Changes in the structure of a Posidonia oceanica meadow and in the diversity of associated decapod, mollusc and echinoderm assemblages, resulting from inputs of waste from a marine fish farm (Malta, Central Mediterranean)

Abstract

Nutrient inputs resulting from fish farming activities located in the vicinity of seagrass meadows can potentially alter the structure and affect the functioning of these ecosystems, however, few studies have addressed this problem. The impact of waste generated by an offshore fish farm in Malta (Central Mediterranean) on the structure of a meadow of the seagrass Posidonia oceanica and on the associated decapod, echinoderm and mollusc assemblages was studied by collecting samples from stations located at distances of 10 m, 30 m, 50 m, 90 m, 170 m and 330 m away from the farm. Meadow morphology and leaf epiphyte load changed with distance from the fish cages, as did the species richness and abundance of macroinvertebrates associated with the seagrass. However, while shoot morphometric measures increased significantly in value over the whole length of the transect (330 m), macroinvertebrate species richness and abundance peaked at an intermediate distance (40–160 m) from the cages. These results suggest that while waste generated from fish farms can severely alter the structure of a seagrass meadow over a large area, nutrient enrichment could increase productivity in certain parts of the same meadow, leading to a localized increase in species richness and abundance of associated macroinvertebrates. These changes result in different 'ecological zones' round the source of nutrient input.