Abrupt changes in Potamogeton and Ruppia beds in a Mediterranean lagoon Abdessalem Shili a,b, * , Naceur Ben Maı ¨z c , Charles Franc ¸ois Boudouresque a , El Bahri Trabelsi c a UMR CNRS 6540 ‘‘Diversite ´, e ´volution et e ´cologie fonctionnelle marine’’, Centre d’Oce ´anologie de Marseille, Campus de Luminy, 13288 Marseille Cedex 9, France b Institut National Agronomique de Tunisie, 43 Avenue Charles Nicolle, 1082 Tunis Mahraje `ne, Tunisia c Socie ´te ´ de Promotion du Lac de Tunis, BP 36, Tunis El Bouhaira, 1080 Tunis Cedex, Tunisia Received 12 August 2005; received in revised form 20 March 2007; accepted 27 March 2007 Available online 25 April 2007 Abstract Until the early 1990s, the Ichkeul lagoon (80 km 2 , Tunisia) was characterised by the seasonal alternation of low and high salinity and by the presence (mainly in summer and autumn) of more or less extensive Potamogeton pectinatus beds (western and south-eastern zones). The yield was largely consumed by wintering waterfowl populations (almost exclusively phytophagous), making this lagoon one of the most important wintering places in northern Africa, and recognized as a high ecological value World Heritage site. Ruppia cirrhosa (together with Chaetomorpha linum floating mats) was confined to a small north-eastern or eastern zone of the lagoon. This is the distribution which the authors observed in 1993, in the course of a study running through 1998, with P. pectinatus covering 37 km 2 (28,236 t DW) and 30 km 2 (20,139 t DW) in summer and autumn, respectively. In 1994, P. pectinatus had disappeared, an event which had already occurred in the past; however, this situation was still apparent in 1998; such a 5 year absence had never been reported since at least the early 1960s. From 1996 onwards, R. cirrhosa and C. linum ‘‘moved’’ to the sites formerly occupied by P. pectinatus. Direct causes (shortage of freshwater input, salinity increase) and indirect ones (construction of dams in the water catchment area of the lagoon, a relative drought) can account for these abrupt changes. The persistence of this situation could threaten the Ichkeul lagoon, as a major site for the wintering of phytophagous waterfowl. # 2007 Elsevier B.V. All rights reserved. Keywords: Brackish lagoon; Potamogeton pectinatus; Ruppia cirrhosa; Cartography; Biomass; Tunisia 1. Introduction The Ichkeul lagoon (80 km 2 ) is located in northern Tunisia (37810 0 N, 9833 0 E). It receives freshwater from a 8000 km 2 catchment area via seven wadis and is linked by a small channel (Tinja) to the lagoon of Bizerte which in turn opens into the Mediterranean Sea (Fig. 1). Because of the high value of the Ichkeul lagoon as a wintering place for large waterfowl populations (180,000–230,000 birds), several national (National Park) and international (Ramsar Convention List, MAB Biosphere Reserve, UNESCO World Heritage List) designations have been applied for protection purposes since the early 1980s (Hollis, 1978; Tamisier and Boudouresque, 1994). The most abundant waterfowl species are phytophagous: greylag goose Anser anser (L.), European wigeon Anas penelope L., common pochard Aythya farina (L.) and European coot Fulica atra L. (Tamisier and Boudouresque, 1994; Tamisier, 1999; Tamisier et al., 2000). Through the early 1990s, autumn and winter rainfalls filled up the wadis and the lagoon with freshwater that overflowed into the lagoon of Bizerte through the Tinja channel (Fig. 1). In summer, high evaporation favoured by hot easterly winds lowered the water level and allowed seawater to enter Ichkeul lagoon. The mean freshwater load was 340 Mm 3 /a (Zaouali, 1974; Lemoalle, 1983a,b; Hollis et al., 1986; Ennabli and Kallel, 1990; Tamisier and Boudouresque, 1994; BCEOM et al., 1995). A primary characteristic of the Ichkeul lagoon thus was an annual alternation of high water levels (up to 2–3 m www.elsevier.com/locate/aquabot Aquatic Botany 87 (2007) 181–188 * Corresponding author at: Institut National Agronomique de Tunisie, 43 Avenue Charles Nicolle, 1082 Tunis Mahraje `ne, Tunisia. Tel.: +216 71 289 431; fax: +216 71 799 391. E-mail address: shili.abdessalem@inat.agrinet.tn (A. Shili). 0304-3770/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2007.03.010