Doctoral thesis, 138 pages (first reviewer)

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English version see below.

With the advance of globalisation, the spread of alien species is continuously increasing due to the removal of dispersal barriers. These species can negatively impact and jeopardise the native species composition and biodiversity. Coastal areas around the world are hotspots of invasion. Protected areas have a special role to play in preserving this endangered biodiversity.

The study area of this study covered the East Frisian Islands and neighbouring areas, the largest part of which belonged to the Lower Saxony Wadden Sea National Park. Occurrences of the invasive species Crassula helmsii have been known on Norderney since the early 2000s. This species is the only known aquatic and invasive neophyte in the study area. Senecio inaequidens has been detected on all islands in the study area and has rapidly colonised new areas on the mainland in recent decades. The species is one of the fastest spreading neophytes. The required site conditions in coastal areas and the possible effects on the native flora have not been investigated or only insufficiently investigated for the two species mentioned above. The data basis for Campylopus introflexus , the third species analysed in more detail, is different. Its impact on biodiversity has been well studied. Individual studies have indicated a decline in the species, but it was not known whether this applies to the study area. The baseline data for this study is a survey of all alien plant species from 2006.

The following questions were answered as part of this study: - How have the populations of Crassula helmsii, Senecio inaequidens and Campylopus introflexus developed? - How do these species socialise? - What site requirements do the species have in coastal ecosystems? - Can the further development of the populations be predicted?

To this end, the distribution of the individual species was first recorded and vegetation surveys were carried out at selected sites in order to quantify the influence on species composition and diversity. Based on this, various environmental parameters such as pH value, soil conductivity, grain size distribution or carbon and nitrogen content were determined at presence and absence sites. These results were transferred to habitat models created using multiple generalised linear models (GLM) in order to identify the environmental parameters that are important for the species and to generate a tool for predicting the further development of the species.

This methodological approach proved successful for Crassula helmsii and Senecio inaequidens . It was possible to record the development of the populations over a longer period of time and to identify endangered species and habitats. In addition, it was possible for the first time to determine the important environmental parameters for both species. It was shown that both species are not realising their full potential and that further expansion is to be expected.

In the case of Campylopus introflexus , it was not possible to create a habitat model. However, the distribution surveys showed that the species is declining in the study area. For the first time, it was shown that sites where C. introflexus has disappeared can hardly be distinguished in their species composition from sites where the species has never been recorded.

The second part of this work focusses on identifying possible new invasive or potentially invasive species on the East Frisian Islands and naming the vectors of introduction to the islands. To this end, the survey of all alien species was repeated on Wangerooge and Mellum and areas on the mainland were also analysed for the first time. It was shown that the majority of the alien species recorded are ornamental and cultivated plants and that humans are the most important vector for their introduction to the island. In addition, Aronia prunifolia, Conyza canadensis and Lonicera involucrata were identified as potential problem species already present on the islands.

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Invasive and potentially invasive neophytes of the East Frisian Islands and neighbouring areas

With the advance of globalisation, the spread of alien species is continuously increasing due to the removal of dispersal barriers. These alien species can negatively impact and threaten native species and biodiversity. Coastal areas around the globe are the hotspots of invasion, Protected areas play a special role by helping to conserve the endangered biodiversity.

This thesis focuses on the distribution, plant communities and habitat requirements of alien and potentially alien species. The study area includes the East Frisian Islands and the surrounding areas, which mostly belong to the Lower Saxony Wadden Sea National Park.

The invasive species Crassula helmsii was first recorded on Norderney in the early 2000s. The species is the only known aquatic invasive alien plant species in the study area. Furthermore, Senecio inaequidens has been recorded on all islands in the study area and has rapidly colonised new areas on the mainland in the last decades. The species is one of the fastest spreading alien species of all. The required habitat conditions in coastal areas and the possible effects on the native flora are insufficiently investigated or not investigated for the previously mentioned species. Unlike that, the impact on biodiversity of the third species, Campylopus introflexus, has been well studied. There are studies, which indicate a natural decline in covered areas, and it was unknown if this was also the case for the study area. A census of all alien plant species from 2006 serves as data basis for this work.

In the context of the present work, following questions were answered: - How did the populations of Crassula hetmsii, Senecio inaequidens and Campylopus introflexus develop since 2001? - In which communities do these species occur? - What are the habitat requirements of the species in coastal environments? - Can the further development of populations be predicted?

First, the distribution of each species was recorded, and secondly vegetation surveys were carried out at selected sites in order to be able to quantify the influence on species composition and species diversity. Based on this, various environmental parameters such as pH value, soil conductivity, grain size distribution or carbon and nitrogen levels were determined at presence and absence sites. These results were transferred into habitat models created by several generalised linear models (GLM) to identify the important environmental parameters for the species on the one hand and to create a tool for predicting the further development of the species on the other.

For Crassula helmsii and Senecio inaequidens, this methodological approach was successful and endangered species and habitats could be identified. Furthermore, it was possible for the first time to determine the important environmental parameters for both species. It could be shown that both species are not fully exploiting their potential and that further expansion is to be expected.

In the case of Campylopus introflexus, it was not possible to create a habitat model, however, by surveying the distribution, it was possible to prove that the species is decreasing in the study area. For the first time, it could be shown that sites where C. introflexus has disappeared are hardly distinguishable in their species composition from sites where the species has never been recorded.

The second part of this thesis focuses on the identification of possible new invasive or potentially invasive species on the East Frisian Islands and the naming of Import vectors. For this purpose, the census of all alien species on Wangerooge and Mellum was repeated and areas on the mainland were mapped for the first time.

It was found that most alien species recorded are ornamental and cultivated plants and that humans are the most important vector for their introduction. In addition, Aronia prunifolia, Conyza canadensis and Lonicera involucrata were identified as potential problem species already present on the islands.

Doctoral thesis, 120 pages (first reviewer)

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German version see below.

Evidence of plastic litter in the marine environment was first found in the 1960s and 70s, yet it was not until the early 2000s when research on marine debris and in particular plastics gained momentum. Over the last 20 years, knowledge of the scope and impacts of this modern environmental pollution has advanced: Plastics have been found in all known marine and freshwater habitats, from the poles to the tropics, while pronounced accumulation particularly occurs in ocean gyres. The material's durability results in slow degradation and fragmentation from macro- to micro- to nanoplastics, estimated to persist for several years and decades, if not centuries. Meanwhile, hundreds of species, including humans, are already confirmed to be impacted through interacting and ingesting plastic litter. And with advancing technologies and a growing body of science, new fields and further research questions emerge.

In the North Sea, litter pollution has been reported since the 1970s. Several environmental organisations have thus surveyed beach debris since the late 1980s, and marine litter has been identified by the European Union as a key polluter that needs to be systematically assessed, addressed, and reduced. Nonetheless, region-specific published literature on the topic is not extensive. The objective of the interdisciplinary research project "Macroplastics Pollution in the Southern North Sea - Sources, Pathways, and Abatement Strategies" at the Carl von Ossietzky University of Oldenburg was therefore to improve the understanding of sources, drift paths, and accumulation areas of floating debris in the German Bight and major tributaries through numerical modelling, field data, and experiments. Furthermore, the team set out to identify relevant stakeholders and to analyse socio-behavioural patterns with regard to waste and littering, The goal was to inform partner Organisation and the public, and to jointly devise abatement strategies. The aim of this thesis within the project was to quantitatively and qualitatively assess anthropogenic litter loads along the German North Sea coast and in the Ems, Weser, and Elbe, while investigating the role of these rivers as pollution pathways. The experimental approach of deploying uniquely identifiable wooden (and several satellite-tracked) drifters should provide further details on litter sources, dispersal, and accumulation that may not be evident from field surveys alone, and would assist in validating the drift models used within the project.

To gain an insight into past and present levels of coastal litter pollution, 25 years of beach litter monitoring data (1991-2016) from 29 sites along the German North Sea coastline were subject of an exploratory analysis in the first study. It was shown that pollution levels at all sites varied greatly over time and for only a few locations a significantly increasing (e.g. Mellum) or decreasing (e.g. Hörnum on Sylt) trend in mean annual litter abundances could be seen. Shorelines of Schleswig-Holstein accumulated less debris than those in Lower Saxony, while site-specific litter quantities were generally higher along the main coastal current, at estuaries, and in the vicinity of man-made coastal structure. Similarities in abundances of different debris types were not consistent, and the effect of wind, tides, and exposure on debris composition appeared to be minimal. Despite best efforts during data validation, unresolved inconsistencies and data gaps are likely to have compromised the full potential of this long-term data set. Human error should therefore be proactively addressed and minimised in ongoing and future monitoring work.

The second study provided a holistic assessment of macrolitter in the transitional waters of the Ems, Weser, and Elbe. Litter surveys were conducted in four river compartments, i.e. along the embankment, on the surface, in the water column, and on the river bed, categorising the recorded debris according to the standardised classification of the "OSPAR Guideline for Monitoring Marine Litter [...]". Similar to coastal litter, riverine litter was also found to vary spatio-temporally, with distinct pollution levels for each compartment. The highest debris quantities were found in the Elbe, the lowest in the Ems. Riverside beach litter was the most abundant and diverse, while river bank vegetation and harbour structures accumulated significantly less, and stony embankments were least (diversely) polluted. Daily emission rates of surface-floating and suspended litter, based on two extrapolation approaches, were combined and scaled to total annual mass discharges, using the mean and median per waste/plastic item weight determined from stow net litter samples. These estimates ranged from 0.2-2.8 ^ty-1 of litter incl. 0.1-1.^6ty-1' of macroplastic emitted via theEms and 0.2-12.01 ^y-1 of debris incl. 0.2-6.3 ^ty-1 of plastic discharged by the Weser to 2.4-801 ^ty-1 of waste incl. 2.2-451 ^ty-1 of macroplastic carried into the North Sea by the Elbe. Previous discharge estimates based on global models were higher for the Ems and Weser and considerably lower for the Elbe. Possible reasons for these discrepancies are discussed. Overall, even the highest emission rates accounted for <1 % of the total mismanaged plastic waste estimated for the respective catchment, leading to the conclusion that most of the unsoundly discarded litter must accumulate on land and/or along the waterways.

The third study describes and evaluates the experimental approach of assessing floating macrolitter drift in the German Bight using uniquely identifiable wooden drifters that members of the public should report once found - a core component of the research project. Details are provided on the manufacturing process, the selection of release locations, field logistics, and the report information provided via the project website. Between October 2016 and July 2019, >27,000 of 63,400 deployed drifters were registered at least once, resulting in a data set of ~33,000 validated reports from thousands of locations in seven countries bordering the North Sea. While report rates varied seasonally and between release sites, >60 % of the records were registered within the first month of deployment, almost 90 % after four. Many reports included positive appraisals of the project. In a 24/7 flotation experiment that was conducted over the course of a year, drifters increased 1.9- to 2.6-fold in mass but remained afloat while their density was similar to conventional plastics after 2-3 months. Drifter findings made >1 year after the deployment confirmed both floatability and durability of material and branding.

In the fourth study, drifter dispersal and accumulation were assessed, based on the validated report data set. Two thirds of the drifters released on the Lower Saxony coast and into the Ems, Weser, and Elbe were first reported <25 km away, demonstrating that macrolitter from coastal and riverine sources appears to mainly pollute adjacent shorelines. In several cases, coastal and riverine drifters crossed distances of several hundred kilometres, comparable to those deployed at sea. Scandinavian and British coasts were occasionally reached within a few days or weeks. Geostatistically significant annual and seasonal clustering occurred at 41 sites, 28 being located in Lower Saxony, some of which were release locations. These findings, limitations to the methodological approach, and points of consideration are discussed.

As public awareness of litter pollution grows, global alliances are formed between scientists, industrial corporations, and various stakeholders to address this environmental threat, and legislation on product regulation and waste management advances, long-term litter monitoring and predictive models will likely increase in importance, particularly in evaluating the success of new policies and public efforts. Currently, there still appear to be many unknown or unassessed variables influencing litter quantities and composition in various environmental compartments. This results in high levels of uncertainty and will restrict the identification of actual trends and change. Future studies should therefore consider expanding litter surveys geographically while factoring a range of potential small- and large-scale influences into the assessment. This will further improve our understanding of litter dynamics which will in turn help tailor the management tools needed to effectively combat litter pollution.

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Anthropogenic macro-litter pollution in the southern North Sea - sources, dispersal and deposition along river courses and coastal stretches

The first scientific evidence of plastic litter in the marine environment dates back to the 1960s and 70s. However, it was not until the early 2000s that more research was conducted on marine litter, especially plastic. Over the last 20 years, knowledge about the extent and consequences of this modern environmental problem has grown: plastic has been found in all known marine and freshwater habitats, from the poles to the tropics; in particular, floating marine litter accumulates in the large ocean eddies. The stability and resilience of plastics in turn results in fragmentation from macro-, to micro-, to nanoplastics - a process estimated to take several years and decades, if not centuries. However, hundreds of species, including humans, are demonstrably affected by the interaction with and consumption of plastics of all sizes. In light of intensive research, optimised measurement methods and technologies and a constant increase in information, new fields of research and questions are constantly opening up.

Environmental pollution with anthropogenic rubbish has been known in the North Sea since the 1970s. Since the end of the 1980s, several environmental protection organisations have therefore set themselves the task of documenting beach litter within their protected areas. Furthermore, litter has been recognised throughout Europe by the European Union as one of the main stressors for the oceans, which must be systematically quantified, evaluated, addressed and reduced. Nevertheless, the region-specific scientific literature on this topic is not yet comprehensive.

Since 2016, the interdisciplinary research project "Macroplastics in the Southern North Sea - Sources, Distribution Pathways and Avoidance Strategies" at the University of Oldenburg has therefore aimed to improve our understanding of the entry points, drift paths and accumulation areas of floating macro litter in the German Bight and in the major tributaries of Lower Saxony by means of numerical modelling, field data and experiments. The identification of relevant interest groups and analyses of social patterns of action with regard to waste and littering should also shed light on the social background. The aim is to inform partner organisations and the public and to jointly develop prevention strategies.

The aim of this dissertation within the project was to analyse the quantitative and qualitative aspects of anthropogenic waste along the German North Sea coast and in the Ems, Weser and Elbe rivers. In addition, the role of these rivers as input pathways for waste was to be evaluated. The experimental approach of deploying individually labelled wooden drifters (and drifting buoys) should provide additional details on the sources, distribution and accumulation of drifting macro-litter that are not available from field surveys and thereby also validate the drift models further developed in the project.

In order to identify past and current trends in pollution levels along the German North Sea coast, the first study exploratively analysed litter mapping data covering 29 sites and a period of 25 years (1991-2016). It was shown that local litter abundance varied greatly over time. Only a few stretches showed a significant increase or decrease in the average amount of litter per year (e.g. Mellum and Hörnum on Sylt respectively). Sections of coastline in Schleswig-Holstein aggregated less litter on average than those in Lower Saxony. Site-specific litter quantities were generally higher in the area of the main current parallel to the coast, at river mouths and in the vicinity of coastal defences. A quantitative correlation of materially, morphologically or density-specific similar types of litter was only occasionally observed. The influence of wind, tides and site exposure on the composition of beach litter appeared to be minimal. Despite rigorous data preparation prior to the analyses, remaining data gaps and inconsistencies probably limited the full potential of this long-term dataset. Human error should therefore be proactively minimised in current and future muller records as well as in data maintenance.

The second study comprised an investigation of macro litter in the transitional waters of the Ems, Weser and Elbe rivers. Litter surveys were carried out in four river areas: along the banks, on the water surface, in the water column and on the river bed. The classification of the collected litter followed the standardised litter categorisation according to OSPAR. As already evident in the beach litter mapping data, river litter also varied both spatially and temporally and showed marked differences in quantity and composition between the areas. The highest levels of litter were found in the Elbe and the lowest in the Ems. The amount of litter on river beaches was the highest and most diverse, while vegetation and harbour structures accumulated significantly less litter; rocky shores had the lowest litter load. The amounts of litter discharged at the river mouth via the surface and water column were extrapolated to total annual emissions using two methods, based on the mean and median of all litter and plastic items weighed from the water column during the haul. These estimates range from 0.2-2.81 ^y-1 macro litter including 0.1-1.61 ^y-1 plastic from the Ems and 0.2-12.0 t ^y-1 litter including 0.2-6.3 t ^y-1 plastic discharged into the North Sea via the Weser to 2.4-S01 t ^y-1 anthropogenic litter including 2.2-4511 ^y-1 macro plastic discharged into the German Bight via the Elbe. Emission values from past studies based on model calculations are significantly higher than the values for the Ems and Weser and significantly lower than those for the Elbe. Possible reasons for these discrepancies are discussed. Overall, even the highest estimates account for <1 % of the total amount of improperly disposed plastic waste generated in the catchment area. Consequently, the majority of this would have to accumulate on land and/or along the waterways.

The third study describes and evaluates a key component of the research project: the experimental approach of simulating drifting macro-litter within the German Bight using individually labelled wooden drifters to draw conclusions about its distribution. Citizens who find these drifters should report them to the project. Both the production process and the selection of the locations where the drifters were released, as well as the field logistics and the find information received via the reporting portal, are described in detail. Between October 2016 and July 2019, >27,000 of a total of 63,400 drifters released were recorded at least once, generating a dataset of -33,000 validated drifter detections from seven North Sea countries. Whilst the detection and reporting rate varied seasonally and between release points, the majority of records were made within one month of release. Together with the reports, the project regularly received feedback from finders. Furthermore, a one-year laboratory experiment in which drifters were permanently floating in artificial seawater showed that their respective mass increased by a factor of l .9 to 2.6, but none of the drifters sank, and their density resembled that of conventional plastics after about 2-3 months. Drifter findings reported over a year after exposure confirm both the buoyancy and the durability of the material and the burnt-in texts.