Nematology, 2008, Vol. 10(3), 323-333 Mussel-farming effects on Mediterranean benthic nematode communities Ezzeddine MAHMOUDI 1,∗ , Naceur E SSID 1 , Hamouda BEYREM 1 , Amor HEDFI 1 , Fehmi BOUFAHJA 1 , Patricia AÏSSA 1 and Pierre VITIELLO 2 1 Laboratoire de Biosurveillance de l’Environnement, Faculté des Sciences de Bizerte, 7021 Zarzouna, Bizerte, Tunisia 2 Centre d’Océanologie de Marseille, Campus de Luminy, Case 901, 13009 Marseille Cedex 9, France Received: 18 September 2007; revised: 22 October 2007 Accepted for publication: 26 October 2007 Summary – Mussel aquaculture activities in coastal areas are growing rapidly throughout the world, inducing an increasing interest and concern for their potential impact on coastal marine environments. We have investigated the impact of organic loads due to the biodeposition of a mussel farm in a lagoonar ecosystem of the Mediterranean Sea (Bizerta lagoon, northern Tunisia) on the benthic environment. The most evident changes in the benthic habitat under the farm were a strong reduction of oxygen penetration into the bottom sediments and a large accumulation of chlorophyll a (concentrations up to 16 μgg −1 ), phaeopigments (concentrations up to 48 μgg −1 ) and total organic matter (concentrations up to 12%). Results from univariate analysis of the nematofaunal data show that the nematode abundance increased in all the stations located inside the mussel farm (I1, I2, I3) and the site I2, located in the centre of the mussel farm, was the most affected. At this site, Shannon-Wiener index H ′ , species richness (d), evenness (J ′ ) and number of species (S) decreased significantly. Results from multivariate analyses of the species abundance data demonstrated that responses of nematode species to the organic matter enrichment were varied: Mesacanthion diplechma was eliminated at the most affected station (I2), whereas the abundances of Paracomesoma dubium, Terschellingia longicaudata and T. communis were significantly enhanced. Responses of free-living nematodes to mussel farm biodeposition (elimination of some species and increase or decrease of some others) could lead to food limitation for their predators that, ultimately, could alter entire communities and ecosystems. Consequently, we suggest that site-specific hydrodynamic and biogeochemical conditions should be taken into account when planning new mussel farms, and meiobenthic communities should be monitored before and after farm development to prevent excessive modifications of benthic assemblage structure. Keywords – biodeposition, free-living nematodes, lagoons, marine nematodes, mussel culture, Mytilus galloprovincialis, nematode communities. Mussel production in the Mediterranean Sea alone is over 700 000 t yr −1 (FAO, 2000). The environmental ef- fects of mussel-farming activities have been extensively studied (e.g., Dahlbäck & Gunnarsson, 1981; Dinet et al., 1990; Miron et al., 2005) but many of these studies have yielded different results, even in similar habitats and with the use of the same taxonomic groups. For example, mus- sel farming is responsible for the intensive biodeposition of faeces and pseudo-faeces that causes strong changes in the physical and chemical characteristics of the sediments beneath the long-lines (Dahlbäck & Gunnarsson, 1981; Kaspar et al., 1985; Gilbert et al., 1997). The effects of mussel farms on benthic environments are likely to be of limited spatial extension and appear less relevant than the impact of intensive fish farm activities employing exter- * Corresponding author, e-mail: ezzeddine.mahmoudi@laposte.net nal sources of organic matter (Mazzola et al., 1999, 2000). Mussel biodeposition affects benthic community structure (Castel et al., 1989; Dinet et al., 1990; Kröncke, 1996) and induces reducing conditions over a wide area of the substrate (Dahlbäck & Gunnarsson, 1981), which could markedly alter the organic matter composition and cycling (Kaspar et al., 1985). Mussel farming is eco-sustainable as it does not alter the coastal marine ecosystem, either in terms of functioning and trophic states, and it does not affect the biota at the different levels of the biological organisation (Danovaro et al., 2004). Many factors may contribute to these seemingly conflicting results; the cul- turing method used, the density of the cultivated mussels, the water depth and the hydrographical conditions in the system being investigated. © Koninklijke Brill NV, Leiden, 2008 323 Also available online - www.brill.nl/nemy