RECONSTRUCTION OF PRISTINE MORPHOLOGY, FLOW, NUTRIENT CONDITIONS AND SUBMERGED VEGETATION OF LOWLAND RIVER SPREE (GERMANY) FROM PALAEOMEANDERS

摘要

The European Water Framework Directive requires the definition of reference conditions for each type of surface waters as a base to establish a classification system in which deviations from this high quality status must be determined. In order to reconstruct pristine conditions in the lower river Spree we investigated palaeomeanders using palaeohydrological and palaeolimnological methods. The settlement history of this region suggests low anthropogenic impact for all periods before ～700 cal AD. Three palaeomeanders representing the conditions of the late Sub-boreal/early Sub-atlantic were investigated. River width and depth at bankfull stage were reconstructed using cross-sections of the meanders. Based on these data and experiments on the recent river, a parametric model was developed to calculate the bankfull palaeodischarge. Reconstructions show narrower and shallower channels for the undisturbed lower Spree (～20 m mean width and 0.8 m mean depth at bankfull stage) as compared to recent conditions (35 m and 1.6 m, respectively). Flow velocities and discharge at bankfull stage have been smaller in reconstructed sub-fossil channels (0.5 ms~(-1) and 8 m~3s~(-1) in the pristine lower Spree as compared to 0.9 m s~(-1) and 52 m~3 s~(-1) in the recent lower Spree) and flow variability was higher. The increase in bankfull discharge was mainly attributed to deforestation and drainage of the catchment as well as channelization, bank protection and river regulation measures. The organic silt at the base of the sediment cores contained well-preserved fossil diatom assemblages. Diatom-inferred total phosphorus (DI-TP) concentrations of 59-73 μgL~(-1) (median 62 μgL~(-1)) indicate eutrophic to hypertrophic conditions and suggest naturally slightly lower nutrient levels than today. These past nutrient conditions, morphology and large numbers of macrofossil remains indicate optimum growth conditions for submerged macrophytes growth. The combination of palaeohydrological and palaeolimnological parameters proved to be a useful approach for the determination of pristine conditions in a lowland river.