Indulge me a little… lets travel through time and (depending on where you are) space. First stop – it’s the 1960s and we’re in Sydney, Australia. We head out to some shallow, subtidal rocky reefs and we see an abundance of seaweed – specifically crayweed (Phyllospora comosa). This crayweed is quite substantial in size, typically reaching 1-2 meters long. Living for around two years, these guys produce all year round, providing not just a high level of canopy cover but also extensive cover. The expanse of crayweed isn’t just limited to Sydney, stretching for some 2,500 km along Australia’s east coast. Crayweed provides an underwater forest, and just like forests the habitat is important for a whole host of other organisms – invertebrates, fish, molluscs. Come down to these waters and you see a fantastic show of marine biodiversity. It’s now the early 1980s and we are back in Sydney. Again we head out to those shallow, subtidal rocky reefs but this time something is amiss. The crayweed is gone. We hear about near-shore sewage outfall in Sydney and how the volume of outfall peaked in the 70s and 80s. Is there a connection? Well possibly. It’s quite tricky to point to a single, definitive cause for such thing, but it turns out that pollutants found in sewage are detrimental to the crayweed’s embryos. No embryo’s means no offspring and with a two-year life-span…well you can see where that ends up.
Moving forward to the 1990s. Sydney has improved its sewage infrastructure. Sewage is now being pumped into the deep ocean; near-shore outfalls are being taken out. Water quality is on the up, and there is hope for the crayweed.
It’s now the mid-2000's. The crayweed has not returned to Sydney’s shallow subtidal rocky reefs. In that 2,500 km band of crayweed that stretches from Port Macquarie (New South Wales) to Robe (South Australia), the 70 km gap along Sydney is about as conspicuous as it gets.
Moving forward now to 2011 and there is still no signs of recovery. Time for Alexandra Campbell from the Sydney Institute of Marine Sciences and colleagues to step in. They plan to try something bold, something controversial – restoration of the crayweed.
The first thing the researchers needed to do was determine if the crayweed could survive in the Sydney area. For this, they transplanted some adult plants from two habitats near the edge of Sydney and placed them in one of three treatment locations. The first group went to Sydney itself (two locations), one to an area where crayweed currently resides (two locations) and finally, one group returned back to the area they were removed from. That final step might sound a little strange, but if transplanted crayweed can’t survive and reproduce where it originally came from there is a problem. It also gives the researchers another benchmark to work from in determining just how well crayweed does in the new areas around Sydney. The transplantation was repeated twice. The first time (between February and May 2011), the researchers visited each of the transplantation sites every -4 weeks and recorded some important survival information. At the end of this stage of the experiment, the transplanted individuals were collected up and their length measured. Some had their ecopyhsiological condition assessed by testing their photosynthetic capabilities. In the second stage of the experiment (August 2011 to January 2012), the transplant sites were visited every 5-10 weeks and again important survival information collected. In addition between February 2012 and August 2012 the researchers also calculated the density of crayweed recruits, and their distance from the transplanted patches. And just to see how they well they were growing, the length of the recruits was also measured.
The results are a bit of a mixed bag, but overall positive. At one of the sites in Sydney transplanted individuals did well - not just surviving but reproducing at a greater rate than their counterparts in their original area. 18 months after transplant around half of the recruits had grown four times their length than they were at just 6 months old. Good stuff. The same can’t be said for the second transplant site in Sydney though. After 6 months, both survival and reproduction was low, with all the crayweed seeming to have suffered from herbivory (they were being eaten by fish). So, some parts of Sydney’s subtidal reefs may be more suitable for transplantation than others. The researchers note that this is not an insurmountable problem though. Excessive herbivory isn’t an issue unique to this study. With a reduction in their predators largely due to overfishing, herbivores can run rampant in many areas. Adding the crayweed provided them with a new, delicious, and compact source of food that was all too quickly gobbled up. One simple answer might be to increase the size of the transplant so that some plants survive becoming dinner. Another problem might be a lack of adult plants that can offer canopy cover to recruits. Even in the area where recruitment was good, most were found on or in the edges of the canopy provided by the adults. If we want the restoration to be even more successful, the authors suggest, more research into reproduction and recruitment needs to be done.
One of the controversies around restoration is the cost. How much should we spend of restoring ecosystems lost or in decline? Well the researchers believe that transplantation of crayweed is a cost-effective measure. They calculated that all in, their method cost AUD 38,000 per hectare – and believe this will likely reduce as methods develop and improve their efficiency. The most direct pay-off for people? These habitats are important for commercial and recreationally caught species. If successful, this sort of restoration could help boost these industries – and the provision of food – for Australia.
This paper is published in the open access journal PLoS ONE. If you fancy having a read of it, you can find it here: http://dx.doi.org/10.1371/journal.pone.0084106
Image: Transplanted seaweed is attached to a reef by a team member. Credit: UNSW, made available with the media release on this paper. Photographer unknown.