Hydrobiologia 279/280: 149-156, 1994.J. J. Kerekes (ed.), Aquatic Birds in the Trophic Web of Lakes.© 1994 Kluwer Academic Publishers. Printed in Belgium.

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Aquatic bird populations as possible indicators of seasonal nutrient flowat Ichkeul Lake, Tunisia

Alain Tamisier' & Charles Boudouresque 2

'Centre d'Ecologie Fonctionnelle et Evolutive, Centre National de la Recherche Scientifique, B.P. 5051,F-34033 Montpellier, France; 2Laboratoire de Biologie Marine et d'Ecologie du Benthos, Facult des Sciencesde Luminy, F-13288 Marseille Cedex 9, France

Key words: Ichkeul, Mediterranean lagoon, waterbirds, bio-indicator, nutrient flow

Abstract

Lake Ichkeul is fed in autumn and winter by 7 main freshwater oueds (rivers) which create an overflowtowards the mediterranean sea. Conversely, seawater enters the lake after the end of the rainy season.Thus, strong inverse variations occur twice a year both in water depth and salinity level, with a majorhydraulic flow to the sea between October and January.

Waterbirds exploit the lake (ca 50 species belonging to 13 families) with a marked seasonal variationin specific richness and diversity; in winter, population size (200000 ducks, coots and geese) and bio-mass are much higher: 92.2 % of the trophic impact occurs between October and March, because of bothnumbers and size of the wintering birds. The bird community is phytophagous for 96.2% of its annualbiomass.,

The data, related to the hydraulic regime of the lake, support the hypothesis of an energetic input (Nand P) which is mostly linked to the freshwater flow and an export brought about by migrating birdsand the harvesting of fish. Nutrients would be a limiting factor for both plant and animal communities,Ichkeul being an atypic (rather oligotrophic) mediterranean lake. New dams around the lake (for drinkingand irrigation water) are changing drastically this energetic balance and will lead to an important lossof the main characteristics of the lake, making Ichkeul an ordinary man-managed wetland.

Introduction

Ornithological data collected on Ichkeul Lake(Tunisia) over the last 9 years revealed a strongseasonal variation in bird populations through theyear. Peak populations of migrant waterbirdsoccur in winter whereas in summer the aviancommunity is quite limited in numbers and spe-cies. There is no indication of accumulation oforganic matter (unconsumed plants) or summereutrophication as occurs on most Mediterraneanlagoons (Belkhir, 1984; Frisoni, 1990). What hap-pens with the trophic resources which support the

huge waterfowl community in winter? Is the ex-cessive organic matter exported outside IchkeulLake, and if so, how? The objectives of our studyis to use ornithological and hydrobiological datain the context of the hydraulic functioning of theecosystem in order to provide some hypothesesaddressing these questions.

Site description and recent changes

Ichkeul Lake (37 ° 10' N 9° 33' E), Tunisia, inthe northeastern portion of the African continent

along the Mediterranean Sea, is a permanent wet-land of 80 km2 surrounded by 30-40 km2 of tem-porary marshes. It collects fresh water directlyfrom rains (annual mean: 60 million m3) and froma drainage basin of 8000 km2 bringing 340 mil-lion m3 by 7 oueds (temporary rivers). It is linkedby a small channel (Tindja) to the Lake of Biz-erte (37° 18' N 09 ° 35' E) which in turn opens tothe Mediterranean sea.

The annual rainfall varies between 234 and1330 mm (mean = 640-670 mm) with a maximumin autumn and winter. Mean monthly tempera-tures are 11-12 C from October to March, and23-26 C from July to September. Autumn andwinter rainfalls fill up the oueds and the lake withfreshwater that overflows into the Lake of Bizertethrough channel Tindja. In summer, high evapo-ration favoured by hot easterly winds lowers thewater level and allows seawater to enter IchkeulLake (Zaouali, 1975; Hollis, 1977; Lemoalle,1983; Ben Rejeb-Jenhani, 1989; Ennabli, 1990).

A primary characteristic of Ichkeul lake is anannual alternation of high water levels and lowsalinity in winter (mean salinity 3 g 1-', meandepth 2-3 m with all marshes inundated) and lowwater level but high salinity in summer (meansalinity 30-50 g 1- , mean depth 1 m, marshesdried up) (Fig. 1). A high annual variability addsto this alternation in depth (Fig. 2) (Bredin et al.,1986; Tamisier, 1987, 1990). The freshwater out-put through the Channel Tindja is 47 to 955 mil-lion m3 per year (mean 305), the mean saltwaterinput is ca 60 million m3 per year. The wide dis-

SALINITY g%o

-40

-4 30

I-20

i- 10

I ._|= I I I

WiN. SPR. SUM. AUT,

Fig. 1. Schematic annual alternation of water level and watersalinity, drawn after Hollis et al. (1986).

WATERLEVEL

m.

2.00-

1,50-

1.,00 -

0.50-

i ./

/.

l/ K -- 8-84... /83-8487-88

'" i /~r ~ /' 85-86- ~.-" 88-89

OCT. NOV. DEC. JAN. FEB. MAR.

Fig. 2. Inter-annual variability of water level for the winterseasons 1982-83 to 1988-89.

equilibrium in the input-output water balance ap-pears as the second characteristic of the lake(Boudouresque, 1990; Floret, 1990).

Because high annual climatic variations arecharacteristic of the mediterranean basin, themeans have usually a lower significance than theextreme values.

The lake is usually close to being saturated withdissolved Oxygen (5-11 mg 1- ). Phosphate var-ies from 0.01 to 2.94 pg 1 1 with higher valueseither in summer or winter times according to the2 annual sets of available data (Ben Rejeb-Jenhani, 1989). Concentrations of nitrate varyfrom 0 to 35 pg I - 1 with strong annual differencesand highest values in February (stream from Ich-keul to Bizerte). Nitrite concentrations remainlower than 1 g 1- . Fluctuations in the N/P ratiolead to an alternation between N and P as thelimiting factor in the phytoplankton production(Ben Rejeb, 1986; Ben Rejeb-Jenhani, 1989).

The aquatic vegetation community is verysimple with large monospecific beds of Potamo-geton pectinatus in the eastern and western partsof the lake, small patches of Zostera noltii andRuppia sp. mostly close to the Oued Tindja andscattered patches of Scirpus maritimus and Sc.lacustris in the marshes. The lake and marshes areseparated by a thin strip of reeds (Phragmites aus-tralis). For the past 15 years, large seasonal andannual fluctuations of the extent (0.5 to 4.5 km2)and the biomass of submerged macrophyte beds

150

WATERLEVEL

m. 2.0 -

1.0-

,

I

i

`···.

151

were observed (Hollis etal., 1986). During thewinter 1991-1992, the phytomass of the lakereached 9000 t (dry weight, Ni6ri et al., 1992).

Breeding bird community is rather low in num-bers and species while waterfowl populationswinter there in large numbers (180 000 to 230 000birds, Tamisier, 1990).

Because of the high identified value of this wet-land unique among numerous sebkhras and saltlakes, several national and international designa-tions have been applied for protection purposessince the early 1980's: National Park, RamsarConvention List, MAB Biosphere Reserve,UNESCO World Heritage List. Nevertheless, aproject was proposed in 1982 to build 6 dams onthe main oueds in order to catch water for drink-ing and irrigation. Today, 2 dams are operatingwhich retain ca 25 % of the freshwater volume ofthe lake, 1 is to be completed in 1993 and theother 3 are still under discussion. When operat-ing, the first 3 dams together can retain ca 70%of the water volume, and the 6 dams 100 % (Hol-lis et al., 1986).

A scientific program was set up in 1982 to sum-marize the existing data and to (1) evaluate therichness of the waterbird populations, (2) exam-ine and model the functioning of the whole eco-system, (3) predict changes likely to occur in thenear future after the completion of the damprojects and (4) develop management proposalsaimed at preventing a full loss of the aquatic eco-system due to a lack of water.

Material and methods

Ornithological data were derived from 2 mainsources.

(1) Many occasional observations were madeby various ornithologists, providing basic infor-mation on the breeding, migrating and winteringaquatic avifauna (Gaultier, 1986; pers. corn.;Skinner et al., 1986; Smart, pers.com.).

(2) Since 1982, regular monthly counts cover-ing the whole of the lake and marshes were madefrom September or October to February orMarch, providing the total size and the species

composition of the wintering waterbird popula-tions as well as their diurnal distribution. Simul-taneously, field observations were performed ontime budget activities of the main 4 waterfowlspecies, and some data were collected on theirmost important food resources (Bredin et al.,1986; Tamisier, 1987; 1990; Tamisier et al., 1992).In spring and summer, additional data were col-lected at the species level (Maamouri & Maatal-lah, 1986).

The annual cycle is divided into 2 main periodswintering from September 15th to March 15thand breeding from March 15th to September15th. The migrating period (4 month) which in-cludes both autumn and spring migrations, over-laps the 2 former periods. These respective du-rations will be used for interperiod comparisonson species diversity and biomass. When dealingwith the annual cycle, calculations are made ac-cording to the following schedule which takes intoaccount the overlap of the successive periods:

1 year = 4B + 4W + 2(B + M) + 2(W + M),

where numbers = number of months;B = Breeding;W = Wintering;M = Migrating.

The same species may occur over several pe-riods. Waterbirds include Grebes (Podicipedidae),Cormorants (Phalacrocoracidae), Herons (Ar-deidae), Ibis and Spoonbills (Threskiornithidae),Storks (Ciconiidae), Geese and Ducks (Anatidae),Rails and Coots (Rallidae), Shorebirds (Haemato-podidae, Recurvirostridae, Charadriidae, Scolo-pacidae) and Gulls and Terns (Laridae). We alsoadded 2 raptors because of their functional linkto wetlands (the Marsh Harrier Circus aerugino-sus and the Fish-Eagle Pandion haliaitus).

Numbers of birds for the winter period comefrom the monthly counts for the years 1982-83 to1988-89 (n = 69 counts): thereafter numbers onthe lake changed drastically as a consequence ofboth the effectiveness of the first 2 dams and anexceptional rainfall deficit. Data after 1989 arenot considered representative of the natural situ-ation at Ichkeul Lake and are excluded here.

152

Table 1. Specific richness according to groups of families andperiods.

Breeding Wintering Migration

Grebes, CormorantsGeese, Ducks, Coots 6 18 2

Herons, Storks etc... 3 6 3Raptors 1 2 -Shorebirds 3 13 14Larids 1 2 3

Total 14 41 22

These numbers were obtained from regular countsperformed by the same experienced team on thesame observation points. Numbers for the peri-ods of breeding and migration are less precise(several independent observers) and documented.They are surely underestimated by lack of ex-haustive observations, mostly for the migratingperiod.

Biomasss of birds were calculated from meanweights for each species (Cramp & Simmons,1977; 1980; 1982). The specific trophic levels arefrom Cramp & Simmons (loc. cit.) and from localobservations, mostly for wintering species.

These ornithological data are examined in re-lation to other environmental information avail-able in the litterature: climatology and hydrology(Chaumont, 1956; Hollis, 1986; Ennabli, 1990;Kallel, 1990; com. pers.), sedimentology(Ouakad, 1982; Soussi, 1981), hydrobiology (BenRejeb, 1986; Ben Rejeb-Jenhani, 1989), aquaticbotany (Belkhir, 1984), malacology and ichthyol-ogy (Zaouali, 1974, 1975; Lemoalle & Vidy, 1983;Kartas & Zaouali, 1990).

Table 2. Specific diversity (Shannon Index) in terms of num-bers and biomass for the 3 periods of the year.

Breeding Wintering Migration

Numbers 3.25 2.02 2.50Shannon Index

Biomass 2.83 2.76 1.04

Results

Within the community of waterbirds we observedabout 50 species belonging to 13 families groupedin Table 1. However his specific richness is rathervariable along the periods. The maximum occursin winter for all families except for those includedin Shorebirds which occur in similar species'numbers in winter and in migration. Converselythe specific diversity index, either calculated fromnumbers or from biomass, was highest during thebreeding period (Table 2).

Total numbers and biomass were much higherin winter than in migration and breeding periods(Table 3). The annual trophic impact derived fromthis biomass is largely concentrated in the wintermonths (92.0 %), and it comes almost totally from4 waterfowl species, Greylag Goose Anser anser,European Wigeon Anas penelope, Common Po-chard Aythya ferina and European Coot Fulicaatra (Fig. 3). Even though there may be large bi-ases associated with our estimations of both mi-grating and breeding populations, the overwhelm-ing importance of winter populations is clearlyevident.

The community is almost exclusively phytopha-gous in winter (96.2 % of the biomass) and mostly

Table 3. Numbers of bird-days and biomass (in t) of phyto- and zoophagous waterbird communities. Monthly mean accordingto the period of the year and annual. The anual total is derived from the formula: 1 year = 4B + 4W + 2(B + M) + 2(W + M) (seein the text).

Breeding Wintering Migration Annual total

N. bird-days 35,670 2307,360 256,650 15154,780

Phyto- 4.8 2041.4 138.0 12829.2 96.2%Biomass

Zoo- 15.1 53.2 23.1 502.2 3.8%

153

MEAN MONTHLY

BIOMASS IN t

(WET WEIGHT)

GREYLAG GEESE

WIGEON

POCHARD

COOT

A M J J A S 0 N D J F M A M

Fig. 3. Scheme of the seasonal variations of biomass of the waterbird communities. Four herbivorous species largely dominatethe annual biomass because of their importance in winter. Drawn after Tamisier (1990).

zoophagous in spring. During the migration pe-riod which overlaps the others, transient birds areessentially zoophagous but most counted birdsstill belong to the wintering community, so thecommunity as a whole is mostly phytophagous.Along the annual cycle, phytophagous species re-present 96.2% of the mean total biomass(Table 3). Among them 58.6% feed on stems andleaves of the major plant species (Potamogetonpectinatus for European Wigeon and EuropeanCoot), 36.4% on bulbs of Scirpus maritimus andPotamogeton pectinatus (Greylag Goose andCommon Pochard respectively). Granivorousspecies (Teal Anas crecca and Pintail A. acuta)account for only 1.3%. Among zoophagous spe-cies, Cormorant Phalacrocorax carbo (fisheater)and Shoveler Anas clypeata (planktonophagous)predominate in winter.

Discussion

The summarized available hydrobiological datasuggest (Boudouresque, 1990) that lake Ichkeul is

fed in P and N (Fig. 4) by oueds and possibly inP by Channel Tindja (seawater) (Ben Rejeb,1986). During the period September-February,additional nutritive enrichment surely comes fromthe defecation of birds and from carcasses (thewinter mortality rate of geese, ducks and cootsusualy reaches values as high as 30-50%, Cramp& Simmons, 1977). Defecation is supposed tohave an impact on the development of the aquaticbeds. Powell et al. (1991) conclusively showedthe positive effect of birds roosting half a day ata subtropical estuary in Florida on the produc-tivity of aquatic beds over a 200 m wide crownaround the colonies (estimated value for a meanyear of bird biomass/surface = 1 t km2). At Ich-keul, the value is about 2 times lower but birdsstay and feed on the spot day and night (Tamisier,1990); so nutrients brought in by defecation mighthave some effect on the aquatic plant beds ofIchkeul. However since these nutrients were takenoff by the feeding birds from the Ichkeul systemitself, it suggests that they are recycled betweenplants and feces.

On the other hand a periodic lack of nutrients

2,000

1,500

1,000

500

154

Fig. 4. Simplified and hypothetical premodel of the functioning of the Ichkeul aquatic ecosystem. The width of the arrows indi-cates the importance of flux (middle, large or very large). The size of the biological blocks relates approximately to their biom-ass. Flux and blocks of suspected lesser importance, as well as the input of mineral N, are not represented. Data after BenRejeb-Jenhani, 1989; Boudouresque, 1990; Ni6ri et al., 1992; Tamisier, 1990; Zaouali, 1974, 1975.

is suspected, which would explain (1)the ob-served monthly and annual variations in biomassof the aquatic beds of Potamogeton pectinatus,(2) the absence of vegetal species characteristic ofeutrophic lakes, mostly benthic algae, and (3) thenon-occurrence of the dystrophic phenomenon(so called 'malaYgue') characteristic of mosteutrophic mediterranean lakes and lagoons insummer (Boudouresque, 1990). The origin of thislack of available nutrients is unknown. Howeverit could include actual deficit, adsorption by sedi-ments, storage in sediments and/or in plants(bulbs of Potamogeton pectinatus and Scirpus mar-itimus) and exportation. Adsorption of phosphateby sediments has been demonstrated (Ben Rejeb-Jenhani, 1989). Moreover the roots of P. pectina-tus which can favor the uptake of nutrients of thesediment, are not found deeper than 0.40 m underthe ground level (Ni6ri etal., 1992). Storage inbulbs would allow the development of aquaticbeds in poor waters. Exportation could involve

the transport of sediments carrying nutrients fromlake Ichkeul to the sea; it also involves removalfrom fish (210 t wet weight year caught by com-mercial fishing, Kartas & Zaouali, 1990) andmostly waterbirds migrating back to their breed-ing grounds: the exported biomass is about 13 000t (wet weight, see Table 3), among which ca 20 to30% (= 2600 to 3900 t) were gained from theaquatic beds of lake Ichkeul between the arrivaland the departure date of the birds.

Moreover, it is suspected that the usual deple-tion of the aquatic beds at the end of the winterseason is directly linked to the part removed bywaterbirds: as long as waterlevels are favorablefor feeding, the date of departure of most ducksand coots varies accordingly to the amount offood resources which are still available in the lake.Once the food depletion happens as early as No-vember or December, birds are forced to leaveand scatter over much less favorable wetlands inTunisia and Algeria; food resources seem to be

155

the limiting factor for ducks and coots that win-ter at Ichkeul Lake (Tamisier, 1990; Tamisieret al., 1992).

So many more questions are raised than an-swers are provided to the functioning of the eco-system. Yet waterbird populations, playing an in-dicator role, put the emphasis on the fact that lakeIchkeul appears as a mediterranean aquatic eco-system where limits are reached every year eitherat the nutrient or at the population level, maybeat both. It differs quite fundamentally from mostother mediterranean lagoons. There, the phyto-mass is not fully exploited by herbivorous species,it can accumulate on the bottom and remain theyear long, showing seasonal variations of abun-dance (Mercier, 1973; Dubois & Lauret, 1991). Itis decomposed by detrivorous species and/orbacteria. In this case, anoxic situations can occurin summer with an accumulation of nutrients inthe system which is more or less eutrophic. Onthe contrary at Ichkeul lake, the phytomass (atleast for macrophytes) is quite completely ex-ploited by herbivorous migrating species. Andthese birds, when they leave, export N and Ptaken off from the lake; consequently they mightmaintain the functioning of a rather oligotrophicsystem.

The perspective of the dam constructionprompts reflection on Ichkeul's unique ecosys-tem: as a matter of fact, lake Ichkeul deprived ofmost of its freshwater and nutrient input will losethe essential components of its originality andbecome a rather standard man-managed im-poundment. In this context, one of the objectivesof the management plan is to focus the remainingenergy of the ecosystem towards a maximum pro-duction of the aquatic macrophyte beds in orderto maintain as much as possible its carrying ca-pacity for the wintering waterbird community.That objective must be attained, since the rate ofdecline of the duck populations wintering in themediterranean basin has already reached 46% inthe last 15-20 years (Van Vessem et al., 1992).This may not avoid impoverishment of the natu-ral functioning of Ichkeul lake nor prevent a majorloss in the biodiversity of the aquatic ecosystemsof the region.

Acknowledgements

We wish to thank the tunisian authorities, inparticular the Agence Nationale pour la Protec-tion de l'Environnement (Ministore de l'Environ-nement), the Direction Gnerale des Forets(Ministere de l'Agriculture) and the Facult& desSciences of Tunis for stimulation and/or permis-sion to carry out the research at Ichkeul Lake,M. M. A. El Hili, President of the Fondation Na-tionale de la Recherche Scientifique, H. Baraket,Director of the Parc National de l'Ichkeul andJ. P. Sallei, Head of the Centre National de laRecherche Scientifique in Tunis. We are gratefulto many collaborators who helped us in severalways, in particular Mrs E. Allouche, M. Gerbal,E. B. Trabelsi and S. Valin, and M. M. A. Al-louche, F. Aubry, V. Gravez, F. Kartas,F. Maamouri, M. Ni6ri, A. Shili and O. Sloeck.

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