Natural and human induced trophic changes in European lowland lakes

Abstract

The European legislation (WFD) and the IPCC 2008 are both acknowledging the relevance of current and future problems with regard to water quality and quantity. Globally, many lakes are suffering from increased nutrient input (mainly phosphorus) leading to eutrophication. Eutrophication is diminishing ecosystems by causing toxic algal blooms, anoxic events, fish kills, and biodiversity degradation. Assessing the total phosphorus concentrations (TP) through the history of lakes is a key towards understanding the nature of eutrophication processes. In this dissertation the TP dynamics in eight Dutch and German lakes were quantitatively estimated for the past 30 to 13,000 years. In Sacrower See (Germany), the relationship between seasonal and interannual variability of modern diatoms and TP concentrations, temperature, and precipitation were assessed using sediment traps. On the basis of this data the fossil diatoms of Sacrower See could be interpreted and information on the length of summer stratification, winter circulation, and on the lake productivity could be inferred. The sediments of the past 130 years of Sacrower See were analyzed for diatom assemblages at a seasonal time-resolution. The results of this high-resolution analyses show that the sensitivity of diatom assemblages to meteorological parameters such as wind and temperature affected the diatoms for the past century. On a longer time scale (13,000 years), the palaeoreconstrution of Sacrower See indicated eutrophic conditions (TP ~70 g l-1) during the Younger Dryas cold event (12,700-11,600 years ago) which was caused by the abrupt temperature decrease that led to long-lasting ice cover and consequently the development of anoxic conditions with phosphorus release from the sediment. With the onset of the Holocene (11,600 years ago) and the increased temperatures, the lake became oxygenated again and became meso- to oligotrophic (TP ~10-30 g l-1), which prevailed throughout most of the Holocene. In the past 2500 years the nutrient concentrations increased as much as during the Younger Dryas, but now due to strong anthropogenic impact. Much in contrast to the foreseen effects of global warming, which imply increasing internal loading, the results from Sacrower See provide an unique example of increased TP concentrations due to climate cooling. In this thesis we carried out six trophic reconstructions of Dutch lakes. The results show small changes in the TP of these lakes during the past 30 to 100 years. The major controlling factor for nutrient enrichment in these lakes is the way they are fed (ground water, seepage,) and their catchment was managed. The reconstruction of past nutrient conditions in Lake De Waay (the Netherlands) shows hypertrophic to eutrophic conditions throughout the past 500 years were always evidenced. These results show that the idea of fitting the ecological status of lakes in the context of their natural state is therefore not recommended for lowland lakes. Overall, all case studies demonstrated that the classification of undisturbed environmental conditions should consider the possible effects of climate change and human influence on lake ecosystems when defining targets for ecological reference conditions or restoration of aquatic ecosystems.