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 Fo rnells Bay, Minorca (Balearic Islands) were studied. Changes in plant and m eadow features (e.g. shoot morphology, shoot density, biomass, rhizome grow th, 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; howeve r, seagrass decline, already reported in a previous study for the period 19 88-1990, was still taking place at the time of sampling (July 1994). Differ ences between stations were very clear; the station closest to the fish cag es showed reduced shoot density, shoot size, underground biomass, sucrose c oncentration and photosynthetic capacities. In contrast, shoots from the mo st polluted station showed higher P-concentration in tissues and higher epi phyte: biomass than the other two. Since water conditions had recovered com pletely by the time of the sampling mission, it is proposed that the persis tence of the seagrass decline was due to the excess organic matter remainin g in the sediment. (C) Elsevier, Paris / Ifremer / Cnrs / Ird.