Caulerpa taxifolia in seagrass meadows: killer or opportunistic weed?

Seagrass habitats are being lost throughout the world and the invasive alga C. taxifolia has often been implicated in seagrass declines. Although C. taxifolia can impact a variety of species, evidence for its effects on seagrasses is largely correlative. This study combined observational studies and manipulative experiments done over many years to test hypotheses about effects of C. taxifolia on two Australian seagrasses, namely Posidonia australis and Zostera capricorni. Results indicated that C. taxifolia is not having adverse impacts on the coverage of these seagrasses in the sites studied. Rather, C. taxifolia appears to be an opportunist, persisting longer and its coverage being greater in previously non-vegetated sediments than amongst seagrasses. C. taxifolia co-existed with P. australis and did not cause reductions in the cover of the seagrass. Outcomes of experimental manipulations of C. taxifolia amongst Z. capriconi were less clear due to losses of Z. capriconi in all plots, regardless of the presence of C. taxifolia. It was possible that C. taxifolia may have enhanced the decline in canopy cover of Z. capricorni, but the presence of alga did not alter the final fate of Z. capricorni. There was also no evidence that long-term areal coverage of P. australis or Z. capriconi has been affected by the introduction of C. taxifolia in the embayments studied. A review of literature on effects of species of Caulerpa on seagrasses provided limited experimental evidence for negative impacts of this genus on seagrass abundance.