Response of Chironomidae (Diptera) to DDT, Mercury, and Arsenic Legacy Pollution in Sediments of the Toce River (Northern Italy)

Abstract

The Toce River (Northern Italy) is characterized by legacy contamination of dichloro-diphenyl-trichloroethane (DDT), mercury, and arsenic deriving from an industrial plant active between ca. 1915 and 1996. Chironomidae taxa assemblages and sediments were collected in 2014 and 2019 upstream and downstream of the industrial area to analyze species responses to toxic substances in a river stretch with relatively uniform natural (i.e., hydro-morphological) characteristics. A total of 32 chironomid taxa were identified. Sediment concentrations reached levels potentially toxic for benthic invertebrates: 15.7 µg kg−1 1% organic carbon for DDT, 197 µg kg−1 dry weight (d.w.) for Hg, and 55.7 mg kg−1 d.w. for As. Canonical Correspondence Analysis (CCA) revealed a predominant seasonal gradient, followed by an upstream-downstream gradient. Partial CCA indicated that 5.2% of the total variation was associated with sediment contamination. Self-Organizing Maps (SOMs) were used to represent species responses to toxicants. Most species appeared to be tolerant, e.g., Chironomus riparius, Micropsectra atrofasciata, Conchapelopia pallidula, and Polypedilum spp. Sensitivity to contaminants was observed in only a few species: Diamesa spp., Sympotthastia spinifera, and Prodiamesa olivacea to DDT; Potthastia longimanus to Hg; Odontomesa fulva and Microtendipes pedellus to As. The chironomid community was characterized in presence of contamination levels commonly observed in freshwater ecosystems.

Response of Chironomids (Diptera, Chironomidae) to Environmental Factors at Different Spatial Scales

“…It is well known that at a very large spatial scale, considering wide global areas (Palaearctic, Nearctic, Oriental, Neotropic, Australian), chironomid species composition is quite different [6], but within the restricted area considered (i.e., the Mediterranean-Alpine area), the spatial factors have no influence, or they are mediated by other factors (e.g., altitude, water temperature etc.). Anthropogenic stress was not a focus of the present research [39], but even if many altered sites were included in the dataset, they did not substantially alter our conclusions about the prominent effect of habitat, although the well-known response of chironomids to oxygen shortage and eutrophication (measured as TP) [2,8,18] was confirmed here (Figure 1a).…”

Response of Chironomids (Diptera, Chironomidae) to Environmental Factors at Different Spatial Scales

“…Discriminant analysis emphasized that there is a very good agreement between the 'a priori' and 'a posteriori' site classification, even if some ubiquitous species are present together specialists. The anthropogenic stress emphasized a clear response of chironomids to oxygen shortage [2,8,18], while it is difficult to establish the effect of toxic substances [35].…”