Abstract
Eutrophication remains an environmental challenge in lagoons along the Southern Baltic Sea. Floating islands planted with emergent macrophytes are an option to remove nutrients from eutrophicated waters. Furthermore, floating wetlands offer other ecosystem services such as the provision of habitats. Numerous scientific studies have been conducted; however most remain on the laboratory scale. This research explores the challenges associated with installations in coastal environments and focuses on sustainability of the island design, the habitat function as well as nutrient removal. Most floating wetland designs use polyethylene, polypropylene, polyurethane or polyvinyl alcohol foam to ensure the buoyancy. For this study an artificial polymer free island design was developed and tested. The floating constructions in the Darss-Zingst-Bodden-Chain were planted with native macrophytes which have the potential to act as ‘biodiversity-supplements’ to the adjacent coastal wetlands: Bolboschoenus maritimus, Carex acutiformis, Iris pseudacorus, Juncus effesus, Lythrum salicaria, Schoenoplectus lacustris, Typha latifolia. The chosen macrophytes survived fluctuating salinities. After three months the above-ground biomass was harvested and analyzed for the nutrient concentrations. Phosphorus concentrations were highest in L. salicaria and nitrogen in I. pseudacorus. Video monitoring and field observations were applied in order to observe animals. Birds did not use the floating wetlands as breeding grounds, but the grey heron (Ardea cinerea) was a common visitor for foraging. Especially surprising was the large amount of juvenile eels (Anguilla anguilla). A diverse and large root network below the floating islands boosts not only nutrient removal but serves as a shelter and refuge for fish such as the endangered eel.
Highlights
As a result of the last glacial period and subsequent postglacial processes the coast of the Baltic Sea is fragmented with many shallow, semi-enclosed water bodies (Hupfer 2010)
Mean phosphorus concentrations ranged between 0.5 g kg−1 dry mass in B. maritimus and up to 1 g kg−1 dry mass in L. salicaria
The relatively high density of juvenile eels inside the floating wetlands is remarkable given the general state of the eel stock in the Southern Baltic Sea and the coastal inlets
Summary
As a result of the last glacial period and subsequent postglacial processes the coast of the Baltic Sea is fragmented with many shallow, semi-enclosed water bodies (Hupfer 2010). These lagoons are prone to eutrophication due to reduced water exchange with the open sea and human pressures such as intensive agriculture in the hinterland or wastewater discharges (Kautsky and Kautsky 2000; Conley et al 2011; Newton et al 2014). Floating wetlands offer other ecosystem services such as the provision of habitats or littoral zone protection (Yeh et al 2015). They serve as a refuge for aquatic species (e.g. Nakamura et al 1998; Huang et al 2017) or offer alternative nesting habitats for birds (e.g. Hancock 2000; Shealer et al 2006; DeSorbo et al 2008; Nummi et al 2013; Overton et al 2015)
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