Abstract
Worldwide, coastal water bodies suffer from anthropogenically elevated nutrient inputs, which led to eutrophication. Sediments in eutrophic systems are assumed to be an important internal nutrient source. The total phosphorus (TP) concentration (mg g-1 dry mass) is widely used as a proxy for the sediment nutrient load. 2-D distribution maps of TP concentrations are used for management plans, where areas of high TP values are marked in red. However, the sediment density is lowered at increasing water content, which can lead to different TP stocks per g m-2. The aim of this study was, to do a re-evaluation of TP concentrations and stocks in the model ecosystem of the Dars-Zingst Bodden chain, a typical lagoon system of the southern Baltic Sea. Sediment cores were taken at eight stations along transects from shallow to deeper parts of the lagoon. Samples were analyzed for TP, water and organic content, as well as density. This data set was compared to results from a sediment survey during the time of highest nutrient inputs (40 years ago) at the same sampling stations. TP concentrations from 40 years ago and today were in the same range. The highest TP concentrations (up to 0.6 mg TP g-1 dry mass) were found in the deeper basins and lowest concentrations in the shallow areas of the lagoon (down to 0.05 mg TP g-1 dry mass). However, normalization over dry bulk density reversed some results. The highest TP stocks (up to 5 g TP m-2) were then found in the shallow areas and lowest stocks (down to 0.2 g TP m-2) in the deeper parts of the lagoon. Some stations did not exhibit any differences of TP at all, even after including the dry bulk density. These findings suggest that there seems to be no up-, or downward trend in nutrient concentrations of sediments even after 25 years of reduced external nutrient inputs. Furthermore, TP stocks point to possible diffuse P entry pathways that counteract external nutrient reductions. These findings can have an impact on possible countermeasures for ecosystems rehabilitation, like sediment removal or nutrient reductions in the adjacent land.
Highlights
Along with the Green Revolution in agriculture in the last 50–75 years in the developed countries, ecosystems worldwide are strongly influenced by high anthropogenic nutrient inputs especially nitrogen (N) and phosphorus (P) (e.g., Smil, 2000; Liu et al, 2008; Schröder et al, 2010; Childers et al, 2011; Lu and Tian, 2017)
Analysis of different characteristics of sediment samples, especially total phosphorus (TP) content, from different transects in the Darß-Zingst Bodden chain (DZBC) pointed to the importance of the land use near the transect, affecting the earlier and present day diffuse P input especially in such an almost closed water system such as the DZBC
Strong differences between sampling points within a transect seem to call for additional effects of P- and/or sediment distribution such as DZBC internal sediment transport or biological sediment mixing by plants and animals
Summary
Along with the Green Revolution in agriculture in the last 50–75 years in the developed countries, ecosystems worldwide are strongly influenced by high anthropogenic nutrient inputs especially nitrogen (N) and phosphorus (P) (e.g., Smil, 2000; Liu et al, 2008; Schröder et al, 2010; Childers et al, 2011; Lu and Tian, 2017). Yan et al (2016) pointed to the fact that P seems to accumulate faster than N in aquatic systems, which means that even reduced P loads can still increase the P background within systems This accumulated P has short and long-term effects in aquatic ecosystems. Elevated P pools are still a burden for restoration, as they can be released massively during suboxia in near-bottom water layers or at a slower rate by diffusion. Such release situations are well described for, e.g., the Baltic proper and sediments of the Baltic coastal zones (Pitkänen et al, 2001). Duarte et al (2009) described this elevated P background as a main cause, why restoration will not work and ecosystems fail to return to the reference status upon nutrient reduction
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