Biological research ems-dollard estuary

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rijkswaterstaat communications

biological research ems-dollard estuary

by a survey of the ecosystem research during 1973 and 1982 by order of

- the ministry of transport and public works - the ministry of housing, physical planning and

the environment

- the ministry of education and science - the ministry of agriculture and fisheries

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all correspondence and applications should be addressed to

rijkswaterstaat dienst getijdewateren hooftskade 1

p.O. box 20907

2500 EX the hague - the netherlands

the views in this artiele are the authors' own.

recommended catalogue entry:

biological

biological research ems-dollard estuary by a survey of the ecosystem research during 1973 and 1982/by biological study of the ems-dollard estuary (BOEDE); by order of the ministry of transport and public works ... [et al.]. - the hague: rijkswaterstaat, 1985. - 182 p.: ill.; 24 cm. - (rijkswaterstaat communications; no 40)

bibliogr.: p. 170-179.

chairman of BOEDE is h. m. klouwen.

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Preface

In the centuries-oId battle of the Netherlands against the sea the estuaries have been the principal battlefields where the great flood disasters occurred and where a great deal of hydrological inventiveness was therefore concentrated. The Zuiderzee has now been enclosed for so long that we are starting to forget that it was once an estuary. Now the Delta Project is nearing completion, which will mean the end of the Zeeland and South Holland estuaries, there remain, not counting the Oosterschelde, which will be left half open, only two estuaries: the Westerschelde and the Wadden Sea. A very characteristic feature of the Wadden Sea is the Ems estuary on account of the complete transition from fresh water to sea water with the extensive brackish tidal area of the Dollard. Even this estuary has suffered serious encroachment by land reclamation works and dike construction since it reached its maximum extent in the l6th century following a series of dike bursts. Whereas the area was formerly used predominantly for land reclamation, nowadays it is used for other purposes such as shipping, fishing, effluent disposal, recreation, etc.

Hydraulic engineering works carried out for the benefit of shipping and for coastal protection such as the dredging and maintenance of navigation channels, the building, extension and maintenance of harbours and the straightening of the coastline by means of dikes, lead to changes in the water balance and thus to alterations in the conditions under which the biotic communities that are present can survive.

Effluent discharges also lead to changes in the biotic communities, depending on the quantity and composition of the effluents.

In the early seventies a need arose for the policy-making bodies to be able to weigh up policy decisions affecting the estuary in terms of the anticipated biological consequences of those decisions. An understanding of the effects of the effluent discharges in the Dollard on the ecosystem was also required because, although these discharges had been going on for decades, it was feared that an increase in the effluent quantities via direct discharge into the estuary from pipelines wou1d lead to drastic changes in the ecosys-tem.

For these reasons the five ministries involved in the management of the estuary estab-lished the Biological Study of Potato Waste Effluent. Soon after the start of the study it was clear that the influence of human activities could only be quantified on the basis of knowledge of the whole ecosystem and the processes that determine the functioning of

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that ecosystem. When an inventory had been taken of the various biotic communities, the centre of gravity of the study shifted to the processes that influence the composition and extent of the biotic communities. Particular attention was paid to the interaction between the biotic communities and the abiotic environmental factors.

To express the change in the objectives of the project, the name was changed in 1977 to Biological Study of the Ems-Dollard Estuary (Biologisch Onderzoek Eems-Dollard Estuarium (BOEDE)).

The knowledge that has been gathered has been disseminated via a large number of reports and publications but, perhaps more importantly, it has also been processed in a mathematical ecosystem model. Although complex, this model, parts of which have already been used as the basis for policy recommendations, is still a greatly simplified and therefore rough approximation to reality. Neverthe1ess, once it has been developed and evaluated, it promises to provide valuable assistance in understanding the complex chains of cause and effect that are the consequence of intervention in an ecosystem. Dr. H. M. Klouwen

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1 Introduction

1.1 Background to the research

This book describes our present understanding of the structure and functioning of the ecosystem of the Ems-DolIard estuary. The research that is described is a response to a problem that has existed since the 19th century: serious water polIution in the canals in the North-East Netherlands.

Into these canals was discharged industrial effluent containing mainly organic material. The self-cleansing capacity of these canals was inadequate, and in fact the canals were used as a water purification installation. The water was anaerobic and stank. To provide a final solution to this unsatisfactory state of affairs the Effluent Authority decided to lay pipelines to discharge the polIuted effluent directly into the Wadden Sea and the Ems-DolIard estuary.

In 1970 the quantity of organic waste to be conveyed through the pipelines was estimated at approximateJy 22· 106_{inhabitant equivalents. The greatest contribution to tbis came} from the potato flour industry with 20· 106_{i.e. [235]. The criterion for selecting the} location at wbich the pipeline should discharge into the estuary was that no harmful changes should occur in the biotic communities. Itwas assumed that tbis would not occur as long as the oxygen saturation remained above the arbitrary limit of 50%. The accuracy of tbis assumption was challenged by biologists who asserted that a persistent 50% oxygen saturation would indeed have harmful consequences and that the Ems-DolIard estuary was very probably not capable of processing these large quantities of effluent. Against the background of the absorption capacity of the Ems-Dollard estuary for effluent from the potato industry [173] there was steadily increasing pressure to find alternative solutions to the potato waste effluent problem, one possible solution being purification at source. In about 1970 the industry came up with the technical means of providing a solution. On tbis basis, and with support from the government, a redevelopment programme was drawn up. That programme is not yet complete. How-ever, there was still a need for a firm core of knowledge that would provide the basis for an understanding of how much the Ems-DolIard estuary could absorb without harmful consequences. Biological research that would enable the effects of effluent discharges to be predicted was regarded as essential for the establishment of criteria for the assessment of tbis absorption capacity. Itis interesting that the term 'harmful' was not defined.

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In view of the consequences of the discharge of such large quantities of effluent into the estuary, the Ministry of Transport and Public Works promised a study which would include a biological examination of the Ems-Dollard estuary together with chemical and hydrographic research into the effect of pollution on the estuary.

In 1971 the Steering Committee on Fen-colony Effluent published a report entitled: Voorstel tot nader onderzoek inzake de behandeling van het Veenkoloniale Afvalwater (ProlJosal for further research into the treatment of potato waste effluent) [235]. The purpose of the anticipated biological study of potato waste effluent (Biologisch Onder-zoek Veenkoloniaal Afvalwater (BOVA» was:

- to provide a qualitative and quantitative description of the estuary in its present condition,

to provide means of predicting changes and alterations that would occur in response to a given loading of the estuary with oxygen-consuming substances.

This research was to be carried out by a separately appointed group of specialists housed by TNO at the Netherlands Institute for Sea Research, which was responsible for coordination, and Groningen University and funded by a number of departments from specially allocated funds. The main anticipated research areas were:

- taking stock of the quantities of a number of specified groups of organisms to enable comparison with other areas such as the Wadden Sea and the dutch Delta area - a study of the reactions of individual species of plants and animals to the

effluent.

Thus anumberof aspects of the biology of the estuarywere left out of consideration: the phytoplankton, the meiofauna, the microfauna, the chemistry of dead organic material (detritus) and model studies. However, the research was gradually broadened to include these subjects, with the exception of the microfauna.

The use of mathematical mode1ling to integrate the various studies was proposed at an early stage in the BOVA group, although such modelling could not actually be imple-mented until a later stage.

As stated earlier, it was decided during the period of the BOVA research (1972-1976) that most of the effluent must be purified at source, i.e. at the factories. The fen-colony effluent pipeline was laid, but its use was restricted to the discharge of part of the purified effluent. But, to judge from the questions which the relevant departments had to deal with, there was still great interest in the data that had been collected (see chapter 2). Meanwhile it had been appreciated that discharges of effluent and possibly other factors that affect the estuary should be seen against the background of the many other functions (see 2.1) that the estuary fulfils. Aspects such as these led to re-assessment of the programme to be carried out: not only the structure of the estuary (which was to be described on the basis of the BOVA programme) but also the processes in the ecosystem

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were regarded as important. Thus there was a change of objective and of name: Bio-logical Study of the Ems-Dollard Estuary (Biologisch Onderzoek Eems-Dollard Estua-rium) (1977-1982: BOEDE). The new objectives were:

I To discover and describe the factors that determine the structure and the functioning of the Ems-Dollard estuarine system. To this end, a qualitative and quantitative description was to be given of the ecosystem in its current state and the contributory processes were to be determined and described.

2 To monitor changes and provide means of predicting changes that might occur in the ecosystem as a consequence of altered conditions with regard to chemical and physical discharges, civil engineering works, etc., making use where possible of mathematical (sub-)mode1s of the ecosystem.

3 The provision, with the aid of I and 2, of material for the formulation of a manage-ment policy for the Ems-Dollard estuary and if possible for similar areas.

1.2 Contents of this report

As an aid to policy and to the management of the estuary, the following chapters provide a summary of the current state of affairs with regard to the research that has been carried out and the insights that have been gained by the BOEDE group. This book is not intended as a scientific publication; it is a short summary of the research and is meant to provide the authorities with as much relevant information as possible. It is based on previously published scientific publications and reports (see list of references), which should be consulted for the base data, methods and justification of the conc1usions. Some of the results that were obtained are still to be published; the conc1usions will be anticipated here. A description and documentation of the mathematical models that were used is likewise still to be published.

Chapter 2 considers the interaction between research on the one hand and management and control on the other hand and summarizes the most important results. Sections of the research programme that could not be implemented are also mentioned.

Chapter 3 gives a general description of the hydrography of the estuary and of a number of hydrographic processes together with a picture of the abiotic conditions in the estuary. By way of introduction the chapter also gives a short characterization of the groups of organisms that were investigated and describes their most important characteristics. Chapter 4 deals with the nutrients and the oxygen balance of the estuary and the factors that influence them.

Chapters 5-7 give a more detailed survey of the results of the research into the flora and fauna.

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Chapter 8 discusses the composition of the large quantity of dead organic material in the estuary and the degradation of that organic material by bacteria.

Chapter9 provides, as a synthesis for various parts of the estuary, a survey of the biomass and carbon flows through the ecosystem.

The first 9 chapters provide a fairly statie description of the structure and processes in the estuary; in chapter10,in contrast, the dynamics of the interaction between structure and function, in time and space, are illustrated on the basis of some mathematical models of the estuary that have been developed. In this way a high degree of integration of the component studies is achieved. The status of the models is assessed on the basis of the results obtained.

Finally, chapter11considers a number of stress factors due to man that might damage the biological functions of the estuary, or that do so already.

1.3 Personnel

The Biological Study of the Ems-Dollard Estuary is assisted by a steering committee and a committee of experts. The present members of the steering committee are:

Dl. H. M. Klouwen, chairman ir. P. de Vos, secretary drs. F. Baerselman drs. J. Bezemer ir. J. F. Deunk ir. J.L.KooIen drs. E. Reckman ir. H. van 't Sant ir. T.A. Sprong Dl. P. de Wolf

Ministry of Housing, Physical Planning and the Envi-ronment

Ministry of Transport and Public Works, Groningen Directorate

Ministry of Agriculture and Fisheries; NBüR Ministry of Education and Science

Ministry of Transport and Public Works, Groningen Directorate

Ministry of Transport and Public Works, N ational Institute for the Purification of Effluents (RIZA) Ministry of Housing, Physical Planning and the Envi-ronment; RPD

Ministry of Housing, Physical Planning and the Envi-ronment; DGMH

Ministry of Transport and Public Works Project coordinator, BüEDE

The members of the committee of experts are:

ir.1. L.KooIen, chairman National Institute for the Purification of

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Dr. K. Essink, secretary Dr. P. H. Nienhuis Prof. Dr. H. Postma Dr. W. J. Wolff Dr. P. de Wolf

National Institute for the Purification of Ef-fluents (RIZA)

Delta Institute for Hydrobiological Research Netherlands Institute for Sea Research (NIOZ)

National Institute for Nature Management, Department of Estuarine Ecology

Project coordinator, BOEDE

This report was written by the BOEDE group and is published under the name BOEDE. The foIIowing authors coIIaborated in the production of this report:

dr. W. Admiraal drs. M. A. van Arkel drs. J. W. Baretta dr. ir. L. A. Bouwman dr. F. Colijn dr. F.B.van Es drs. V. N. de Jonge dr. R. W. P. M. Laane drs. P. Ruardij drs. H. <J.J. Schröder A.Stam dr. P. de Wolf Benthic diatoms Macrobenthos Zooplankton

+

models Meiobenthos Primary production Aerobic mineraIization Hydrography, nutrients, silt Detritus chemistry

Mathematical models Anaerobic bacteria Fish and fisheries Project coordinator

AIthough these authors are each responsible for the parts of the research listed after their name, is must be emphasized that this report is the result of cooperative effort. The authors were assisted in their investigations by members of the technical staff: J. Beukema mrs. A. Bol-den Heijer mrs. drs. K. <Jrooters-Romeyn mrs. M. de Jonge-Swieter <J. Kamstra ing. A. Kop ing. H. Malschaert M. Mulder H. PeIetier mrs. L. A. H. Venekamp L.A. Villerius

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and by former members of the technical staff: mrs. W. E. Lewis J. Nijboer H. Pelleboer C.J. Pieters mrs. M. Rademaker mrs. W. Uitman-Kno1 mrs. G.T. Visser

In concluding this report it is a p1easure to thank the many individuals and organizations that contributed to the success of the Bio10gical Study of the Ems-Dollard Estuary. Thanks are due above all to the members of the steering committee and the committee of experts.

Many thanks are owed to the members of the technical staff who have contributed to this project with constant commitment and enthusiasm and to the 80 or so students and trainees who he1ped with the research for shorter or 10nger periods.

An important factor that contributed to the success of this research was the accommo-dation provided by the Netherlands Institute for Sea Research (directors Prof. Dr. H. Postma and Dr. J. J. Zijlstra) and two institutes of Groningen University: the Marine Bio10gy Group (Prof. Dr.C.van den Hoek) and the Microbio10gy Labora-tory (Prof. Dr. H. Veldkamp). The accommodation in these institutes and the constant avai1ability of facilities, advice and opportunities for discussion were of great value. Moreover, the study benefited greatly from constantly avai1ab1e data and stimulating discussions with staff of

- the National Institute for Nature Management, Department of Estuarine Eco1ogy, Texel

- the Survey and Consultancy Department of the Ministry of Transport and Public Works, Delfzijl

- the National Institute for the Purification of Effluents and - the Waterways and Shipping Authority, Emden.

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2 The Biological Study of the Ems-Dollard Estuary and

management of the estuary

2.1 Whyresearch is done as an aid to management

Throughout the ages, virtually everywhere in the world, the estuaries have always been the part of the sea that is most influenced by human activities. For a long time there was unlimited use of estuaries without any serious damage to the natural environment and without any conflict between the interests of the various users. But as a consequence of the increase in the population density, industrialization and in particular the increase in scale of all manner of activities the natural environment came under increasing pressure and the various activities were no longer compatible with each other. Management with specific objectives therefore became necessary. The memorandum for the preparation of the National Physical Planning Key Decision for the Wadden Sea gives a number of examples of human activities in the Wadden Sea, all of which are equally applicable to the Ems-Dollard estuary:

- Coast management - Land reclamation - Traffic and transport

- Use of harbours and industrial areas - Detection and extraction of minerals - Discharge of effluent

- Excavation - Recreation - Fisheries

- Military exercises.

Since the Ems-Dollard estuary forms an integral part of the Wadden Sea as far as administration is concerned, the criteria by which it is managed are those defined in the National Physical Planning Key Decision for the Wadden Sea. The key decision char-acterizes the Wadden Sea as a unique natural area whose value lies in:

- its very dynamic character - its ecological significance

- its importance for the fish stocks of the North Sea - its importance for the exceptional beauty of the landscape - its importance for scientific research.

The general objectives of the management of the Wadden Sea are the protection, the maintenance and where necessary the restoration of the Wadden Sea as a natural area.

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Existing and proposed activities are tested against these criteria, weighing the social desirability of the activity against the probable consequences of the activity for the natural environment of the Wadden Sea. Itis explicitly stated that account must be taken of the cumulative effect of various activities.

To obtain a prognosis of the consequences of a given action, information about the composition and functioning of the biotic communities is necessary. In practice this means that one must know the size of each of the functionally different groups of organisms and the place it occupies in the biotic communities, how the incidence of these organisms varies in space and time and how they influence each other, whether in predator-prey relationships or in other ways, such as mutual competition, positive or negative influence on each other's environment, etc. In addition it is necessary to know how the organisms depend on the physico-chemical environment. Each element of the ecosystem depends on other elements of the system, so that the ecosystem forms a network of relationships between the elements. Although the Wadden Sea environment is evaluated positively by man on account of isolated elements (birds, seals), all the elements must be known before arealistic assessment can be made of the consequences of a given intervention in the environment. This is all the more true where inconspicuous elements such as bacteria, algae, microfauna and meiofauna account for the greater part of the biological activity of the system.

2.2 Developments in the study of the estuary

When the BOVA-BOEDE study began, relatively litde was known about the Ems-Dollard estuary in comparison with other parts of the Wadden Sea. Neither the Dutch nor the Germans had done much research in the area.

One of the oldest publications on the area is the description of the physical geography of the Dollard which Stratingh and Venema published in 1855 [234] and which, owing to the comprehensive description that it contains, is still useful as a reference from the period before the large flow of effluent into the Dollard began. In 1907 Lohmeyer [198] published a survey of the fish fauna that occurred in the estuary at that time. On the German side,Kühland Mann [192; 193] carried out a hydrographic and biological study of the river Ems and the estuary in the early 1950s as part of a study of the rivers of the western area of the North German Plain. The biological research was of a qualitative nature and many organisms are only very broadly treated. On the Dutch side, scientific studies were carried out in the estuary in the years 1954-1956 by researchers from various disciplines. The biological research provided a great deal of information about the composition of flora and fauna but was not designed to collect quantitative data.

The results were published in 1960 [242]. The sedimentological and hydrographic work described therein is still useful today. In 1965 Eggink [173] used the hydrographic model

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of the estuary described in that publication as a basis for his ca1culations on the effects of effluent discharges on the oxygen balance of the estuary. The same model also provided the basis for the present mathematical model for the simulation of transport phenomena (see 10.3). Finally, zoological research has been done by the German Research Institute for the Protection of Islands and Coasts in Nordemey and hydrographic and biological measurements have also been carried out by staff of the Netherlands Institute for Sea Research.

Nevertheless, in the early seventies the available information on the estuary was very fragmentary.Itwas logical therefore that the primary aim of the BOVA study should be to provide a complete description of the estuary, not only qualitatively but also quan-titatively (see 1.1.). The original research was directed towards the following groups of organisms: algae in and on the estuary bed, bacteria in the water, bacteria in the estuary bed, larger animals in the estuary bed (macrobenthos), zooplankton, fishes, crabs and shrimps. Experimental research was also carried out into the influence of effluents on these organisms.

When the first phase of this research was carried out it became evident that a different type of information was necessary in order to establish why the organisms in the estuary are there as they are. Research was necessary into the interdependence of the various organisms and the effect of the environmental conditions. Italso became evident that information on phytoplankton, small animal organisms in the sea bed (rnicro- and meiobenthos) and detritus was desirabie. Finally there was a need for mathematical integration of the many biological data. The consequence of all this was that the research was placed on a broader footing and, as the objectives of BOEDE (see 1.1) show, became generally more directed towards the structure and function of the ecosystem of the Ems-Dollard estuary.

Although the research was in practice divided into a number of separate subjects, it actually formed a single whole since the work was based on a shared concept of the ecosystem. The diagram in figure 9.3 gives an outline of this concept. The groups of organisms that form a single functional unit are indicated by boxes and the arrows give the interactions between these groups. The actual number of interactions is much greater, but only the most important are given here.

The ecosystem can be approached from various angles. Thus it can be characterized by a description of the quantities of carbon of which the groups of organisms consist and the size of the carbon flows between these groups. But such an approach does not take account of every facet of the ecosystem, and so it is necessary for example to consider simultaneously the influence of physico-chernical environmental factors on organisms and conversely the modification of these factors by organisms (lowering of oxygen content by biological consumption, sediment structuring, etc.). From all these facets the

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structure and relative importance of the components can be presented in the form of diagrams such as figure 9.3. But such diagrams invariably represent a transient situation or an average situation. What is required in addition is a representation of how the influence of a first quantity on a second quantity can lead to feed-back which modifies the first quantity and of how the influence of a quantity is exerted via achain reaction on other quantities: in short, a representation of the dynamic element of the system. This can only be done by describing the system with a mathematical simulation model. Such modelling has therefore received much attention in recent years.

Although the research is of an applied nature, it has come about through interaction with developments in pure research in estuarine and marine biology and has in its turn made its own contribution to pure research. This interaction was encouraged by the fact that the staff were housed at research organizations (Netherlands Institute for Sea Research and Groningen University).

Some of the new insights and questions that came to the fore during the research were adopted in the research programme, but other questions were left unanswered owing to lack of time or financial resources or owing to the absence of suitable research meth-ods.

By working on the basis of a common plan, effective integration of the separate parts of the research was obtained and the various practical activities harmonized with each other. An important feature here was the fact that the organizational structure was flexible enough to allow the research to be modified in the course of the study. Such an approach to ecosystem research is achieved by only a few groups in the Netherlands and elsewhere.Itis probably superfluous to stress that the result that is obtained is much more than the sum of a number of individual results.

2.3 Results of the study and their application in management

The results of the study can be divided into two categories, firstly the basic data that are necessary to provide a description of the ecosystem and secondly the results of the research into the functioning of the ecosystem. As regards the results of the first category, information is now available on the distribution of the most important orga-nisms, the composition of the populations and the chemical composition of the organic material.Aninsight has also been obtained into spatial variability, seasonal variation and variation from year to year. This is necessary so that when comparisons are made, either with other areas or with subsequent research, the differences that are found can be distinguished from the natural variations that are always present in populations. It has recently become apparent that the microbenthos (unicellular animals in the bed) and the microzooplankton (uni- and multicellular animals in the water, smaller than 200p.m) may play an important role in the ecosystem. No research has yet been done into these

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groups. No research into birds and seals has been done within BOEDE, but information on these species is available from elsewhere (National Institute for Nature Management and Groningen University).

In addition, information is available on the influence on organisms of physical and chemical environmental factors such as salt content, sediment composition, temperature regime, suspended material, currents and the exposure of the mud flats. Little is known however of the influence of e.g. heavy metals, chlorinated hydrocarbons and radioactive isotopes. For a number of substances in the first two groups it is known that their content in animal organisms in the Ems-Dollard is higher than in other parts of the Wadden Sea (data from the National Institute for the Purification of Effluents (RIZA) and the Netherlands Institute for Sea Research (NIOZ)). This indicates heavy loading of the estuary with these substances but there has been no further research into the conse-quences of this for the ecosystem and thus it is impossible to say anything about those consequences, even with a simulation model. Finally, insight has been obtained into the relations between the various organisms. This is based partlyon results of BOEDE research and partlyon the results of research carried out elsewhere.

Itis apparent from the research that there is a distinct difference in the structures of the biotic communities in various parts of the estuary. The Dollard is a species-deficient brackish system whilst further out to sea the number of species increases and the ecosystem becomes more similar to that of the Wadden Sea. Functionally, the Dollard is dominated by the benthic system, the principal energy source being the influx of organic material from elsewhere. In the outer area, in contrast, the pelagic system predominates and the local production of organic material by algae is the principal source of energy for the ecosystem.

The results are documented in the publications and reports of BOEDE (chapter 13), of which the present report forms a summary. In addition, a large part of the existing knowledge has been integrated in the form of a mathematical simulation model of the ecosystem.

The fact that new insights have been obtained into the functioning of the Ems-Dollard ecosystem and the role played by the various elements is at least as important as the collection of concrete data that provide a description of the ecosystem. Examples of these insights are: the fact that the great majority of the conversions in the biological system are performed by the very smallest organisms; the role of the detritus in the ecosystem and the fact that only part of it is biologically useful; the possibility of establishing an oxygen balance whereby the relative importance of various processes in this balance can be determined; the relationship between the contributions of aerobic and anaerobic organisms to the total mineralization process.

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While the BOEDE research was still in progress, the results were already being used for management purposes. Data were used for the following recommendations, reports and activities in the sphere of environmental impact assessment:

- Collaboration on the RIN report on: the ecological consequences of the deflection of the Ems via the Dollard [155].

- Making data available to the consulting engineers Dwars Heederik en Verhey BV for a report on the possible consequences of the establishment of a petrochemical complex at Eemshaven.

- Reports on the consequences of laying the potato waste effluent pipeline at Bierurn (Gr.). [BOEDE reports 21; 53; 61; 84].

- A report on the environmental consequences of the possible installation of an LNG terminal at Eemshaven [169].

- Use of BOEDE data in a report commissioned by NV Gasunie on the environmental consequences of the construction of a coal gasification installation at Eemshaven [153; 243].

- A report on methods for measuring quantities of silt in suspension in connection with the dumping of spoil in the Ems at Delfzijl[30].

- A publication on the amount of dredging and the quantity of material in suspension in the water [77].

- Provision of information required for management planning for the Wadden Sea. - Collaboration on the provisional committee assessing the environmental impact of

large-scale deposition of spoil in the Rhine mouth area.

- Research into the sea-bed fauna of the North Sea in the dumping area for Ti0₂waste acid, carried out in accordance with EEC guidelines (Public Works Department, North Sea Directorate) and reporting of the results [107].

- Advising on the possible consequences of laying a pipeline for unpurified effluent from the potato flour factory at Ter Apelkanaal to Nieuwe Statenzijl, for the oxygen content of the Dollard [116].

In addition to the problems that arose in these recommendations there are many more human activities that threaten or interfere with the ecosystem. In chapter 11 the influence of individual examples (discharge of potato waste effluent, increased turbidity, eutrophication) on the ecosystem is discussed. It emerges that besides the direct influence on organisms, indirect influences can also occur as a result of chain reactions in the system.

In addition to the direct use of experimental results as a guide to management policy, the bodies responsible for management of the estuary have also benefited indirectly from the research.Ithas contributed to their understanding of the functioning of the ecosystem, as is demonstrated by the expansion of the research objectives during the transition from BOVA to BOEDE (see 1.1).

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For guidance of future policy-making the simulation model of the ecosystem is available. This is the most compact summary of a large proportion of the results.Itmust be realized though that application of the simulation model requires expert knowledge if a correct interpretation of tbe results is to be obtained. Tbe usefulness of the simulation model is not confined to the problems of the Ems-Dollard estuary. Owing to its universal design and because it is largely based on general scientific data, it can be modified for other estuarine areas provided that sufficient descriptive data are available. A first test of this has already been carried out. A model was set up for a Canadian estuary (Cumberland Basin, Bay of Fundy) and the first version gave a broadly satisfactory picture of the ecosystem there.

Looking back over the BOEDE research it can be conc1uded that a coordinated mul-tidisciplinary design and flexible planning and organization permitting modifications during the course of the research are essential for a successful outcome. From the practical viewpoint it is important that the research extends over at least a number of years and there should preferably, as at BOEDE, be little turnover of staff.

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3 Hydrographic and biological survey

3.1 The estuary

3.1.1 Introduction

The Wadden Sea is a shallow coastal sea characterized by the exposure of large areas of mud flats as the tide ebbs. The ebbing water flows through a heavily branched system of channe1s into the main channels. The major tidal channe1s find an outlet to the sea through the gaps between the islands. The process is reversed during the flood tide. The Frisian Islands form a natural barrier against the North Sea. The Wadden Sea is in fact divided into a number of tidal basins which are separated from each other by relatively high-lying systems of mud flats behind the islands (tidal watershed). Furthermore the area is intersected at a number of places by river mouths. The estuary of the river Ems (see map at back of book) forms such an intersection.

In an estuary the movement of water and the behaviour of suspended material are complex phenomena. This is partly due to interaction with the often complicated topography.

These physical processes are important for the ecosystem because in many respects they create a characteristic environment which forms the boundary conditions within which the biological processes take place. A brief outline of the morphology and hydrody-namics will therefore now be given and the relationship between hydrodyhydrody-namics and the transport of suspended material will be discussed. In 3.2 a description of the estuarine biotic communities will be given.

3.1.2 Morphology

The estuary of the river Ems is characterized by channel systems, mud flat systems, salt marsh, islands and dikes as the fixed boundary of the coastline (see map at back ofbook). The boundarywith the North Seaisformed by the linejoining Rottumeroog to Borkum. The vertical boundary is formed by the mean high water level.

Geographically the area under investigation can be divided into three portions: the so-called outer area is formed by the region between the boundary with the North Sea and Eemshaven, the 'rniddle area' is situated between Eemshaven and the Mond van de Dollard, and the 'inner area' comprises the Dollard, the Emder Fahrwasser and the river

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channel of the Ems as far as Pogum. Between part of the Emder Fahrwasser and the rest of the Dollard is an artificial barrier in the form of a bank constructed as a guide to navigation, which however has gaps at a number of places. The area of the Ems estuary between Borkum and Leer is approximate1y 500 km2_._{The total area of the flats that are} exposed at the mean low water level is approximately 250 km2•The proportion of the estuary that on average is exposed at low water is approximately 45% in the outer area, 50% in the middle area and 80% for the Dollard. The extensive mud flats in the Dollard are bordered by a belt of salt marshes with an area of approximately 9 km2

.In contrast to the other parts of the Wadden Sea, the salt marshes in the Ems-Dollard estuary con-stitutes a significant proportion of the total area: 2%. Table 3.1 gives a number of morphological data for the three areas of the estuary.

Table 3.1 Dimensions of the three distinct areas of the Ems-Dollard estuary

Area of channels (m2)

Mean channel depth (m)

Total water volume of channels (m3)

Area of intertidal flats (m2₎

* Including the Ems as far as Pogum Outer 145.104 6.6 955.106 IlO· 106 Middle 56.4· 106 7.45 420.106 53.1· 106 Dollard* 19.7.106 6.2 120· 106 82.6.106

3.1.3 Distribution and transport of water and dissolved and suspended material

The movement of water in the Ems estuary is caused principally by the twice-daily tides and to alesser extent by the entry of rivers and by wind. The tidal wave moves through the area with a mean period of 12 h 25 min. As this wave passes the water rises and fal1s (vertical tide). The mean difference between high and low water for the last five years is 2.30 mat Borkum, 3.16 mat Emden and still slightly more than 3 m at Nieuwe Statenzijl. The rise and fall of the water level is accompanied by water displacement (horizontal tide). This horizontal tide can be characterized in various ways. First of all it can be expressed as the quantity of water flowing in and out through a given cross-section in the area. At Borkum 900-1000.106_m3 _{water passes during an ebb or flow period. The} decreasing storage in basins means that at the Mond van de Dollard this value is only 150-200· 106_{m3 .}

Another way in which the horizontal movement of water can be expressed is the tidal path. This is the mean distance travelled by the water between two consecutive tides. This distance varies and is approximately 12 km in the Dollard and approximately 17kmin the outer area. The rate of flow is approximately 0.3-0.4 m·S-l.Dnder normal weather conditions the maximum rate of flow varies from Im·S-l in the Dollard to

2 m·S-Iin the outer area. Accurate analysis of the flows measured at many places by the

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that not only does the water flow backwards and forwards but also there are channels that displace more water during, the flood than during the ebb (flood channels) and vice versa (ebb channels). These ebb and flow channels normally occur together in pairs, setting up a water circulation which leads to intensive mixing. And within the flood and ebb channels are similar circulatory pairs, of which figure 3.1 gives an example.

DOL

A_~~-S

-10 B, 106m3 oSleb,?-

n

os]floOd

Bocht v.Watum Oost - Friesche Gaatje

Figure 3.1 Example of a channel system with ebb and flood channels. The distribution of residual flows (B) is given for the cross-sections (A) of the Bocht van Watum and the Oost-Friesche Gaatje. D.O.L.= Dutch Ordnance Level

U nder the influence of wind strength and wind direction, so-called drifts can also occur. They cause a water circulation which is of particular importance in the Dollard and the outer area of the estuary. In the Wadden Sea considerable quantities of water can be transported behind the islands across the tidal watershed during periods of strong wind [160]. Measurements performed by the Survey and Consultancy Department of the Public Works Department at Delfzijl on the flats along the Oude Westereems, the flats along the Ransel and in the Oostereems (see map at back of book) show that this process of drift currents also occurs at low wind speeds but that the water transport is of little quantitative importance. This means that water exchange occurs between the Ems estuary and the bordering parts of the Wadden Sea, depending on wind strength and direction. Thus in the outer area the boundaries of the estuary cannot be sharply drawn.

In addition to the circulations described above there is also a vertical water circulation. This occurs in the area where river water comes into contact with brackish estuary water. Because it is less dense, the fresh river water tends to flow to sea above the brackish water whilst the salt sea water, owing to its greater density and the movement of the tides, moves beneath the lighter fresh water (salt wedge). Meanwhile the two masses of water become partially mixed under the influence of the turbulent flow. The result is a net transport of the salt water in the upstream direction. This mixing eliminates the vertical density differences. This estuarine circulation is maintained by the influx of fresh water.

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Animportant factor in the mixing of different masses of water is the turbulent diffusion that results from differences between the flow rates in different parts of the cross-sections of channels. The above is a brief outline of some of the many types of water circulation that all contribute to the mixing of salt and fresh water which manifests itself as a salt gradient (figure 3.3) in which the gradient of the curve is primarily determined by the size of the influx from rivers [69].

F or modelling purposes the mixing of the water in the estuary can be expressed simply in terms of diffusion constants (see below). These diffusion constants actually represent the sum of all the circulations and exchanges occurring in the estuary and are only partially associated with molecular diffusion or turbulent diffusion. By means of a water trans-port model [69] the state of mixing of salt and fresh water can be calculated for various run-offs from the river Ems and the Westerwoldsche Aa (chapter 10), and similar calculations can in principle also be performed for other substances dissolved in the water such as nutrients, oxygen, carbon dioxide and micropollutants. Further charac-teristics such as the mean residence time of the water in the estuary and the life-time of the water can also be calculated. The residence time is the time taken for a particle starting at a given place (compartment) in the estuary to disappear from the estuary. The life-time relates to water that comes from outside (North Sea, river Ems or Wester-woldsche Aa) and represents the time for which the water has been in the estuary. For instance, for an Ems run-off of approximately 55 m3 . S-l and a Westerwoldsche Aa run-off of 9 m3 •S-I the residence time of the water is approximately one week in the

outer area and approximately seven weeks in the Dollard. The mean residence time of the water can vary widely, depending on the influx of fresh water.

The movement of estuarine water outlined above is in equilibrium with the morphology of the estuary [231; 77]. This means that changes in the morphology as a result of e.g. dredging lead to changes in the movement of water and consequently to changes in the parameters that are directly associated with this water movement. This subject is dis-cussed in 11.3.

The monthly run-off from the river Ems varies between extremes of 25 m3 •S-lin the

summer and 380 m3 . S-Iin the spring (see figure 3.2). The influence of this is expressed directly in the variation of the salinity gradient over the year (figure 3.3). In the spring isohaline 15 is situated on average half-way along the Oost-Friesche Gaatje and the Bocht van Waturn. In the summer period, when river run-offs are low, this isohaline is displaced approximately 20 km towards the Dollard and at Pogum further up-stream into the river Ems.

The influx of suspended material from the river and the presence of suspended material in the sea do not in themselves lead to a gradient in suspended material. The charac-teristic gradient arises as the result of the estuarine water circulation in the transition

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-3 -1 m 'S 300 200 100 1975 1976 1977 1978

Figure 3.2 Mean monthly water run-offs from the Ems at Pogum and from the Westerwoldsche Aa for the years 1975-1978

salinity

30

20

Figure 3.3 Salinity distribution along the axis of the Ems estuary and the Dollard for two different river discharge values:

1: Ems 53.3 m3_'s-1_{and Westerwoldsche Aa 8.8 m}3_·s I;

2:Ems 320 m-3 • S-Iand Westerwoldsche Aa 31 m3 •s I

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region from freshwater to brackish water (figure 3.4). Under the influence of the decrease in flow rate in this transition region, some of the suspended material that is introduced with the river water is deposited. This material thus finds its way into the salt wedge that is flowing upstream. Once they have entered this salt-water layer, the particles can be carried up to the fresh-water top layer again by turbulent diffusion and then the cycle can begin again. A similar situation pertains for the material that is carried upstream from the sea by the salt wedge [216]. This mechanism leads to accumulation of suspended material in this zone, which is known as the turbidity zone and moves backwards and forwards between Emden and Leer depending on the phase of the tide and the river run-off. Tbe river run-off has a direct influence on the maximum content of suspended material in this zone [214]. This maximum is low for high river run-off and high for low river run-off. From the BOEDE data bank it is evident that the mean annual content of suspended material between Emden and Leer is virtually constant.

suspended material (mg·I-') 400 300 200 100

•

\

•

O-+--..,.---r--r---r--..,.----,-..,.-...,...--r---r--r---r---r----10 20 30 40 50 60

t

70

t

80 90 100 distance (km)

Borkum Delfzij I Knock Emden Leer

Figure 3.4 Gradient of content of suspended material along the axis of the estuary for the period 1975-1976. The so-called turbidity zone lies between Emden and Leer

The tide itself is also the reason why, independently of the influx of fresh water, a gradient in suspended material occurs [208; 210; 211; 232; 233]. This is associated with the mechanism that leads to the accumulation of sand and silt in estuaries. The tide, which is virtually symmetrical in the open sea, becomes asymmetrie in estuaries because of their shape and shallowness. During the flood tide the maximum speed at which the tidal wave enters the estuary is therefore greater than the rate of flow during the ebb. Since the capacity to transport sand and silt increases with velocity, the flood Can bring

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in more silt than the ebb can take out. An additional factor that is particularly relevant to mud flats is that around high tide material can be sedimented without the water velocity at the onset of the ebb being sufficient to resuspend the material. This happens because the water velocities necessary to resuspend sedimented material are greater than the velocity at which the same material can be sedimented.

The silt gradient shown in figure 3.4 is liable to change because the contents vary with the flow rates (figure 3.5). This leads to considerable variations in the concentrations of suspended material. At half tide (greatest flow rates) the concentrations are anything up to a factor of 5 higher than at high and low tide (minimum flow rates). üwing to the different specific gravities, these differences are much greater for sand (the fraction larger than 55 p.m diameter) than for silt (the fraction smaller than 55p.m).Moreover the concentrations near the bottom (figure 3.5) are 3 to 5 times greater for silt and 5 to 10 times greater for sand than at the water surface.

In addition to the variation caused by the tide there is also a seasonal variation in the concentration of suspended material. This is mainly due to an increase in the resus-pension during periods of strong wind.

In the fourth quarter the mean concentrations of suspended material are consequently approximately 1.5 times as high as in the rest of the year. The increases in concentration due to wind disappear rapidly (in about two days) when the wind abates.

The influence of wind and waves is particularly strong at high water since the mean depth of the water is small and, owing to the long distance over which the wind can act, the waves have a strong effect on the sediment of the flats. This leads to exchange of sediments and other material (e.g. diatoms) between flats and channels (see chapter 5) and to a certain sorting of the sediment. The exposure of the flats with respect to wind direction plays an important role here.

The mean content of suspended material has been found to vary over a number of years (tabIe 3.2). These variations were established by BüEDE on the basis of 50 to 175 annual measurements and by the Public Works Department on the basis of more than 500 measurements. The differences in the annual means are small. The annual fluctuations in the contents of suspended material are also reflected in the optical attenuation coefficient K_d(see 5.3), which is a measure of the c1arity of the water. Possible reasons for the year-to-year variations are discussed in 11.3.

The composition of the suspended material in the estuary is analysed by means of a partic1e counter. Figure 3.6 shows by way of example the distribution of the material over various partic1e size categories. Although they are relatively uncommon, the large partic1es still constitute a considerable fraction of the total volume of material. These spectra have been measured at various places in the estuary.Ithas been found that the relative compositions in the inner and outer area are not very different from each other,

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surf. de ep sand

1=

180.3 118.6 114.0 ebb 20 80 60 40 cm·sec-1 flood _"\ /

'---

,

0+-'----'4.---,-,0<.,.:-1

_,

./

"

/ / '--/ 100 , o+-,..L!...----"

-=_-=::c..::.-_----"'--'-"--""-:::

_~""

o+-r';:-'r-r-.--.--....-.--.----.--.--r=:rr 5: 7 9 11: 13 15 17:hours I.w hw I.w

Mond van de Dollard

Figure 3.5 Variations in the current velocity in cm· s-' and the content of suspended material over the tidal cycle. The highest sand (particles > 55p.m)and silt (particles < 55p.m) con-centrations are attained in the ebb and f100d tide close to the bottom (deep); the lowest concentrations occur during slack water and at the water surface (surf.). Measurements from the Mond van de Dollard.

despite the striking differences in the total content of suspended material.

The Wadden Sea is subject to a continual accumulation in parallel with the faIIing of the sea bed with respect to sea level. Reenders and Van der Meulen [219] estimate that the annual deposition of silt in the Dollard amounts to ca. 8 mmo From research in the western Wadden Sea this accumulation appears to vary from year to year, a normal value being 7 mm [179].

For the Ems estuary an annual accumulation of 7 mm requires a good 5.106 _ton sediment per year. This quantity cannot possibly be supplied by the river Ems, which discharges only 0.1 . 106_{ton [183]. The material that is sedimented in the Ems estuary} must therefore be predominantly of marine origin, which is consistent with the geolog-ical observations of Salomons [225], Rudert and MüIIer [224] and Favejee [178]. Blöcker and Thomsen [157] and Ittekkot et al. [186] have compared concentrations of suspended material from individual estuaries with each other. It emerged that the content of suspended material in the turbidity zone of the Ems-Dollard estuary was a good 1.5 times as high as the values measured in the estuaries of the rivers Elbe and Weser [77]. The mean concentration of suspended material in the Ems-DoIIard estuary outside the turbidity zone is also a good 1.5 times as high as in the WesterscheIde, according to RIZA data. The Ems-DoIIard estuary therefore contains, on average, more suspended material per unit volume than the other estuaries that have been mentioned.

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Tab1e 3.2 Annual mean concentrations of suspended material in the Ems-Dollard estuary measured by [1] Postma [209] and Manue1s& Rommets [200]; [2] BüEDE and [3] The National Institute for the Purification of Effluents (public Works Department). The clarity is expressed as an optical attenuation coefficient Kd

Year Mean content of suspended (1) material (mg·I-I) (2) (3) 1954 1970 1971 1972 1975 1976 1977 1978 1979 1980 45.8 61.3 60.4 90.4 58.2 57.6 68.3 96.6 76.5 91.5 82.5 98.3 2.94 3.18 3.69 3.37 3.93 cumulative volume in % 100 / / / / 50 / / /

""

,,-I / /

"

r' .::..,1 ,----...,.._,.:.-~..JJ+ 0 15 20 25 30 40 diameter Cu m)

Figure 3.6 Size distribution of suspended particles near Emden showing the volumes of individua1 fractions (7--40I-Lm)and their percentage contribution to the total volume (p10tted on a cumu1ative scale)

The consequences of tbis will be discussed in chapter 11.

3.1.4 Sediment

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differences are primarily in chemical and mineralogical composition and in the size of the individual particles. In addition to the mineral particles the sediment also contains organic material of both animal and vegetable origin (8.4).

Between the separate particles is the interstitial or pore water of the sediment. This water contains dissolved substances (nutrients, oxygen, chloride, sulphate, dissolved organic carbon compounds, free sulphide).

The mineral particles can be classified according to size (Wentworth scale): a coarse sand fraction (2-0.25 mm), a fine sand fraction with a particle size between 250 and

62.5JL, a silt fraction (62.5-3.9JLm) and a clay fraction

«

3.9JLm). Most sediments consist of a mixture of different size classes. Thus the soft muddy sediments at the inland end of the Dollard contain considerable quantities of silt and sand in addition to a considerable percentage of clay. Sand usually forms the main component of the sedi-ment. The sand and silt fractions consist of different mineraIs, quartz (Si02) invariably being dominant. The clay consists mainly of aggregates of plate-like clay minerals; to these adheres mainly organic material that thus contributes to the aggregation. The way in which the material suspended in the water undergoes net transportation towards the coast against the concentration gradient has been described above. This accumulation mechanism, together with resuspension under the influence of waves and sedimentation (depending on flow rate), leads to a certain sorting of the sediment fractions. Thus a sediment that is heavily exposed to wave action has a sandy character whilst more sheltered and higher-lying sediments, particularly those lying more inland and tnwards the coast, attain a much higher clay content. There is thus a pattern of gradients from sands with a low clay content to clay-rich sediments.

Since these resuspension, sedimentation and accumulation mechanisms also apply to organic particles, the gradient pattern of organic carbon in the sediment is roughly the same as that of clay [121]. The particle size pattern is coupled to the pattern of the specific gravity of the sediment, the porosity (content of pore water as a percentage by volume of the sediment) and the permeability, on the understanding that a sandy sediment has a high specific gravity, a low porosity and a high permeability.

The black colour of deeper sedimentary layers is due to iron sulphide which is formed by the precipitation of free sulphide dissolved in the interstitial water with iron ions to form watery, colloidal iron sulphides (FeS· nH₂0) The sulphide originates through bacterial reduction of sulphate and in the form of iron sulphide can be reoxidized (figure 8.11). Sulphide also occurs as the very stabIe pyrite (FeS2)which is lighter in colour. Owing to its great stability, pyrite can also occur in the aerobic top layer of the sediment when it is transported there by the digging activity of worms, particularly of larger worms. Iron sulphide plays an important role as an anaerobic buffer in the sediment (4.2.1) and free sulphide is important because it is a toxic substance which can poison organisms even at

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3.2 Characterization of the groups of organisms

3.2.1 Energy flows

As in any ecosystem, the biotic communities in the Ems-Dollard estuary depend on the presence of biologically consumable energy. Sunlight is the primary energy source because during photosynthesis by the so-called primary producers, which in the Ems-Dollard estuary are mainly unicellular algae, solar energy is used to produce organic material- new algae cells - from carbon dioxide, water and inorganic salts. The algae are eaten (grazed) by innumerable herbivorous organisms which use their food partly as fuel to provide their own energy and partly to form their own biomass (secondary produc-tion). Besides primary production a second food source is also important in the estuary: organic material transported into the estuary by the rivers and from the North Sea, especially in the form of dead organic material (detritus). Detritus is (sometimes par-tially) broken down by bacteria, thus re-creating living organic material (or bio-mass).

Bacteria are in their turn eaten by larger organisms and thus support the food chain in the estuary. Dead algae and animals are also broken down by bacteria and their organic matter is thus partly returned to the biotic community. Within the overall food chain a distinction can therefore be made between a so-called detritivorous branch based on bacteria and aherbivorous branch based on the photosynthetic algae. In an estuary these two components of the food chain are c10sely interwoven. Itis the task of ecosystem research to chart the flows of organic material through the populations of algae and bacteria and to analyse the transfer of material in the food chain. Like the density of organisms, the food flow is expressed in terms of organic carbon so that the concen-trations of various organisms and their food and their consumption rate, respiration and growth can be compared with each other (see chapter 9).

3.2.2 Pelagic and epibenthic organisms

Figure 3.7 shows some common representatives of the groups of organisms that occur in the plankton.

The principal primary producer in the water, the phytoplankton (see chapter 5) consists of unicellular siliceous algae (diatoms) and mobile phytoflagellates, of which the most conspicuous is the colony-forming species (Phaeocystis pouchetii) which blooms in the outer area in the late spring. The composition of the phytoplankton by species fluctuates sharply in the course of the year; altogether we are dealing with approximately 50 common diatom species and at least 20 species of phytoflagellates. Some species of diatoms bIoom in the early spring when the concentration of dissolved silicate is high; silicate is an indispensable element for the construction of the diatom skeleton.

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Other species of algae bIoom in the summer when the temperature and light are optimal for growth.

The zooplankton, which largely consists of calanoid copepods and in certain seasons of the larvae of bottom-dwelling fauna (see chapter 6), forms the most important group of secondary producers. They feed mainly by sieving the water, capturing food partic1es such as algae cens and detritus fragments together with the associated bacteria. The distribution of the 8 species of calanid copepoda in the estuary shows a c1ear corre1ation with the salinity gradient in the estuary and moreover shows seasonal variations. The microzooplankton, organisms that pass through a net with a mesh size of 55 p.m such as ciliates and flagellates, have not received much attention within the BOEDE. In recent years however it has become steadily c1earer, in the literature too, that the microplankton is a group of organisms that forms a quantitatively important link between the phyto-plankton and the bacteriophyto-plankton on the one hand and the larger zoophyto-plankton on the other. Under favourable conditions these microscopically small organisms can multiply very rapidly (in hours to days) by division and thus bIoom explosively.

The nekton consists of organisms such as fish, shrimps and crabs that are caught with a net with a mesh size of 0.5 cm. They are divided into pelagic and epibenthic species (see chapter 7) according to the main place where they forage. Herring, young smelt and sprat are examples of pe1agic species that feed on zooplankton or other macroplankton, whilst all flat fish, shrimps and crabs belong to the epibenthic group which feeds on benthic fauna. Altogether approximately 40 fish species, 7 crab species and 2 shrimp species occur in the estuary. Their generation time varies from one year (shrimps) te several years (fish). Nevertheless their population density can vary rapidly in response to short-term variations in e.g. the food supply: eggs and larvae are produced in great excess and their chance of survival depends strongly on the prevailing food condi-tions.

Pelagic fauna too seek high food concentrations, performing mass migrations which can mean large fluctuations in the population density of a particular area or sub-area. The top carnivores in the Ems-Dollard estuary, birds and sea mammals (now only seals) were not studied within the BOEDE. Sufficient information on this group of organisms is available from elsewhere (Netherlands Institute for Sea Research [NIOZ], National Institute of Nature Management, department of Estuarine Ecology (RIN), Groningen University), however, (see chapter 7) to allow them to be incorporated in general ecological considerations of the Ems-Dollard estuary and in considerations of possible consequences of human intervention in the estuary (see chapter 11).

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Q,1mm

Figure 3.7 Examples of organisms living in the water. Ctenophora (Ct), Herring (H), Noctiluca (N), Copepods (C), Mysids (M), Crab larva (L), Tintinnidae (T), Diatoms (D), Flagellate algae (F), Protozoa (P).

3.2.3 Benthic organisms

Figure 3.8 shows some common bottom-dwelling organisms that populate the intertidal flats of the Ems-Dollard in large numbers.

The primary producers are represented by roughly 100 common species of diatoms, whilst several species of blue-green algae are present (see chapter 5).

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The benthic fauna is subdivided into microfauna, meiofauna and macrofauna (see figure 3.8). This classification depends primarilyon the size of the organism: macrofauna cannot pass through a sieve with a mesh size of 1 mmo Meiofauna is smaller and as multicellular organism is distinct from the unicellular microfauna.

Each of these fauna groups comprises organisms that are taxonomically very different. The meiofauna, for example, includes nematodes, harpacticoid Copepoda, Oligochaeta, Ostracoda and Turbellaria. The micro- and meiofauna lives mainly between the sand grains (interstitial) whilst the macrofauna can make passages in the sediment. The turning over of the flat by all these organisms (bioturbation) is important e.g. for the oxygen balance of the flat (see 4.2).

The concentration of flora and faunainthe top layer of the flat is very high and the food interrelationships are complicated (see 3.2.4).

Owing to the large differences in environmental conditions within the sediment, bacteria are represented by a large number of metabolic types (figure 3.9).

Inthe oxygen-rich part of the sediment the aerobic heterotrophic bacteria are particu-larly important in the mineralization process (see 8.5.1). Under anaerobic conditions anaerobic bacteria take the place of the aerobic heterotrophic bacteria in the degrad-ation of organic material. The hydrogen produced by the degraddegrad-ation of organic mate-rial is coupled by aerobic heterotrophic bacteria to molecular oxygen, which thus functions as a terminal hydrogen or electron acceptor. This is aerobic respiration. There is also anaerobic respiration in which oxygen atoms in inorganic compounds such as NO)- (nitrate) or S042_- _{(sulphate) function as terminal hydrogen acceptors (figure 3.9).} The bacteria that carry out these processes are known as denitrifying and sulphate-reducing bacteria respectively. There are also fermenting bacteria which use organic molecules as terminal hydrogen acceptors, excreting the reduced organic molecule. The excreted molecules are usually low-molecular organic carbon compounds such as lactic acid, which cannot be fermented further, although they can be mineralized further by anaerobic sulphate-reducing bacteria.Inaddition to the aerobic heterotrophic bacteria the anaerobic sulphate-reducing bacteria are of particular importance in the sediment of the estuary not only because they account for a sizable portion of the total mineralization (8.5.2) but also because they produce poisonous sulphide or hydrogen sulphide on a large scale. This sulphide may however be precipitated with iron to iron sulphide giving the sediment a black colour. Sulphide can also be used as an energy source by aerobic sulphide-oxidizing bacteria which oxidize the sulphide to sulphate with oxygen. With the energy that is gained they can fix CO₂and thus build their cell material (figure 3.9). Together with the sulphate-reducing bacteria they form the sulphur cyc1e, which is maintained by the supply of degradable organic material from surface sediment layers.