Chironomids (Diptera) in river floodplains: their status and potential use for palaeoenvironmental reconstruction purposes

Abstract

The chironomid subfossils from a single sediment core in the floodplain of the Aa river (Saint-Omer basin, Northern France) were analysed. This study was a pioneering attempt to use sedimentary chironomid assemblages and lithostratigraphic analysis to reconstruct the palaeoenvironment and the past hydrodynamic conditions in a river system during the mid-Holocene period. In this paper, a chironomid subfossil typology is proposed, based on the current scientific knowledge about the present distribution of chironomid species through the cross-section of a floodplain (from lotic to more lentic habitats). A hydrological status is attributed to 27 subfossil chironomid taxa (limnophilous taxa and rheophilous taxa) from the Saint-Omer sediment sample. Based on these chironomid hydrological statuses and in regard with the lithostrati-graphic data, the dynamics of chironomid subfossil assemblages showed the occurrence of some fluctuations between periods with lower (lentic periods SMch-1, SMch-3 and SMch-5) and higher (lotic periods SMch-2 and SMch-4) Aa river discharge rates during the Late-Atlantic and Subboreal periods. The lotic periods were correlated with several climatic events known to have occurred during these periods in Europe. Several hypotheses are put forward and discussed, as to the potential relationships between the levels of precipitation and the hydrological conditions in the Saint-Omer basin. Although chironomid analysis in river systems is still in its developmental phase, the conclusions drawn from the present findings on subfossil chironomids show that these can provide a useful tool for palaeoenvironmental reconstruction purposes.