Field transplantation of seagrass (Posidonia oceanica) seedlings: Effects of invasive algae and nutrients

Seedlings are a key life stage in seagrasses, providing genetic diversity and being a useful tool for restoration. We examined the influence of increased sediment nutrients and the presence of the invasive macroalga Caulerpa cylindracea on the success of in situ transplanting Posidonia oceanica seedlings in a six-month experiment. Our results indicate that one-year old seedlings successfully survive in the field and their survival and growth are positively affected by the presence of C. cylindracea. Furthermore, nutrient addition in the sediment had positive effects on both C. cylindracea (increasing its cover) and seedlings (increasing leaf development), and the increased C. cylindracea cover did not result in detrimental effects on seedlings. Therefore, biological invasions and nutrient addition do not reinforce each other in the short term to negatively impact transplanted seedlings, which highlights facilitative interactions between invasive algae and native seagrass and provides useful information for successful strategies of seagrass restoration.