Wadden Sea tidal basins and the mediating role of the North Sea in ecological processes: scaling up of management?

Abstract

a b s t r a c tThe scale of human pressures on marine ecosystems gradually increased from local to global effects during the last centuries. Global warming, sea level rise and trans-oceanic spread of alien species are now or will be major drivers of ecological change in the Wadden Sea. Eutrophication may rank as a superregional effect as the organic matter turnover in the Wadden Sea is driven by import from the North Sea. These effects will not necessarily be uniform across the entire Wadden Sea. A major challenge will be to understand the interactions between global factors with local conditions, both changed by human interference. A conceptual understanding of factors leading to regional differences within the Wadden Sea is only slowly evolving. Based on our understanding of the eutrophication process and a regional comparison of the eutrophication status of the Wadden Sea we propose that wider tidal basins (with a large distance between barrier islands and the mainland) generally have a lower eutrophication status than narrower tidal basins (with a short distance between barrier islands and mainland). This may be explained by a "dilution" of the imported organic matter in wider tidal basins. The interactions between global and super-regional effects on local ecological developments in the Wadden Sea will be complex and pose a major challenge to science, Wadden Sea management and the assessment of ecosystem health as required by EU legislation. The implications of integrating a North Sea view into Wadden Sea management are discussed and we conclude that the river basin approach of the EU Water Framework Directive is not appropriate for the Wadden Sea.

Maximum sinking velocities of suspended particulate matter in a coastal transition zone

“…By contrast, in deeper offshore regions, SPMC is lower and the flocs are looser and more organic. This general pattern of changing SPMC and composition from coast to open waters is typical for our research area, the German Bight (Eisma and Kalf, 1987), and is observed worldwide in estuaries (e.g., Fugate and Friedrichs, 2003) and across coastal seas (e.g., van der Lee et al, 2009). Since both SPMC and turbulence control the w s of cohesive material (Pejrup and Mikkelsen, 2010), it is likely that these cross-shore SPM gradients induce considerable spatial variability in w s and thus affect the transport and fate of SPM in coastal marine systems.…”

“…Applying the aforementioned concept of the transition zone as an off-coastal closing mechanism for nutrient cycling to the two contrasting German Wadden sea regions -East Frisian Wadden Sea and North Frisian Wadden Sea (particularly the Sylt-Rømø Bight) -may help to better understand regional differences in nutrient concentrations. Lower nutrient concentrations and thus lower eutrophication levels in the Sylt-Rømø Bight compared to the East Frisian Wadden Sea are regularly observed (van Beusekom et al, 2009). In this case, the maximal w s calculated in front of the Sylt-Rømø tidal inlet amounts only to 4 × 10 −4 m s −1 , about half the magnitude found off the other Wadden Sea regions.…”

“…For boundary conditions in coastal waters, we therefore assume an average D p = 4 µm due to the dominance of cohesive sediment (Winterwerp, 1998), D = 100 µm (Fettweis et al, 2006), ω = 0.9 to account for a loss on ignition (LoI) of ≈ 0.04 (Fettweis, 2008) and d f = 2.0 as an average coastal fractal dimension (Winterwerp, 1998). By contrast, for open waters we presume the following: D p = 10 µm for algaedominated particles; D = 500 µm since we assume similar trends as in van der Lee et al (2009) for the Irish Sea; ω = 0.05, to parallel a LoI≈0.89 (Eisma and Kalf, 1987); and d f = 1.6 as a typical value for fluffy aggregates (Logan and Alldredge, 1989;Li and Logan, 1995). The densities of primary particles are set to ρ p = 2650 kg m −3 (Maggi, 2009) and ρ o = 1100 kg m −3 , which is computed as an average of the range given in Fettweis (2008).…”

“…7. As a result, retention efficiency is likely reduced and potentially contributes to the observed lower nutrient concentrations compared to the East Frisian Wadden Sea (van Beusekom et al, 2009).…”

“…Tidal and wind-induced currents are the major driver for resuspension and subsequent horizontal transport, while biological processes, such as algae growth and bio-induced sediment stabilization (Black et al, 2002;Stal, 2003), interfere and thus shape the complex distribution of SPM in coastal and estuarine systems. Typically, SPMC and composition show cross-shore gradients (Tian et al, 2009;van der Lee et al, 2009;Li et al, 2010). In shallow waters near the coast, where turbulence and thus resuspension are high, SPMC consists of flocs that are mainly composed of mineral particles with high densities.…”