FROM POLDER TO SALTMARSHSALTMARSH RESTORATION IN NOARDERLEECH (WADDENSEA)
AN EXPERIMENT
FROM POLDER TO SALTMARSHSALTMARSH RESTORATION IN NOARDERLEECH (WADDENSEA)
AN EXPERIMENT
FROM POLDER TO SALTMARSHSALTMARSH RESTORATION IN NOARDERLEECH (WADDEN SEA).
AN EXPERIMENT.
Published by:
Olterterp 2007
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1 Summary................................................................................................................................3
2 Struggle and Rise ...................................................................................................................5
3 Historical Background.............................................................................................................6
4 Reclaim and Retreat................................................................................................................8
5 Saltmarsh Dynamics..............................................................................................................10
6 Saltmarsh Restoration Elsewhere ..........................................................................................13
7 Management of Noarderleech ..............................................................................................14
8 The Intervention ...................................................................................................................15
9 Research...............................................................................................................................17
10 Monitoring ...........................................................................................................................18
11 Tidal Flooding and Salinisation .............................................................................................19
12 Vertical Accretion .................................................................................................................21
13 Vegetation............................................................................................................................22
14 Birdlife..................................................................................................................................25
15 Evaluation and Perspectives ..................................................................................................27
16 Sources.................................................................................................................................28
CONTENTS
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Human habitation along the edges of the WaddenSea dates back to long before Roman times. Duringthe Middle Ages, the inhabitants of this regionbegan to reclaim land from the Wadden Sea. Fromthe seventeenth century onwards ditch systemswere developed to stimulate accretion on the saltmarshes, and this essentially continued as a‘farmer’s method’ of land claim well into the twen-tieth century. By embanking the elevated saltmarsh-es, between 1897 and 1939 summer polders withan area of 1100 ha were created in Noarderleech.Land reclamation was undertaken in an ever moreprofessional manner and, eventually, responsibilitywas assumed by the national Directorate-General ofPublic Works and Water Management, theRijkswaterstaat. During the nineteen seventies andeighties, people’s ideas and attitudes changed asland reclamation became increasingly expensive andthe natural values of unembanked saltmarsheswere appreciated more and more; so much so, thatnew large-scale targets were formulated to restoresummer polders which had been reclaimed fromthe sea back into saltmarshes, and to open themup to tidal influence again. Noard-FryslânBûtendyks (North Friesland ‘outside the dikes’)would then become the largest saltmarsh area inEurope. Processes such as accretion and erosion inrelation to the sediment balance of the whole ofthe Wadden Sea are very complex and rarely ob-vious. Because the consequences of saltmarshrestoration cannot be predicted with any certainty,the nature conservation society It Fryske Gea beganan experimental saltmarsh project in 2001 in anarea measuring 135 ha. Researchers carefully moni-tored the results until the end of 2005 to learnabout future de-embankments, adding to the bodyof knowledge about saltmarsh restoration alreadygained in various countries around the world. ItFryske Gea would like to see rich and varied salt-marsh vegetation being created in Noard-FryslânBûtendyks with good foraging opportunities forgeese; to achieve this a semi-natural grazed salt-marsh would be the most appropriate land use. In2001, the summer dike was breached in threeplaces after a very natural-looking but artificialwinding creek system had been dug to stimulatethe supply of seawater and sediment. Through
these breaches, the area will be flooded fourteentimes a year on average during storm tides. A monitoring programme was set up to study thesalinisation of the groundwater and soil and thevertical accretion in grazed and ungrazed condi-tions, in the abiotic environment. Monitoring of thebiotic environment involved studying the vegetationdevelopment and assessing the numbers of winter-ing and staging geese and breeding birds in relationto trends in the wider environment.
Against all expectations, salinisation of the ground-water, soil and soil moisture proved to evolve onlygradually. After four years the levels of salinity werestill below those seen in the adjacent natural salt-marshes. There was no massive die-off of non-salttolerant vegetation as a result of ‘salt shock’, andno salinisation was established in the adjacent summer polder. Vertical accretion varied from 0.3 –36.7 mm per year. It was highest in the lower-lyingparts of the trial area and higher in ungrazed areasthan in grazed areas, because taller ungrazed vegetation traps more sediment. Vertical accretionin Noarderleech is more than adequate to keeppace with land subsidence and the rise in sea level.
Over time the vegetation gradually changed intothat which is typical of a saltmarsh with many salt-tolerant species, resulting in a clear increase of thespecies targeted by the restoration project. Grazinghas proved to be a very important factor in attain-ing the target scenario. In ungrazed areas (exclo-sures) the plant cover developed towards species-poor Couch-grass vegetation. The availability offood plants for Brent and Barnacle Geese still lagsbehind expectations and was lower than it was inthe adjacent summer polders and saltmarshes. Thefood situation will probably recover as the salt-marsh vegetation continues to develop. No changeswere observed in the numbers of breeding birds inthe trial area.
In summary, the experimental saltmarsh restorationin Noarderleech can already be considered a success, and the prospects for continued saltmarshrestoration in Noarderleech and the rest of Noard-Fryslân Bûtendyks are good.
SUMMARY1
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Traditionally, the Dutch have had to fight the sea tosurvive and they are known for this characteristic allover the world. The motto of the province ofZeeland, Luctor et emergo – I struggle and rise –could well be their national motto; it is a source ofnational pride. The Dutch have been exporting theirknowledge of drainage and land reclamation forcenturies and Dutchmen are involved in drainageand embankment projects across the globe. Butnow, out of the blue, they are breaching dikes inseveral places in their own country to allow the seainto land that was reclaimed from it earlier at greateffort and cost. How on earth can this be? It is nowonder that many people, especially in theNetherlands, find it hard to understand; and nowonder that, deep in their hearts, many people arehostile to it, especially those who live in placeswhere it is happening, where families have strivenfor generations to claim and secure farming land,and where older people have sharp memories ofcatastrophic dike breaches and floods. We mightjust have a thing or two to explain to them.
The world around us is changing, but it’s not justthat: we ourselves are changing as well and ourviews on nature, agriculture and water manage-ment have changed. In the Netherlands until half acentury ago, nature was something that had to beovercome. Woods and heaths were described as‘waste lands’, just lying there waiting to be culti-vated. Moorland had to be excavated for peatextraction, stinking bogs drained and the sea keptin check. Mudflats were described as cheerless greyexpanses where the only sounds heard were thedismal cries of the occasional shorebird, so …reclaim, impolder!
Towards the end of the twentieth century itbecame clear that nature, agriculture and watermanagement could not be treated separately any-more. Nature was becoming scarce, was soughtafter and therefore precious; agriculture was nolonger profitable in many places because of therising cost of labour; and water management cameinto everything. If everybody continued to draintheir land without limit – abroad as well, in theupper reaches of the main Dutch rivers – theNetherlands would soon be confronted with insoluble problems of excess water. For the firsttime, water management and water storage were
thought about in relation to agriculture, nature andother uses, i.e., in terms of what we call integratedwater management. Interestingly, and perhaps typi-cally, the Dutch have managed to turn this into aglobal export product too, as other countriesincluding the USA and China also have enormouswater management problems; the Dutch ‘polder-men’ have again taken the lead in finding solutionsas they continue to struggle and rise. It is in keep-ing with this change in attitude to breach dikes andexpose agricultural land to the influence of rivers,estuaries and inland seas again, replacing poldersby saltmarshes in a managed way, which is oftenreferred to on the European continent as de-embankment (or depoldering, ontpolderen inDutch).
Saltmarshes in the Dutch part of the Wadden Seacan be found on the mainland side of the WaddenSea islands and along the mainland coasts ofFriesland and Groningen. The largest continuousarea of saltmarsh in the whole of the Wadden Sealies in the Noard-Fryslân Bûtendyks area. It FryskeGea has obtained extensive tracts of this area overthe past few years for the purpose of improvingtheir natural values. To achieve this, the plan is tobreach summer dikes in certain places on a largescale to allow the Wadden Sea back into summerpolders that were previously embanked. An experimental summer polder was opened in 2001by breaching a dike in three places. Researchershave monitored the consequences for five yearsand their scientific findings have been set out in thereport Proefverkweldering Noard-Fryslân Bûtendyks(Van Duin et al. 2007). The most important conclu-sions from this report will be presented in thisresearch article.
STRUGGLE AND RISE2
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Human habitation of the Wadden Sea coast goesback to about the sixth century BC, when small settlements were founded on the higher leveesalong the many intertidal creeks in the coastalregion of present-day Friesland and Groningen. Thesea level was falling at that time, and so humanoccupation of the vast, expanding saltmarshes continued in a seawards direction. When the seabegan to rise again in about the third century BC,the saltmarsh inhabitants found themselves underthreat and responded by constructing mounds (terpen). Over the following centuries the sea ate itsway deeper and deeper into the land, in particular,swallowing up sites where peat soils were farmedand where early drainage systems had been laidout, because these activities depressed the soil andmade it easier for the sea to enter. Deep baysresulted such as the Zuiderzee, the Middelzee, theLauwerszee and the Dollard.
The oldest summer dikes date from Roman times,but it was during the Middle Ages, when the threatof floodings became greater, that people began toconstruct simple dikes around the mounds on alarger scale to protect their farming land. An un-intended side effect was that new saltmarshes wereformed, because accretion took place on the sea-ward side of the new dikes. From the tenth centuryonwards longer dikes were built to protect the hinterland. By the thirteenth century, when the seahad stopped rising, all the mainland of theNorthern Netherlands along the Wadden Sea coasthad been diked with dikes following the contours
of the inlets and inland seas. Subsequently, theseinlets were also reclaimed, bit by bit, by embankingthe saltmarshes. In Friesland, the main land recla-mations in the Middelzee were carried out betweenabout 1200 and 1500 AD. In the seventeenth century and the first half of the eighteenth century,some narrow polders were added which borderonto the present Noard-Fryslân Bûtendyks areawithin the seawall.
In the seventeenth century, farmers on the Frisiancoast began to stimulate accretion of the salt-marshes by digging ditches. It is not known exactlyhow these ditch systems developed, but it seemslikely that the farmers discovered, first, that ditchesin the vegetated part of the saltmarshes stimulatedaccretion between the vegetation and, then, thatextending the ditches to the bare mud approxi-mately 100 metres beyond the vegetation linefostered the accretion of the saltmarshes. In thenineteenth century in particular, elaborate systemsof ditches running parallel to the coast and con-nected to shore-normal drainage channels evolved.The ditches had to be cleared every year. Thismethod of claiming land, known as the ‘farmers’method’, continued well into the nineteen thirties.
The first summer polder in the Noarderleech areawas made in 1897 outside the seawall by the con-struction of a summer dike. A summer polder onlyaffords protection against floodings during the rela-tively calm summer season. During autumn andwinter storms the sea may occasionally spill over
the low summer dike so that the summerpolder is flooded and a new layer of sediment isdeposited. The grass sward is not harmed bythese incidental winter inundations when thepolder is empty of livestock. The construction ofsummer dikes continued until the year 1939, bywhich time the total surface area of all thesummer polders amounted to 1100 ha.
During the first decades of the twentieth centu-ry, land claim by farmers tailed off because ofsocio-economic circumstances. Cheap labourwas scarce and the prospects of making short-term profit from claiming land were not good.In addition, there were many legal disputesabout the rights to the new land. Instead ofaccreting, in some places saltmarshes even
HISTORICAL BACKGROUND3
eroded, threatening the integrity of the seawalls.Then came the great economic recession of thethirties when, making a virtue of necessity, the government took land reclamation into its ownhands and set large numbers of unemployed people to work in the heavy clay of the intertidalmudflats. A new method of claiming land wascopied from Germany, the so-called Schleswig-Holstein method. Employing this method, sedimen-tation fields are constructed which are enclosed bylight brushwood groynes consisting of double rowsof stakes driven into the clay which are in-filledwith brushwood fences. The top of the brushwoodgroynes usually stand approximately 30 cm abovethe mean high tide level. The advantage of usingbrushwood groynes is that seawater can flowthrough them freely so that no great differences ofstress are caused on either side, whilst wave energyis dissipated which accelerates the settlement of siltand clay particles. The sedimentation fields used inGermany measured 200 x 400 or 200 x 200 m, butin the Netherlands larger fields of 400 x400 m were made which extendedbeyond the vegetation line into the baremud. Usually, two or three rows of fieldswere constructed behind each other. Inthe groynes running parallel to the coast,openings for water drainage were madeat 200 m intervals.
The work in the heavy clay, often insevere weather conditions, was extreme-ly hard and, in retrospect, there is gener-al agreement that it was actually toohard for the less physically developedunemployed men from the big cities,who were in fact exploited.
During the Second World Warthe land reclamation workscame to a halt and much of therecently gained land was lostthrough erosion. The work wasresumed in about 1946.Gradually, after 1950, most ofthe excavation work becamemechanised. In the late sixties,the maintenance of the ditchesbecame disproportionatelyexpensive because of economicgrowth and rising wages, and in1968 digging ditches was dis-continued in the seaward facingrows of sedimentation fields. Inthe vegetated fields ditch main-tenance was continued every
two to six years instead of annually until the nine-teen nineties. In 1980 a new target for the landreclamation works was added, namely, the protec-tion and restoration of nature conservation values,and the term ‘land reclamation’ was replaced byterms like saltmarsh restoration or conversion, ecological restoration, etc.. Seaward expansion ofthe saltmarsh area is no longer the objective; theprimary goal now is maintenance and preservation.
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The changed purpose of the saltmarshes and sum-mer polders in Noard-Fryslân Bûtendyks did notcome about without a struggle; the nineteen seven-ties and early eighties, in particular, were turbulent.As prescribed in the Delta Act, which was passed in1958 following the 1953 flood disaster in Zeeland,all sea defences in the Netherlands including theembankments of all the major rivers had to beraised to a certain minimum safe height (the ‘deltaheight’) to prevent such disasters from recurring. Inthe early seventies it was the Frisian Wadden Seacoast’s turn to be protected in this way, and a decision had to be made about how the Act shouldbe implemented. The simplest solution was to justraise the existing seawall but, as the opportunitywas there, a good alternative would be to constructa new dike surrounding the accreted saltmarsh areaand thus reclaim a sizeable area of intertidal mud-flats where seed potatoes could be grown. Four different designs were made, each with their ownsmall band of supporters and opponents. Plan Aconsisted of raising the existing seawall, Plan Bincluded the summer polders and a narrow strip ofthe vegetated saltmarsh outside them, Plan Cincluded all of the vegetated saltmarsh, and Plan Denvisaged a new dike being constructed all aroundthe land reclamation plots, which would constitutean impolderment of some 4000 ha of saltmarsh.
In practice, Plans B and C did not receive muchattention and debate focused on the pros and consof Plans A and D. The supporters of Plan D regarded the occasion as a unique opportunity forconcluding the land reclamation projects in style,and pointed out the advantages in terms of agricul-tural use, employment and other regional economicbenefits. The advocates of Plan A emphasised theinternational importance of the area as a naturearea within the Wadden Sea and questioned theagricultural significance and the increase in employ-ment which was expected by their adversaries. Thebattle between them flared up at times and almostfifteen years of demonstrations and protest meet-ings followed. Stacks of thick reports were writtenwhich together would fill many desk drawers, andheated discussions took place between the localinhabitants and nature conservationists, but even-tually feelings calmed down and people could seeeye to eye again. At first it seemed as if all the parties would agree to Plan B, a compromise, but inthe end the tenacity of the Wadden Sea Society(Vereniging tot Behoud van de Waddenzee) wonout, and in 1986 the Council of State declared that:“There are no compelling reasons that would justifythe impoldering of this nature area.” So, Plan A itwas: not potatoes but avocets.
RECLAIM AND RETREAT4
The decision by the Councilof State may seem surprisingand partial, but it does notstand on its own. InDecember of 1979 the government had published itsKey Planning Decision on theWadden Sea (PlanologischeKernbeslissing Waddenzee, orPKB-Waddenzee), which wasadopted by the Lower Houseof Parliament in January of1980 (and has been revisedseveral times since, lastly in2006), and which states as itsprimary goal: ‘the sustainableprotection and developmentof the Wadden Sea as anature conservation area’. InNoard-Fryslân Bûtendyks theboundary runs along the toeof the seawall so that thewhole area outside the seawall including the sum-mer polders falls within the PKB area. This decisionmade by Parliament had, in fact, already virtuallyprecluded the adoption of Plan D. The Zeitgeist wasnot with the farmers.
During the implementation of the PKB, throughoutthe nineteen eighties, the protected status of theWadden Sea was elaborated in greater detail,adopted and placed within a legal framework atthe national, provincial and municipal levels.
The protection of the Wadden Sea also has conse-quences internationally. The Netherlands have madea proposal that the Wadden Sea, including thesummer polders and saltmarshes, be designated aswetlands of international importance under theRamsar Convention; countries who are signatoriesto this Convention have a duty to protect wetlandswith this designation. The area has also been designated an EC Birds Directive area and an ECHabitats Directive area, and the EU imposes a dutyon its member states to protect areas that meetcertain criteria in accordance with these Directives.The ministers of the three Wadden Sea countriesinvolved (the Netherlands, Germany and Denmark)have also made agreements about the conservationof the Wadden Sea, the central concern being thatNoard-Fryslân Bûtendyks is the largest continuoussaltmarsh area in the whole Wadden Sea.
Large parts of the area have, in the meantime,been bought by It Fryske Gea with subsidies fromthe European Community, the Dutch governmentand the province of Friesland, to be managed by it
as a nature conservation area. It Fryske Gea nowoversees approximately 3500 ha of the 4000 ha ofNoard-Fryslân Bûtendyks (saltmarshes plus summerpolders), almost 90% of it. Converting the summerpolders into saltmarshes is not automatic, and thelocal people quite naturally have shown resistanceto it. For this reason, It Fryske Gea decided to firstcarry out an experiment by converting an area of135 ha in size into a saltmarsh and to carefullymonitor the effects over a number of years.
This research was carried out with financial contri-butions from the EU (the LIFE-Nature programme),the Ministry of Agriculture, Nature and FoodQuality, the Directorate-General of Public Worksand Water Management (Rijkswaterstaat), theprovince of Friesland, the Prince Bernhard CultureFund, the University of Groningen, and It FryskeGea.
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As noted above, whatever is done in relation towater affects water management somewhere else,in the form of an excess or deficiency of water. Tomanage water sensibly all the various aspects(agriculture, nature, drinking water, safety, recreation, et cetera) must be considered in relationto each other, in other words, water managementmust involve integrated practise. The same appliesto sand and silt. The Wadden Sea is a ‘sand-sharing’ system in which sand is transported by thewind, tides and currents, but in the end alwaysretains more or less the same spatial distribution. A local excess of sand will eventually be distributedacross the whole Wadden Sea, and the sameapplies to any local deficiencies. Thus, land subsi-dence (or sea-level rise) should lead to a chronicdeficiency of sand resulting in receding coastlines.As we don’t want the Wadden islands to shrink,what is done these days is to suck sand up fromthe North Sea to reinforce them (beach and fore-shore renourishment). The law of the sand-sharingsystem then ensures that the excess sand from theWadden islands is distributed across the wholeWadden Sea area during autumn storms.
Actual reality is, of course, more complex. Thesand-sharing Wadden Sea is not a closed system.Along the whole of the Dutch North Sea coast, thedominant wind directions and sea currents causenet sand transport from the south-west to thenorth-east. Thus, sand enters the Wadden Sea fromthe west through the Marsdiep and other tidalinlets, and on the eastern side sand is carried on tothe German and Danish parts of the Wadden Sea.If the supply and loss of sand balance each otherout, the sand balance in the Wadden Sea is main-tained. Unfortunately, the balance has been disturbed by the construction of Maasvlakte 2, aport and industrial zone near Rotterdam, which hasresulted in an enormous area of sedimentation,called the Voordelta, which is now a beautifulnature area, but which has also caused a deficiencyof sand supply in the Northern Netherlands.
That is not all. Following the natural sand flow, theWadden islands ought to migrate towards the eastand south, because they are subject to constanterosion on the exposed north-west side and accreteon the sheltered south-east side. On the island ofSchiermonnikoog, for example, the village ofOosterburen (‘oost’ means east) now lies on the
western side of the island, whilst Westerburen hasended up on the bottom of the sea halfwaybetween Schiermonnikoog and the island ofAmeland. In the eastern part of the Dutch WaddenSea, Rottumeroog will in time fall over the edge ofthe deep channel which leads to the Dollard. Onthe western side, the Haaks shoals between themainland city of Den Helder and the island of Texelare growing into a new Wadden island, althoughthis is being obstructed because Texel is no longermigrating. Action has been taken to prevent Texelfrom being displaced, in the same way as the otherislands are being kept in place, so that villages likeWesterburen will disappear into the sea no more.
On the eastern sides of the Wadden islands,Rijkswaterstaat has constructed long sand-driftdikes to reinforce the coastal defence function ofthe islands. On the sheltered Wadden Sea side,beautiful ‘natural’ saltmarshes have formed, such asthe Oosterkwelder on Schiermonnikoog and theBoschplaat on Terschelling. According to the sand-sharing law this should result in less accretion onthe other side of the Wadden Sea, that is, on thecoast of Friesland and Groningen.
By their nature saltmarshes are never stable. Theyare subject to constant change, growing throughvertical accretion if sediment is withdrawn from thesand-sharing system, and shrinking when sedimentis returned to the system through erosion. The relationship between accretion and erosiondepends on the balance between the supply anddischarge of sediment in the Wadden Sea as awhole, on phenomena such as sea-level rise andland subsidence, and on structural changes inweather patterns. At times the wind blows harderand more often from the north-west for a numberof years, resulting in higher mean high water levelsfrom damming up.
A natural saltmarsh forms if a mud or sand flat risesthrough vertical accretion until it lies just below themean high water level so that it is submerged twicea day for several hours and is dry the rest of thetime. If this is the case the flat will be colonised byprimary vegetation – the pioneer plants CommonCord-grass and Glasswort. The zone where theseplants grow is called the pioneer zone. When thetide rises, the plants standing in the water impedethe flow of water and augment the sedimentation
SALTMARSH DYNAMICS5
of silt particles. Next, Common saltmarsh grass mayestablish itself which promotes the trapping of sediment even further. This second zone, which liesjust above the mean high water level and is notsubmerged at high water every time, is called a lowsaltmarsh. As the saltmarsh grows and becomeshigher, it is flooded less and less often, and newplant species establish themselves until a middlemarsh is formed which is flooded only occasionallyduring storm tides.
Figure 1. Section of a saltmarsh showing vegetation zones in relation to flooding
duration and frequency.
In theory, one would expect the vertical accretionrate to decrease as the saltmarsh gains elevation,because the frequency and duration of inundationsdecrease, but in reality things are more complex.Because vegetation traps sediment much betterthan bare mud does, accretion is not greatest in thelowest zone but a little higher up where there ismore grass. As a result, the saltmarsh’s slopetowards the sea becomes steeper; which is not aproblem until the saltmarsh becomes so steep thatwave action cuts a cliff in it. If this happens, under-cutting will cause the steep saltmarsh edge toretreat more and more inland. The saltmarsh iseaten up by the sea and in time may disappear.Sometimes this process stops halfway and a newsaltmarsh forms in the lower zone on the seawardside of the cliff edge.
The sedimentation pattern is also subject to seasonal effects. All the sediment which collectsduring the calm summer months within and justoutside the pioneer zone may be stirred up duringautumn and winter storms and be deposited on thesaltmarsh. A natural saltmarsh is characterised by a highlybranched creek system, which the seawater flowsback through into the sea after high tide. During
high tide, seawater enters these creeks so that thewater spills over the banks, depositing coarser sedi-ments just outside the banks and more finelygrained sediments farther away. Thus levees arecreated, just as they are along rivers. Levees originate at two different levels of scale. Apart fromthe small-scale levees which form along the creeks,storm surges may form broad levees consisting ofcoarser sediment along the full length of the saltmarsh parallel to the outer coastline.
The Frisian saltmarshes,which have resultedfrom land reclamationprojects, are less compli-cated. Instead of havingnatural creeks, they aredrained by means ofditches during thefalling tide. When theditches silt up, they arecleared and the mud isdispersed evenly overthe plots. Differences inelevation caused by levees are thereforeabsent.
Finally, the role of animals must be mentioned inconnection with the vertical accretion process.Above the mean high water level, plants assist sedimentation and below this line molluscs lend ahand. They filter enormous amounts of seawater asthey hunt for the digestible organic particles whichthey feed on. Most suspended particles which theyingest, however, are indigestible clay or silt particleswhich they need to eject again and which they doin the shape of pseudofaeces: tiny mud excretionspackaged in a slimy mucus layer. They are not realfaeces because they have not passed through adigestive tract, hence the name pseudofaeces.These tiny mud packages settle more easily thanindividual silt or clay particles do and if they arestirred up from the bottom of the sea by a stormthey behave more like coarse sediment, which settles more rapidly.
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The experimental saltmarsh restoration undertakenby It Fryske Gea is not the first instance of salt-marsh restoration and does not stand on its own.Saltmarsh restoration is also receiving attentionelsewhere in the Netherlands as well as abroad,where it may also be referred to as managedrealignment or managed retreat, often with anemphasis on coastal defence, which is not the casein the Dutch situation. The Dutch may have been atthe forefront internationally for centuries in the artof land reclamation and, as noted above, are theleaders with regard to integrated water manage-ment, but they have some catching up to do vis-à-vis countries such as Great Britain and theUnited States of America in terms of the practicalimplementation of saltmarsh restoration.
In England, for example, managed realignment hasbeen implemented on the Essex coast as an economic and sustainable solution for coastaldefence against erosion and the risk of flooding inthe event of sea-level rise. In many cases, the inter-tidal habitats in question originate from accidentaldike breaches that were not repaired. In these areasthe level is raised by vertical accretion, which provides better protection of the hinterland againststorm tides. Another aspect which is importantnowadays is compensation of lost habitats. An essential difference between the English andDutch situations in general is that in England thecoastal strips which are protected by the dikes arevery narrow: only 175 metres on average! Thismeans that, because of the cost of labour, main-taining the dikes far exceeds any revenue from thenarrow strip of arable land protected by it. In theNetherlands, the area protected by dikes againstflooding is usually a lot larger and so, generallyspeaking, dikes are far more precious. In Englandsaltmarshes are being developed because they provide good resistance to wave action, they keepup with sea-level rise, require little maintenanceand are widely accepted as green nature areas.
In the USA many saltmarsh conversion projects arecarried out within the framework of the MitigationAct. Under this Act, preventing damage to wetlands is a requisite precondition for gettingplanning permission or, if this is not possible, anydamage caused must be compensated. Many saltmarsh conversions, such as those in SanFrancisco Bay, have resulted from such compensa-tion schemes. In the past, only the surface area ofthe re-flooded area was taken into account in relation to the disruptive intervention but,
increasingly, measurable nature conservation out-comes are set as a requirement which are based onspecific losses in the affected areas. In conse-quence, there is a tendency to restructure the losthabitat exactly as it was by copying the relief usingmachinery, by planting the desired plant species,often applying intensive fertilisation, and by intro-ducing animals. But on the whole, there is littleunderstanding of the dynamics and the naturalprocesses in these areas.
In the Netherlands and Belgium, the de-embank-ment of polders, or ‘depoldering’, is a highly sensi-tive subject, especially in the south-westernprovince of Zeeland where the disastrous flood of1953 is a living memory. The problem centres onthe Western Scheldt, because the EU has attachedcertain conditions to the intended deepening of theshipping channel to Antwerp; compensation measures must now be taken if any natural areasare affected, and returning polders to the sea isseen as the main tool. The EU is making many millions of euros available, but this is scorned as amess of pottage in the region. Returning land tothe sea is almost a taboo topic in Zeeland. Still, onecase of de-embankment has taken place within theWestern Scheldt, when a dike in the Selenapolderwas not repaired after it had been breached. Thearea was bought by the regional nature conser-vation organisation Zeeuws Landschap which hasdeveloped it into an intertidal nature area with asurface area of 100 ha called Sieperdaschor. De-embankments are also under considerationalong the banks of the Western Scheldt on theBelgian side, however, mainly for safety reasonsbecause the risk of flooding is increasing as a resultof sea-level rise, reduced floodwater storagecapacity because of embankments, and the increasing frequency of hard westerly winds.
In the north of the Netherlands, two polder areason the north-east coast of Friesland have beenrestored to saltmarsh. After the dike of one of thetwo summer polders in the Peazemerlannen naturearea was breached during a storm surge in 1973, itwas agreed that the breach should not be closedand that the polder should be allowed to developinto a natural saltmarsh of about 100 ha. At thepier at Holwerd a small polder measuring 37 hawas deliberately exposed to tidal action by openingthree tide gates permanently from 1989 onwards,another breach in the summer dike being added in1995. The experience gained from these two pol-ders has been very important for the set-up of the
SALTMARSH RESTORATION ELSEWHERE6
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experimental saltmarsh in Noarderleech.It Fryske Gea’s management aims for Noard-FryslânBûtendyks are to create sufficient foraging oppor-tunities for geese, to prevent the increase of tall-growing plant species and to delay the ageingof the saltmarsh as much as possible by grazinghorses, cattle and sheep on it during the summer.These animals behave very differently from eachother and thus have different effects on the vege-tation. Together they create a varied pattern of vegetation.
If spring grazing is applied there is always anuneasy balance between grazing livestock andbreeding birds. Grazing is necessary to maintain thebreeding habitat but trampling of the nests isinevitable. Horses cause most damage because theyhave a way of charging across the fields in groupsnow and again, covering great distances and leaving many footprints in proportion to their gazing needs. Horses are a particular threat in areaswith gregarious birds, because it almost seems as ifthey enjoy charging right through the bird colonies,causing a lot of damage on the way. Cows aremuch calmer animals with behaviour which is muchmore predictable, and they trample the least num-ber of nests proportionately although young cattlecan also run riot at times. Sheep also trample fewnests individually and they have smaller hooves,which reduces the chance of their hitting nests, but
because more (small) sheepare needed than (big) cowsto achieve the same grazingdensity, the damage fromtrampling caused by sheep isgreater than that caused bycows.
In principle, the grazing sea-son in the summer poldersof Noarderleech runs from15 May to 15 October, butthe animals are put out topasture later in some placeswhere management agree-ments in the context of theNature Management SubsidyScheme (SubsidieregelingNatuurbeheer 2000) are inplace. These agreementsprotect the nests of meadow
birds during the nesting season. Grazing may alsobe delayed by bad weather or flooding. The grazingseason in the Noarderleech saltmarsh runs from 1July to 1 October.
The grazed saltmarsh areas have never been treatedwith manure or fertiliser, but the summer polderscreated by private farmers are treated intensivelywith combinations of liquefied manure, fertiliserand dung. On the parcels of land leased to privatefarmers by It Fryske Gea, treatment is less intensiveand no fertiliser is used. The summer polders managed by It Fryske Gea have not been treatedsince 1998; this also applies to the trial saltmarsh.
In the summer polders, thistles are combatedmechanically. The polders are drained by a systemof drains and ditches which connect to tide gates inthe summer dike. In several places, however, outside the trial saltmarsh as well, the tide gateflaps have been lost and have not been replaced sothat seawater can also enter into other summerpolders in a small-scale way without the dikesneeding to overflow.
MANAGEMENT OF NOARDERLEECH7
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The plan for Noard-Fryslân Bûtendyks involves thecreation of one of the largest saltmarsh areas inEurope by de-embanking 1100 ha of summer polders. The plan is co-funded by the EU LIFE-Nature programme with additional funding fromthe national and provincial governments. The aim isto restore the natural character of the area as muchas possible, but expressly without excluding humaninfluence. From the preceding chapters it should beclear that to strive for a wholly natural system inthis area would be a utopian dream, because thewhole area would be lost through erosion if theinfluence of the restored saltmarshes wereremoved. The human influence which must bemaintained is the continuation of the restored saltmarshes on a modest scale, and the grazing.Research in the Netherlands and Germany hasshown that without grazing, saltmarsh vegetationdevelops into a dense and monotonous vegetationdominated by Sea Couch. Such vegetation is poorin biodiversity and is unattractive to many birdspecies including typical saltmarsh breeding birds,such as the Avocet, Common Redshank andCommon Tern, and grazing flocks of geese duringthe winter season. It Fryske Gea envisages a semi-natural, grazable saltmarsh where managed naturalprocesses can take place as much as possible andwhich is of a sustainable character.
Noard-Fryslân Bûtendyks as a whole can bedescribed as a ‘semi-natural landscape intended tohold a varied vegetation cover containing as manyplant and animal species as possible that naturallybelong in saltmarshes’. The qualification ‘semi-natural’ is essential for the making of managementchoices.
Three basic principles apply to the trial saltmarshproject:• Tidal water must be able to flow into and out of
the trial area freely so that typical halophytic vegetation grows up. However, the experimentmay not lead to a higher water level in the adjacent areas or to their salinisation, because theowners of the adjacent summer polders must notexperience any disruption.
• Sufficient vertical accretion must take place tomeet the target scenario and to keep up with soilcompaction, sea-level rise and land subsidence,but not too much or the vegetation will age toosoon and exhibit less biodiversity. The saltmarshfronting the trial area may not suffer any adverseeffects from the changed sediment balance.
• Safety must be guaranteed. The flooding frequency and duration in the adjacent summerpolders must not change, and the breaches in theouter summer dike must not erode any further.
THE INTERVENTION8
Figure 2. Plan for the experimental saltmarsh in Noard-Fryslân Bûtendyks undertaken by It Fryske Gea. After a drawing by Oranjewoud.
In preparation for the exper-iment, the tide gates in thesummer dike have beenopen permanently since1997. Since that time,therefore, the inflow of saltwater has been higher and,along with it, the inflow ofseeds and the vegetativeparts of saltmarsh plants.Small sluices exist at bothends of the summer polder.The one on the west side isopen permanently and theone on the east side hasbeen closed.
In the course of the year2000 the drainage channelswhich connected to the former tide gates were filledup and three artificial creek systems were dugwhich were to have a role in the supply and discharge of seawater and sediment. The creekswere designed in a winding pattern to give them anatural appearance. Their initial width was between5 and 10 m. The soil dug up was used to raise thedike behind the saltmarsh along the adjacent sum-mer polder in case of an increased risk of flooding.
The summer dike was finally breached on 14September 2001. It was dug away to ground levelin three places over a width of 20-40 m at themouths of the creeks dug earlier. At the centrebreach, the initial opening of the 5-10 m wide
creek was later limited to 2 m by means of aculvert. A bridge has been built across the creekmouth of the western breach which, however, doesnot directly affect the flow opening.
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The formation of a saltmarsh is a complex processwhich is always attended by insecurity about (therate of) vertical accretion and vegetation develop-ment. Many developments may seem likely but nevertheless cannot be predicted with any certainty.Similarly, it is not possible to know in advance whateffect a saltmarsh conversion will have on thefronting lower saltmarsh or how the changes in thearea will affect its use by geese and breeding birds.The Noarderleech trial saltmarsh was, therefore,regarded as an important experiment which was toprovide insight into the process of saltmarshconversion and its impact on the flora and fauna inthe area. Because the insight gained can informand support other saltmarsh restorations in thefuture, it was very important to monitor the trialproject carefully, and a comprehensive monitoringresearch programme was set up which was continued until the end of 2005.
The research, which was commissioned by It FryskeGea, was undertaken by a collaboration betweenAlterra-Texel (since 2006: Wageningen IMARES),the ecological research consultancy Koeman enBijkerk, Altenburg & Wymenga (A&W) ecologicalconsultants, and the Wadden Sea Birds WorkingGroup (Wadvogelwerkgroep) of the Frisian Societyfor Field Biology (Fryske Feriening foar Fjildbiology,FFF). Alterra coordinated the research, the results ofwhich have been published in the report by VanDuin et al. (2007) referred to above. The influx ofplant seeds by floodwater was studied by theUniversity of Groningen.
The principal objective of the saltmarsh trial was:“To gain insight into the abiotic and biotic changesthat occur when a summer polder is converted intoa saltmarsh.” This led to the following researchquestion being formulated for the monitoring pro-gramme: “Do the proposed design and manage-ment measures, i.e., de-embankment, cessation ofmanuring, abandonment of ditch maintenance andextensification of grazing, allow a varied saltmarshvegetation to develop in the trial saltmarsh?”
In addition to the principal objective, the followingancillary objectives were formulated:• The project must provide insight into the develop-
ment of the elevation and the vegetation in thetrial saltmarsh;
• The project must provide insight into the possibleeffects of the conversion on the adjacent naturaland man-made saltmarshes;
• Any effects of the saltmarsh conversion on adjacent (agricultural) areas/summer polders mustbe recorded;
• The project must provide insight into the effectson the fauna, focusing in particular on staginggeese and breeding birds;
• Experience with this kind of nature developmentmust be gained to enable advice to be given onthe design and management of future saltmarshrestoration projects and to enable making predictions about them.
Seven hypotheses were formulated which were tobe verified on the basis of the results of the moni-toring research:• De-embankment will cause a ‘salt shock’ in the
trial saltmarsh, which will lead to the die-off ofthe halophobic vegetation and create opportuni-ties for saltmarsh plants to establish themselves.During the transition phase the lower-lying eastern part of the saltmarsh will temporarily bepoorly vegetated and wet.
• After the de-embankment, rapid vertical accretionwill take place on the low eastern part of the trialsaltmarsh because of the high inundation frequency in this area. The inundation frequencyin the higher-lying western part of the polder willbe lower so a lower rate of vertical accretion isexpected there.
• Vertical accretion in the trial saltmarsh will notaffect the development of the surface elevationof the adjacent natural and man-made salt-marshes.
• Surface elevation and the degree of drainage willbe adequate for the formation of grassy salt-marsh vegetation.
• Vertical accretion will also be influenced by thevegetation structure and, thus, indirectly, by thekind and intensity of grazing too.
• The establishment of saltmarsh vegetation is conditional upon the influx of seeds with flood-water (research by the University of Groningen).
• Use of the converted summer polder by geeseand breeding birds will not diminish.
RESEARCH9
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Biological fieldwork consists, to a large extent, ofmaking accurate counts and measurements whichmust be repeated ad infinitum. A brief outline ofwhat was counted and measured in the trial salt-marsh follows below.
A vegetation survey involves marking out plots andthen meticulously recording the composition andnumber of plant species occurring in them. A plotwhich is monitored annually is called a PermanentQuadrat (abbreviated as PQ). In the Noarderleechtrial saltmarsh 72 PQs, each measuring 4x4 m, weremarked out. Vegetation surveys were carried out inthese PQs for five years in combination with verticalaccretion measurements so that changes in surfaceelevation could be monitored. In addition, threetransects were set up to obtain a better picture ofthe development of the vegetation in the wholearea. In these transects a total of forty individualplant species were mapped. Twenty of them weredesignated as ‘target species’ for the new salt-marsh.
To measure the effects of grazing, exclosures weremade. An exclosure is a small piece of land within agrazed area which is fenced to keep the livestockout.
To monitor groundwater salinisation, salinity wells(pipes) were placed in the trial saltmarsh and theadjacent summer polder to measure groundwaterlevels and water salinity. Changes in salinity werealso measured in the top 5 cm of the soil and inthe soil moisture contained in this top layer.
Changes in surface elevation were measured usinga so-called Sedimentation/Erosion Bar in the follow-ing way. Two stakes were driven firmly into the soiland the sandy subsoil, 2 m apart and with theirtops exactly level at ca. 35 cm above the soil surface. To take the readings an aluminium bar wasplaced across the tops of the stake heads, a rodwas placed through holes in the bar and the heightwas measured from the top of the bar to the soilsurface. Vertical accretion was also measured usingso-called sedimentation plates: steel 30 x 30 cmplates which were placed in the soil horizontally ata depth of 10 cm. Using a thin iron ruler, the depthof the plates to the surface could then be read.
Goose counts of Brent and Barnacle Geese whichwere taken every two weeks in the1996/97, 1997/98 and 1998/99 seasons, form the basis for the baselinesituation. After the breach creating thesaltmarsh had been made in 2001,counts were carried out in 2001/02,2002/03, 2003/04 and 2004/05.
To determine the effects of the salt-marsh conversion on the geese moreaccurately, pellet counts were carriedout. Because geese defecate at veryshort and regular intervals, the densityof the droppings within a fixed circle (a pellet plot) is the measure of thegeese’s grazing pressure. In total, 105pellet plots were used. Because thespecies composition of the grass swardis important for the geese’s choice offood, additional vegetation surveys werecarried out in the pellet plots.
Nests and habitats of breeding birds in and aroundthe saltmarsh conversion were mapped from 1999up to and including 2005.
MONITORING10
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Inundation frequency in the trial saltmarsh was notmeasured directly but can be deduced from theinformation we have about the height of the summer dike before and after the intervention, incombination with the automated water level measurements performed every ten minutes byRijkswaterstaat at the Nes station on the island ofAmeland. From these measurements the high waterlevels have been selected for the period from 1997-2005, that is, starting more than four and a halfyears before the intervention on 14 September2001 and continuing until almost four and a halfyears after it, a total of nine years.
Before the intervention, the lowest point of theouter summer dike stood ca. 2.72 m aboveAmsterdam Ordnance Datum. At all tides above2.7 m, salt water could therefore flow into thepolder, which would have happened only threetimes during the four and a half years before theintervention started. During the four and a halfyears after the intervention not a single tide washigh enough to wash over the dike. Since the inter-vention, the polder is flooded completely at waterlevels exceeding ca. 1.9 m above AmsterdamOrdnance Datum. In the period preceding the inter-vention, 62 tides would have been high enough.
From the date of the intervention until 31December 2005, in theory 64 inundations tookplace so that, on average, the polder was, and willbe, inundated about fourteen times a year.
Against all expectations, the inundations which infact took place after 14 September 2001 did notcause a salt shock; salinisation proved to proceedvery gradually. In the low-lying eastern part of thesaltmarsh, the shallow groundwater became moresaline at a faster rate than the groundwater in thehigher western part, and no levelling off could beobserved after four years. In the drier western part,levelling off was observed from the third yearonwards. In the deeper groundwater (120 cm) theincrease in salinity levels, measured in terms of electrical conductivity (EC values), was very gradual(Figure 3).
In the top 5 cm of the soil and in the soil moisturecontained in it, salinity content also increased onlyvery gradually. After four years, chloride concen-trations were about 60% of the values for naturalsaltmarshes (Figure 4). Changes in soil moisture, inparticular, are relevant with regard to the vegeta-tion.
Figure 4. Changes in the chloride concentrations inthe soil moisture, indexed by the value for natural
saltmarshes.
No indications were found that any salinisationoccurred in the adjacent summer polder as a resultof the saltmarsh conversion. In a number of places,a decrease in salinity was even observed, in particular, in the deeper groundwater.
TIDAL FLOODING AND SALINISATION11
Figure 3. Increase of the EC value of the ground-water in the trial saltmarsh near the dike breaches
in the western (Location 1) and eastern parts(Location 11), during the first four years after
the de-embankment.
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The measurements using the Sedimentation/ErosionBar show that vertical accretion in the lower eastern part of the trial saltmarsh occurred morerapidly than in the higher western part. Verticalaccretion was more pronounced close to the artificial creeks than it was farther away from them.Grazing had great effect. Vertical accretion clearlytook place more rapidly in ungrazed conditionsthan in grazed conditions (because of sedimenttrapping by taller vegetation). In grazed areas therate of vertical accretion varied from 0.3 mm/yearto 31.5 mm/year, while the rate in ungrazed areaswas 10.7 mm/year to 36.7 mm/year.
As a result of new sediment being deposited, theburied sedimentation plates gradually came to lie ata deeper level. In the summer period (April –August), when normally no inundations occur, surface elevation decreases as a result of soil compaction. During the storm season (September –March) an increase occurs as a result of sedimen-tation and, perhaps, the swelling of the wet soil.Figure 5 describes the progression of the depths ofthe sedimentation plates at one of the locations.Conditions at the various locations differed but, onthe whole, observations throughout the whole areashowed a similar trend.
Figure 5. Progression of the depth of the sedimentation plates. For clarity’s sake, the depth in
the baseline situation has been set at 0.
On the basis of the specific volumeand the vertical accretion figures, arough estimate can be made of theannual sediment supply in the trialsaltmarsh. In the higher part of thearea, vertical accretion amounted to6.0 mm/year at a sediment supply of4.8 kg per square metre per year. Inthe lower part of the saltmarsh it was14.8 mm/year at 10.4 kg. In bothcases, the rate of vertical accretionwas more than adequate to keeppace with phenomena such as landsubsidence and sea-level rise. Thelevel of the mean high tide shows anupward trend of approximately 2.6mm per year, allowing for changes inweather systems.
VERTICAL ACCRETION12
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Rapid changes occurred in a number of individualplant species after the de-embankment, but thevegetation as a whole changed only very gradually.It was expected that the salt shock resulting fromexposure to seawater would lead to the die-off ofvegetation and would cause bare patches, but thisdid not happen. Even after four years, changes aregradual and in keeping with general trends.
The salt-intolerant species (glycophytes) and themore salt-tolerant species (brackish) species showeda large, gradual decline compared to the baselinesituation. A remarkable appearance in the first andsecond years after the de-embankment was SeaClubrush, which was seen in the creek dug in thehighest transect, but in the third and fourth years itwas not found here anymore, probably because ofthe continuing salinisation of the groundwater
running off through the creek. Creeping Thistledeclined strongly and, in the course of only a fewyears, has virtually disappeared from two of thethree transects. Interestingly, Red Bartsia andStrawberry Clover established themselves in someparts of the old, higher ‘farmer’s method’ salt-marsh.
Salt-tolerant species (halophytes) established them-selves soon after the de-embankment and spread.The majority of these species, in the baseline situa-tion, were already present on a modest scale in oralong ditches and, in particular, near tide gates thathad already been removed in 1997. After the dikehad been breached they spread rapidly in the fields,where they had not occurred before.
VEGETATION13
Figure 6. Distribution and cover of Common saltmarsh grass inTransect 2 in the year preceding the de-embankment (year –1)
and the four following years (year +1 to year +4).
Figure 7. The percentage of target species foundper transect from one year before de-embankment
until four years after.
Only Common Cord-grass and Sea Purslane wereabsent in the baseline situation, but establishedthemselves after the de-embankment. The mostsurprising and remarkable new arrival wasCommon saltmarsh grass, which appeared ‘out ofthe blue’ immediately, and en masse, during thefirst year after the new saltmarsh was created. Figure 6 shows the change in coverdensity in Transect 2.
With the exception of Grass-leaved Orache,all the mapped halophytes can be classed astarget species for the saltmarsh restorationprogramme. Figure 7 shows how the percentage of target species increased in thethree mapped transects.
In grazed sites in the higher parts of the trialarea, the types of vegetation characteristic ofa saltmarsh edge (the natural transition zonefrom the middle marsh to the hinterland)continued to dominate for some time afterthe de-embankment. The Perennial Ryegrasstype was replaced by the Silverweed typeand a relatively species-rich type of Couchgrass. In the lower sections the vegetationtypes of the saltmarsh edge were replacedby typical saltmarsh vegetation much morequickly.
In ungrazed situations the variety of vege-tation types was less. In the third year afterthe de-embankment, Perennial Ryegrass vegetation had largely been replaced by aspecies-poor type of Couch grass from the
middle marsh. It is expected that Couch grass willeventually give way to Sea Couch. The natural finalstage of an ungrazed saltmarsh is a monotonous,species-poor vegetation dominated by Sea Couch.Sea Couch was first observed in the trial saltmarshin the second year of the experiment, but by thefourth year it already covered almost 20% of theungrazed permanent quadrats. This is evidence thatgrazing is an important management tool for themanagement of a varied, species-rich saltmarsh.
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Wintering geese (Brent and BarnacleGeese) use the salt-marshes, the summer polders andthe adjacent land within the dikes.To get an overview of the availabilityof a feeding habitat for geese, cropping plan maps from 1996 and2004 were compared which provedthat the saltmarsh conversion wasnot the only change in the geese’shabitat. A marked shift towardsagriculture took place in the areawithin the dikes at the expense ofcultivated grassland. In the summerpolders, a spectacular shift from‘cultivated grassland with low vege-tation’ to ‘cultivated grassland withtussocky vegetation’ occurred whenlarge parts of the area came intothe ownership or management of ItFryske Gea, after which extensifica-tion took place and fertilisation wasstopped. Cultivated grassland was replaced by salt-marsh vegetation in the trial saltmarsh, as describedabove.
Brent and Barnacle Geese are both abundant inNoard-Fryslân Bûtendyks with average seasonalmaximums of 18,000 and 93,000, respectively, inthe period 2000-2005. Converted into goose days,the number of Barnacle Goose days was 4 to 5times higher than the number of Brent Goose days.Brent Goose numbers peaked in April and May,accounting for 70% of the total number of goosedays. Barnacle Goose numbers peaked twice, inOctober-November and in March-April, the latterpeak being the highest.
In the period from 1996-2005 the number of BrentGoose days in Noard-Fryslân Bûtendyks graduallydecreased every winter season from over one mil-lion to fewer than 200,000. This development followed the national trend. The number ofBarnacle Goose days in the same period was lowestin 1996/97, at fewer than 6 million. In the follow-ing years the number fluctuated between 9 and 7.5million. Brent Geese favoured the saltmarshes,while Barnacle Geese visited both the saltmarshesand the summer polders, reaching their highestdensities in the summer polders.
To date, the food supplies available for geese havedeclined because of the saltmarsh experiment. Thecover of plant species which are important forgeese lay between 65 and 80% in the summerpolders and between 45 and 85% in the salt-marshes. In the trial saltmarsh, however, the coverdid not exceed 37% during the first years after theintervention. This may change as the saltmarsh vegetation continues to develop.
Despite the temporary deterioration of availablefood, the changes in the summer polder have notled to large numbers of geese moving to areaswithin the dikes.
The differences in food availability between thesaltmarsh, cultivated grassland and the trial saltmarsh are reflected in the grazing pressure,which was measured by means of pellet counts(expressed as the number of droppings per squaremetre per day). The pellet counts do not allow anydifferentiation between Brent and Barnacle Geese.Figure 8 shows the grazing pressure for the varioushabitats, which is higher in spring than in autumn.
Birds breeding in the summer polders were chieflythe Avocet, Lapwing and Oystercatcher and, insome years, a lot of Black-Headed Gulls. The numbers of breeding birds remained fairly stable
BIRDLIFE14
over the years. In the saltmarsh, Avocets andOystercatchers were the most numerous, with largenumbers of Black-Headed Gulls and Common Ternsbreeding there in some years. The Godwit was rare,and Meadow Pippits and Lapwings occurred only inlow numbers. Remarkably, Larks increased, goingagainst the national trend.
Avocets and Oystercatchers were the most abundant birds in the trial saltmarsh. Oystercatchernumbers remained constant, as did the numbers ofGodwits, Larks and Meadow Pippits. The number ofAvocets fluctuated strongly without any clear trend.The Common Redshank appeared to be recoveringafter an initial decline following the saltmarsh con-
version. The Common Tern declined in numbers,but they were not numerous in the first place andthe decline was slight.
In general, the numbers of breeding birds in thetrial saltmarsh remained more or less stable. It wasexpected that Lapwings, Godwits and Larks woulddecline in number and that Avocets, CommonRedshanks, Black-headed gulls, Common Terns,Arctic Terns and Meadow Pippits would increase,but this was not borne out by reality as the numberof breeding species decreased slightly in the trialsaltmarsh.
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Figure 8. Grazing pressure in spring in various habitats, measured by droppings density.
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To what extent have the predictions that were formulated at the start of the research come true?The main outcomes are, in brief summary:
• In the first year after de-embankment, thevegetation changed considerably but, because nosalt shock occurred no massive die-off took place.In many species the change was so gradual thatthe changes still followed trends in the fourthyear.
• Vertical accretion in the western part of the areawas lower than in the eastern part. In ungrazedsituations the difference was 11 mm annually.Differences in sediment supply caused by thecreek forms may have played a role.
• Vertical accretion was less in grazed areas. In thehigher part of the trial saltmarsh, sediment trap-ping in ungrazed conditions exceeded that ingrazed conditions. In the low-lying part nogreater sediment trapping was measured in theexclosures.
• There are no indications that vertical accretion inthe trial saltmarsh had an effect on the surfaceelevation development in the adjacent naturaland man-made saltmarshes.
• Compared to the existing grazed saltmarshes, theuse of the trial saltmarsh by geese in autumn wasconsiderably less. Use by geese in spring hasincreased substantially since 2001/02. No trendcan be observed as regards the number of breeding birds.
EVALUATIONS AND PERSPECTIVES15
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The information about the monitoring research and the
research results were taken from the comprehensive
research report:
Van Duin, W.E., P. Esselink, D. Bos, R. Klaver, G.
Verwey & P.-W. van Leeuwen, 2007.
Proefverkweldering Noard-Fryslân Bûtendyks.
Evaluatie kwelderherstel 2000-2005. (The Noard-
Fryslân Bûtendyks Trial Saltmarsh Restoration.
Evaluation of the Saltmarsh Restoration 2000-2005.)
Report C020/07, Wageningen IMARES, Den Burg,
Texel. Koeman & Bijkerk report no. 2006-045,
Koeman & Bijkerk, Haren. A&W report 840,
Altenburg & Wymenga, Veenwouden, 209 pp.
Other sources used:
Dijkema, K.S., A. Nicolai, J. Frankes, H. Jongerius & H.
Nauta, 2001. Van landaanwinning naar kwelderwerken.
(From Land Reclamation to Saltmarsh Restoration.)
Rijkswaterstaat Directie Noord-Nederland (Directorate-
General of Public Works and Water Management, North
Netherlands Department), Leeuwarden. ALTERRA Green
World Research, Texel, 68 pp.
Esselink, P., 2000. Nature Management of Coastal Salt
Marshes. Interactions between anthropogenic influences
and natural dynamics. Doctoral thesis, University of
Groningen. Koeman & Bijkerk, Haren, 256 pp.
Hosper, U.G. & J. de Vlas (eds.), 1994. Noord-Friesland
Buitendijks. Beschrijving en toekomstvisie. (Noard-Fryslân
Bûtendyks. Overview and Outlook.) Report by the ‘Noord-
Friesland Buitendijks’ Working Group. It Fryske Gea,
Olterterp, 80 pp.
Jager, S.J. & S. Rintjema, 2003. Beheerplan Noard-Fryslân
Bûtendyks. (A Management Plan for Noard-Fryslân
Bûtendyks.) Working document 2003-228. It Fryske Gea,
Olterterp, 66 pp. + annexes.
Oevelen, D. van, E. van den Bergh, T. Ysebaert & P. Meire,
2000. Literatuuronderzoek naar ontpolderingen.
(A Literature Study of De-embankments.)
Report IN.R.2000.7, Institute of Nature Conservation,
Brussels, 55 pp.
SOURCES16
PUBLISHER’S IMPRINT
This publication was financed by: It Fryske Gea, Olterterp
Text: Albert Beintema, Biowrite, Gorssel
Text editors:Peter Esselink, PUCCIMAR, Vries
Willem van Duin, IMARES, Texel
Daan Bos, Altenburg & Wymenga, Veenwouden
Henk de Vries, It Fryske Gea, Olterterp
Photographs: It Fryske Gea archives
Figures: Peter Esselink and Oscar Langevoord
Layout and printing: Brandsma Offset, Ferwerd
Print run: 200 x
2007
Altenburg & Wymenga Consultants,VeenwoudenP.O. Box 32, 9269 ZR Veenwouden, The Netherlands
A&W Report 1020
It Fryske GeaVan Harinxmaweg 17, 9246 TL Olterterp
P.O. Box 3, 9244 ZN Beetsterzwaag (NL)
Telephone: 05126-1448, Fax 2973
Email: [email protected]
Website: http://www.itfryskegea.nl
Giro account no.: 895212
