Different effects of Caulerpa racemosa on two co-occurring seagrasses in the Mediterranean

In this study, we present the results of a 14-month field experiment on the effects of the introduced alga Caulerpa racemosa on two co-occurring native seagrasses, Cymodocea nodosa and Zostera noltii. The experiment was conducted in a mixed shallow seagrass bed along the Tuscan coast (Italy) that has been invaded by the alga since 1995. Treatment consisted of the manipulation of the presence of the alga (presence vs. removal). Response variables considered for Cymodocea nodosa were shoot and flower (both male and female) density and for Zostera noltii were vegetative and reproductive shoot density, the first species being dioecious and the latter monoecious. Results have indicated that the presence of the alga influences shoot density of both plants. With Cymodocea nodosa the shoot density was lowered and with Zostera noltii it was increased in areas invaded by the alga. Both Cymodocea nodosa flower density and Zostera noltii reproductive shoot density were found to be significantly higher where Caulerpa racemosa was present relative to where it was removed. Hence, different effects, either positive or negative, of the alga were found depending on the species and response variable. Potential mechanisms of interaction are discussed.