How did the nutrient concentrations change in northeastern German lowland rivers during the last four millennia?

Abstract

This study focuses on the feasibility of floodplain sediments and fluvial sediments in paleomeanders and ox-bows of two lowland rivers (River Havel, River Spree, Brandenburg State, Germany) as archives for quantitative paleolimnological reconstructions and potential basis of future river management strategies. The results presented provide a mean to differentiate between the natural and cultural eutrophication of rivers. Available transfer functions of littoral diatom assemblages in 84 Brandenburg lakes and river sites, and total phosphorus (TP) and total inorganic nitrogen (TN) were used to infer nutrient changes in the Rivers Havel and Spree since the last 4,000 years. In the River Spree near Platkow, fossil diatoms indicated moderate eutrophic TP- and TN-concentrations between 1300 and 1850 AD (TP: 36 µg dm-3, TN: 1,000 µg dm-3). During this time period, the human impact on the nutrient status of the River Spree was more or less indirect via increases of runoff from the catchment as a result of deforestation. In the second lowland river, the lower River Havel, diatom inferred TP-concentrations were 80 µg dm-3 in the late Subboreal (2,000 to 500 BC). That means that the natural diatom flora of this river was eutrophic; mesotrophic conditions even in times without intensive land use did not occur. Furthermore, the fossil diatom flora revealed a potential nitrogen limitation during summer times (till 1400 AD: TN 1,600 to 1,700 µg dm-3). Anthropogenic eutrophication impact on the River Havel can be detected since approximately 800 year ago. The diatom-inferred nitrogen/phosphorus-relation highlighted different trends in eutrophication history within the study area. Without human activities the ratio of both nutrient components was relatively constant. Anthropogenic changes in the catchment area led to a declining TN/TP ratio in the last 1,000 years with changes in algal communities, such as increases of nuisance cyanobacteria blooms in the last decades.