pH < 3). The biogeochemical functioning of these lakes is mainly governed by cycling of iron. This represents a relevant ecological problem and intensive research has been conducted to understand the involved biogeochemical processes and develop bioremediation strategies. Despite some studies reporting the presence of living organisms (mostly bacteria, algae, and macro-invertebrates) under such acidic conditions, and their trophic interactions, their potential impact on the ecosystem functioning was poorly investigated. The present study aimed to assess the influence of chironomid larvae on oxygen dynamics and iron cycle in the sediment of acidic pit lakes. In the Mining Lake 111, used as a study case since 1996, Chironomus crassimanus (Insecta, Diptera) is the dominant benthic macro-invertebrate species and occurs at relatively high abundances in shallow water. A 16-day laboratory experiment using microcosms combined with high resolution measurements (DET gel probes and O2 microsensors) was carried out. The burrowing activity of C. crassimanus larvae induced a 3-fold increase of the diffusive oxygen uptake by sediment, indicating a stimulation of the mineralization of organic matter in the upper layers of the sediment. The iron cycle was also impacted (e.g. lower rates of reduction and oxidation, increase of iron-oxidizing bacteria abundance, stimulation of mineral formation) but with no significant effect on the iron flux at the sediment-water interface, and thus on the water acidity budget. This work provides the first assessment of bioturbation in an acidic mining lake and shows that its influence on biogeochemistry cannot be neglected.
Highlights
As a consequence of mining activities in lignite areas, the filling of abandoned sites by groundwater and weathering water frequently leads to the formation of acidic lakes
The aim of the present study was to investigate the role of Chironomus crassimanus larvae on the biogeochemistry of the Mining Lake 111 (Lusatia, Germany), a lake which has been used as a study case since 1996
A laboratory experiment was carried out using intact sediment cores and organisms directly collected in the Mining Lake 111 (ML-111), in the Lusatian lignite mining area in eastern Germany
Summary
As a consequence of mining activities in lignite areas, the filling of abandoned sites by groundwater and weathering water frequently leads to the formation of acidic lakes. Compared to most natural lakes, these ecosystems are characterized by low primary production and nutrient concentrations, a high solubility of metals and simple food webs (Geller et al, 1998). In Germany about 500 lignite pit lakes exist of which about 50% have been initially acidic (Schultze et al, 2009). The acidity of lake water is a result of the weathering of pyriteand marcasite-enriched surrounding dumps through the production of SO24−, Fe2+ and protons. Subsequent oxidation of Fe2+ leads to the formation of Fe(III) minerals that precipitate and settle on the sediment surface. Under anoxic conditions, microbial reduction of Fe(III) and to a lesser extent SO24− removes protons from the water which creates alkalinity.
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