Restoring fen water bodies by removing accumulated organic sludge: what are the effects for aquatic macroinvertebrates?

摘要

Pristine freshwater fens harbour many species of aquatic macroinvertebrates. Effects of eutrophication and desiccation have strong negative impacts on macroinvertebrate assemblages. To restore degraded fens, the removal of accumulated organic sludge by dredging seems a necessary step. However, degraded fens may harbour relic populations of rare and characteristic species as was found for raised bogs and shallow soft water lakes. This study investigates the effectivity of dredging by comparing dredgedand undredged water bodies in two areas (SW & MP). To help interpret the observed differences, a third least impacted area is sampled in addition (WD). Abiotic conditions clearly differed between areas, but when comparing dredged and undredged water bodies, only turbidity was lower in dredged water bodies. Coverage of submerged vegetation was higher in dredged water bodies, especially in MP. For aquatic macroinvertebrates, strong differences between dredged and undredged water bodies were found for both SW and MP. Dredged water bodies in MP resembled WD most strongly, in abiotic conditions, vegetation, and invertebrates. Nevertheless, a number of species commonly occurring in WD were mainly associated with undredged water bodies, indicating incompleterestoration of certain key factors. Results indicate that dredging contributes to ecological restoration of fens. To maximise effectiveness of dredging, internal and external supply of nutrients should be minimized, removal of organic sludge should be near-complete, while retaining small patches of vegetation and recesses as sources of individuals to facilitate recolonisation. Furthermore, this study shows that taking fauna into account can yield new information which is not uncovered by researching solely abiotic conditions and vegetation. In contrast to raised bogs and shallow soft water lakes, no relic populations of rare and characteristic species -were found in degraded, undredged fen water bodies. These differences may be related to differencesin ecosystem functioning, with characteristic fen species having a lower persistence and a higher recolonisation rate.