Assessment of long term interaction between the endemic seagrass P. oceanica and the exotic green alga C. cylindracea in the South Eastern coast of Spain (Murcia Region)

The green alga Caulerpa cylindracea has rapidly spread throughout the Western Mediterranean during the last 20 years. Compared with other native benthic communities, Posidonia oceanica meadows seems to be highly resistant to the colonization by the alga. Nonetheless, it is suggested that in the long-term C. cylindracea could affect the seagrass altering its vitality, structure and functions, however little is known about the interactive effects between these two structuring species. To this end, the abundance of both macrophytes was quantified and monitored in invaded and non-invaded localities of the South Eastern coast of Spain (Murcia Region) over an 8-year period (2007-2014). Results indicate that no differences were highlighted between invaded and no invaded meadows and all the monitored meadows showed stable or progressive trends in shoot density, meadow cover and net population growth. Regardless of the depth, in all of the invaded localities C. cylindracea biomass present inside the seagrass leaf canopy was about 10 to 50–fold lower than that measured just outside the leaf canopy. In summary, our results do not support the existence of a long-term competitive interaction between the invasive alga and the native seagrass, at least in the studied meadows and at the meadow level. C. cylindracea forms huge biomass gradients associated to the seagrass meadow edges that are stable with time, which suggests the existence of highly limiting conditions for algal growth and survival under the P. oceanica leaf canopy. Future research on such limiting factors could help us to understand the invasive capacity of the alga and the factors involved in the resistance of native habitats to bioinvasions.