Does the subfossil Chironomidae in sediments of small ponds reflect changes in wastewater discharges from a Zn–Pb mine?

Abstract

This study was aimed to show whether subfossil Chironomidae (Diptera) may be useful tool for assessing toxicity of heavy metals in the aquatic environment. Investigations were carried out in subsidence ponds affected by the activity of metal mining: (1) the older ones formed before mining activity and (2) the younger one formed after the mine was closed. Waters of ponds were analyzed for physico-chemical parameters, whereas sediments were studied for metals (Cd, Pb, and Zn), pH, organic matter, nutrients (TOC, N-tot, and P-tot), and subfossil Chironomidae. High concentrations of Cd 6.7–612 μg g−1, Pb 0.1–10.2 mg g−1, and Zn 0.5–23.1 mg g−1 were found in the seven analyzed sediment cores. In total, 374 head capsules of Chironomidae larvae, belonging to four subfamilies Chironominae, Orthocladiinae, Tanypodinae, and Prodiamesinae, were determined. Both the diversity and density of Chironomidae change in particular sediment cores and layers. However, these changes were not related to metal concentrations, as shown in the statistical calculations (dendrogram of similarities, Mann-Whitney test, Spearman correlations). The only exception was the negative correlations between Cd, Pb, and Zn concentrations and the density of head capsules of Polypedilum sp. We found that organic matter and nutrient contents were important factors that control the spatial distribution of subfossil Chironomidae, mainly Orthocladiinae. Other factors which may influence subfossil Chironomidae distribution in sediment cores are discussed. To sum up, despite very high Cd, Pb, and Zn concentrations, the Chironomidae community was not clearly altered, therefore subfossil Chironomidae taxa appeared to be not useful for reconstructing dozen-years scale changes in the toxicity of the aquatic environment.