Palaeoecological and documentary records of recent environmental change in Garaet El Ichkeul: A seasonally Saline Lake in NW Tunisia

Biological Conseroation 58 (1991) 275-295

Palaeoecological and Documentary Records of Recent Environmental Change in Garaet E! Ichkeul: A Seasonally Saline Lake in NW Tunisia A. C. Stevenson Department of Geography, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK

& R. W. Battarbee Palaeoecology Research Unit, Department of Geography, University College London, 26 Bedford Way, London WC1H 0AP, UK (Received 8 September 1990; revised version received 18 January 1991; accepted 19 January 1991)

ABSTRACT A palaeoecological and documentary study of the RAMSA R site of Garaet el lchkeul in N W Tunisia demonstrates that the ecology of the lake has undergone significant changes over the last 400-500 years. 21°pb dating of short sediment cores from the lake shows a significant increase in sediment accumulation rates over the last 150 years. This is associated initially with an increase in agricultural intensity within the catchment by French settlers in the late 1800s. Subsequently, sediment accumulation rates increased further as a result of anthropogenic modifications to the course of the Oued Djoumine. Pollen evidence suggests that Potamogeton, the major food resource of wintering waterfowl, only became established in the lake in c. AD1890. This establishment appears to be associated with changes to the hydrological regime of the adjoining Lac de Bizerte by construction of the Bizerte Ship canal in .4D 1895. The current importance of lchkeulfor wintering waterfowl is probably a direct consequence of these anthropogenic modifications and the dramatic reduction in wetland areas in the Mahgreb over the last 100 years. 275 Biol. Conserv.0006-3207/91/$03-50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

A. C. Stevenson, R. W. Battarbee

276

INTRODUCTION Garaet el Ichkeul is a National Park lying near the Mediterranean coast of NW Tunisia (Fig. 1)"which is internationally important for its wintering waterfowl (Carp, 1980; Scott, 1980; Hollis, 1986). This 90km 2 lake is freshwater (total salinity 4 g litre-1) in winter when the rivers are in flood and saline (total salinity 35 g litre- ') in summer because of evaporation and the inflow of seawater. The 30 km 2 freshwater marshes around the lakeshore are maintained by winter inundation and the continual input of river water. Up to one-third of the lake's area is occupied by extensive beds of Potarnogetonpectinatus (nomenclature follows Tutin et al., 1964-1980). This is the major food for over wintering pochard Aythya ferina (120000 maximum), wigeon Anas penelope (112 000 maximum) and coot Fulica atra (188 000 maximum) (Hollis, 1986). The marshes are dominated by Scirpus maritimus, whose new shoots and bulbous root parts provide the staple diet

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Environmental change in Garaet el lchkeul

277

of the 17 000 greylag geese A n s e r anser that have been known to overwinter here. While anthropogenic interference in the catchment has occurred since at least Roman times (Stevenson, unpublished data), recent and proposed river channelisation, dam installations on inflowing rivers and agricultural improvement schemes have had and are having a profound effect on the lake's hydrology, water quality and bird populations (Hollis, 1986). In addition, the agricultural development of the surrounding Plain of Mateur will lead to increasing use of pesticides and fertilisers, which will be transferred by the drainage system into the lake (Hollis, 1986). The extent to which these current and potential threats will affect the bird populations is speculative. An extensive programme of ecological studies (G. E. Hollis, pers. comm.) is now considering how Ichkeul might respond to various dam management strategies and ameliorative measures including construction of a sluice in the outflow. However, little attention has so far been paid to longer term changes and the natural variability in salinity and water level at Ichkeul. In order to assess the inherent natural variability of the lake in recent historic times (AD 1500 to present), a palaeolimnological study was conducted on a series of short sediment cores from the lake. A search of the available historical documentary evidence concerning Ichkeul was also conducted at archival collections in the British Museum, Royal Geographical Society and Aix en Provence, France. The site

The Ichkeul National Park, created by the Tunisian government in 1977 in recognition of the important international role the area plays as a wintering ground for waterfowl (Scott, 1980; Hollis, 1986), lies at the southern end of the Mogods mountains on the Mediterranean coast of NW Tunisia (Fig. 1). The Park contains a large, shallow, euryhaline lake (87 km 2 at its summer minimum) dominated on the south shore by a limestone mountain, Djebel Ichkeul. The catchment lies at the northern end of the Mateur depression and is underlain by a Quaternary sedimentary basement (Bonniard, 1934). The main lithologies within the lake catchment are generally of Tertiary age, comprising calcareous mudstones and microcrystalline limestones. Eocene gypsiferous strata outcrop within the Oued Melah and Oued Doumiss catchments. The lake is surrounded by Quaternary marine terraces of RissWurm age (Ennabli, 1967) and important, fossil-rich, Villafranchian beds are located on its northern shores (Arambourg et al., 1949). Djebel Ichkeul comprises a steeply dipping complex of fine-grained limestones and

278

A. C. Stevenson, R. W. Battarbee

dolomites interbedded with schistose siltstones of Triassic to Cretaceous age. The region has a sub-humid pluviothermic Mediterranean climate (Emberger et al., 1963). Rainfall is strongly seasonal with 60-70% of the annual rainfall (600 mm) falling during the winter months. Temperatures are typically Mediterranean with average winter minima of 7.6°C and average summer maxima of 31.1°C. Hydrology

Five major rivers feed the lake (Fig. 1) of which the Sedjenane, draining 580 km 2 of the southern Mogods, is the most important, accounting for 43 % of the total water input. The rivers flood after the first winter rains and, except for the Sedjenane, dry up completely during the summer months. Ichkeul is drained by the Oued Tindja to Lac de Bizerte, a tidal marine bay of the Mediterranean. The Oued Tindja is unusual since its flow reverses in the summer when the level of Ichkeul falls below that of Lac de Bizerte. This results from the cessation of flows in the major inflowing rivers and high evaporation rates from the lake. As a consequence saline, thalassic water enters Ichkeul and the lake fluctuates in level and salinity throughout the year (Lemoalle, 1983; Hollis, 1986). The lake basin is very shallow with mean summer and winter depths of 1-4 and 2.5 m, respectively. Significant changes in the hydrological regime of the lake are now taking place as a result of the dam and agricultural improvement scheme (Hollis, 1986). Three dams are complete and hydrological simulation models demonstrate that without any management of the water resources the level and salinity regime within the lake will alter significantly (Hollis, 1986). Significant increases in mean winter salinities have already been recorded; the mean winter salinity for 1988/89 was 24.5 glitre-1 (G. E. Hollis, pers. comm.). Models predict that inundation ranges will contract and mean winter salinity will rise from the present average of 5-7 g litre-1 to 15-16 g litre- 1. These changes will radically alter the present plant communities at Ichkeul and destroy the food base for the current wintering waterfowl (Hollis, 1986). Lake chemistry

Few complete measurements of lake water chemistry exist. It is apparent that the annual variation in the hydrological cycle is the biggest influence on the lake water chemistry (Table 1). Summer ionic concentrations are on average twice those of the winter with sodium and chloride dominating the ionic composition.

279

Environmental change in Garaet el Ichkeul TABLE 1 Lake Chemistry: Mean Winter and Summer Ion Concentration (mg iitre- 1) and Total Dissolved Salts (TDS) (5 years data 1972-1977 pre-dam)

Winter Summer

Ca

Mg

Na

CI

S04

C03

pH

TDS

174 274

278 629

2 165 4822

3 717 8246

854 1 796

73 97

8"3 8.1

6409 17211

Vegetation The plant communities of the National Park have been extensively studied (Fay, 1980; Kirk, 1983; Hollis, 1986). Several wetland ecosystems comprise the major vegetation types and are the reason why up to 250 000 waterfowl use the lake as a wintering site each year. The most important and extensive of these are fresh/brackish water marshes (20 km 2) dominated by Scirpus rnaritimus and S. litoralis. On more saline soils vegetation is either absent, or extensive populations of Arthrocnemumfruticosum develop. The lake is at present dominated by extensive beds of the saline-tolerant Potamogeton pectinatus (30km 2 in area) with limited amounts of Ruppia cirrhosa, Lamprothamnium papulosum and Zostera noltii.

Fauna Invertebrate diversity in the lake is low because of physiological stresses imposed by a fluctuating saline environment and because the lake bed is constantly disturbed by wind action resulting in high water turbidity (secchi disk c. 15-20 cm). Those invertebrates that do occur are present in great abundance. They include polychaete worms such as Nereis diversicolor, bivalves such as Sphaeroma hookeri, Cerastodermaglaucum, Idothea balthica, the gastropod Hydrobia ventrosa, and crustacea such as Gammarus aequicauda, Corophium volutator (Conservation Course, 1977). The lake also supports a productive fishery of Mugil cephalus, M. ramada and Anguilla anguilla eels. METHODS In April 1980 a preliminary stratigraphic study of one core from the lake indicated that at least four mollusc layers were present within the top metre of sediment. Thirteen sediment cores were subsequently taken to examine the distribution of these layers over the lake and variations in their depth (Fig. 1). On each occasion a simple piston corer operated from a small boat was used. The cores were either extruded in the laboratory or field at 1-cm intervals and the depth, identity and position of molluscan layers noted.

A. C. Stevenson, R. W. Battarbee

280

TABLE 2 Core Lengths

Core number

Length (cm)

and Range

Mollusc

of Analyses

Physical analyses

1

28

+

.

2

50

+

+

4

75

+

6

70

+

7

90

+

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Pollen

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.

.

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.

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on Each Core

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8

600

+

10

20

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11

34

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.

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12

34

+

.

.

.

.

13

34

+

.

.

.

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14

34

+

.

.

.

.

15

34

+

.

.

.

.

16

90

+

.

.

.

.

Ostracoda/ Foraminifera

Three cores were subdivided more finely for measurements of wet density, percentage dry weight, percentage loss on ignition and for pollen stratigraphy (Table 2). Pollen preparation involved an initial decalcification in 10% HC1 followed by sieving the sample through a 10-/am acid-resistant mesh to remove 60-70% of the fine clay minerals (Cwynar et al., 1979). The retained fraction was then subjected to 2 h hydrofluoric acid (HF) (150°C) in PTFE beakers before acetolysis. Mounting procedure followed Moore and Webb (1978). At least 400 pollen grains from land species were counted from each level. One core was dated after subsamples of dry sediment were measured for 21opb according to the methodologies of H~is/inen (1977). Another core was subsampled and wet-sieved through a 65-#m mesh for extracting Ostracoda and Foraminifera (Table 2). Diatom analysis was attempted on the cores but while the surface sediments contained a characteristic brackish-water flora, dissolution of the diatom silica (probably because of the high pH, salinity and temperature of the lake) resulted in their disappearance below 15 cm. RESULTS The gross stratigraphy of all cores is similar and consists of a surface 2 cm composed of reddish flocculant silt/clay overlying darker more cohesive material. Colour changes were minimal and varied from pale olive (5Y 6/3) to olive (5Y 5/3) below the surface layer. The percentage dry weight of the analysed cores (Fig. 2) increased rapidly down to 5 cm but then stabilised at

Environmental change in Garaet el Ichkeul

281

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approximately 40-45% dry weight. Textural analysis revealed the sediment to be very uniform (median size 10-11 phi) down core. The coarse fraction ( > 4 phi) amounted to less than 1% of the dry weight and comprised mainly fragments of ostracods, Foraminifera and occasional chironomid head capsules. Euhedral gypsum and framboidal pyrite (identified by X-ray diffraction) were common mineral components. Preliminary X-ray diffraction analysis of the < 2-/~m fraction indicated a uniform clay sequence comprising kaolinite, illite and smectite in roughly equal proportions. Percentage loss-on-ignition at 450°C provides a crude estimate of organic matter in the core (Fig. 2) and falls within the 5-9% range. However, the variations are not thought to be significant because of problems with differential clay dehydration; the minima recorded at 18-19cm and 26-27 cm represent the mollusc beds.

Dating The 21°pb flux for core 2 is low (c. 0.13 pCi cm- 2 year- 1) when compared to other N African lakes, e.g. Dayat-er-Roumi (0.31 pCi cm-2 year-1), Dayat Affougah (0.54 pCi cm- 2 year- 1) and Dayat Azigza (0.58 pCi cm- 2 year- 1) (Flower et al., 1989). Since the profile is non-monotonic the constant rate of supply model (Appleby & Oldfield, 1978, 1983) was used to construct sediment chronologies (Fig. 3 and Table 3). It is apparent that the sediment accumulation rate has changed significantly throughout the time period covered by the core (Fig. 3, Table 3). From 1821 to 1892 the accumulation rate was low at about 0"28 mm year- 1 but then increased some fourfold to 0.97 mm year- 1 between 1892 and 1923. Subsequently, the accumulation rate doubled to 1.5 mm year-1 between TABLE 3 21°pb Chronology for Core 2 Calculated according to the CRS Dating Model Depth

Date

(cm)

(Ao)

S e d i m e n t a c c u m u l a t i o n rate g cm - 2 y e a r - 1

0.0 0-5 4-5 8-5 12.5 16-5 20.5 24-5

1982 1981 1974 1967 1954 1925 1892 1821

-0.295 0.282 0.416 0-089 0.093 0-059 0.015

cm y e a r -

-0-597 0-512 0.695 0.145 0-152 0.097 0.028

Environmental change in Garaet el lchkeul

283

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1923 and 1954 and after 1954 it increased about six times to its current value of around 5-6 mm year- ~. Dates were subsequently transferred to the other cores by biostratigraphic correlation using the pollen and molluscan evidence. MOLLUSCA, OSTRACODA A N D F O R A M I N I F E R A The core survey of the lake revealed the presence of several layers of the brackish-water bivalve Cerastodermaglaucum (Table 4, Fig. 4). The number 0

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Fig. 4.

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A. C. Stevenson, R. I~. Battarbee

284

TABLE

4

D e p t h o f the M a i n M o l l u s c a n L a y e r s o f Ichkeul

Core number

Core length (cm)

Depth of C. g l a u c u m layers First (cm)

Second (cm)

20-21? 23-25 bc

2 4

28 50 75

14-17 13-16 8 - 9 bc

6 7 8

70 90 600

. 8-15 14-18 b

10 11 12 13 14

20 34 34 34 34

12-18 b 10-11 b 8-10 b 11-13 b 17-18 b

1

Cerastoderma glaucum in

15

34

10 b

16

90

19-20 b

Third (cm)

--

.

. 22-32 27-29 b * 25-27 b 19-22 b 19-23 b 24-27 bc --

25-27 b

,a

45-46 50-60 b

Fourth (cm) ,

* 69-73 b

. * 55 b * * * * * *

54-57 b

* 60-62 a * * * * * *

62-70 b

a , , Below core depth. b Hydrobia ventrosa peak. c Scrobicularia plana peak.

of layers in each core varied from four (cores 4, 8 and 16), three (core 2), two (cores 11-14) to only one (cores 1, I0 and 15). Molluscan layers were absent altogether from core 6 on the lake margin. This variation in numbers of beds between cores is the result of at least two factors. Some cores (10-15) were too short to penetrate the deeper beds, and the beds may not have been continuous in their lateral extent. In addition, only the uppermost two molluscan beds included articulated valves indicating in-situ formation of the bed rather than the result of erosion and redeposition. The disarticulated valves in the bottom two beds of cores 4, 8 and 16 are thought to be the result of erosion from the early Holocene marine terraces since a x*C date of 12 100 _+ 100aP was obtained on one of these in core 8 from a depth of 60 cm (Beta-13483). While the exact depth of these molluscan layers varies from core to core because of differing sediment accumulation rates it is readily apparent that all four beds occupy roughly the same depths, with the youngest one restricted to 10-19 cm, the next oldest to 20-30 cm and the erosional bands between 50-60 and 60-70 cm. In addition to beds of C. glaucum, peaks of both the brackish-water gastropod Hydrobia ventrosa and Scrobicularia

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plana, where present, are coincident with the Cerastoderma layers (Fig. 4, Table 4). Since the bottom two beds are not thought to represent in situ deposition of the mollusc layer these will not be discussed further. The dating of the molluscan layers from core 2 indicates that the upper one was formed between 1925 a n d 1954 with the lower one around the end of the 19th century (c. AD 1890). In addition to the Mollusca bands, remains of both Ostracoda and Foraminifera were found in the cores. Figure 5 presents the results for these two groups for core 8. Ostracod numbers peak at 67, 47 and 31 cm and are chiefly represented by the euryhaline species Cyprideis torosa and the saline species Loxoconcha elliptica. After the peak at 31 cm numbers decline and remain generally low to the top of the core. A peak of the marine foraminferan Ammonia batava occurs after the 31-cm ostracod peak at 26 cm with a further minor peak at 13 cm. These latter peaks are coincident with the uppermost two beds of C. glaucum. Pollen The palynological record of the cores affords additional insight into the past history of Ichkeul. The non-aquatic pollen record of all three cores shows the loss from c. AD 1890 of an extensive mediterranean scrub dominated by Pistacia lentiscus and its replacement by many ruderal and pastoral indicators such as Liguliflorae, Plantago coronopus and Rumex spp. This change presumably reflects the spread and implementation of agricultural practices within the Ichkeul catchment since the mid-late 19th century. In addition to changes in the catchment vegetation, major changes are also seen in the lake macrophytes. Analysis of the upper sediment of three cores (cores 2, 6, 8--Figs. 6(a)-(c) shows that Isoetes gives way to Ruppia and Potamogeton communities which in core 2 are dated to c. AD1892. In both cores 2 and 8 this change lies in a similar position to the first and second molluscan beds and confirms that the dating appears consistent between cores. Unfortunately, a similar confirmation cannot be obtained from core 6 because molluscan beds are absent from this core. However, the pattern of a Pistacia decline followed by a Potamogeton rise seen in cores 2 and 8 is also recorded here, although the change occurs at 55 cm, indicating a very high sediment accumulation rate since c. AD 1890. This is not surprising given that the core is from the lake margin, taken through one of the extant Potamogeton beds, which act as effective sediment traps during the growing season. In the context of the present ecology of the lake the change from Isoetes to Potamogeton would conventionally be taken as a change from fresh to the

Enoironmental change in Garaet el Ichkeul

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present brackish-water conditions. However, the molluscan, ostracod and foraminiferal record suggests that Ichkeul has experienced some degree of saline influence for all the time represented by the analysed cores. The Isoetes record must therefore either represent communities of Isoetes hystrix growing in extensive freshwater marshes (the replacement of these communities by Potamogeton indicating further brackish influence), or result from river-borne input of the spores from Lac Sedjenane. This is a site situated some 60 km upstream of Ichkeul, from which the plant collectors E. Cousson and G. Barratte recorded abundant Isoetes velata in 1848 (British Museum (Nat. Hist.) Herbarium sheet). The large reduction of Isoetes in the pollen diagram would then be the results of the draining of Lac Sedjenane in the early 1920s and the consequent loss of habitat rather than inherent changes within Ichkeul. It is interesting to note that the Isoetes decline is not coincident with the Potamogeton rise but is later and is dated to the 1920s in core 2. Core correlation

Since the major changes in Pistacia, Isoetes and Potamogeton lie in the same relative positions in cores 2 and 8 to the molluscan beds, a firm biostratographic correlation between all cores can be established (Fig. 7). Although sedimentary depths of the various events change markedly between cores this is inevitable because of non-uniform sediment accumulation rates, a process exacerbated by the shallow basin and wind action.

DISCUSSION Sediment accumulation rate

The changes in the sediment accumulation rate over the recent past can be related to a series of agricultural and channelisation schemes in the Ichkeul catchment. Although agricultural intensification within the Ichkeul catchment has been occurring since Roman times (Stevenson, unpublished data), the pollen evidence from cores 6 and 8 demonstrates that the past 100-200 years has seen the largest change with the replacement of the former Pistacia scrub by agriculture. The timing of this scrub removal and consequent soil erosion is coincident with the early rises in sediment accumulation recorded within the lake sediments.

Environmental change in Garaet el lchkeul

291

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Changes in the course of the Djoumine river from pre-AD 1948 to AD 1982.

The recent rises in the sediment accumulation rate are linked to modifications of the course of the Oued Djoumine in order to reclaim a large amount ofmarshland to the south of Djebel Ichkeul. Prior to 1947 the Oued Djoumine used to flow and deposit its sediment load on the marshland to the south of Djebel Ichkeul (Fig. 8). In the mid-late 1940s a new channel was constructed so that the Oued Djoumine flowed directly onto the Djoumine marsh, allowing the river's sediment load to be deposited directly into the lake during the winter. The date of this channelisation coincides almost exactly with the sixfold increase recorded in the sediment accumulation rate

292

A. C. Stevenson, R. ~ Battarbee

in 1954 in core 2. This phase is also coincident with an accelerated movement of the Phragmites band and general progradation of the shoreline at the edge of the Djoumine marsh associated with increased sediment supply (Hollis et aL, 1983).

Water quality The documentary record reveals that even as far back as the 1 lth century water flow in the Oued Tindja was observed to undergo reversal at certain times of the year (Bonniard, 1934). Subsequent writers have also noted this phenomenon but, depending on the time of year of their observation, this has led to conflicting accounts within the documentary record. Thus a French geographer G. A. Peyssonnel, writing in November 1724 about a recent visit to the area, observed that the Oued Tindja underwent reversal and was saline at the time of his visit (Dureau de le Malle, 1838). On the other hand, Lieutenant Spratt on a hydrographical survey in May 1845 not only provided the first accurate bathymetric map of Ichkeul but observed that Ichkeul was fresh and fit to drink (Spratt, 1846). Despite the documentary evidence reliable information concerning the length of time of reversal is not available and the past salinity regime of Ichkeul cannot be fully elucidated. Although the documentary evidence suggests that Ichkeul has been under some form of saline influence over the last 700 years the sediment cores indicate that major changes have taken place in the flora, fauna and water quality of the lake. The most prominent and significant of these is the change recorded by the macrophyte pollen which shows the rapid increase in the Potamogeton and Ruppia since c. AD 1892, although until recently it had been assumed that the present Potamogeton community, on which the waterfowl feed, had been a permanent feature of the lake. The present international status of Ichkeul for waterfowl may thus be based on an ecosystem of very recent origin, unless a previous food source was available but not recorded in the pollen record. This change in lake water quality towards somewhat more saline conditions is confirmed by the molluscan and foraminiferan evidence; the second molluscan layer in core 2 is similarly dated to c. AD 1892, and can be traced across most of the lake bed (Fig. 7, Table 4). In addition, in core 8 a peak in the marine foraminiferan Ammonia batava is associated with both molluscan layers. The most obvious historical event with which this floral and faunal change can be correlated is the construction of the Bizerte ship canal in 1895, connecting Lac de Bizerte to the Mediterranean (Bonniard, 1934). Prior to the construction of the canal many authors had observed that Lac de Bizerte was non-tidal during the winter months while lake levels in both Ichkeul and

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Lac de Bizcrte were high, but became tidal during the summer months (Dureau de le Malle, 1838; Spratt, 1846; Bonniard, 1934). This non-tidal regimc occurred because the connection between Lac de Bizerte and the Mediterranean prior to the construction of the canal was a narrow, shallow channel through the littoral dune systems (Bonniard, 1934). Consequently, the summer period of flow reversal in the Oued Tindja may have been shorter than now and both the mean salinity regime and salinity maxima may have been markedly lower. The establishment of tidal conditions within Lac de Bizerte all year round probably led to Ichkeul having a higher mean salinity regime than in the past. The upper bed of Cerastoderma glaucum, associated with peaks in Hydrobia ventrosa, ~Scrobiculariaplana in the majority of cores and the marine foraminiferan Ammonia batava in core 8, suggests a further more recent phase of increased salinity in the lake between 1925 and 1954. This corresponds to a period when the meteorological records of the nearby Merjerda catchment (1939-1944) show a series of five years of belowaverage rainfall, including two with only 50% of the average (R. Kallel, 1982, pers. comm.).

CONCLUSION There has been considerable natural and human-induced variations in the past and throughout the last 200 years the lake has always been saline with some marine influence. The spread of Potamogeton in the last decades of the 19th century as an ancillary effect of the construction of the Bizerte Ship Canal in AD 1895 probably heralded the point at which Ichkeul became a bird wintering ground. It is clear that the occasional runs of saline years, especially in the 1940s when even the winters remained saline, provide the best analogues to the future. While the ecosystem in its broadest sense has undoubtedly been resilient to these short-lived changes, it is unlikely to cope with the predicted continually saline conditions. Already, preliminary analysis of satellite images for summer 1989, taken after a period of high winter salinities for the winter of 1988/1989 and after completion of the Djoumine and Rhezala dams, demonstrates a dramatic reduction in the extent of Potamogeton in the lake from 30 km 2 to 0.5 km 2. This has been followed by a large fall in mean bird numbers from 100000 to 20000 in the winter of 1989 (A. Tamisier, 1990, pers. comm.). In the future, re-coring of the lake should be carried out to allow the changes in Potamogeton and mollusc populations to be monitored and documented as conditions become more saline. Coring Lac de Bizerte should help to elucidate the effects of the canalisation schemes.

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ACKNOWLEDGEMENTS The authors would like to thank Dr R. J. Flower, M r S. Phethean, Ms J. Foster, Ms J. Dempsey and M r N. Chisnall for help with the fieldwork; Dr J. E. Robinson and Dr A. Lord for identification o f the ostracods and foraminifera; Ms A. Rooke for cartographical work. This work was partly funded by the Hayter Travel F u n d of the University of L o n d o n and a contract from the E E C - E N V 676 (UK) H.

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