Characteristic regional differences in trace element pattern of 2014 German North Sea surface Wadden sediments – A judge and assessment
Graphical abstract
Introduction
The Wadden Sea of the North Sea extends from Den Helder peninsula in the Netherlands, through the estuaries of the different German rivers (Ems, Weser, Elbe), along the Frisian Islands, to its northeastern boundary at the island of Skallingen in Denmark. Within these boundaries, its length is about 500 km and its maximum width is approximately 40 km, covering an area of up to 10,000 km2 (Van den Brink and Kater, 2006; Wolff, 1988). Hence, the coastal area of the Wadden Sea represents Europe's largest wetland biotope and the largest mudflats area in the world (Lotze, 2007). Consequently, three national parks and three biosphere reserves have been recognized as World Heritage Sites by the Organization of United Nations for Education, Science and Culture (UNESCO) since 2009, and are therefore under its protection (Gittenberger et al., 2016).
The North Sea in its entirety is subjected to an enormous anthropogenic influence because it is surrounded by one of the most industrialized areas in the world and is heavily used for shipping and off-shore constructions (Enemark, 2005). Thus, construction measures such as dike building, draining of areas or pre-washing of sand for coastal protection cause a continuous change of natural conditions, which, among other things, also affects the ecosystem of the (German) Wadden Sea (Kabat et al., 2012; Lotze et al., 2005). Furthermore, the North Sea is affected by inputs of waste from highly industrialized neighboring countries, which enter the water body via the atmosphere, sewage and especially via the rivers flowing into the German Bight (Freese et al., 2008).
A class of pollutants which are of high concern for the environment due to their toxicity are metal(loid)s like As, Cd, Cr, Cu, Hg, Ni, Pb and Zn. Discharges of this class of pollutants have decreased in recent decades, but inputs are still noticeable (Logemann et al., 2022). Quantitatively, the highest input of metal(loid)s origins from major rivers discharging into the North Sea. For example, the input of Cd from the Elbe River into the Wadden Sea has decreased from about 10 tons per year in 1986 to about 3 tons in 2000 (Essink et al., 2005b). However, regular extreme events such as flooding result in the transport of large amounts of dissolved and particulate bound pollutants originating from legacy pollutant deposits present in the different river catchments, like the Elbe River (Pepelnik et al., 2005).
After the release of (heavy) metals and metalloids into a water body, only a small amount of the metal(loid)s ions is present in the water phase due to hydrolysis, adsorption as well as co-precipitation. By far the biggest part of the metal(loid)s ions is retained in the sediment due to particulate properties and chemical composition (Bastami et al., 2014). The accumulation of metal(loid)s ions in the sediment, or the ability to re-dissolve and thus be dispersed by the water stream, depends on a variety of factors, such as redox potential, pH, salinity or the presence of organic ligands. Therefore, sediments can also serve as source, apart from sink, for a wide range of pollutants including metal(loid)s (Bastami et al., 2014; He et al., 2012; Wang et al., 2014). Due to the fact that they usually serve as sinks, sediments can be used to monitor and assess the status of water bodies regarding metal(loid)s (Chabukdhara and Nema, 2012; Ioannides et al., 2015). Due to the toxic and non-biodegradable nature of many metal(loid)s, the determination of their mass fraction in various environmental matrices like sediment, suspended matter or biota is of great interest. In a large area like the Wadden Sea, the elemental composition and quality of sediments is influenced by different parameters like the adjacent North Sea or river discharges (Postma, 1981). Hence, sediments are defined by natural characteristics, which are based on their geographical location, e.g., protection by a barrier island, its geological origin or anthropogenic impacts. A wide variety of parameters are required to fully characterize a sediment sample in terms of its nature and geographic location, which makes both the analytical process, and the following data processing, much more demanding.
Developments in the field of analytical chemistry enabled the rapid digestion of sediment samples and the determination of their elemental composition e.g. by ICP-MS. The automatization in terms of sample preparation, as well as sample introduction allows for the generation of large datasets, which can be subjected to cross examination for multivariate statistics. Methods, like cluster analysis or principal component analysis, allow for the interpretation of large amounts of data. Consequently, they can be put into context in terms of geographic location and anthropogenic influence. The benefits of this combined approach have already been exhibited by Reese et al. (2019). In the study statistical approaches, based on elemental mass fractions and isotopic signatures of selected elements, were used to unravel tracers for dynamic processes like sediment transport and mixing. A similar approach was used by Deng et al. (2021) for the Weser river.
In this study, we investigated the elemental composition of surface sediments (grainsize fraction ≤20 μm) taken from the Wadden Sea and incorporated different statistical approaches (cluster analysis, principal component analysis and discriminant analysis), to evaluate suspected geological and anthropogenic influences. Furthermore, the obtained data is compared with older Wadden Sea sampling campaign data, which took place in the years 1984 to 1991.
Section snippets
Materials, reagents and standards
For sample digestion hydrochloric acid (30 % w/w, Suprapur; Merck KGaA, Darmstadt, Germany), nitric acid (65 % w/w, Suprapur; Merck KGaA, Darmstadt, Germany), hydrogen peroxide (30 % w/w, Ultrapure; Merck KGaA, Darmstadt, Germany) and tetrafluoroboric acid (48 % w/w; Sigma Aldrich Corp., Missouri, USA) were used. Hydrochloric acid and nitric acid were double sub-boiled in quartz stills (AHF Analysentechnik, Tübingen, Germany) under clean room conditions. After digestion, samples were
Results and discussion
The sampling sites were chosen based on their geomorphological and hydrodynamic characteristics, representing the different characteristic mud flat areas within the German Wadden Sea. Therefore, four different zones were defined: i) high and constant salinity, sheltered back tidal flats in west/east direction (East Frisia), with offshore barrier islands, narrow intertidal, and high turbidity (nine samples; Number 1–9 (Fig. 1a); ii) estuarine tidal flats with variable salinity, high exposure and
Conclusion
This study demonstrates the value of multivariate statistics when applied to environmental datasets. The methods used were able to distinguish different geographical locations based on their elemental fingerprint and proved that these fingerprints are significant for the studied areas and the analyzed ≤20 μm grain size fraction. Considering a second dataset obtained in 1984–1991, it was shown that the fingerprint changed over time and that sediment quality significantly improved during the last
CRediT authorship contribution statement
Marcus von der Au: Investigation, Validation, Formal analysis, Writing – original draft. Tristan Zimmermann: Investigation, Visualization, Writing – review & editing. Ulrike Kleeberg: Data curation, Writing – review & editing. Wolf von Tümpling: Conceptualization, Writing – review & editing, Supervision. Daniel Pröfrock: Conceptualization, Resources, Writing – review & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank the crew of the RV Ludwig Prandtl for their help during the campaign in 2014, as well as the Wadden Sea authorities to allow the sampling in the different Wadden Sea Areas. Carlo van Bernem, Ina Frings, Stephan Lassen, Burkhard Erbslöh, Anika Retzmann and Rainer Jablonski are acknowledged for fruitful discussions and assistance during sampling and sample preparation.
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