Tolerance of the invasive alga Caulerpa taxifolia to burial by sediment
Since Caulerpa taxifolia was first discovered in New South Wales in April 2000, it has invaded 9 waterways (some sparse infestations, others dense) and now covers approximately 8.1 km2 of the bottom of these waterways. The seaweed is found primarily in soft sediments at depths of 0.5 – 10 metres. Regular mapping of infestations in NSW has shown that the shapes and sizes of beds of C. taxifolia are very dynamic and sometimes areas of C. taxifolia can disappear over weeks or months. The exact causes for such changes are unclear, but may be related to decreases in temperature and salinity and increases in turbidity.
It is also likely that sections of C. taxifolia beds may seemingly disappear because they are buried by sediments. In some Sydney waterways, large sections C. taxifolia beds have been buried by sediment (either partially or totally) after many days of heavy rainfall and runoff of sediment. In this paper, we investigated the ability of C. taxifolia to survive burial by sediment and so assessed whether beds of C. taxifolia could fluctuate in size due to burial. C. taxifolia beds may be reduced in size due to sediment killing the seaweed or, if the seaweed is resistant to burial, beds may seem to increase in size when overlying sediment is removed. Based on results of an overseas study on a similar seaweed, we predicted that C. taxifolia would be capable of surviving complete burial for a maximum of 6 days, but that the growth of the seaweed would be diminished. We also predicted that partially buried C. taxifolia (i.e. such that some parts of the seaweed were exposed) would survive for greater than 6 days and that its growth would be less affected than if it had been totally covered with sediment.
Experiments were set up in the laboratory and we examined the responses of C. taxifolia fragments to burial to different degrees (partial vs total) and for different durations (2, 6, 17 days). Once uncovered, fragments in all treatments began to grow and by the completion of the experiment (92 days), partially buried fragments were not different from undisturbed controls (that had not been buried) for some of the variables measured. Thus, the effects of partial burial on C. taxifolia are minimal. Fragments buried totally were severely affected and their growth was greatly diminished compared to controls and partially buried fragments. Nevertheless, 35% of these fragments survived total burial for 17 days and began to recover when the sediment was removed.
C. taxifolia can therefore persist for long periods under sediments, meaning that, in areas where burial is likely, the distribution of the seaweed may often be underestimated. These findings indicate that any potential control technique must penetrate sediments to be totally effective. The current method of control in NSW involves covering C. taxifolia with coarse sea salt. To date, there has been mixed success of large-scale salting trials, and we suggest that this may be in part because buried C. taxifolia was not seen by divers and so not treated with salt. It is also possible that salt may not always penetrate sediments to a sufficient depth to kill all the seaweed and studies are currently being planned to investigate this.