Hydrobiologia 282/283 : 1-14, 1994 . P. H . Nienhuis & A . C. Smaal (eds), The Oosterschelde Estuary . 1994 Kluwer Academic Publishers . Printed in Belgium .
1
The Oosterschelde estuary, a case-study of a changing ecosystem : an introduction
P. H . Nienhuis' & A . C . Smaal' 'Netherlands Institute of Ecology, Vierstraat 28, 4401 EA Yerseke, The Netherlands ; 'National Institute for Coastal and Marine Management/RIKZ, P .O . Box 8039, 4330 EA Middelburg, The Netherlands
Oosterschelde estuary, civil engineering works, storm-surge barrier, hydrographical changes, long-term ecological changes, community and ecosystem responses Key words :
Abstract During the period 1980-1990 long-term physical, chemical and ecological studies were carried out, to study the changes induced by the building of a storm-surge barrier in the mouth of the Oosterschelde estuary and two large auxiliary compartment dams in the rear ends of the estuary . The storm-surge barrier was constructed in the mouth of Oosterschelde estuary (SW Netherlands) during the period 1979-1986 . The barrier allows the tides to enter the estuary freely, and, on the other hand, the barrier guarantees safety for the human population and their properties when a stormflood threatens the area . Oosterschelde estuary is isolated from the river input, the rear ends of the ecosystem were separated from the estuary by sea-walls and the strongly decreased tidal exchange with the North Sea induced sheltered circumstances . The Oosterschelde changed from a turbid estuary into a tidal bay, and yet primary production responses appear to be robust and resilient, and the biological communities showed only quantitative shifts from the dominance of specific species assemblages to other assemblages . In many cases predicted changes in the structure of the biological communities could not be verified owing to the large natural variability mainly caused by physical factors (e .g . temperature) .
The Delta Project - Storm flood protection in the Oosterschelde The Netherlands Delta Project covers the socalled Delta area, created by the rivers Rhine, Meuse and Scheldt . Like most delta-estuarine environments, this area represents, in its natural state, complicated ecosystems consisting of a complex hydrodynamic regime, with fast-flowing water masses in tidal channels, changing estuarine configurations, inhomogeneous tidal and subtidal sediments, and salt-marsh areas subject to periodic flooding . The history of the SW Netherlands is marked by a continuous struggle
between man and the sea . Since the year 1000 man reclaimed salt-marsh areas and transformed those into agricultural land . But irregularly occurring stormfloods broke the man-built seawalls and recaptured parts of the gained land. On February 1, 1953, a northwesterly storm induced tides to 3 m above normal levels, breached approximately 180 km of coastal-defense dikes and flooded 160000 hectares of polderland . 1835 people lost their lives in this large storm flood, more than 46000 farms and buildings were destroyed or damaged, and approximately 200000 farm animals were lost . The Delta Project, formalized in 1957 by an act
2 of the Dutch parliament, was conceived as an answer to the continuous risk of flooding, which threatens lives and property in this low-lying region . Because of the low mean elevation and premium on space in The Netherlands, the Dutch have a long tradition of coastal-defense construction and land reclamation . The need for continuous coastal construction has intensified over the years as a result of population growth, land subsidence and rising sea levels . The core of the Delta Project called for the closure of the main tidal estuaries and inlets in the SW Netherlands, except for the Westerschelde where the existing dikes have been raised, for reasons of continued international shipping access to Antwerp (Huis in 't Veld et al., 1984) . A prerequisite for the construction of the primary sea-walls in the mouths of the estuaries was the need to reduce tidal-current velocities in the estuaries, before the construction of the primary barriers could be undertaken . Tidal velocities were lowered by constructing secondary compartmentalisation barriers (Zankreekdam, Grevelingendam and Volkerakdam ; Fig . 1) to reduce the extent of the Delta area subject to tidal influence . This resulted, in turn, in a reduced tidal volume and, therefore, lower current velocities through the main estuaries . The former estuaries Veersche Gat and Grevelingen were closed off from the North Sea by high sea-walls in 1961 and 1971, respectively, and turned into non-tidal lakes or lagoons filled with brackish or saline water, whereas the Haringvliet was closed in 1970 by the construction of large sluices, meant to function as an outlet for the rivers Rhine and Meuse (Fig . 1) (Knoester, 1984) . The original plan for the Oosterschelde estuary called for a dam across the mouth of the estuary, a distance of 9 km, to be finished in 1978 . The tidal basin would than have been changed into a stagnant lake filled with - polluted - water from the river Rhine . But the final form of the present barrier differs drastically from the simple dam that has been envisaged originally. Through the 1960's and early 1970's, conservationists provoked an awareness in many people of the need to protect the area's outstanding natural resources
and its unique tidal habitat, including an extensive shellfish industry, the only one in The Netherlands (Smies & Huiskes, 1981) . The Dutch government decided to change the design of the dam in 1974. After several years of desk studies the Dutch parliament accepted in 1976 a compromise solution : a storm-surge barrier . The barrier allows the tides to enter the estuary freely, thus safeguarding the tidal ecosystem, including the plant and animal communities . On the other hand the barrier guarantees safety for the human population and for the properties of the inhabitants when storm floods threaten the area. This design meant a turning point in the Dutch political decision-making process with regard to the natural environment (Knoester et al., 1984). The storm-surge barrier was constructed between 1979 and 1986 in the western mouth of the estuary (Fig . 2) . A series of 65 prefabricated concrete piers (pylons) form the framework of the barrier. The piers support 62 steel gates which can be dropped like portcullises to close off the estuary when danger threatens . The piers and the beams which tie them together have been built inside coffer-dams, and were transported by a special vessel, a pier transporter, to the barrier alignment across the estuary . The piers have base dimensions of 25 x 50 m 2 and a maximum height of 43 m ; they weigh 18 000 tonnes each . The piers are located on the bottom of the sea on thick foundation mattresses, to prevent erosion of the bottom sediments . The largest part of the entire construction is permanently under water . To provide for the long-term stability of the installed piers, the base of each pier has been covered with rubble and large stone blocks, and the piers were joined together with beams to make a single construction . The final superstructure of the storm-surge barrier carries a road across the estuary mouth (Fig. 3) . The steel gates in between the piers can be raised and lowered, but for most of the time they will be suspended above the waves . But when extremely high tides - more than 3 m above Mean Sea Level - are predicted they will be lowered into the water, once or twice a year, to block off the estuary from the North Sea
3
Fig . 1 . Map of the Delta area of the rivers Rhine, Meuse and Scheldt in the SW Netherlands, with various waterbodies as re-
sulting from the Delta project engineering scheme . 0 = Kreekrakdam, 1867 ; 1 = Zandkreekdam, 1960 ; 2 = Veersegatdam, 1961 ; 3 = Grevelingendam, 1964 ; 4 = Volkerakdam, 1969 ; 5 = Haringvlietdam, 1970 ; 6 = Brouwersdam, 1971 ; 7 = Oosterschelde stormsurge barrier, 1986 ; 8 = Philipsdam, 1987 ; 9 = Oesterdam, 1986 . Markiezaatsmeer has been closed off from Zoommeer by Markiezaatsdam in 1983 .
(Watson & Finkl, 1990 ; Van Westen & Colijn, 1994) . Although topographically separated from the main sea-wall, two auxiliary compartment dams, built between 1977 and 1987, form an indissoluble
entity with the storm-surge barrier . The Oesterdam is an 11 km long dam in the rear end of Oosterschelde estuary, separating the saline seaarm from the eastern freshwater compartment, the Markiezaatsmeer . The Zoommeer in between
4
RHINE - MEUSE GREVELINGEN Philipsdam 2
Storm ..,, surge barrier
1 K q'j
9
VOLKE R
K
0
= sea-wait = salt-marsh - =mean low-water line
F
-~'Q Oesterdam
VEERSE MEER H
NORTH
ZOOMMEER O d F
SEA
E NETHERLAND
6 Km >/ x OELGI M
Fig. 2 . Oosterschelde estuary in the SW Netherlands . R = Roompot; S = Schaar ; H = Hammen . Krammer-Volkerak contains river water, Oosterschelde contains sea water. Locks in Philipsdam and Oesterdam indicated .
functions as an important shipping route between Rotterdam and Antwerp . The Philipsdam is 6 km long and separates the northern branch of Oosterschelde estuary from the newly created freshwater lake Krammer-Volkerak . The dam contains two large shipping sluices of 280 x 24 m and a smaller lock (Fig. 2). The construction of the storm-surge barrier together with the compartment dams took place between 1977 and 1987 . These 10 years can arbitrarily be divided in 3 periods, after 1984 roughly indicated by changes in the tidal amplitude (Fig . 5) : (1) The pre-barrier period (1977-1984) ; (2) the construction period in which the prefabricated elements were positioned in the mouth of the estuary (1985-April 1987), and (3) the post-barrier period (April 1987 and later). The Delta Works, and especially the stormsurge barrier, represent the state of the art of
storm-surge protection . Newly developed designs and techniques, particularly with regard to foundation preparation and the use of foundation protection mattresses, reflected both innovative thought and effective communication between scientists specialized in hydrodynamics and geomorphology and civil engineers . The use of prefabricated components and methods to install heavy constructions in open water, subject to the effects of waves and currents, represent an important breakthrough in marine civil engineering (Watson & Finkl, 1990) .
A case study of a changing ecosystem The execution of the large hydrotechnical works in and around the Oosterschelde estuary offered ample opportunities to study the changes in the
5
Schaar
Roompot
720 m
1440 m -
Hammen!n
b
4
675 b
4
m
A = sill (sediment) B = tidal water C = pier D = steel gate Fig. 3. Scheme of the storm-surge barrier in the seaward mouth of Oosterschelde estuary (see Fig. 2: main tidal branches Roompot, Schaar and Hammen).
aquatic ecosystem. Early biological inventories by collaborators of the Delta Institute for Hydrobiological Research (DIHO), and some other institutions, revealed the outstanding ecological qualities of the estuary (Elgershuizen et al., 1979; Duursma et al., 1982; Saeijs, 1982). In 1980 research groups of the Environmental Section of the Delta Department (now Tidal Waters Division) of the Ministry of Transport and Public Works and the DIHO (now NIOO) decided to start an ecosystem study, aiming at an analysis of the structure and functioning of the changing Oosterschelde estuary, related to the future management of the water body (project BALANS, from 1980 to 1988; project EOS, from 1988 to 1991). This project is unique. It offered ample opportunities to carry out research in various spheres of interest: hydrography, hydrochemistry, autecology, community ecology, ecosystem ecology and fisheries. Integration of the knowledge gained, raises the project to a case study of a changing estuarine ecosystem. It demonstrated the effects of human interference in a non-polluted estuary,
compared with other marine ecosystems (Nienhuis et al., 1994). Long-term responses of animal and plant communities and populations to environmental stress showed that discrimination between responses to human-induced stress and responses to natural stress is difficult to establish quantitatively. The majority of the cause-effect relations described come from correlative, descriptive work, which is not intended to offer causal explanations. A considerable number of documented fluctuations in population dynamics of dominant plants and animals cannot be directly attributed to the construction of the civil engineering works (Nienhuis & Smaal, 1993). The present volume of Hydrobiologia comprises a number of research papers devoted to Oosterschelde estuary. The papers are mainly based on investigations carried out during the period 1980- 1991, sustained by older investigations and long-term reviews. The core of the volume is distracted from the project EOS (1988-1991). This project meant a close cooperation between 25 to 30 scientists and water managers of several State Services, Universities and Private Agencies
6
(Tidal Waters Division, M inistry of Transport and Public Works; Delta Institute for Hydrobiological Research (now Netherlands Institute of Ecology), M inistry of Science and Education; Netherlands Fisheries Research Institute, M inistry of Agriculture and Fisheries, State University of Utrecht; State University of Ghent (Belgium); Environmental Consultancy Agencies).
Hydrography and hydrochemistry The history of the Oosterschelde estuary is characterized by an increasing isolation from the river influences. The estuary changed in the course of time from a coastal plain estuary into a tidal bay. Already in 1867 the freshwater load from the Schelde river was diverted to the Westerschelde, by the closing of the Kreekrakdam. In 1969 the Volkerakdam was closed (Fig. l), depriving the Oosterschelde from extensive amounts of fresh Rhine water: during winters with extremely high fresh-water discharges, salinities near Yerseke dropped to 10 promille chloride (Bakker, 1967).
I53 W m
Krammer Volkerak Grevelingen Zoommeer Veerse Meer
The water balance over the period 1980-1989 (Fig. 4) shows that before 1987 the main freshwater load (50 m3 s - ‘) entered the Oosterschelde via Krammer-Volkerak, through the sluices in the Volkerakdam, mainly derived from the river Rhine, and to a far less extent from the river Meuse. When the Philipsdam was finished (April 1987)the fresh-water load was further reduced to 10 m3 s - ’via the Krammer sluices. The brackishwater load from the adjacent Grevelingen lagoon (Grevelingenmeer)was less than 5 m3 s- ‘, except in the period 1985-1986, when the tidal range in the Oosterscheldeestuary has been reduced drastically (Fig. 5) owing to manipulations with the flood gates in the storm-surge barrier. During that period the load from Grevelingen increased to 20 m3 s - ’ via the siphon in the Grevelingendam. Diffuse loadings of slightly brackish water from agricultural run off - polder discharges - around the Oosterschelde played a m inor role (5-8 m3 s - ‘), Precipitation and evaporation compensated eachother roughly. The water balance is closed by a net transport of water to the North Sea, not shown in Fig. 4. This transport amounted to m
polder discharge
m EZI
rainfall evaporation
25
-25 4. Water budget of Oosterschelde estuary. The gains and losses of water (inflow and outflow) are indicated. The dominant factor, the tidal exchange (1230 x lo6 m3 tidal volume pre-barrier and 880 x lo6 m3 post-barrier; Table 1) is omitted from the figure (data Tidal Waters Division, Middelburg).
Fig.
7
250 200 150 ' Q) Q)
100 50 0 -50
i= -100 -150 -200 I
1
-250 1980
1981
1982
1983
I 1984 1985
pre-barrier period
I 1986
construction period
1987
I 1988
1989
1990
post-barrier period
Fig . 5 . Running average of mean high-water (MHW) and mean low-water (MLW) data, measured daily at Yerseke (central Oosterschelde) . Data from Tidal Waters Division, Middelburg . The pre-barrier, the barrier construction and the post-barrier period are indicated .
50 (summer)-100 (winter) m 3 s - t , and dropped sharply to 0-50 m 3 s -1 after April 1987 . Table 1 summarizes data on the changes in the hydrography of Oosterschelde estuary after the completion of the storm-surge barrier and the two auxiliary compartment dams . The total surface area of the saline tidal ecosystem decreased by 22%, mainly owing to building of the auxiliary dams . The surface area of the intertidal flats decreased by 36% and the area of salt marshes, mainly situated in the rear end of the estuary, by 63% . A new equilibrium between the strongly decreased tidal volume of water (minus 28 %) and the cross section of the estuary (owing to the building of the barrier minus 78%) is developing . The former tidal gullies are too deep and the slopes of the intertidal flats are too steep now . A continuous erosion of the intertidal area occurs, and excess sand and silt are deposited in the tidal gullies . This process will continue for many years, leading to a predicted loss of 15% of the present surface area of the intertidal flats and to 15% of
the present salt marshes over a period of thirty years (Oenema, 1988 ; Smaal et al., 1991 ; Smaal & Nienhuis, 1992 ; Vroon, 1994 ; Mulder & Louters, 1994 ; Ten Brinke et al., 1994). Table 1 also shows the decrease of the current Table 1 . Main hydrodynamic characteristics of the Ooster-
schelde Estuary before and after the completion of the coastal engineering works (data Tidal Waters Division, Middelburg).
Total surface, km2 Water surface, (MWL) km 2 Tidal flats, km Salt marshes, km2 Cross section, barrier in open position, m 2 Mean tidal range, Yerseke, m Max. flow velocity, m s - ' Residence time, d Mean tidal volume m 3 x 10 6 Total volume, m 3 x 10 6 Mean freshwater load, m 3 s - '
Pre-barrier
Post-barrier
452 362 183 17 .2 80000
351 304 118 6 .4 17900
3 .70 1 .5 5-50 1230 3050 70
3 .25 1 .0 10-150 880 2750 25
8 velocities after the building of the barrier, and the strong increase of the residence time of the water ; both parameters express themselves most prominently in the remote rear ends of the estuary . From place to place the tidal amplitude shows minor differences . At Yerseke (Fig . 5) the tidal range was on average 3 .7 m before 1985, and contracted to an average of 3 .25 m after April 1987 . Mean low water is approximately 20 cm higher now, than in the pre-barrier period, and mean high water is approximately 20 cm lower . From summer 1985 to April 1987 the tidal range has been manipulated by flood-gate control to permit the execution of hydraulic works in the back end of the estuary . During that period the tidal amplitude was artificially contracted to a minimum of 2 .25 m (Fig . 5) . Table 2 summarizes data on the hydrochemistry of Oosterschelde estuary in the pre- and postbarrier period . The general tendency reveals an overall decrease of concentrations of inorganic nutrients, seston, particulate organic carbon, chlorophyll a and heavy metals as a consequence of the increased isolation from the influence of the main rivers . Table 2 shows average values, but in reality a wide variation over time is characteristic for most parameters . POC concentrations (Fig . 6) are given as an example : data ranging
4
3 .2 +
West
Chloride Seston POC ChI-a Silicate Nitrate, NO, + NO 3 Phosphate Cadmium Mercury Lead Copper
East
80/84
1988
A %
80/84
1988
A
16 .9 26 .8 1 .5 6 .95 0 .39 0 .52 0 .06 0 .08 <0 .01 <0 .4 1 .5
17 .1 12.9 0.8 3 .82 0.32 0.42 0.05 <0.01 <0.01 <0.1 1 .2
+1 - 50 - 45 - 45 -20 - 20 - 15 - 88 0 -75 - 20
15 .4 22 .0 1 .3 6 .86 0 .55 0 .82 0 .08 0 .19 <0 .02 <0 .6 2 .4
16 .7 5 .9 0 .7 4 .84 0 .29 0 .33 0 .06 0 .02 <0 .01 <0.1 1 .6
+8 - 75 - 50 - 29 -45 - 60 - 25 - 89 - 50 -80 - 33
+
+ + 2 .4
+
+
+
+
++*
+
+ +
E U 0 a
+
++++
+
+
+++ 1 .6
+
+
+ *+
+ +
+
+
# *++~*+
+
+
+
0 .8
4+ + +
#
0
I
I
1980
+*+ + t+ +, +'V + +
I
81
I
82
83
I 84
I 85
86
87
88
89
3.5+ + 2.8-
B
+ + +
2.1-
+ +
*
+ 0.7- +
Table 2 . Average annual concentrations of some waterquality parameters in western and eastern part of the Oosterschelde Estuary in 1980-1984 and 1988 (until October) . All units in gm -3 except for chloride (kgm -3 ) and chlorophyll-a (mg m -3 ); heavy metals consider the dissolved fraction only (data Tidal Waters Division, Middelburg) .
A
+
+
+ * +
+
+ +++++ ++
++
+
+
+++,
+
++
r-# +* ++-V%
*
43t++ +
+++#
0 1980
81
82
83
84
85
86
87
88
89
Fig. 6 . Measured POC concentrations (mg l - ~) in the Oosterschelde western compartment (A) and eastern compartment (B) (data Tidal Waters Division, Middelburg) .
between 0 .2 and 3 .5 mg l -1 showed both a decrease in average values as well as in the overall range after mid 1987 . Secchi disc visibility as an expression of the penetration of sunlight into the water column, varied between 0 .5 and 3 .5 m in the western compartment, and between 0 .5 and 3 m in the eastern compartment in the pre-barrier period, and increased to 1 to 4 .5 m, only in the sheltered eastern compartment (Fig . 7). On average the water is clearer now than before the build-
9
A 4
E
U) 2 V
v Q) U)
I
I
I
1980
1985
1990
1980
1985
1990
Fig. 7 . Averaged Secchi disc visibility in the Oosterschelde western compartment (A) and eastern compartment (B) (data Tidal Waters Division, Middelburg) .
ing of the storm-surge barrier . Already in 1984, before the final construction of the barrier, but after the fixation of the underwater sill in the mouth of the estuary, a decrease in the range of the extinction coefficient could be detected (Bakker & Vink, 1994) .
silicified diatoms to smaller diatoms and flagellates . The phytoplankton community changed from a typical turbid, estuarine community into a tidal bay or lagoonal community (Bakker et al., 1990 ; Bakker et al., 1994). II. The intertidal sand- and mud flats (Fig. 2) . The
Responses at the ecosystem level Four main habitats have been distinguished in Oosterschelde estuary (Smaal & Nienhuis, 1992) : I. The open water mass . Following the changes in
hydrodynamics, a shift in the phytoplankton assemblage has been observed from the heavily
within-habitat diversity of the intertidal sand- and mudflats, determined by the dominant macrobenthic communities, shows large year-to-year changes, depending a .o . on the availability of food, the success of the spatfall, the presence of predators and the occurrence of severe winters . The changes in hydrography and hydrochemistry, superimposed on the natural variability, resulting in a substantial decrease of the total surface area
10 of the intertidal flats and a shift in sediment composition, did not alter the benthic communities qualitatively (Smaal & Nienhuis, 1992) . III. The supratidal salt marshes (Fig . 2) . The
strong decrease of the tidal range during prolonged periods in 1986 and early 1987 (Fig . 5) has irreversibly changed the supratidal salt marshes . Desiccation and aerobic mineralization of organic matter occurred, and a gradual shift in the zonation pattern of the halophyte communities was observed (De Leeuw et al., 1994 ; De Jong et al., 1994 ; De Jong & van der Pluijm, 1994 ; Vranken et al., 1990). The changed tidal range, which introduced the process of erosion of the salt marsh cliffs, will continue in the near future, giving rise to the loss of 4 ha y -1 of salt marsh. IV. The hard substrates of seawalls and dikes (Fig. 2) . The restricted area of artificial stone sub-
strates, covering the seawalls surrounding the estuary carries a diversified flora and fauna . The fixed zonation pattern changed its position, owing to the contracted tidal amplitude. The increased overall shelter of the estuary allows sessile animals and plants to grow in spots previously inaccessible (De Kluijver & Leewis, 1994 ; Meier & Waardenburg, 1994) . The dominant food chain in Oosterschelde estuary consists of phytoplankton (80% of the total primary production) on the primary level, benthic filterfeeding molluscs on the secondary level and carnivorous waterbirds (mainly waders) and man (mussel- and cocklefisheries and cultivation) on the tertiary level . A substantial part of the Hydrobiologia volume is devoted to primary production processes (Wetsteyn & Kromkamp, 1994) and to the structure of the pelagic community (Bakker et al., 1994 ; Bakker & Vink, 1994 ; Bakker, 1994 ; Bakker & van Rijswijk, 1994 ; Tackx et al., 1994). Changes in the structure of the phytoplankton assemblage reflect the concomitant changes in the environment more adequately than phytoplankton primary production . When averaged over the entire Oosterschelde estuary, from the pre-barrier to the post-barrier period, phytoplankton primary production ap-
peared to be a robust, integrated process, without notable changes (Wetsteyn et al., 1990 ; Herman & Scholten, 1990 ; Smaal & Nienhuis, 1992) . It is hypothesized that the decreased loading with nutrients after April 1987, has been compensated by the increased light transmittance through the water column, resulting in approximately the same level of annual primary production before and after the construction of the storm-surge barrier (Wetsteyn & Kromkamp, 1994) . The relation between the phytoplankton standing stock and the benthic filter feeders is of prime importance in the Oosterschelde estuary . For that reason much research has been devoted to the structure of the soft bottom macrozoobenthos communities (Meire et al ., 1994 ; Seys et al ., 1994), to the fluctuations in standing stock, growth rates and mortality rates of blue mussels (Mytilus edulis) and cockles (Cerastoderma edule) (Coosen et al., 1994 ; Van Stralen & Dijkema, 1994), and to the nutrient regeneration capacity of filter feeders (Prins & Smaal, 1994) . Filter feeders act as a driving force in the turnover of phytoplankton biomass and nutrients in the estuary . Filter feeders strongly reduce phytoplankton biomass . On the other hand filter feeders contribute significantly to the mineralization of deposited organic matter . Roughly 20 to 75% of the total nitrogen mineralization in soft sediments of Oosterschelde occurs in musselbeds, although these communities cover only 10% of the bottom of the estuary (Prins & Smaal, 1990) . Hypothetically benthic filter feeders have the potential to enhance primary production in the overlying watercolumn (Prins & Smaal, 1990 ; Asmus & Asmus, 1991) . The standing stocks of the two main filter feeders, blue mussel and cockle, are largely manipulated by man . Each year thousands of tons of small mussels are harvested in the Wadden Sea and brought into the Oosterschelde, and stocked on specific mussel cultivation plots . A large percentage of these mussels dies before they reach their second or third year . A smaller number survives, and is harvested by mussel farmers (Dijkema, 1988) . Roughly 50% of the mussel standing stock is annually harvested by man, and roughly 10% is consumed by water birds, mainly
11 waders (Schekkerman et al., 1994 ; Meire et al., 1994). Natural cockle beds in Oosterschelde estuary are increasingly exploited by man : from roughly 100 tons ash-free dry weight in 1982 to roughly 1000 tons in 1989 (Smaal & Nienhuis, 1992) . Waders consume roughly 15% of the cockle standing stock on an annual basis . When cockle standing stocks are low, as is the case after a series of mild winters, competition may arise between man and waders for the same food source . The carrying capacity of the feeding grounds in Oosterschelde estuary for waders has decreased strongly, owing to the reduction of the intertidal area, and the reduced foraging period per tidal cycle . Increased competition for shellfish between waders and man is a further limit to the carrying capacity for waders (Smaal & Nienhuis, 1992) . Besides the main foodchain (phytoplankton filter feeders - waders), several other aspects have been studied in the estuary, although not as intensively as the main compartments : microphytobenthos (De Jong et al., 1994), zooplankton (Bakker, 1994 ; Bakker & van Rijswijk, 1994), meiozoobenthos (Smol et al ., 1994), hard substrate algae and macrofauna (De Kluijver & Leewis, 1994 ; Meier & Waardenburg, 1994), epibenthic fauna (Hostens & Hamerlynck, 1994), demersal fish (Hamerlynck & Hostens, 1994) and seals (Mees & Reijnders, 1994) . A substantial amount of work has been put in the construction of a mathematical simulation model of the main compartments of the estuary (model SMOES ; Klepper, 1989 ; Klepper et al ., 1994 ; Scholten & van der Tol, 1994) . Both averaged annual carbon models (Fig . 8) as well as sophisticated dynamic models have been presented (Scholten et al., 1990), in order to predict ecological changes induced by the execution of the storm-surge barrier and the auxiliary dams . Figure 8 presents the average annual carbon budget of the main compartments of Oosterschelde estuary, in the pre-barrier and in the post-barrier period . The first impression of the model is a robust, stable carbon budget in which only minor changes have taken place (size of biomass boxes) . Two significant changes in the carbon flows have
POST-BARRIER
_ <25 25 - 50 50 - 100
gC
rv
2yr
I
microphyto benthos
100 - 200
Fig . 8. Annual carbon budget of the Oosterschelde estuary
in the prebarrier period and the post-barrier period . Boxes= biomass g C m - 2 ; Arrows= fluxes g C M -2 Y -1 (Smaal & Nienhuis, 1992) .
to be mentioned . (1) The carbon flow between herbivorous zooplankton and phytoplankton increased, due to the improved quality of the food, containing less silt in the post-barrier period than in the previous period . (2) The role of detritus as food for filter feeders decreased slightly . In the pre-barrier period turbulence kept much detritus suspended in the water column, and consequently much detritus was consumed by the filter feeders, but this was also disposed again as pseudofaeces and faeces . The Oosterschelde project is one of the largest aquatic civil-engineering schemes of recent times .
12 The research investments in this project offer a unique series of documents from which an integration of physical, chemical and biological aspects at the level of the entire ecosystem can be put together . The picture arising, however, is far from complete. Oosterschelde estuary is isolated from the river input, the rear ends of the ecosystem were separated from the estuary by seawalls and the decreased tidal exchange with the North Sea induced sheltered circumstances . The Oosterschelde changed from a turbid estuary into a tidal bay, and yet primary production responses appear to be robust and resilient, and the biological communities showed only quantitative shifts from the dominance of specific species assemblages to other assemblages . In many cases predicted changes in the structure of the biological communities could not be verified owing to the large natural variability mainly caused by physical factors (e .g . temperature, insolation) (Nienhuis & Smaal, 1993 ; Nienhuis et al., 1994) .
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