tunisia, sw mediterranean

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Diatom Research (2009), Volume 24 (1), 175-190

BLOOMS OF THE DIATOM /GENTJS PSEUDO-NITZSCHIA H. PERAGALLO IN BIZERTE LAGOON (TUNISIA, SW MEDITERRANEAN) Ines Sahraoui 1,2,·, Asma Sakka Hlaili 1, Hassine Hadj Mabrouk" Claude Leger 2 & Stephen S. Bates 2 1 Laboratoire

de Cytologie Vegetale et Phytoplanctonologie, Departement des Sciences de la Vie, Faculre des Sciences de Bizerte, Universite 7 novembre a Carthage, 7021 Zarzouna, Bizerte, Tunisie 2

Fisheries and Oceans Canada, GulfFisheries Centre, P.O. Box 5030, Moncton, NB, Canada E1 C 9B6

The phytoplankton composition, including the potentially toxic diatom genus Pseudo-nitzschia, and related physico-chemical water properties were determined at four stations in Bizerte Lagoon (SW Mediterranean Sea) monthly, from March 2004 to March 2005. Total phytoplankton abundance was generally low (1.50-7.12 x 105 cells L- 1), but peaked in April 2004 (19.4 ± 2.9 x 105 cells L-1), July 2004 (19.9 ± ILl x 105 cells L- 1) and March 2005 (12.2 ± 4.8 x 105 cells L-I), and was dominated by Plagioselmis spp., Pseudo-nitzschia spp. and Thalassiosira spp., respectively. The contribution of Pseudo-nitzschia spp. to the total algal community was generally moderate (0.6-8.9%, when detected). The genus Pseudo-nitzschia was detected in 70% of the samples and thus appeared as a regular component of the phytoplankton that develops in the surface waters of Bizerte Lagoon. Distribution patterns of Pseudo-nitzschia species in the "delicatissima" group « 3 ).Lm in width) showed a strong seasonality and were correlated with summer conditions (when temperature, salinity and silicate concentration increased), although they were present during at least seven months of the year, at all stations. Pseudo-nitzschia species in the "seriata" group (> 3 ).Lm in width), conversely, revealed a narrower spatio-temporal distribution and appeared uncorrelated with the environmental factors measured. The causative species of the July peak was identified by scanning electron microscopy as Pseudo-nitzschia calliantha. Two out of four isolates of P. calliantha from Bizerte Lagoon produced the neurotoxin domoic acid (causative agent of Amnesic Shellfish Poisoning) in batch culture. Our findings suggest the possibility of domoic acid contamination of bivalve molluscs in Bizert Lagoon, one ofthe main shellfish aquaculture areas in Tunisia, and therefore the need for continued vigilance.

INTRODUCTION Reports of harmful algal blooms (RABs) have become more frequent in recent years (Smayda 1992, Fehling et al. 2005). The toxins produced by some algae are among the most potent naturally occurring compounds (Graneli & Legrand 2001). Toxin accumulation in filter-feeding organisms can have devastating effects on higher level predators such as fishes, sea birds and marine mammals (Maneiro et al. 2005, Busse et al. 2006, Smith et al. 2006). HAB events may seriously impact public safety, fisheries, wildlife and aquaculture, resulting in considerable economic losses. * Corresponding author: Ines Sahraoui ; tel.: + 216-71-889-330; fax: + 216-71-889-330. e-mail address:[email protected]

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1. SAHRAOm', A. SAKKA HLAILI, H. HADJ MABROUK, C. LEGER & S.S. BATES

Domoic acid (DA), which was responsible for three deaths and the poisoning of over 100 people in November and December 1987 in Canada (Bates et af. 1998), has now been discovered in many parts of the world (Trainer et af. 2008). DA, responsible for a neurotoxic illness called Amnesic Shellfish Poisoning (ASP), is naturally produced by some species of the diatom genus Pseudo-nitzschia H. Peragallo. Among the ~30 known morpho-species of the Pseudo-nitzschia genus, 11 to 12 are known to be toxic, although not always (Bates & Trainer 2006, Quiroga 2006, Trainer et af. 2008). Most of the toxigenic Pseudo-nitzschia species are cosmopolites (HasIe 2002), although an increasing number of cryptic species are being discovered (Orsini et aZ. 2004, Amato et aZ. 2007). In field work, Pseudo-nitzschia species have conventionally been divided into morphological categories, based on their cell widths as observed using light microscopy (e.g. Hasle & Syvertsen 1997): the "delicatissima group" (short and narrow cells) and the "seriata group" (long and wide cells). It is difficult to generalize about the ecology of the genus Pseudo-nitzschia because of the species' diversity and differences among strains of presumably the same species. For example, different clades of P. delicatissima were found in the cold eastern Atlantic (Kaczmarska et aZ. 2008) and in the warm waters of the Mediterranean Sea and Gulf of Mexico (Orsini et aZ. 2004). The occurrence of the Pseudo-nitzschia genus is well documented in the Mediterranean Sea (Orsini et aZ. 2002, 2004, Kaninou-Grigoriadou et aZ. 2005, Quiroga 2006, Amato et aZ. 2007, Quijano-Scheggia et aZ. 2008). In Tunisian (SW Mediterranean Sea) coastal waters and lagoons, Pseudo-nitzschia spp. were first noted and described by Turki & El Abed (2001). In spite of that, the history of toxic Pseudo-nitzschia blooms is poorly known; in fact, no ASP event has yet been recorded in Tunisian waters. This may be do in part to deficiencies in the harmful algae monitoring program, especially concerning DA-producing Pseudonitzschia spp., which are not monitored on a regular basis (DG (SANCO)/8622/2002, http://ec.europa.eu/food/fs/inspections/vi/reports/tunisia/viJep_tuni_8622~2002sum_en.pdf). Bizerte Lagoon is considered among the most important aquaculture areas in Tunisia, where aquaculture has existed for 40 years (Medhioub 1993). The lagoon is used almost exclusively for the culture of Pacific oysters (Crassostrea gigas Thunberg) and Mediterranean mussels (Mytilus galloprovincialis Lamarck). Clams (Tapes decussatus Linne) are also very abundant and are commercialized within the region and for export (Dellali et aZ. 2001, Trigui EI-Mnif et af. 2005). Recently, mussel production reached> 100,000 tons per year (Khessiba et af. 2001). Therefore, the incidence of harmful microalgae may be an obstacle to the further development of shellfish farming in the area, especially since several potentially toxic algae (such as Pseudo-nitzschia spp.) have been identified several times in the Bizerte Lagoon (Turki & El Abed 2001, Sakka Rlaili et af. 2006, 2007). The establishment of monitoring programs for harmful phytoplankton species and their phycotoxins is considered a useful tool in the early warning of RAB events (Rhodes 1998, Graneli & Legrand 2001). In this context, we investigated the spatial and temporal distribution of the potentially toxic genus Pseudo-nitzschia in Bizerte Lagoon. Data is provided here concerning the main Pseudo-nitzschia groups ("delicatissima and "seriata"), including their seasonality and bloom dynamics. Also documented was the toxicity of P. calliantha Lundholm, Moestrup et Hasle isolated during one of the blooms. MATERIALS AND METHODS Study site and sampling Bizerte Lagoon (37° 8'-37° 14' N, 9° 48'-9° 56' E) is located on the northern coast of Tunisia. (Fig. 1). The lagoon is connected to the Mediterranean Sea through a 7-km long, 12~m deep channel. The ecosystem encompasses an area of 150 km2 and has a mean depth of 8 m. Large volumes of water are exchanged between Bizerte Lagoon and the Mediterranean

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PSEUDO-NITZSCHIA BLOOMS IN BIZERTE LAGOON

ea. Water transport is strongly modulated by tidal currents. A semidiumal tide (0.02-0.13 m f amplitude) requires> 1 year to renew the total water volume of the lagoon (Sakka Hlaili tal. 2008). 37"20' N Mediterranean sea

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