Nutrient availability in the sediment and the reciprocal effects between the native seagrass Cymodocea nodosa and the introduced rhizophytic alga Caulerpa taxifolia

Two reciprocal experiments testing for the effects of nutrient addition in the sediment and competitive interactions between the native seagrass Cymodocea nodosa (Ucria) Ascherson and the introduced alga Caulerpa taxifolia (Vahl) C. Agardh were performed. This study was conducted for 13 months (August 1995 until September 1996) in a bay on the south coast of Elba Island (Italy). Each experiment consisted of the manipulation of the level of nutrients (addition vs. control) and the manipulation of the neighbours (presence vs. removal). Response variables were blade density and size for one experiment and shoot density and leaf length of seagrass in the other. Results indicated that the presence of Caulerpa taxifolia did not affect significantly Cymodocea nodosa shoot density and the increased nutrient availability in the sediment did not alter this pattern. Neither the removal of the canopy of the seagrass nor the fertilization of the sediment has influenced significantly the density of the alga. Both species, where co-occurring, show larger size than where the neighbour is removed. Hence, results of this study suggest that the two species on the long term are likely to coexist and that the high nutrient supply of the sediment would not enhance the probability of success neither of the seagrass nor of the alga. Predictions made on the basis of short-term results, that high nutrient loads of the substratum would have represented an even more suitable condition for C. taxifolia to colonize C. nodosa beds and that on the long-term the alga has a high probability of success, did not occur.