Short-term effects of nutrient enrichment of the sediment and interactions between the seagrass Cymodocea nodosa and the introduced green alga Caulerpa taxifolia in a Mediterranean bay

Nutrient addition experiments provide a means for testing ecological theories concerning the effects of nutrient availability on community composition and development. Here we present the results of two reciprocal short-term (4 months) experiments testing for the effects of nutrient addition in the sediment and competitive interactions between the seagrass Cymodocea nodosa and the tropical green alga Caulerpa taxifolia. This study was conducted from 1 June to 30 September 1995 in a bay on the south coast of Elba Island (Italy) recently impacted by the alga. Each experiment consisted of the manipulation of the level of nutrients (addition vs. control) and the manipulation of neighbours (presence vs. removal). Response variables were blade density and length of C. taxifolia in one experiment and shoot density and leaf length of C. nodosa in the other. Results indicate that during the growing season the canopy of C. nodosa did not have any effect on C. taxifolia growth while nutrient addition in the sediment favoured colonization (higher blade density) of this alga independently of the presence of the seagrass. Conversely, C. taxifolia had a negative effect on shoot density of C. nodosa independently of nutrient availability. Hence, colonization by the introduced alga C. taxifolia may provide an explanation for the regression of C. nodosa beds occurring at the site. Furthermore, we demonstrated that C. taxifolia was favoured by high nutrient loads in the sediment, while C. nodosa was not; this might affect the outcome of the interactions among these species in the long-term. (C) 1997 Elsevier Science B.V.