Effects of fish farming on seagrass (Posidonia oceanica) in a Mediterranean bay: seagrass decline after organic loading cessation

The effects of fish farming on a seagrass (Posidonia oceanica) meadow at Fomells Bay, Minorca (Balearic Islands) were studied. Changes in plant and meadow features (e.g. shoot morphology, shoot density, biomass, rhizome growth, nutrient and soluble sugars concentrations...) in three stations along a transect from a disturbed (organic pollution due to fish cultures) to an undisturbed site were assessed. The fish culture had ceased in 1991; however, seagrass decline, already reported in a previous study for the period 1988-1990, was still taking place at the time of sampling (July 1994). Differences between stations were very clear; the station closest to the fish cages showed reduced shoot density, shoot size, underground biomass, sucrose concentration and photosynthetic capacities. In contrast, shoots from the most polluted station showed higher P-concentration in tissues and higher epiphyte biomass than the other two. Since water conditions had recovered completely by the time of the sampling mission, it is proposed that the persistence of the seagrass decline was due to the excess organic matter remaining in the sediment. 0 Elsevier, Paris / Ifremer / Cnrs / Ird aquaculture I seagrass I Posidonia oceanica I eutrophication I sediment characteristics R&urn6 Effets de Nlevage de poissons sur les herbiers de Posidonia oceanica dans une baie m6diterrant5enne. Les effets de l’elevage de poissons en cages flottantes sur un herbier de Posidonia oceanica ont et6 CtudiCs dans la baie de Fornells (Minorque, iles Baleares). Des modifications dans les caracteristiques de la plante et de l’herbier (morphologie et densitt des faisceaux, biomasse, croissance des rhizomes, teneurs en nutriments et en sucres solubles) sont observees en trois stations d’une radiale allant d’une zone poll&e (par la mat&e organique due aux elevages de poissons) a une zone non pollube. Bien que l’elevage ait cesse en 1991, la degradation notee en 1988-1990 s’est poursuivie jusqu’au moment de l’tchantillonage d&it ici (millet 1994). Les differences entre les stations sont tres nettes : la proximite des cages est associte aux plus faibles valeurs de densite et taille des faisceaux, biomasse, teneur en saccharose et taux photosynthetique, tandis que dans cette m&me zone polluee, la concentration du phosphore darts les tissus et la biomasse des epiphytes sont maximales. La colonne d’eau ayant retrouve son &at initial apt& la disparition des Blevages, la persistance de la degradation pourrait $tre lice ?I des modifications du sediment dont la teneur en mat&e organique est restee tres ClevCe. 0 Elsevier, Paris / Ifremer / Cnrs / Ird aquaculture I phan&ogame marine I Posidonia oceanica / eutrophisation I caractkristique du skdiment * Correspondence and reprints Oceanologica (1999 1 22, 1, 109-l 17 Q Elsevier, Paris I fremer I Cnrs I Ird 109