The historical development of three man-made reservoirs in a mining region: A story told by subfossil chironomids

Chironomid stratigraphic changes were studied in three man-made reservoirs situated in a region intensely used for mining in the past (Banska Stiavnica, Central Europe). The reservoirs were built in the first half of the 18th century and are remnants of a former hydro-energetic system that provided water for mining and smelting facilities until the 1920s. Sediment cores spanning the last 170-200 years were sampled and dated. A total of 58 taxa were identified, with the most common taxa being Cladotanytarsus mancus-type, Polypedilum nubeculosum-type, Procladius spp. and Tanytarsus spp. Changes in the subfossil assemblages suggest that all reservoirs followed similar developmental trends. The low number of chironomid remains and the prevalence of littoral and rheophilic taxa characterize the initial phase, with severe water level fluctuations and unproductive conditions. A dramatic shift in both the qualitative and quantitative structure of the chironomid assemblages accompanied the decline of mining activities in the region. The second period of reservoir development was linked to changes in sediment composition and an increase of sediment organic content. In this phase, chironomids indicate nutrient enrichment, and the development of a hypoxic profundal zone, and complex littoral habitats. Since the 1970s, changes in the assemblage structure indicate a reduction in the profundal fauna and an increasing importance of the littoral taxa. Despite the significantly different usage of the reservoirs after the mining phase (recreation and angling vs drinking water supply) the changes in chironomid assemblages were fairly similar and indicated nearly identical developmental trajectories.