The Trilateral Monitoring and
Assessment Program (TMAP)

Wadden Sea Ecosystem No. 7, 1997. Assessment of the Wadden Sea Ecosystem by. J.F. Bakker, N. Dankers, F. De Jong, C. Gätje, T.F. Pedersen, P.Potel & K. Van Berkel, 1997. Common Wadden Sea Secretariat, Trilateral QSR Group & Trilateral Monitoring and Assessment Group, Wilhelmshaven.

INTRODUCTION

This assessment of the status of the Wadden Sea ecosystem was drafted on the occasion of the 8th Trilateral Governmental Wadden Sea Conference, Stade, Germany, 22 October 1997.

The report is based upon material, contributed by a large number of experts in the past two years in the framework of the drafting of the 1997 Quality Status Report. These contributions will be published in the full QSR.

The evaluation of the basic material was carried out by the Trilateral Quality Status Report Group, which is responsible for the contents of this Report.

The Report is structured according to the habitats as defined at the 7th Trilateral Governmental Wadden Sea Conference, Leeuwarden, The Netherlands, 1994. It concerns the tidal area, the salt marshes, the estuaries, the beaches and dunes and the North Sea offshore zone. Each of these chapters starts with a definition of the habitat and the relevant ecological targets. A separate chapter on birds and seals was added because these species groups use more than one habitat. The habitat type 'rural area' is addressed in this chapter as well. The report starts with a chapter on the quality of water, sediment and biota.

The actual assessment focusses on the status of the ecotargets.

In the last chapter the main issues of concern, identified in the 1993 Quality Status Report, are re-evaluated on the basis of new knowledge and insights.

Suggestions and comments to the Report are welcome because they will contribute to the quality of the full Quality Status Report, which will be published after the Stade Conference.

1 Quality of water, sediment and biota

Ecological Targets:

- a Wadden Sea which can be regarded as a eutrophication non-problem area.

- background concentrations of natural micropollutants in water, sediment and indicator species.

- concentrations of man-made substances as resulting from zero discharges.

1.1 Nutrients and Eutrophication

In the period 1985-1995 a significant decrease in phosphate concentrations occurred in the southwestern Wadden Sea from Den Helder to the Jadebusen and in the northeastern Wadden Sea from the Eider estuary to the Sylt-Rømø area. A significant decrease in ammonia concentrations was found for the Ems Estuary and the Niedersachsen Wadden Sea.

The decreasing trends partly reflect the reduction in phosphorus and ammonia loads of Lake IJssel and the Elbe (Figure 1). The riverine reductions can be attributed to the progressive implementation of waste water purification schemes.

For total nitrogen and nitrate an ongoing reduction in the period 1985-1995 could not be observed, but the 1995 nitrogen and nitrate loads of the Elbe were lower than the 1985 loads (Figure 1).

1.1.1 Eutrophication Effects

The decreased concentrations of phosphate have not resulted in decreased average chlorophyll levels. Several explanations have been offered, namely an increase in visibility, delivery of phosphorus from the sediment, import of organic material from the offshore zone and nitrogen as the limiting factor. The reduction in the inputs of phosphorus compounds has been much higher than of nitrogen compounds, as a result of which the ratio between dissolved inorganic nitrogen and phosphorus has increased considerably since 1990 in some parts of the Wadden Sea, especially in the low salinity parts (10 psu) of the estuaries. Changes in N/P ratio may result in shifts in phytoplankton species composition and favor the development of toxic algae. The changed N/P ratio has so far not resulted in significant changes in the occurrence of the nuisance algae Phaeocystis. The abundance of potentially toxic species has remained low in the first half of the 1990s.

The large areas with anoxic sediment surface (black spots) in the Niedersachsen Wadden Sea in the spring of 1996 was most probably caused by a coincidence of hydrodynamic, meteorological and biological phenomena of which also a few cases from the past are known. High mortality rates of benthic organisms and a large amount of planktonic material, together with a sudden rise in temperature exceeded the remineralisation capacity of the benthic system and caused mass mortality of benthic animals. The questions remain whether the affected area has a more than average sensitivity for large amounts of organic material and whether the event must be judged as a sign of an exceeding of the benthic remineralisation capacity in general.

The results of long term monitoring programs on the macrozoobenthos of the tidal flats show large fluctuations in biomass between years. Much of the inter annual fluctuation is caused by the group of bivalves. There does not seem to be a direct link between eutrophication and benthic biomass. In the western part of the Dutch Wadden Sea benthic biomass has increased at the same time that nutrient levels increased. In other areas benthic biomass has always been on such high levels. The recent decrease in eutrophication has not been followed by a decrease in benthic biomass or production. The weather seems to be the major factor influencing the benthic biomass.

1.2 Hazardous Substances

1.2.1 Riverine Inputs

The Elbe is by far the major source of hazardous substances input to the Wadden Sea but the inputs of metals (cadmium, copper, mercury, zinc), lindane and PCBs (Figure 2) significantly decreased between 1985 and 1995. Lead inputs decreased strongly since 1993.

In terms of fresh water discharge the Weser is the third most important river in the Wadden Sea area. The Weser loads of mercury, lead and PCBs are however as high as the Elbe loads. For lead there was even a significant increase in input since the beginning of the 1990s (Figure 1). For the Ems a significant cadmium input reduction occurred, while Lake IJssel shows a significant reduction in lindane.

1.2.2 Concentrations in Sediment, Blue Mussel and Bird Eggs

Generally over the last ten years metal concentrations in sediment have significantly decreased. Only mercury remains a problem in the western and southern Wadden Sea.

PCBs in sediment show a more or less strong downward development

In Blue Mussel metal concentrations generally do not show a significant reduction between 1985 and 1995. Only in the Elbe estuary zinc levels seem to decrease, but this metal is regulated by Blue Mussel and consequently difficult to assess. Generally PCB levels in mussel are decreasing with the exception of in Ems-Dollard and Elbe estuaries, where levels are very variable and on average two times higher than in other parts of the Wadden Sea.

Mercury levels in eggs of Oystercatcher and Common Tern significantly decreased in the past decade, except in the Ems-Dollard and the Halligen area. PCBs and organochlorine pesticides in bird eggs showed a decrease in all investigated areas (Figure 2).

Pesticides and TBT are not part of routine monitoring programs. Surveys in the Dutch and German coastal waters have revealed that many pesticides and TBT are present in relatively high concentrations. Effects of pesticides on phytoplankton and zooplankton could be detected in research projects. Herbicides interfere with phytoplankton photosynthesis. Zooplankton grazing is hampered by certain pesticides. Pesticides may also be a factor in the decline of intertidal seagrass.

TBT is toxic in very low concentrations. The (almost) extinction of the dogwhelk (Nucella lapillus) in the Wadden Sea may be attributed to the effects of TBT. Also the Blue Mussel is sensitive to TBT but is unclear whether TBT has played a role in the decline of intertidal mussel beds.

1.3 Oil

Beached Bird Surveys (BBS) are suitable to show the specific risk of groups of birds to become oil contaminated at sea and furthermore to provide information on temporal changes or spatial differences in the occurrence of (chronic) oil pollution at sea. Standardized, long-term registrations of oil rates - the percentage of all beached individuals found to be oiled - have proven to be a valuable indicator of the level of marine oil pollution. A decline in the oil rate is thus believed to be indicative of a decline in oil pollution at sea. The results of chemical analysis of oil from polluted feathers suggest that fuel residues from shipping are mainly responsible for the pollution along the Danish German-Dutch Wadden Sea and North Sea coasts.

According to long-term BBSs in the North Sea area, oil rates along the Dutch and Danish coasts of the Wadden Sea are generally higher than along the German Wadden Sea coastline.

In The Netherlands the results of BBSs over the past 30 years show consistent declines in oil rates in all areas, especially in the Dutch Wadden Sea, for all seasons and for virtually all species. The results of Dutch investigations provide no evidence for a sudden improvement since MARPOL Annex I came into effect in 1983. The gradual implementation of MARPOL may only have contributed to the continuation of the declining trend.

In Denmark, the oil rates are only partly decreasing, due to the illegal tank washing procedures of ships in the Kattegat and Skagerrak after leaving the Baltic Sea, which is a Special Area according to MARPOL Annex I.

After a decline in the oil rates in Germany in the years 1988 to 1991 - which ran parallel to the period in which reception facilities were free of charge in all German harbours - the 1991/92 data show an increase. A possible explanation is an increased illegal dumping of oil after the gradual phasing out of the free of charge disposal in 1991.

2 Salt Marshes

The habitat type salt marsh includes all mainland and island salt marshes, including the pioneer zone.
Also the brackish marshes in the estuaries are considered part of this habitat type.

Ecological Targets:

- an increased area of natural saltmarshes;

- an increased natural morphology and dynamics, including natural drainage patterns, of artificial salt marshes, under the condition that the present surface is not reduced;

- an improved natural vegetation structure, including the pioneer zone, of artificial saltmarshes.

Generally a differentiation is made between island salt marshes, mainland saltmarshes and halligen salt marshes. There are three sub-categories of mainland saltmarshes, namely the actual mainland saltmarshes, summer polders and estuarine marshes. The sizes of the different salt marsh categories are given in Table 1.

2.1 Natural Salt Marshes

At present most of the island salt marshes and those along the peninsula Skallingen have a natural structure. On several islands the salt marshes developed in the shelter of sand dikes.

An increase of natural salt marsh area is possible through:

- natural accretion. Mainly as a result of the shortening of the main dikes for coastal protection and land reclamation purposes, there are few locations left along the mainland coast where a net sedimentation without the aid of brushwood groynes and other sediment trapping techniques takes place;

- reducing human interference in existing natural salt marshes, for example by reducing grazing pressure.

- the outbankment of summer polders. Reclamations in the past for agricultural purposes have led to a net reduction of the area of theoretically possible salt marshes. A secondary effect has been a reduction in the number of fresh-salt transitions. Through the outbankment of summer polders new semi-natural salt marshes can be created which would also help to stabilize the total salt marsh area. Also new fresh-salt transition zones would be created.

2.2 Artificial Salt Marshes

Most of the present mainland salt marshes developed in front of the dikes with the aid of sediment trapping techniques. In most of these artificial salt marshes the drainage channels are being maintained for agricultural and/or coastal defence purposes. The salt marshes are generally protected by groynes and in some cases hard substrate.

It is possible to increase the naturalness of these salt marshes by the (experimental) reduction of drainage channels and a cessation of systematic drainage in the salt marshes and the accretion zone.

The protection of the salt marsh edges against erosion will in most cases remain necessary. Brushwood groynes are amongst thw0nvironmental techniques for this purpose.

2.3 Flora and Fauna

The Wadden Sea salt marshes consist of salt tolerant pioneer vegetation, grasslands and dwarf shrub communities including thousands of invertebrate species, many of them insects. Saltmarshes provide resting, breeding and feeding grounds for great numbers of birds.

The adjacent wetlands behind the dikes - often a result of past reclamation - are important habitats for breeding waders. Further information on birds is given in chapter 7.

A reduction of grazing pressure by domestic animals can result in a more differentiated vegetation pattern. Also the above mentioned reduction of active drainage will increase the naturalness of the vegetation and consequently the related fauna.

3 Tidal Area

The tidal area covers all tidal flats and subtidal areas. The border to the North Sea side is determined by an artificial line between the tips of the islands. The borders to the estuaries are determined by the average 10 isohaline at high water in the winter situation.

Ecological Targets:

- a natural dynamic situation in the tidal area;

- an increased area of geomorphologically and biologically undisturbed tidal flats and subtidal areas;

- an increased area of, and a more natural distribution and development of natural mussel beds,Sabellaria reefs and Zostera fields;

- viable stocks and a natural reproduction capacity, including juvenile survival, of common seal and grey seal;

- favorable conditions for migrating and breeding birds.

3.1 Natural Dynamics and Geomorphology

Sea level rise causes an increased net sand import into the Wadden Sea. The sand originates from the offshore zone up to the 20 m isobath. This in turn will cause a steepening of the foreshore of the barrier islands. An increase of the tidal volume of the Wadden Sea proper, for example through the outbankment of summer polders, may reduce the sand hunger of the tidal area.

In the past sea level rise has caused a moving of the islands in the direction of the mainland. The Wadden Sea has become smaller and the possibilities for sedimentation of fine grained material have become less. This phenomenon is aggravated by the straightening of the coastline for coastal defence purposes.

There is an urgent need for an integrated approach to these issues, taking into account the overall Wadden Sea sediment balance and the role of fixed coastal defence structures.

3.2 Sabellaria, Seagrass and Mature Mussel Beds

3.2.1 Sabellaria

The reef building polychaete Sabellaria sp. is probably still occurring on two locations in the German Wadden Sea. Extensive reefs can be considered extinct, although it is not clear whether they ever occurred in the Dutch part of the Wadden Sea. In Germany most of the reefs have disappeared possibly because of fishing activities with sediment disturbing trawls and dredges. Regeneration may be possible in areas which are closed for fishery with bottom gear. It is not known whether these reefs will redevelop if fishery is stopped. Pilot studies will be necessary before Sabellaria can be included in the TMAP.

3.2.2 Seagrass

In the past subtidal populations of Seagrass (Zostera marina ) covered vast areas in the entire Wadden Sea. After an epidemic disease, caused by the protozoan Labyrinthula macrocystis, in the early 1930s, the subtidal seagrass beds completely disappeared and never reestablished. In the intertidal, a severe decline of the area covered by seagrass has been observed during the last years (Table 2). In the Dutch Wadden Sea, only a few scattered seagrass stands have survived until today. Although recently a local reestablishment of another seagrass species, Zostera noltii, has been observed, the overall decrease has continued.

The causes for the decline are still unknown. Eutrophication, phytotoxic pollutants like herbicides, increased turbidity and macroalgal blooms are discussed as possible factors.

3.2.3 Blue Mussel beds

Recent studies have confirmed that Blue Mussel beds play an important role in the ecology of the Wadden Sea, both in sediment dynamics, nutrient dynamics, biodiversity and as food source for birds.

The area covered by intertidal mussel beds is still very small. In the Netherlands only a few percent of the Dutch reference value of 5000 ha is present and consists of young immature beds which are vulnerable to winter storms. In Niedersachsen, the area of intertidal mussel beds has decreased from 5000 ha in 1975 to some 790 ha in the beginning of 1996. The reduction in biomass is even more dramatic. From 1996-1997 a moderate recovery to 1200-1400 ha was observed. In Schleswig Holstein the area of intertidal beds is stable since fishery has been restricted to the subtidal areas but many beds are still immature and vulnerable. In Denmark mussel beds, both in areas closed for fisheries and areas with regulated fishery, developed between 1995 and 1996 after a decline between 1991 and 1995. Ice coverage is the most important regulating factor for the development of mussel beds in the Danish Wadden Sea.

As new beds seem to settle in places where mature beds used to occur, protection of developing beds is a precondition for the implementation of the target.

3.3 Birds and Seals

The feeding area for most birds are the tidal flats with their rich supply of benthic animals. The food availability for birds is partly dependent on the situation with regard to wild mussel beds. The food source and their distribution, as well as other factors such as the availability of undisturbed breeding and roosting sites (eg. disturbance both by man and predators) mainly determine the distribution and size of the bird populations in the Wadden Sea area.

Further information on breeding and migratory birds and on the common seal and grey seal and is given in chapter 7.

4 Beaches and Dunes

Beaches and dunes include beaches, primary dunes, beach plains, primary dune valleys, secondary dunes and heath land behind the dunes.

Ecological Targets:

- increased natural dynamics of beaches, primary dunes, beach planes and primary dune valleys in connection with the offshore zone;

- an increased presence of a complete natural vegetation succession;

- favorable conditions for migrating and breeding birds.

Beaches and dunes develop through landward sand transport by wind and water and subsequent sediment sorting by currents. Thus especially beaches and young dunes are characteristic features of the dynamic of the Wadden Sea ecosystem.

As a result of anthropogenic influences, especially agriculture, coastal protection and recreation and tourism, natural and undisturbed beach and dune areas have become rare. In Figure 3 the total dune area for several Dutch and German islands is presented together with the percentage of white dunes.

4.1 Natural Dynamics

Coastal protection limits the dynamic of sand transport and conservative measures preserve actual zonation patterns. Most of the older dunes are consolidated, partly covered by pine wood plantations. They are erosion free now without the natural renewal of secondary dune formation. An intensified geomorphological dune turnover will improve community patterns on the benefit of complete series without emphasis on older stages.

4.2 Natural Vegetation

The present dune vegetation is strongly anthropogenically determined. The major factor is the above described stabilization by coastal protection measures as a result of which the natural early as well as the oldest successional stages are underrepresented.

Past activities like grazing and turf cutting have caused an enrichment of early successional stages and prevented wood formation.

The dunes are affected directly by tourism mainly by trampling which causes destruction of sensitive dune vegetation.

A more indirect effect of tourism probably is the increased groundwater extraction. Considerable effects on the community composition of in particular of wet dune valleys have been reported. The plantations of Pinus sylvestris also place a burden upon the available ground water.

The rose species Rosa rugosa was originally brought to many of the islands for decorative purposes. This species does however not integrate in the local plant communities but superimposes by covering large areas.

4.3 Birds

Bird species mainly breeding in dune areas are for example Common Eider (Somateria mollissima), Herring Gull (Larus argentatus) and Hen Harrier (Circus cyanus). Great Ringed Plover (Charadrius hiaticula), Kentish Plover (Charadrius alexandrinus) and Little Tern (Sterna albifrons) build their nests on beaches, primary dunes and sandpits.

Natural beaches and primary dunes have not only become a rare commodity due to the decreased dynamics. In addition the remaining primary habitat sites suffer from increased human disturbance, mainly from tourism and recreation.

These areas are important whelping areas for grey seals (in the winter) and indispensable breeding areas for specialized and still endangered birds like the Kentish Plover and the Little Tern. Additional information on birds is given in chapter 7.

5 Estuaries

Estuaries include the estuaries of the rivers with a natural water exchange with the Wadden Sea. On the landward side, estuaries are delimited by the mean-brackish-water line. On the seaward side, the border is the average 10 isohaline at high water in the winter situation.

Ecological Targets:

Valuable parts of Estuaries will be protected and the river banks will remain and, as far as possible, be restored in their natural state.

5.1 Morphological Changes

Anthropogenic modifications of estuaries have been accompanied by changes and losses of habitats and ecological functions. During the last century, riparian forests, large marsh and semi-terrestrial areas bordering the rivers have been embanked for coastal protection and for agricultural purposes (Table 3). The diking of freshwater/brackish marshes and reeds and the subsequent drainage for agricultural use has destroyed areas of major importance for breeding and migratory birds. Another consequence is the reduction of the natural retention function as these inundation areas have served as reservoir for water masses during storm surges.

The still remaining natural or nearly natural areas require special protection. There are still possibilities to restore estuarine habitats that have been destroyed by diking. The first step could be an inventory of the most suitable sites for de-embankment.

The construction of tidal weirs and sluices has led to a lack of a natural salinity gradients. These constructions hinder or prevent the migration of invertebrates and fish species and hamper the tidal influence in the upper reaches of the estuaries. As a consequence, fish species like sturgeon, houting, salmon, trout are now extinct or are in danger of extinction. Restoration of smooth gradients of salinity and tidal amplitude in small creeks along the Wadden Sea coast and in the estuaries would improve the conditions for long term maintenance of endangered migrating species.

The morphology of the Elbe, Weser and Ems estuaries has been altered in order to optimize their function as shipping routes. Their depths have been and still are continuously adjusted to the increasing size of vessels. The channelization has resulted in a loss of intertidal and shallow subtidal areas and in an irreversible increase of the tidal range (Table 4, Figure 4).

It is not expected that the effects of most of the above outlined impacts will be reduced in the near future. On the contrary, on the basis of ongoing and anticipated projects a further impact on the natural dynamics will occur through dredging (Elbe, Weser) or the planned construction of a surge barrier (Ems). In the Netherlands no tidal rivers have remained. Sluicing regimes will however be adapted to a more natural situation and it will be investigated to what extent salt fresh transitions can be restored.

The Varde Å in Denmark and the Gödelniederung on Föhr are the only natural estuaries in the Wadden Sea area. The banks of the Varde Å are used for intensive agriculture, but recently an extensification scheme has been initiated.

5.2 Water Quality

Inputs by rivers are the main cause for elevated nutrient and contaminant concentrations in the Wadden Sea. The inputs of industrial and municipal waste water as well as land runoff from the riverine catchment areas are responsible for the high riverine loads of nutrients, heavy metals and organochlorines. Part of these substances is deposited in the estuaries by removal processes and may enter the estuarine food web. The concentrations of nearly all monitored pollutants have decreased significantly in the estuaries of Ems, Elbe and Eider (Chapter1). Nevertheless, some chemicals are still found in bird eggs in concentrations capable of impairing bird reproduction.

Long time data series of systematic monitoring only exist for hydrological and water quality parameters. Biological monitoring programs have been set up a few years ago. They have to be maintained and intensified in order to evaluate the anthropogenic impact on community structures and ecological processes and to detect long term changes.

6 The Offshore Zone

The offshore zone ranges from the 3-sea-mile line to an artificial line connecting the outer tips of the islands. The border between the offshore zone and the beaches on the islands is determined by the average low-tide water mark.

Ecological Targets

- an increased natural morphology, including the outer deltas between the islands;

- a favorable food availability for birds;

- viable stocks and a natural reproduction capacity of the common seal, grey seal and harbor porpoise.

6.1 Geomorphology

The offshore zone up to a depth of 20 meter forms one morphological system with the tidal area. This is best illustrated by the sand displacement between barrier islands: sand and silt are transported back and forth between the tidal and the offshore area (Figure 5). The Wadden Sea system which consists of tidal inlets, tidal basins, outer deltas and neighboring island coasts has a closed sand economy. The consequence of this dynamic equilibrium is that disturbances will be compensated until a new equilibrium is reached. Sea level rise and bottom subsidence both cause a deepening of the tidal basin resulting in an increased net sand import from the offshore area. In the end this sand originates from the North Sea side of the islands. Together with a net sedimentation on the mainland side this results in a landward movement of the islands. However most of the inhabited islands have been (partly) stabilized by fixed coastal defence works, and in this way hamper the natural compensation for sea level rise. As a consequence the foreshore of the islands will steepen. Ultimately the tidal flat system may disappear because no sufficient sand can be delivered.

For reasons outlined above the use of foreshore sand suppletion for the protection of the coasts of the islands is preferable to the use of solid coastal construction works. It also follows that the suppletion sand should derive from outside the 20 meter isobath and that removal of sand from the system, for example through commercial sand extraction, should be limited as much as possible.

6.2 Biology

Also biologically the Wadden Sea and North Sea are intimately linked. Phytoplankton, especially dinoflagellates, is transported from the offshore area to the Wadden Sea proper and is remineralized here. Several shellfish, like the Cockle and Blue Mussel may restock the Wadden Sea from deep water refuges in the North Sea, after a severe winter has decimated the population of the exposed flats. Motile animals, like fish, shrimps and crabs largely leave the Wadden Sea in autumn to survive the winter in the relatively warm waters of the North Sea. Several species of fish also take advantage of both seas. Without the Wadden Sea, the stocks of such fish would be greatly reduced.

The total quarterly macrozoobenthos biomass off Norderney showed a significant increase since 1988 which can be explained by the synergistic effect of mild meteorological conditions and eutrophication.

Several groups of birds occur in internationally important numbers in the area. Sea ducks take advantage of the relatively shallow waters that hold rich banks of bivalve food species, particularly Trough Clams (Spisula subtruncata and S. solida). Spisula fishery may interfere with the food situation of the sea ducks. Through proper management a negative effect on the bird populations can be prevented.

Local breeding fish eaters, the gulls and terns breeding at Wadden Sea islands, as well as autumn (Gannet Sula bassana, Great Cormorant Phalacrocorax carbo, terns) and winter visitors (divers, auks) specifically seek out these waters that are so rich in small fish. In total, the coastal zone of the North Sea holds a specific seabirds assemblage in all seasons, and may be seen as a specific ecotope that is not found further inshore (Wadden Sea) or offshore (the open North Sea).

The sizes of the populations of the gull species Herring Gull (Larus argentatus), Lesser Black backed Gull (Larus fuscus) and Great Black-backed Gull (Larus marinus) are to a considerable extent determined by discard and offal from fisheries in the coastal area.

The Grey Seal (Halichoerus grypus) and the Common Seal (Phoca vitulina) use both the Wadden Sea and the North Sea. In winter, when feeding conditions deteriorate for the seals in the Wadden Sea, they follow the fish into the North Sea in order to forage.

The Harbour Porpoise (Phocoena phocoena) is the most common cetacean species, with many thousands present in the area in winter. Highest densities have been calculated for the area off Sylt, Amrum and southern Rømø. This area has been identified as a breeding and nursing ground for this species.

7 Birds and Seals

Ecological Targets

- viable stocks and a natural reproduction capacity, including juvenile survival, of common seal and grey seal;

- favorable conditions for migrating and breeding birds:

= a favorable food availability;

= a natural breeding success;

= sufficiently large undisturbed roosting and moulting areas;

= natural flight distances.

7.1 Birds

The Wadden Sea is important as breeding and staging areas for numerous birds which feed, moult and roost in the region. Ten to twelve million individuals of more than 50 species utilize the area throughout the year. The population size of birds is determined by mortality and breeding success. Both are governed by the availability of food, staging and breeding sites and factors such as predation, weather and pollution. The various habitats of the area have their specific significance for these birds and are linked and depending on each other. To avoid energy loss, birds try to avoid unnecessary flights, i.e. they choose safe roosting sites which lie closest to their feeding areas. On the mudflats of the Wadden Sea the food availability is not simply the amount of food present but is very much determined by the behavior of prey species, weather conditions, season, winter severity and human activities.

7.1.1 Breeding birds

The Wadden Sea is an important reproduction area for more than 30 species of breeding birds. The Black-headed Gull (Larus ridibundus), the Herring Gull (Larus argentatus) and the Oystercatcher (Haematopus ostralegus) are the most common breeding species at present. The most important habitats for breeding birds are salt marshes, dunes and beach plains on the islands. For 22 species at least 1 % of the NW-European population breeds in the Wadden Sea and for 15 of these it holds more than 5 % of the NW-European breeding bird population.

During the last decades the population of many of the bird species breeding in the Wadden Sea have significantly increased (Common Gull Larus canus (Figure 6), Arctic Tern Sterna paradisaea, Lesser Black-backed Gull Larus fuscus), some are stable (Avocet Recurvirostra avocetta, Great Ringed Plover Haradrius hiaticula , Black-headed Gull Larus ridibundus) or have decreased (Kentish Plover Charadrius alexandrinus, Ruff Philomachus pugnax), some species have extended their distribution range (Eurasian Spoonbill Platalea leucorodia, Great Cormorant Phalacrocorax carbo,) and luckily non have become extinct. The reason for the increase and decrease in population size and the factors threatening the populations are manifold. They range from the positive effects of conservation measures to global climate change.

Many breeding bird species are recovering from drastic declines in the past caused by persecution (e.g. hunting, egg collecting), recreation and pollution and have by no means reached their previous population levels.

7.1.2 Migrating and Staging Birds

The Wadden Sea including its tidal flats and salt marshes constitutes one of the worlds most important wetlands for migratory waterbirds (ducks, geese, waders, gulls, terns and others) which breed in northwest Europe, the western and central parts of Siberia, on Iceland, Greenland and the north east part of Canada. The Wadden Sea area is of outstanding international importance as a staging, moulting and wintering area for at least 52 populations of 41 species using the East Atlantic Flyway, e.g. Grey Plover (Pluvialis squatarola), Siberian Knot (Calidris C. canutus), Dunlin (Calidris A. alpina).

High numbers of moulting ducks and geese, in particular Shelduck (Tadorna tadorna) (Figure 7) and Common Eider (Somateria mollissima), are present on the tidal flats every year. For the Shelduck the Wadden Sea is the only moulting area for the North European populations.

Migratory birds, such as some waders, ducks and geese species are also using rural areas on the islands and on the mainland behind the dikes during their stay in the Wadden Sea area. Meadows, pasture land and arable land are utilized by Golden Plover (Pluvialis apricaria), Common Lapwing (Vanellus vanellus) and also Ruff (Philomachus pugnax) and Whimbrel (Numenius phaeopus) as roosting sites mainly in spring and autumn. Wigeon (Anas penelope), Barnacle Goose (Branta leucopsis) and to a lesser extent also Brent Goose (Branta bernicla) as herbivores, are also using meadows and arable land as feeding areas during autumn and spring.

7.2 Seals

With some 11,000 counted individuals in 1996, the common seal population is now larger than before the 1988 epidemic. In the near future the increasing population will demand more suitable habitats for hauling out. This will require a better tuning with other users of the area.

Although hunting has stopped and large areas are protected, the populations are currently far from being unaffected by human activities, such as disturbance, change in physical habitat, interaction through fisheries, pollution and treatment and release of weakened seals. According to recent studies disturbance in any form interferes with survival of the pups and site choice of the adults.

Also the grey seal population is growing. There are now two reproductive colonies in the Wadden Sea with a respective size of 315 (NL maximum in 1996) and 30 to 40 (SH) animals. Grey seals need undisturbed high sands or beaches and salt marshes for whelping and nursing.

8 Evaluation and Conclusions

In the 1993 Quality Status Report concern was amongst others expressed about developments with regard to

• pollution with nutrients and hazardous substances;
• the impact of sea level rise and increase in temperature;
• the impact of fisheries on characteristic Wadden Sea communities.

In this chapter these issues are re-evaluated on the basis of knowledge that has become available in the past four years.

8.1 Nutrients and eutrophication

As possible negative effects of the high nutrients loads entering the Wadden Sea, the 1993 QSR mentioned increase in toxic and nuisance phytoplankton blooms, increase in biomass and abundance of macrozoobenthos, a shift in species diversity and composition, increasing abundance of macroalgae and oxygen deficiency in the sediment surface.

The relationship between nutrient loads and the above listed phenomena has however proven to be even more complex than already thought. The observed reduction in dissolved phosphorus and, in some cases, nitrogen compounds has not caused a reduction in average chlorophyll levels in any part of the Wadden Sea. In the Marsdiep the intensity of blooms is still high whereas the duration of Phaeocystis blooms at Norderney has considerably reduced in the period 1993-1996.

On the basis of an analysis of zoobenthos long-time series it was concluded that weather conditions, especially ice-winters, are probably the most determining factors in zoobenthos development. Macroalgal coverage which had increased since 1989, has reduced again.

The burial of macroalgae has been identified as the main cause for the anoxic spots in the sediment of the east Frisian Wadden Sea (black spots). The large anoxic areas which occurred in the same area the spring of 1996 were most probably the result of a number of factors (high mortality of benthic animals, high amounts of planktonic material, sudden temperature rise), through which the remineralisation capacity of the sediment was temporarily exceeded.

Four possible causes for the continuing high primary production in the Wadden Sea are given, namely increased visibility, delivery of phosphorus from the sediment, import of organic material from the North Sea and nitrogen as the limiting factor.

8.2 Hazardous substances

With regard to hazardous substances the 1993 QSR identified as issues of concern the stagnation in the reduction of contaminant inputs, the fate and distribution of substances which are not routinely monitored, amongst others most pesticides, and the effect of contaminants on the immune system of mammals and fish.

On the basis of the latest data it can now be concluded that riverine inputs of heavy metals, PCBs and HCHs have significantly declined in the period 1985-1995. This has resulted in decreased concentrations of these substances in water, sediment, Blue Mussels and bird eggs in most parts of the Wadden Sea.

Surveys for hazardous substances, not routinely monitored, have made clear that many of these substances occur in the Wadden Sea in relatively high concentrations. It concerns here mainly TBT and pesticides. There is increasing evidence that certain pesticides hamper the grazing ability of zooplankton. Pesticides of the herbicide type interfere with the photosynthesis of phytoplankton. Pesticides may be a factor in the decline in intertidal seagrass.

TBT is a special case because of its very high toxicity for several marine organism, most notably Dogwhelk (Nucella lapillus).

Recent studies have substantiated that hazardous substances may interfere with the immune system of seals and fish but up till now a direct cause-effect relationship could not be established.

8.3 Sea level rise and temperature increase

In the 1993 QSR the acceleration of the increase in mean high water level (MHW) and mean tidal range (MTR) in the last decades was attributed to climatic changes. More recent analyses point to long-term cyclic processes. The increase in intensity and frequency of storm surges as a result of climatic changes, suggested in various publications, could not be substantiated in recent analyses of long time series. Since 1994 a more comprehensive picture has emerged about the possible consequences of sea level rise for the Wadden Sea. Although the Wadden Sea tidal system will be able to cope with a moderate sea level rise, the North Sea coasts of the barrier islands will steepen. If not sufficient sand is delivered from the offshore zone the Wadden Sea tidal flats may disappear.

So far no increase in average temperature of North Sea water has been observed. The increase in macrofaunal biomass off Norderney since the late 1980s is amongst others attributed to a series of mild winters.

8.4 Mussel beds and eelgrass beds

The decreasing trend in the number of mature mussel beds and the area covered by eelgrass, has continued in recent years.

In the Dutch Wadden Sea less than 100 ha intertidal mussel beds have remained. The reference area is some 5000 ha. In Niedersachsen the area of intertidal mussel beds had decreased from 5000 ha in 1975 to some 790 ha in the beginning of 1996. The reduction in biomass was even more dramatic. From 1996-1997 a moderate recovery to 1200-1400 ha was observed. In Schleswig-Holstein the area of intertidal beds is stable since fishery has been restricted to the subtidal areas but many beds are still immature and vulnerable. In Denmark mussel beds, both in areas closed for fisheries and areas with regulated fishery, developed between 1995 and 1996 after a decline between 1991 and 1995.

Storms, ice winters, seed mussel fishery and foraging birds were the main factors in the decline of intertidal mussel beds. The low abundance of mature mussel beds make recovery problematic because settlement occurs mainly on old beds or their remains.

In the 1993 QSR the decline in eelgrass in the Wadden Sea was extensively described. Especially in the Dutch Wadden Sea the decrease has been dramatic. Recently more information has become available about declines in the German Wadden Sea. In Niedersachsen the area covered by eelgrass stands decreased from 35.5 km2 around 1970 to 8.2 km2 in 1993/94. In Schleswig-Holstein a gradual decline occurred from 128.8 km2 in 1980 to 30 km2 in 1996. As possible causes for the decline amongst others eutrophication, reduced visibility, meteorological conditions and herbicides have been mentioned.

8.5 New issues

In the 1993 QSR the habitat types 'Beaches and Dunes', 'Estuaries', 'Offshore Zone' and 'Rural Area' were not addressed. In this Assessment the reduced dynamics of most dune areas, the lack of sufficiently protected beaches as whelping and breeding areas for grey seal, Kentish Plover and Little Tern have been identified as pressing issues.

Furthermore the essential role of the offshore zone in the Wadden Sea sand balance and the biological importance as feeding area for birds and nursing area for fish are underlined.

Only five estuaries have remained in the Wadden Sea area. Three of these have been modified considerably by diking and deepening.