The Zuiderzee: transformation of a brackishwater ...

CHAPTER 2

The Zuiderzee: transformation of a brackishwater ecosystem W. VAN VIERSSEN & A.W. BREUKELAAR

1. Introduction 2. The Zuiderzee ecosystem 2.1. Physico-chemical characteristics 2.2. Animal communities 2.3. Macrophyte communities 3. After the closure 3.1. Physico-chemical characteristics 3.2. Animal communities

3.3. Macrophyte communities 3.4. Differences between the brackishwater and the freshwater ecosystem 4. Restoration efforts and present-day management 4.1. Long-term management goals 4.2. Ways to reach the water quality goals 5. Acknowledgements 6. References

Abstract In 1932 the Zuiderzee, a brackishwater embayment was closed off from the sea. It was an ecosystem with a salinity gradient ranging from 30 %o Sat the entrance to the Wadden Sea to 8%o S in the central part and freshwater conditions at the mouth of rivers discharging into it. Dominant aquatic macrophytes were Zostera and Ruppia species with Potamogeton pectinatus L. occurring in the lower salinity ranges. Fisheries played an impdrtant role, especially on Anchovy, Herring, Smelt and Eel. The communities in the Zuiderzee differed quite considerably in species composition from those in the more isolated brackish waters along the coast of The Netherlands, but were similar to those of the brackish Baltic Sea. After the closure, the Zuiderzee ecosystem completely changed into a freshwater ecosystem within 15 years. After the empolderment of parts of the former Zuiderzee, now called Lake IJssel, Lake Veluwe came into existence as one of the Border Lakes in 1956. Originally, this shallow lake (40 km', mean depth 1.3 m) was inhabited by a variety of submerged macrophytes and a rich associated aquatic macrofauna. However, eutrophication caused a decline in diversity in the early seventies. Towards the mid seventies this development had resulted in a very poor water quality. Only 5% of the lake area was still vegetated by one single species, P. pectinatus . The once abundant charophyte meadows had disappeared completely, as well as a number of other species. The lake seston was dominated by the cyanobacterium Oscillatoria agardhii Gomont. Macrofaunal communities declined and waterfowl numbers showed a sharp decrease. More than 80% of the number of birds originally present disappeared. The present ecosystem diversity is low compared with the situation during the early days of Lake IJssel. It can also be concluded that the present species richness of Lake Veluwe is less than that of the former Zuiderzee, in spite of the generally low diversity of brackish waters. A compensation of the quality of the brackishwater ecosystems which disappeared will only take place if a dramatic improvement of the water quality of the present Lake Veluwe-is achieved.

1. Introduction

brackish Zuiderzee embayment was closed off from the Wadden Sea by the IJsselmeer Dam (`Afsluitdijk' in Dutch) and became Lake IJssel (Fig. 2.1). We will restrict ourselves here mainly to the shallow-water littoral communities and discuss changes in submerged macrophytes, macroinvertebrates, fish and birds. Biological information on the situation before and after the closure is gathered in the monographs of Redeke (1922, 1936) and De Beaufort (1954). Only after several years of debate the Dutch

Because Lake Veluwe plays an important role in this book as the field site of much of the work, an outline of the recent natural history of the lake and its communities was considered worthwile. In this chapter we will briefly sketch the development of this lake from a section of the former Zuiderzee littoral, and set a perspective for the long-term goals of lake management. The lake had its origin in the early thirties of this century. In May 1932 the 5

W. van Vierssen et al. (eds), Lake Veluwe, a Macrophyte-dominated System under Eutrophication Stress, 5-19. © 1994 Kluwer Academic Publishers. Printed in the Netherlands.

Lake Ussel

Enkhuizen

Houtribdijk

Lake Marken Eastern Fievelancl

Marken

NHarderM

5 kilomete4' Fig. 2.1. The Lake Ussel area and its Border Lakes.

parliament finally decided in 1918 to close off the Zuiderzee and to reclaim some of the enclosed area (Thijsse 1972). The main reason for closing off the Zuiderzee and reclaiming the land was initially not based on a need for more agricultural land. In the beginning of the 20th century, agricultural productivity showed a sharp increase because of the introduction of artificial fertilizer. Moreover, steamships allowed the transportation of large quantities of Argentinian grain to Europe. Both factors kept prices at a relatively low level, also because the demands were amply met. The closure of the Zuiderzee was mainly pursued for safety reasons. Storms and high tides had caused much damage

over the years and closing off the Zuiderzee from the Wadden Sea was seen as one way to prevent this in the future. The exact plans changed considerably over the years. Thijsse (1972) summarizes these developments in his historical overview of 'half a century of Zuiderzee works'. One of the most conspicuous differences between the first plan by Van Diggelen from 1849 and later alternatives is the fact that the Wadden Sea islands were only included in the first (Fig. 2.2a). A second and adapted proposal from 1877 (Fig. 2.2b) was rejected because it would have necessitated the heightening of the dikes along the coastline of the remainder of the Zuiderzee. The

7 ..•

Fig. 2.2. Different plans for land reclamation in the Zuiderzee area. The shaded areas indicate empoldered land (after Thijsse 1972).

8 relatively shallow, as they form the original littoral zones of Lake IJssel and the Zuiderzee. Therefore, the Border Lakes can be considered as extensive littoral systems, while Lake IJssel must be seen as a shallow, but pelagic system. Lake IJssel presently makes up for approximately 60% of the volume and 50% of the area of Dutch surface waters.

2. The Zuiderzee ecosystem 2.1. Physico-chemical characteristics

The most important differences between the former Zuiderzee and Lake IJssel are salinity (and its fluctuations) and the influence of strong local tidal currents (up to 1.5 m s--1 in the former Zuiderzee). The salinity of the water in the Zuiderzee showed a clear gradient (Havinga 1954a), being high at the entrance to the Wadden Sea (salinity approximately 30%0 S), intermediate at Urk, Marken and Lemmer (11%o, 10%0 and 8%o S, respectively) and low ( 10 m-1, compare Blom et al., chapter 10). 3.2. Animal communities Within 15 years, the transformation of the faunal communities was completed (De Beaufort 1954). A number of fish species survived the change and even extended their distribution. This especially per-

tained to the salinity-tolerant freshwater fish that already occurred in the less saline parts. Initially, the River Lamprey (Lampetra fluviatilis (L.)), the Atlantic Salmon (Salmo salar L.), Trout (Salmo trutta L.) and Smelt (0. eperlanus (L.)) thrived well under the new regime. However, due to the presence of a physical barrier (the IJsselmeer Dam), and the deteriorating environmental conditions in the. Dutch and other European rivers, the catches of these species declined. Immediately after the closure in 1932, the amount of landed Smelt and Flounder decreased sharply (Fig 2.7). Eel fisheries, on the other hand, improved and freshwater fisheries in general flourished. Following the closure, the invertebrate species composition changed completely. During the first years, nutrient availability was probably controlling the number of species to a large extent. Further, the number of invertebrate species as well as the -number -of individuals per species was positively correlated with the number of macrophyte species (De Vos 1954). A detailed description of the freshwater faunal communities in the littoral zone of Lake IJssel and Lake Veluwe (including the location of the experimental set-up in the period 1986-1988, compare Fig. 2.1 and Van Vierssen et al., chapter 9) is given

(b)

(a)

6000 Pikeperch • Perch •

Bream

4000

5,

>7,

C O

to C 0 2000

0 1925

year

1935

1945

1955

year

Fig. 2.7. Amounts of marketable fish from the Zuiderzee and Lake IJssel before (a) and after (b) the closure (data from Havinga 954b). Freshwater fish: Pike-perch, Stizostedon lucioperca (L.); Perch, Perca fluviatilis L.; Bream, Abramth brama (L.) Roach, Rutilus rutilus (L.)), brackish and marine species, see text.

14 by Dresscher (1954), who recorded over 140 invertebrate and invertebrate species. In the area between the mouths of the rivers IJssel and Zwarte Water, the communities were much richer in species than in Lake Veluwe (Table 2.1, columns I versus II). He postulated that this was partly due the higher exposure to wave action of the latter habitat, but mainly because of the fact that the (then slightly) eutrophic waters from the rivers IJssel and Zwarte Water provided nutrients to enhance the development of macrophytes. Especially the groups of the Coleoptera, Mollusca and Hydrachnidae were less abundant in area II. This may have been due to the lack of abundant marsh and submerged vegetation (cf. Vermaat, chapter 13). The most abundant species were freshwater snails, i.e. Bithynia tentaculata (L.), Hydrobia jenkinsii Smith and_Tyinnaea peregra The importance of these ecosystems for waterfowl is well documented. In the Lake Velure area, the most abundant bird species was the Coot (Fulica atra (L.); thousands). The Pintail (Anas acuta L.) was also quite abundant (more than 1200 individuals each year). The same applies to the Mute Swan (Cygnus olor (Gmelin)), Bewick's Swan (Cygnus bewickii (Yarr.)) and the Wigeon (Anas

penelope (L.)) which occurred in numbers exceeding 500 (Ten Kate 1936; Brouwer & Tinbergen 1939). From Fig. 2.8 (after Bick & Van Schaik 1980), it can be concluded that waterfowl numbers in Lake Veluwe declined between the mid-sixties and the mid-seventies. This is the same period during which the aquatic macrophytes declined (Scheffer et al., chapter 3). The total number of birds that annually visited the lake declined with almost 85%. Except for Teal, Shoveler and Goosander (Mergus merganser (L.), not given in Fig. 2.8, its number increased sevenfold, from 20 to 140), all bird species declined sharply in numbers. In the case of the decline of Bewick's Swan, it seems quite probable that the sharp drop in P. pectinatus caused this decline, since it feeds on the plants' tubers (Timmerman 1977). Though poorly documented, the decline in number of invertebrates probably also coincided with IhEd-edlihe in macrophytes (Leentvaar 1961; W. van Vierssen, personal observations, see Fig. 2.7). The present invertebrate fauna is still rather poor in species, in spite of the slow but clear recovery of the P. pectinatus vegetation (personal observations; compare Scheffer et al., chapter 3).

Table 2.1. Taxonomic composition of animal communities in the littoral zone of Lake Ussel at the mouths of the rivers Ussel and Zwarte Water (area I) and between Doornspijk and Harderwijk (area II). Given are the number of species in a higher order taxon, area I had a total of 168 species, area II had 73 species. From Dresscher (1954).

area:

I

Agnatha Amphibia Amphipoda Arachnoidea Bryozoa Cladocera Coleoptera Copepoda Diptera Heteroptera Hirudinea

taxon

n species

taxon 3 4 1 1 1 17 20 8 4 6 13

I

II

2 1 0 7 12 4 3 8

Hydrachnidae 20 2 Isopoda 2 Lepidoptera 41 Mollusca 1 Neuroptera 3 Odonata 7 Oligochaeta 5 Pisces Porifera 1 6 Trichoptera 2 Tricladida

II n species 7 2 1 21 1 2 2 -

15 circinatus Sibth., Elodea canadensis Michx., P. pectinatus, Potamogeton perfoliatus L., Z. palustris L. 2 Pintail s.1., Lemna minor L. and Lemna trisulca L. Leentvaar (1961) investigated Lake Veluwe dur3 Mute swan ing the late fifties and early sixties. He described the 4 Coot zonation pattern in Lake Veluwe as follows. In the 5 Smew zone between Eastern Flevoland and the deeper 6 Gadwell navigation channel (depth of 4 m) he found a dense 7 Bewick's Swan vegetation of Chara. Near the deeper fareway, P. 8 Pochard perfoliatus was dominant. Only occasionally he 9 Mallard found Myriophyllum spicatum L., E. canadensis and R. circinatus. It is remarkable that Leentvaar (1961) 10 Wigeon observed no P. pectinatus in the part of Lake 11 Tufted Duck Veluwe he visited. Scheffer et al. (see chapter 3 for a 12 Teal map) state that at the end of the sixties this species 13 Shoveler was very abundant. According to Leentvaar (1961) eutrophication was still not having adverse effects —110 —60 —10' 6-on the ecosystem in the early sixties. We would like Percentage change to return to one of the conclusions of Dresscher (1954) in this respect. He held the low trophic status Fig. 2.8. Relative changes in wintering bird numbers in area from the mid-sixties to the mid-seventies. Average of that particular area in Lake IJsselmeer responnumber of wintering birds in the mid sixties (in brackets): 1. sible for the fact that the vegetation was very thin. Bucephala clangula (L.) (250), 2. Anas acuta L. (1280), 3. Cygnus During the years he visited Lake Veluwe, the olor (Gm.) (600), 4. Fulica atra L. (5500), 5. Mergus albellus L. eutrophicated waters from the river IJssel enhanced (170), 6. Anas strepera L. (150), 7. Cygnus bewickii (Yar.) (730), the diversity of the ecosystem. 8. Aythya ferina (L.) (70), 9. Anas platyrhynchos (L.) (4430), 10. Anas Penelope L. (530), 11. Aythya fuligula (L.) (90), 12. Anas Parallel with increased nutrient loads crecca L. (250), 13. Spatula clypeata (L.). (from Bick & Van culminating in an almost permanent Oscillatoria Schaik 1980). bloom in the seventies, the vegetation deteriorated. P. pectinatus appeared to be the most resistent 3.3. Macrophyte communities species, but even this species almost completely disappeared in the mid seventies. In 1975 the lake The two Zostera species disappeared soon, most was in a very poor condition. Only 5% of Lake probably because they were not able to tolerate the Veluwe was still vegetated (Scheffer et al., chapter lower salinities. Other macrophytes were able to ex3). It is important to state that Lake Veluwe was not tend their distribution. In first instance, Ruppia and an exception. In many other lakes in The NetherZannichellia species became more abundant. Later, lands the situation was quite similar (see e.g. Parma these had to give way to freshwater species. 1980; Best et al. 1984). Dresscher (1954) described the aquatic vegetaMacrophyte inventories are carried out regularly tion in the littoral zone of Lake IJssel. He observed in the Lake IJssel area. Doef et al. (1991) report that that in the area, presently known as Lake Veluwe since the late eighties, macrophytes are recovering (area II of Table 2.1), the submerged vegetation was following restoration measures (see section 4.2) and much thinner than near the mouth of the river IJssel are colonizing new areas in the littoral zone of Lake (area I). For Lake Veluwe, Dresscher (1954) deIJssel and most of the Border Lakes. scribed a beach-like littoral zone with sparse vegetation consisting of Phragmites australis (Cay.) Trin 3.4. Differences between the brackishwater and the ex Steud., Scirpus maritimus L. and some Eleocharis freshwater ecosystem palustris (L.) R. et Sch.. In the open water the vegetation was also sparse and the only species he We will limit ourselves here to a comparison of the explicitly mentioned to occur there is P. pectinatus. littoral communities and consider Lake Veluwe repIn ditches and shallow inlets he found Ranunculus resentative for the other Border Lakes. We will use 1 Golden eye

16 species richness and uniqueness of the systems as quality criteria. The Zuiderzee was an ecosystem with relatively many macrophyte species fitting into a series of similar large-scale brackishwater ecosystems along the West European coast. As argued above similar brackish embayments still occur in Denmark and Germany nowadays. The plant communities in the Zuiderzee were characterized by a species combination of marine (Zostera), brackish (Ruppia, Zannichellia) and sometimes freshwater taxa (such as P. pectinatus). The faunal community was dominated by marine and brackishwater species. The number of macroinvertebrate species was not extremely high, but was in the same range as known from the Baltic brackish embayments. This combination of communities was extremely rare and is nowadays virtually absent in The Netherlands. In Lake Veluwe, the aquatic vegetation consists of a few macrophyte species only (mainly P. pectinatus and P. perfoliatus) and is thin in many places. Presently, recovery of the macrophyte beds is reported (Doef et al. 1991; Scheffer et al., chapter 3). Although no extensive background material is available, it has to be concluded that the aquatic invertebrate and vertebrate fauna is relatively poor in species, as compared to the communities present in the lake thirty years ago as well as to littorals of other shallow Dutch lakes. In retrospect, we must conclude that an invaluable habitat with the closure of the Zuiderzee was destroyed in 1932.

4. Restoration efforts and present-day management 4.1. Long-term management goals The water quality management policy in The Netherlands has been to define a Basic Quality for waterbodies (a set of verbal statements, supported by a series of water quality parameters; Second Water Action Programme, compare Parma 1988). Specific uses (e.g. drinking water, nature conservation, swimming water) can lead to the formulation of stricter standards. This approach has been refined in the Third Water Action Programme with a differentiation according to 15 water types (e.g. lakes, ditches, streams, rivers, canals etc.). Basic

Quality criteria are: summer averages of chlorophyll a < 100 mg m-3, transparency > 0.4 m, total P < 0.15 mg 1-1, total N < 2.2 mg 1-1, and no permanent dominance of bluegreen algae. As eutrophication had rapidly led the Lake Veluwe system away from this state, resulting in highly turbid Oscillatoria-dominated waters, lake management concentrated on eutrophication abatement through a reduction of phosphorus loading. The (inter-related) long-term management goals were a significant reduction of the phosphorus load, a termination of the blooming of bluegreen algae and an increase in transparency to a Secchi depth of 1 m (short-term 0.5 m; Hosper et al. 1986). Evaluating the different uses of the lake, Hosper et al. (1986) argued that littoral zones with abundant submerged and emergent vegetation are an attainable goal for Lake Veluwe. It can be argued from earlier sections that, - -ecologically, the long-term goal for the development of the littorals of the Border Lakes should be sought in the situation in the fifties and sixties, as described by Dresscher (1954) and Leentvaar (1961). This ecologically desirable long-term goal is in agreement with the goals set by the lake management (Hosper et al. 1986) as described above. 4.2. Ways to reach the water quality goals Parallel with the development of water quality management programmes, the importance of the Lake IJssel system was recognized and an increasing amount of research was carried out (Bick & Van Schaik 1980; Schoorl 1985; Brocades Zaalberg 1986; Oonk 1988; Anonymus 1990; Verdonschot 1990). Also, the shallow Border Lakes have been the subject of many studies (a.o. Berger 1975a,b; Zevenboom et al. 1982; Berger 1983; Kerkum 1983; Berger & Bij de Vaate 1983; Brinkman & Van Raaphorst 1986; Hosper & Meijer 1986; Hosper et al. 1986; Breukelaar 1990; Meijer et al. 1990a; Boers et al. 1991). For Lake Veluwe, the restoration programme initially consisted of (a) the installation of a phosphorus removal step in the largest waste water treatment plant discharging into the lake (Harderwijk, about 175,000 population equivalents in 1983), and (b) flushing of the lake with polder water of high alkalinity and low phosphorus content. Indeed, P-loading has been reduced and since 1985

17

P

e e

e

if 0

h

the continuous Oscillatoria bloom was broken. Water transparency, however, remained persistently low (Hosper et al. 1986; compare Blom et al., chapter 10). Several additional measures have been proposed, i.e. increased P-removal to further decrease algal light attenuation, whole-lake foodweb manipulation to reduce zooplankton predation and bioturbation of sediments, and creation of sheltered areas to reduce wind-induced sediment resuspension. An evaluation of the different approaches has been attempted (e.g. Hosper & Jagtman 1990; Meijer et al. 1990b), but largely depends on the conceptual model framework of the ecosystem that is used (e.g. Moss 1990; Scheffer 1990). This topic will be returned to in later chapters.

e

5. Acknowledgements e S

r

The Zuiderzee Museum (Enkhuizen, The Netherlands) is acknowledged for making available Figs 2.4 and 2.6 and their permission to publish them. Dr M.J.M. Hootsmans and Dr J.E. Vermaat critically read the manuscript.

6. References e

g g e

r E. a r

r

Anonymus. 1990. Natuur: zoete wateren. (Nature: fresh waters, in Dutch). Base report Third Water Action Programme, The Netherlands Ministry of Transport & Public Works. Berger, C. 1975a. De eutrofiering en het voorkomen van Oscillatoria agardhii Gomont in de randmeren van Flevoland (Eutrophication and the occurrence of Oscillatoria agardhii Gomont in the Border Lakes of Flevoland, in Dutch). H2O 8: 340-348. Berger, C. 1975b. Occurrence of Oscillatoria agardhii Gom. in some shallow lakes. Verh. int. Ver. Limnol. 19: 2689-2697. Berger, C. 1983. Biomassa en licht als regulerende factoren in Oscillatoria meren (Biomass and light as controlling factors in Oscillatoria lakes, in Dutch). H2O 16: 178-182. Berger, C. & Bij de Vaate, A. 1983. Limnological studies on the eutrophication of Lake Wolderwijd. A shallow hypertrophic Oscillatoria dominated lake in the Netherlands. Schweiz. Z. Hydrol. 45: 458-479. Best, E.P.H., De Vries, D. & Reins, A. 1984. The macrophytes in the Loosdrecht Lakes: a story of their decline in the course of eutrophication. Verh. int. Verein. Limnol. 22: 868-875. Bick, H. & Van Schaik, A.W.J. 1980. Oecologische visie Randmeren (An ecological outlook on the Border Lakes, in Dutch). Natuurwetenschappelijke Commissie, Natuurbeschermingsraad, Staatsbosbeheer, 291 pp.

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