De Levende Natuur nummer 3 van 2012 (English summary)




DLN 2012-3

Primary producers of the Wadden Sea

C.J.M. Philippart, J.C. Kromkamp & P.M.J. Herman

In shallow seas, such as the Wadden Sea, phytoplankton and microphytobenthos are at the base of the food chain and are the source of food for most other marine organisms. Statistical analyses of long-term field observations indicate that changes in nutrient loads were followed by changes in biomass, in species composition, and in the productivity of phytoplankton. The spatially and temporally scattered observations to date on primary production and biomass of higher trophic levels suggest that the carrying capacity of coastal ecosystems, such as the Wadden Sea, is largely under bottom-up control. Despite the eminent role of primary production in setting the upper bound to the carrying capacity of the Wadden Sea, consistent measurements of pelagic primary production are limited to a single station only, whereas data on benthic primary production are virtually lacking. The present development and deployment of new techniques, including automated equipment and satellite images, is expected to quickly enhance our knowledge on variation and underlying causes of primary productivity in time and space.

The development of bivalve shellfish stocks on the tidal flats of the Dutch Wadden Sea

K. Troost, J. Drent, E.O. Folmer & M.R. van Stralen

Bivalve shellfish are of key importance in estuarine and coastal sea ecosystems. Common species in the Wadden Sea are the Cockle (Cerastoderma edule) and Baltic tellin (Macoma balthica) who live hidden in the sediment, and the Mussel (Mytilus edulis) and Pacific oyster (Crassostrea gigas) who form complex reef-like structures on top of the sediment. By building these complex structures, Mussels and Pacific oysters increase the diversity in habitats and thereby increase the biodiversity in the Wadden Sea. Invertebrate animals living on the tidal flats have been monitored since about 40 years, at the Balgzand, Piet Scheveplaat and Groninger Wad tidal flats in respectively the western, central and eastern parts of the Dutch Wadden Sea. For the management of shellfish fisheries, stocks of these bivalves have been monitored annually since 1990, and the contours of mussel and oyster beds are mapped since 1995. We use these time series to show the long-term development in stocks of the common bivalves mentioned above, for the eastern and western parts of the Dutch Wadden Sea.

The long-term development of the cockle stock is mainly determined by a combination of years with a successful recruitment and the occurrence of severe winters that lead to an increased mortality among adult animals. The development of the Baltic tellin stock is highly fluctuating in space and time, but shows an overall decline in recent years. The cause of this decline is still not known but it appears to be related to a warming climate. Mussel- and oysterbeds are still found in exactly the same locations as in 1969 and 1975. The Pacific oyster was introduced in the 1970’s, and became increasingly dominant after 2000. Since 2007/2008 the Pacific oyster stock appears to be stabilized. A remarkable development is the increasing settlement of mussels in Pacific oyster beds. Shellfish fisheries on the tidal flats of the Wadden Sea have been restricted in 2004. Nowadays, these activities consists of hand-raking for Cockles and hand-picking of Pacific oysters. An effect of these activities on the development of shellfish stocks is probably negligible.

Developments of the fish fauna and fisheries in the Dutch Wadden Sea

I. Tulp, P.A. Walker & L. Bolle

The Wadden Sea is a nursery area for many fish species that contribute to the commercial fisheries in the North Sea. Moreover it is a home to residential species and provides feeding habitat and passage to migrants and seasonal visitors. Recent developments show a dome shaped pattern in total fish biomass, notably in the marine juvenile guild  peaking in the 1980s. Also resident species show the same pattern. Patterns are more pronounced in the Western Wadden Sea than in adjoining areas. A comparison between feeding guilds shows that planktivores started to flourish at the time when benthivores decreased.  The nursery function of the area seems drastically altered as compared to the mid 1980s, when fish biomass was booming, but also compared  to the beginning of the monitoring series in 1970. Climate change and fisheries both inside and outside the Wadden Sea are most likely responsible for the observed changes, but knowledge on the exact mechanisms is still limited. This knowledge is needed to underpin any initiative to restore the nursery function of the area. Historically fisheries in the Wadden Sea have been regulated through nature conservation legislation. This has led in the past to conflict and controversy. The current approach is to identify long-term goals and opportunities for sustainability. Plans for the future include a regionally embedded, integrated and adaptive management system.

Invasive species in the Wadden Sea

A. Gittenberger & M. Rensing

In 2009 and 2011 two inventories were done in the Wadden Sea focussing on non-native and cryptogenic species. This was done by searching as many habitats as possible with a large variety of different monitoring methods. As a result the number of these species that is known (in literature) for the Dutch Wadden Sea has increased from 54 to 72 species in three years’ time. We assume that many of these species were already present in the Wadden Sea prior to 2009 but were not noticed before, because no inventory had ever specifically targeted non-native species in the Wadden Sea before. Dikes, shellfish areas and pleasure craft harbours were all intensively searched; by far most of the non-native species were found in the harbours. Within these harbours most species were found on the floating docks, indicating that hull-fouling on pleasure crafts may be the most important import vector of non-native species in the Wadden Sea. The present research indicates that until recently relatively little was known about the number of different non-native species in The Wadden Sea. Now that we have a better idea, we may have better opportunities to prevent new species from being introduced. Knowing how these non-native species have spread and impacted other areas where they were introduced, this may also enable us to predict in more detail what effect exotic species in general will have at present or in the near future on the Wadden Sea ecosystem.

Waddensleutels: Mussel bed and other ecosystem engineers as the foundation for a healthy Wadden Sea?

T. van der Heide, E.J. Weerman & H. Olff

Project Waddensleutels (‘Wadden Keys’) investigates the importance of ecosystem engineers for ecosystem functioning in the Wadden Sea. The project is characterized by two main research components that will eventually converge in a document that describes possibilities and methodologies for optimizing ecological restoration. One part examines the current the importance of ecosystem engineers on the biodiversity and food web structure in the Wadden Sea and develops methodologies for monitoring changes in the food web. The second component is a large-scale experiment that investigates limiting factors for settlement and survival of intertidal mussel beds – one of the most important ecosystem engineers in the Wadden Sea. Here, we present a preliminary analysis of the first-year outcomes from this experiment. Results indicate that mussel bed recovery is affected by several factors. Hydrodynamics caused severe losses of adult mussels, while effects of bird predation seemed of less importance. Predation exclosures demonstrated that spatfall and survival of young mussels is most likely hampered by a lack of stable substrate (e.g., adult cockle and mussel beds) and strong predation by shrimp and crab. Future research in this project will further investigate the factors responsible for mussel bed persistence and examine the importance of ecosystem engineers in the Wadden Sea food web.

Seagrasses in the Wadden Sea

M.M. van Katwijk

Two species of seagrass occur in the Wadden Sea: (1) Zostera marina (two morphotypes: (1a) flexible, annual and intertidal and (1b) robust, perennial and subtidal, since the 1930s extinct) and (2) Zostera noltii (perennial, flexible, intertidal). In the Dutch Wadden Sea restoration of the intertidal seagrasses (1a and 2) has been successful, but not permanent. A larger scale of transplantation, via seed, started in September 2011. Restoration of the subtidal seagrass beds (type 2b) may be feasible at only a few, carefully selected sites (limited salinity fluctuations, sheltered location), but only with additional measures to reduce turbidity until the transplant is large enough to clear the water itself. Seagrass transplantation successes are not guaranteed because nutrient loads may still be too high, in spite of recent reductions in the Dutch Wadden Sea.

The MOSSELWAD program

J.M. Jansen, N. Dankers & B.J. Ens

The MOSSELWAD program studies the faith of littoral and sublittoral mussel beds (Mytilus edulis) in the Wadden Sea. With field surveys, -experiments and lab-work, researchers from EUCC, NIOZ, UU and IMARES combine forces to unravel which factors determine the development and survival of mussel beds. Key factors under investigation are hydrodynamics, various forms of predation and mussel bed quality parameters. To achieve its ambitious goals, the team is strengthened by 5 PhD students and advanced equipment, such as camera poles on intertidal mussel beds and a 3D laser scanner. The project aims to produce knowledge that can be used in mussel bed restoration and will try to develop a littoral mussel bed by the end of the program.

Transition of the Dutch mussel sector

J.M. Jansen, M.R. van Stralen, P. Kamermans & H. Sas

Mussel spat harvesting by bottom trawling in the Dutch Wadden Sea has been subjected to a long history of controversy. In 2008, this culminated in a ban by the highest Dutch court on a large part of the bottom harvest. Negotiations between government, nature conservationists and the mussel sector led to an agreement on the gradual abolishment of bottom trawling, to be replaced by mussel seed collectors, suspended in the water column. The agreement was reached in 2009. Since then, space for mussel collectors was gradually extended and harvests increased. Concurrently, several mussel seed beds were closed to fishery. Monitoring has shown that predation has led to a strong decrease in mussel coverage in one of the beds, whereas the other is still going strong. Biodiversity is low, but that can be due to the young age of these mussel beds. A factor which causes great concern is the failure of spat fall in the last two years, causing a sharp decline in the seed stocks and consequently in the income of the fishermen. This, in turn threatens the required extension of the seed collector area, since the sector needs this income in order to do the necessary investments.

Salt marshes: applied long-term monitoring

K.S. Dijkema & W.E. van Duin

50-year monitoring of mainland salt marshes (levelling and vegetation recording) taught us how to restore pioneer marsh zones to a successful defence zone against erosion for the benefit of both the salt marsh and the coast. Due to succession and decrease in grazing the area of climax-vegetation with Elytrigia atherica has increased.

Climate change, tidal flooding risks and breeding shorebirds

M. van de Pol, B.J. Ens, J.P. Bakker & P. Esselink

Bird populations are declining in the Wadden Sea. The contribution of climate change has been understudied, but recent evidence suggests that increased flooding frequencies during the breeding season are already affecting bird populations that breed on salt marshes. These effects are expected to get worse in the near future if birds do not adapt their nest-site selection, suggesting that the importance of low-lying breeding areas for shorebirds could decline substantially in the coming decades. The crucial question now is to what extent birds may adapt their habitat selection to the changing environment. Although several studies suggest that the adaptive potential of birds to more frequent flooding events is limited, this is based on anecdotal evidence and systematic long-term studies are now needed. Furthermore, such studies will inform us about the factors that constrain birds from selecting higher nest sites (vegetation, food, predators), such that efficient management strategies may be identified.

The impact of subsidence by gas production on the Wadden Sea region

G.J.M. Wintermans

In the Dutch Wadden Sea region gas is produced from sandstone layers at about 2.5 km depth. The compaction of the underground causes subsidence in the Wadden Sea itself, the North Sea coastal zone and nature areas on the island of Ameland (ca 33 cm) and the mainland (ca 10 cm). The impact of subsidence on the ecology of the nature areas involved is monitored by a number of research institutes over a period of 23 years. The monitoring addresses both abiotic and biotic parameters.

Within the tidal basins of the Wadden Sea subsidence by gas production cannot be measured as a lowered elevation. Natural sediment dynamics appear to dominate. Measurable effects were observed in dune slacks and salt marshes, where sedimentation rates are lower than the annual subsidence.
The most relevant impacts are an increase in the frequency and duration of flooding by sea water (salt marshes and dune valleys) and a rise in the ground water table and longer period of immersion by fresh water(dune slacks). Due to these impacts changes have been taken place in the vegetation of salt marshes and low dune valleys. The vegetations of the middle and high salt marsh locally show retarded succession and in a small area regression. The vegetation of low dune valleys reflects impacts of longer periods of inundation and, in some years, of siltation.

Explorative modeling of impacts of subsidence on benthic life and birds show potential impacts if subsidence is not compensated by sedimentation on the tidal flats and in breeding areas on salt marshes. In 2013 the results of more elaborated calculations with these models will be presented and discussed.

Focus on the Ems-estuary ecosystem

M.J. Firet, D. Bos, R. Postma & L.A. van Duren

The ecosystem of the Ems-estuary is out of balance.  The core problems are caused by the strongly artificial morphology of the estuary, which developed under human influence over long temporal and large spatial scale. These changes in morphology acted upon the water movements and the sediment transport and have resulted in a ‘regime shift’. The current ecological situation differs strongly from the natural situation. Especially the high levels of turbidity, the regular oxygen deficiencies in parts of the tidal river and the limited quantity and quality of estuarine habitats provide reason to act.
In order to accomplish ecological improvement, and go to a desired target state, the potential measures need to be of sufficient scale and impact. Options for ecological improvement thus almost necessarily affect the system properties. Most options have consequences for the dimensions of the system. It needs to be considered what can be changed to the boundaries of the system, basically the length, the width and the depth. Those options that move the ecosystem in the direction towards a more natural reference state are preferred above others. The ‘Inspiration-map’ shows which measures could be taken where.
Entrepreneurs, NGO’s and governmental organisations are working together to achieve integral spatial plans and agreements on measurements that combine economical and ecological development. How far the development will come depends strongly on the degree to which the stakeholders will look for integral solutions. 

The Banc d’Arguin in Mauritania as a source of knowledge and inspiration for managers of an ecologically richer Wadden Sea

O. Overdijk & T. Piersma

In this contribution we introduce the Banc d’Arguin, a worldwide unique coastal intertidal system with ca. 500 km² of intertidal flats covered to a large extent with seagrasses (especially  Zostera noltii) and gracefully managed by Mauritania as one of its two national parks, in the context of a comparison with the Wadden Sea, especially the Dutch part. The Banc d’Arguin is well known as wintering ground for many shorebirds, Spoonbills and other waterbirds that pass through the Wadden Sea using the East Atlantic Flyway. Although lying in the subtropics, it shares many biota other than migrating birds with the Wadden Sea. Its distinguishing feature is the fact that so many positive ecological interactions are at play of the kind that, through human-intervention such as dredging, are impossible in the Wadden Sea nowadays. Such positive interactions may be reciprocal, such as the seagrasses harbouring the shellfish resources on which Red Knots (Calidris canutus) depend, and the Red Knots facilitating or even ensuring the persistence of seagrasses through specific predation processes. The Banc d’Arguin makes clear that for biodiversity to again beget biodiversity, the managers of the Wadden Sea must ensure the boundary conditions for such enriching webs of interaction to take hold. This is complicated, as it necessitates a lighter human hand.

The programme ‘Towards a rich Wadden Sea’

H.J.W. Sas & T. van den Heiligenberg

The programme ‘Towards a rich Wadden Sea’ was created in 2010. Its origins were the conflict about mussel fishery and the demand by nature conservationists and regional parties for a more coordinated approach towards nature restoration in the Dutch Wadden Sea. Since its start, the Programme has set up a wide variety of activities. First and foremost, is the transformation of the seed mussel fishery, form bottom trawling towards application of seed collectors in the water column. Elsewhere in this magazine, a separate article is devoted to this transition process. Besides, we have mediated between the cockle fishery, government and nature conservationists in order to reach a long-term agreement; this was concluded in May 2011. Also we initiate a variety of other projects, such as mitigation of the effects of the Afsluitdijk, creation of saltwater marshlands, diminishing the effects of dredging, flyway management of migratory water birds and a trilateral policy to prevent the introduction of invasive species into the Wadden Sea.