River flooding and landscape changes impact ecological conditions of a scour hole lake in the Rhine-Meuse delta, The Netherlands

摘要

A 400-year sediment record from an 18 m deep scour hole lake (Haarsteegse Wiel) near the Meuse River in the Netherlands was investigated for past changes in water quality, flooding frequency and landscape change using geophysical, geochemical and micropaleontological information. The results are highly significant for determining long-term trends of water quality, the impact of atmospheric (as SCP, spheroidal carbonaceous particles) and industrial (chromium) pollution on the terrestrial and aquatic flora, and the impact of river floods. The studied sediment record was dated by combining 137Cs activities, biostratigraphical ages, micro-tephra layers, and historically documented floods indicated by the magnetic susceptibility. The oldest flooding event is indicated at AD 1610 when the lake was created by water masses bursting through a dike. Large historical river floods are well documented in regional chronicles and thus may provide reliable age calibration points. Based on assumptions about the timing of flood events and constant rate of sedimentation, it appears that sedimentation rates in Haarsteegse Wiel declined after ca. AD 1880. This decline might be a result of a widespread change from wheat cultivation to pasture land from around AD 1875 as a direct result of falling wheat prices and intensified cattle farming linked to the agricultural crisis in the last quarter of the nineteenth century. Water quality changes and absolute phosphorus concentrations were reconstructed using a diatom-based transfer function. Results show that the currently nutrient-rich lake has mostly been in a mesotrophic state prior to ca. AD 1920, with the exception of several apparently sharp eutrophication events that were coeval with river floods. River flooding also impacted the vegetation composition by importing allochthonous components, and indirectly by the influx of nutrients which had a clear influence on the composition of the water plant communities and aquatic species diversity. Magnetic susceptibility changes and pollen data show that within the period AD 1610–1740, within the Little Ice Age period, several undocumented floods may have occurred. Thus, documentation of geophysical, geochemical, and biological flooding signals in a high-resolution archive present the possibility to detect flooding regimes further back in time.