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

Subfossil chironomids in man-made reservoirs

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 18^th century and are remnants of a former hydro-energetic system that provided water for operating mining and smelting facilities until the 1920s. The dated sequence of sediment cores span the last 170–200 years, however the oldest sediments were not identified. A total of 58 taxa were recorded 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 of reservoirs connected with severe water level fluctuations and unproductive conditions. A dramatic shift in both qualitative and quantitative structure of the chironomid assemblages correspond to the documented decline of mining activities in the region. The second period of the reservoir development is also connected to changes in sediment composition and an increase of sediment organic content. In this phase, chironomids indicate nutrient enrichment, developed profundal zone with oxygen depletion and a complexity of littoral habitats. Since the 1970s, changes in the assemblage structure indicate the reduction of the profundal fauna and increasing importance of littoral taxa. Despite the significantly different usage of the reservoirs after the mining phase (recreation and angling vs drinking water supply) the chironomid succession trajectories were fairly similar.