The evolution of a mining lake - From acidity to natural neutralization

•Originally acid water lake had poor phyto- and zooplankton populations.•Process of natural neutralization lasted approximately 23 years.•Presently, lake's ecosystem is similar to other shallow lakes in the region.•Changes in the lake are representative for other mine lakes.

Along the border of Poland and Germany (central Europe), many of the post-mining lakes have formed “an anthropogenic lake district”. This study presents the evolution of a mining lake ecosystem (TR-33) based on subfossil phyto- and zooplankton, isotopic data (δ13C, δ15N), elemental analyses of organic carbon and nitrogen (C/N ratio and TOC) and sedimentological analyses. Recently, lake TR-33 became completely neutralized from acidification and an increase in eutrophication began a few years ago. However, the lake has never been neutralized by humans; only natural processes have influenced the present water quality. From the beginning of the existence of the lake (1920s) to the present, we can distinguish four stages of lake development: 1) very shallow reservoir without typical lake sediments but with a sand layer containing fine lignite particles and very poor diatom and cladoceran communities; 2) very acidic, deeper water body with increasing frequencies of phyto- and zooplankton; 3) transitional period (rebuilding communities of diatoms and Cladocera), meaning a deep lake with benthic and planktonic fauna and flora with wide ecological tolerances; and 4) a shift to circumneutral conditions with an essential increase in planktonic taxa that prefer more fertile waters (eutrophication). In the case of lake TR-33, this process of natural neutralization lasted approximately 23 years.

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