Is the invasive alga Caulerpa taxifolia impacting native seagrass beds in Sydney, Australia?
Non Technical Summary
Caulerpa taxifolia is a marine green seaweed that is endemic to tropical and sub-tropical regions around the world. An invasive strain of the seaweed was discovered in the Mediterranean Sea in 1984 and has since spread along the French coast to Italy and Tunisia. Outbreaks of a strain of C. taxifolia were first discovered in NSW, Australia, in April 2000 and the alga is now present in 9 waterways, growing primarily in soft sediments at depths of 0.5 - 8 metres. It is not yet clear whether the N.S.W. strain is the same as that in the Mediterranean, but in both places C. taxifolia is often found growing near or amongst seagrasses. It has often been assumed that C. taxifolia has severe negative impacts on native seagrasses, but there is little published empirical evidence. Surveys in N.S.W. indicate that dense meadows of C. taxifolia are found where there is no seagrass (Zostera capricorni or Posidonia australis), or where the cover of seagrass canopy is sparse. Far less C. taxifolia is found amongst dense beds of seagrass. These patterns could be caused because (i) C. taxifolia slowly invades and kills seagrass of any density and it has only just begun to invade dense seagrass in N.S.W. or (ii) C. taxifolia has negative impacts on sparse seagrass, but not dense seagrass, or (iii) the seaweed co-exists with seagrass and has no negative impacts. Here I describe an experiment designed to distinguish between these alternatives. It involved transplanting small fragments of C. taxifolia into patches of sparse or dense seagrasses and then monitoring the growth of the seaweed and the seagrasses. Experiments were done in areas where C. taxifolia was already abundant and so the seaweed was transplanted very short distances. Results so far indicate that C. taxifolia is growing well amongst sparse seagrass,but not amongst dense seagrass. As yet, there is no indication that either sparse or dense seagrass is being affected by C. taxifolia, but the experiment will need to continue for at least 1 year before conclusions can be drawn. Results of these experiments will be vital for determining how we should allocate our control efforts and whether additional efforts should be devoted to protecting seagrass beds from invasion by C. taxifolia.