sites targeting the period 9000–6000 years ago, with some in- terpretation and data usage caveats.
Once a first WaddenSea had formed, apart from sea-level rise, several other processes played their role in the further evolution of the WaddenSea. Sediment from the hinterland by the rivers Rhine, Vecht, Ems and Weser/Elbe had been delivered to the North Sea floor in the Last Glacial and before, and thus was abun- dantly available for recirculation from the moment the North Sea transgressed the region. Then, beginning c.9000 years ago, tidal and wave-driven currents started shifting these sandy sedi- ments. By 8000–7000 years ago, this resulted in the first barrier islands at positions in the immediate offshore of the present system (Jelgersma, 1979 ). Then, coeval with barrier develop- ment and depending on distance to main sediment feeds to the coastal plain, the tendency of underfilled tidal systems behind the barriers to trap large volumes of sediments in next millennia came into play (Beets & Van der Spek, 2000 ; Oost et al., 2012 ; De Haas et al., 2017 ). By 6000–5000 years ago this had cul- minated in partially to fully filled tidal basins (switching from ‘transgressive’ to ‘high stand’ system modes). Then, again coeval with barrier formation and back-barrier tidal basin filling pro- cesses, the last process to consider is the tendency of marsh and swamp vegetation to create peaty substrates in the most inland parts of the coastal plain, over the pre-transgression substrate (basal peat references earlier in this section) as well as on top of tidal deposits in silted-up basins (Beets & Van der Spek, 2000 ; Oost et al., 2012 ; Vos, 2015 ; Pierik et al., 2017 ). Altogether this has allowed for a lot of sediment accommodation in back-barrier space. In the Zeeland and Holland sectors, that accommoda- tion left fewer back-barrier waters open,than in the WaddenSea sectors.
The morphological stability of the tidal basins in WaddenSea is disturbed by sealevel rise and human influences, which is most pronounced in Marsdiep Basin after the closure of the Zuider Sea in 1932. After this closure large changes have been observed in the morphology of the basin and its channels and flat areas as well as in the ebb-tidal delta and adjacent coast line. Although the major effects of this closure have appeared in the first 40 years after the closure, the adjacent coastline is still eroding extensively. Also recent studies on the field data in the Marsdiep (e.g. Elias et, al, 2003), show that the sediment import to this basin is more than expected. Therefore it seems that this basin did not reach the stable morphological condition and the effect of construction of the Afsluitdijk and closure of the Zuider Sea is not dissipated yet. On the other hand the channel/shoal ratio in the Marsdiep and close-by basins suggests that this area is far from equilibrium and needs more sediment to reach its morphologically stable condition (Elias, 2006).
Abstract In the southern North Sea, coastal people commenced with habitat conversions 1,000 years ago. Partly interrupted in late medieval times by large-scale inundations of marshland, progressive embankments transformed the landward half of the amphibic transi- tion zone between a limno-terrestric and a brackish- marine ecosystem into arable land and freshwater lakes. Sea walls rigidly separated the land from the sea. Dy- namic transitional habitats have vanished. Areal loss has diminished the capacity of the WaddenSea to dissipate wave and tidal energy. A coastal ecosystem once rich in marsh plants, seagrass and diatoms on mud ﬂats became transformed into one with less autochthonous photo- troph production, dominated by sandy tidal ﬂats, and dependent primarily on allochthonous plankton supply. The large estuaries have been dredged to serve as ship- ping canals, and have lost most of their former retention and ﬁlter capacity. Riverine loads are now ﬂushed right into the North Sea. Symptoms of a syndromatic coastal habitat degradation are diagnosed, leading to a decline in natural habitat diversity. The conventional on-line coastal protection may not achieve a sustainable coastal habitat conﬁguration. At sedimentary coasts immobi- lised by dikes and petriﬁed shores, a more ﬂexible re- sponse to sealevel rise is recommended.
It is important to consider that nonlinear dynamics play an important role in shallow basins and in tide-surge interactions. Under storm conditions the simplifications of the model are becoming less realistic, the water level elevation is no longer small in comparison to the mean water depth and linear friction is becoming less accurate with high flow velocities. This is also noted by Spencer et al. (2015) in their study on the impact of the 2013 storm on the Southern North Sea coast of the UK, according to them: "Storm surge impacts are not simply linearly related to maximum water level but rely on more complex, nonlinear interactions between tide-surge conditions". Furthermore Prandle and Wolf (1978) showed that quadratic friction is the dominant interaction mechanism between tides and storm surges on the Thames and Horsburgh and Wilson (2007) confirmed this and presented a mathematical explanation for surge clustering on the rising tide, a tidal phase shift combined with the modulation of surge production due to water depth. By including nonlinear dynamics it also becomes possible to investigate the effects of the simplifications (i.e. linear friction and assuming the water level elevation is small compared to the water depth) on the results. A possible method is given by Alebregtse and de Swart (2016) using a twofold expansion, a harmonic truncation combined with a perturbation expansion, although it should be changed so as to incorporate wind.
The fyke scheme run by NIOZ since 1960 has shown that many species from the western WaddenSea are declining ( Van der Veer et al., 2015 (in press) ; van der Veer et al., 2011 ). Trends in the DutchWaddenSea as a whole, based on a demersal fish survey (DFS) carried out by IMARES, were analysed before ( Tulp et al., 2008 ). In both lower and higher levels of the ecosystem contrasting trends have been found between tidal basins within the WaddenSea ( Ens et al., 2009 ; Tulp et al., 2008 ) (P. Herman pers.comm). The tidal basins greatly differ in sediment, nutrients, salinity water visibility and stoichiometry. Therefore a basin approach in time series analysis may provide better inside in potential drivers. This notion gave rise to a re-analysis of the WaddenSea fish data per tidal basin. Adjoining coastal areas North of islands and along the main coast are included in the comparative analysis as well to provide a reference for the patterns observed within the WaddenSea. In course of the survey period the timing of the DFS survey in the WaddenSea as advanced (ca 1 month in 40 years) because of practical planning reasons. The changed timing of the survey could partly explain trends in fish species: if the residence period of fish in the WaddenSea has changed, such phenological changes in combination with a change in timing of the survey may lead to time trends that do not reflect true
The numerical model, Delft3D, used data and physical parameters in order to reflect the real conditions. In addition to that, the literature researches and data from flume experiments had been used to convert the biological activity into physical parameters. In general, the biological activity is more complex than the implementation in this model. The biological parameters in this research were based on the effect of different benthos on the erosion of fine sediment as measured in the individual-flume experiments. Actually, these experiments are a simplification of the natural condition because in nature, several benthic organisms are present in the same place and the abundance of these organisms is influenced by several factors, for instance bed level changes, interaction between each other, pollution, nutrients and etc. The mussels in this research presented as young mussel bed, therefore no seed beds could take place in the other cells in this model and no emigration could occur in this numerical model as described by Van Leeuwen et al. (2010). Moreover, the biomass of each grazer in this research was equal for all basins, in fact there spatial variation in the biomass of every grazers in basins. In addition, other types of benthic organisms, for example Mya arenaria, Crassorstrea gigas, Ensis directus and etc. were also presented in the DutchWaddensea but this model could not deal with their impact of fine sediment dynamics because the limited data for the biomass of species and lack of both field and laboratory experiments. In spite of these shortcoming in the assumptions and the uncertainties, the extended model highlight the promising usefulness of the biological activity in prediction more accurate results because it identified key potential parameters that could improve the numerical models in the future.
With respect to the fate of the fresh water, two impor- tant aspects have still to be addressed. The first aspect is the quantification of the actual export of fresh water through the different tidal inlets. Although it was previously shown that the fresh water goes towards the Texel and Vlie inlets (Zimmerman, 1976), no estimates (from measurements or numerical models) of the net export exist. The model results to be presented here will shed light on the residual transport through the inlets and across the Terschelling watershed. In general, this exchange can be expected to show a lot of vari- ability, depending on the freshwater discharge rate, wind di- rection and speed, water level surges, and on the phase in the spring–neap cycle. This variability also makes the applica- tion of the concept of flushing time less obvious, as will be discussed below. A second aspect concerns the unsuitability of salinity as a tracer, which is compounded by the fact that the fresh water from the two sluices is fundamentally indis- tinguishable.
Dover Channel was stressed. They showed that, despite the decreasing riverine loads in phosphorus, this element still might play a signiﬁcant role in fuelling the primary production in the western DutchWaddenSea. How- ever, the increased productivity of the WaddenSea is not only a direct eﬀect of nutrient-rich river water being advected into the area but also, and perhaps even mainly, an indirect eﬀect due to an increased import and remineralisation of organic matter from the adjacent coastal zone (Postma 1954; de Jonge and Postma 1974; van Beusekom et al. 1999). Changes in the seasonal cycle of nutrients might be used as a direct indicator of changes in organic matter import and remineralisation rates. De Jonge and Postma (1974) already explored this strategy and inferred a tripling of the organic matter import from the North Sea into the WaddenSea from 1950 to 1971–1972. On the other hand, Helder (1974) did not observe any clear-cut change in the annual nitrogen cycle between 1960–1962 and 1971–1972 in the DutchWaddenSea. Van Beusekom et al. (2001) and van Beusekom and de Jonge (2002) linked the shape of seasonal cycles of ammonium and nitrite to the eutro- phication status of the WaddenSea. They inferred a two- to threefold increase in the eutrophication status of the western DutchWaddenSea since the earliest mea- surements of nitrogen compounds in 1960–1962 (Post- ma 1966).
On the other hand, Sentinel-1 is a C-band radar satellite mission funded by the European Union and operated by the European Space Agency (ESA). It comprises a constellation of two satellites, Sentinel-1A launched in April 2014 and Sentinel-1B launched 2 years later in April 2016. These Sentinels have a repeat period of 6 days for the twin constellation. We selected Sentinel-1A images because they are readily available for our investigated area. Level-1 products are recommended for land cover analysis. For a fair comparison with TerraSAR-X, we chose the interferometric wide (IW) swath mode and its GRD (ground range detected) option containing medium-resolution data. We limited ourselves to single polari- sation (VV) data.
For the integration of the socio-economic and ecolog- ical aspects of the entire WaddenSea system, the Wad- den Sea Policy Supporting System (WADBOS) has been developed. The system has a dynamic structure, using feedbacks and connections between all relevant factors and processes at every (micro- or macro-) level. This has been done in order to structure the relationships between human activities and ecosystem mechanisms, and to ex- plore the effects of different policy options in terms of engineering works, type and intensity of fishery, boating, pollution, eutrophication, and so on, on both the socio- economic and the natural parts of the integral system (Fig. 3). The first panel gives the general structure of the described relationships between ecosystem and socio- economic system while the second panel gives some of the impacts of activities in the first panel on items such as “environmental quality” and “economy, landscape, experience and pressure”.
Large areas of the Netherlands were situated below sealevel in the past, today it is up to 6,7 meters minus NAP near the city of Rotterdam at ‘Zuidplaspolder’ [NAP the general sea-level of the North Sea]. Climate-change will make this situation worse in the coming decennia, the seawater level will rise and storms will become more severe (Vousdoukas et al., 2017). In 2016 the EO local television broadcast organization presented a drama series in the Netherlands concerning ‘What would happen when the dikes break’ [Dutch; Als de dijken breken’] by Johan Nijhuis creator and Hans Herbots cine- aste. This series ushered in a period of more interest by the Dutch people on the topic of seawater rise dangers as a result of climate-change and included a call to the national government for better infor- mation concerning the personal risks of people and their houses. The result was that the national ‘Rijkswaterstaat’ government organization, among other things responsible for the national water defence system, consulted experts, started-up informational processes, and opened a website through which every house could review what the current risks are and how far the house is below sealevel [www.overstroomik.nl]; in addition, a report on the risks of flooding for common people became available (Vergouwe, 2016), see figure 4. The maps of figure 4 show the threats of climate-change for the Netherlands come from both rising see levels and heavy rainfall. Concerning the rainfall, the in- creasing intensity of rainfall will not only increase the water influx from the East, but also the ‘polders’ maintenance will also difficulties with too little pumping capacity.
First, as mentioned earlier, there are 3 countries governing the region and coordinating activities, if including the German “länder” Lower Saxony and Schleswig-Holstein even five. All make their own decisions in regard to policies. None of those is superior to the others. Therefore, none of those governments can be the policy broker who keeps conflicts in limits, balances different views and finds compromises, at least not resulting in one final policy for the whole of the concerned region. It is of primary interest to them to guarantee a well-functioning and beneficial management of their national territory, not of the region as a whole. A reason for this are re-elections. Therefore, a highly relevant motive of governing parties and their policies is the probability of being re-elected. They try to satisfy as much interest groups, that is to say voters, as possible to increase their chances to stay in office. E.g. for the German government, it is of minor importance to give consideration to Dutch and Danish citizens because they will not influence the voting outcome. For such a multinational policy regime a policy broker could only be an institution or organisation that brings together different national coalitions. Nevertheless, governments are highly relevant actors as members of the advocacy coalitions.
It is clear that the locations that were chosen based on the habitat suitability maps and expert judgement in the field are indeed suitable. The fact that the two observed “new” locations are also very close to sites that according to the habitat suitability map (‘zeegraskansenkaart’, De Jong, 2005), indicates the value of this map. One of the factors determining habitat suitability for eelgrass is elevation. As the WaddenSea is a highly dynamic area with shifting gullies and mudflats one has to observe some level of caution in using such a map from 2006. On a very small scale (tens of metres) suitable areas may have shifted. However, on a larger (hundreds of metres) scale we still expect locations that were deemed suitable in 2006 to be in roughly the same location.
The current national policy (LNV 1999) is to re-establish a mussel bed area of about 4,000 ha. One measure to enable this is to close part of the WaddenSea tidal flats for fishery activities. Nowadays, about 25% of the intertidal area is closed for any economic activity; in the rest of the area fishery is strictly regulated according to a number of rules. A certain amount of shellfish is re- served as food for migrating birds; fishery is not allowed if the food stock is lower than this threshold level. Also, it has been decided to stop intertidal mussel bed fishery but in some exceptional cases, to forbid cockle fishery there where mussel spatfall is observed and to addition- ally close those areas for fishery that are most suitable for new mussel bed establishment. The last decision presently concerns about 6% of the DutchWaddenSea intertidal area, in addition to the 25% which is already closed.
We hypothesize that a key factor in the occurrence of sea- grass beds in the study area is sediment stability. The spa- tial pattern of seagrass beds in the northern WaddenSea as shown in Fig. 2 may be explained by (1) sediment mobility too high for seagrass to occur on sandy Xats exposed to a long fetch from the prevailing southwesterly and westerly winds, (2) islands and high sands providing shelter against wave disturbances originating from that direction, (3) clay from former salt marshes underlying soft tidal sediments and facilitating sediment stability and Wrm rooting, and (4) sediment accretion rates being too high for seagrass in the nearshore zone along the mainland coast. To con W rm this, a W eld survey on sediment dynamics needs to be conducted. An emphasis on sediment stability shall not reject other fac- tors often discussed for seagrass distribution such as sedi- ment type, tidal level, hydrodynamics, light availability, salinity, nutrient over-supply and herbivory. However, for the spatial pattern in the northern WaddenSea these other factors or any combination of them do not seem to provide a consistent cue to the observed pattern.
Coastal Sensitivity to Sea-Level Rise: A Focus on the Mid-Atlantic Region (CCSP 2009) provides a detailed description of some of the likely impacts of sealevelchange in the Mid-Atlantic region. Some of these are summarized here. Along the coast, environmental health is closely linked to sealevel. Many environments, including beaches, barrier islands, wetlands, and estuarine systems, adjust to increasing water level by growing vertically, migrating inland, or expanding laterally. If the rate of sealevelchange accelerates significantly, coastal environments may not be able to respond accordingly and will decrease in size or be submerged. These changes can fundamentally change the state of the coast.