Alex Meehan - PhD candidate, Wollongong University
Good morning ladies and gentlemen.
Today I'm going to talk to you a little bit about some of the changes that have occurred to the seagrasses in Port Hacking over the last 50 years or so. This is a summary of some of the research that I've done with Rob Williams of NSW Fisheries. Just to give a brief outline of my presentation, I'll talk a little bit about what a seagrass is and why seagrasses are important. I'll then move on to the seagrass history of Port Hacking, give an overview of the changes that have occurred throughout the Port and then I'll focus in on three case studies which illustrate some of the impacts that have occurred. I'm then going to talk about some of the actions we might take to preserve the biodiversity of seagrasses in the future and then make a few brief conclusions.
A seagrass is an aquatic plant adapted to life in the marine environments. Most species have a similar morphology in that they consist of a well developed rhizome system which gives rise to strap-shaped leaves and forms a useful habitat for a variety of fauna. The distribution of seagrass is determined by a number of different factors. Obviously light is thought to be of primary importance. Seagrass is a plant that needs light to photosynthesise, so we might expect seagrasses to grow to a greater depth in nice clear waters than we would in highly turbid waters. Another factor that influences their distribution is salinity. Some species of seagrass prefer to grow in open marine conditions where salinity is close to that of the ocean, whereas other seagrass species are quite happy to grow in brackish water conditions. Another factor that affects the distribution of seagrasses is wave activity along the NSW coasts. Wave activity is generally too harsh in the open waters and we tend to find seagrass in our estuaries and in some sections of our sheltered embayments.
There are three species of seagrass that occur in Port Hacking. We have Posidonia australis , Zostera species and Halophila species. Posidonia is obviously a stable seagrass. It doesn't undergo large seasonal changes in density for example but it is very slow growing. To give you some idea, lateral extension is about 20cm a year and its ability to grow from seed is quite poor so it's a very vulnerable species. Zostera is not quite so vulnerable as it expands at a greater rate, and Halophila is probably the least vulnerable of the seagrasses in Port Hacking.
Seagrasses are important for a variety of reasons. They provide habitat and nursery grounds for juvenile fish species and produce large amounts of organic material for the estuarine food chain. They also stabilise sediments, and by doing so hopefully increase water quality, particularly given that they can act as a baffle to water and cause sediment to settle-out rather than being continually redistributed.
"The history of Port Hacking seagrass" is a study we did using historical aerial photographs which we mapped using a GIS. We were able to find coverage for 1930 and 1942 although we couldn't get complete coverage of the Port. But we were able to get complete coverage for the years 1951, 61, 75, 77, 85, and 1999. We endeavoured to map the distribution of seagrass in terms of area and distribution, but also in terms of the species composition where possible. These 2 photos give you some indication of the source materials that we were dealing with. On the left we have Port Hacking in 1930, and Port Hacking in 1999. This just shows Gunnamatta Bay and Burraneer Bay. You can obviously see there's been quite a significant amount of development on the northern shore of Port Hacking. There also been some development on the southern shore in the form of the townships of Maianbar and Bundeena. What I'd also like to point out is that there's been a substantial amount of change to this area. You can see that Deeban Spit, which in 1999 consists of this large sand wall, did not exist in 1930. In 1930 it was basically an isolated sand bar. And Cabbage Tree Basin was much more open to the ocean. This is a map of Port Hacking showing the zones that we divided the Port into to examine the changes in seagrass distribution at a finer scale. There were 9 zones in total. We looked at changes in 'The Entrance', in Gunnamatta Bay, in Burraneer Bay and Cabbage Tree Basin, in the middle Port area, in South West Arm Creek, central mud basin, Yowie Bay and the Hacking River. I don't have time to talk about all of these zones today, I'm just going to focus-in on three case studies: Gunnamatta Bay, Cabbage Tree Basin, and the Hacking River. Just before we do that, I'm going to give an overview of what's happened in seagrasses.
Most historical studies throughout the world have documented declines in seagrasses. Unfortunately, Port Hacking is no exception. We've lost 55% or about 95 hectares between 1951 and 1999. You can see that as soon as the mapping begins we go straight down to 1961 and 1975 where seagrass is reduced to about half or just under half. There's been some recovery up to the mid 1980s but what you can notice is that basically there's some period of stability, and a period of initial decline. The seagrass declined in 7 of the 9 zones I mentioned earlier so it was fairly consistent throughout the Port. The only places where seagrass was stable was South West Arm Creek and in 'The Entrance' area.
The first case study is Gunnamatta Bay. We've had a decline in seagrass in this Bay by about 52%, or 13 hectares between 1930 and 1999. It was fortunate in this case that we were able to go back right to the 1930s and 1940s. The reason why this is useful is that we were able to document a period of considerable stability between 1930 and 1951, where seagrasses were doing quite well. Once we get to 1951 however, the seagrasses tend to fall dramatically almost to about half their area. Thereafter we have a period of stability, followed by a decrease, an increase and once again another decrease. The 10-year period between 1951 and 1961 is really the 10 year period where most of the loss occurred. The reason we think this occurred is that there were 10 category 'X' storms during this ten year period which is a far greater number than occurred prior to this, or after this, and so this leads us to believe that storm damage has basically been responsible for the death of the seagrasses in Gunnamatta Bay. However, the Bay was also closed to bait digging in 1961. This was done in response to residents' concerns about the impact on some of the shallow water sandy areas. And so the coincidence is that the Bay was closed to bait digging in 1961, and in 1961 the seagrasses stopped declining, so perhaps that's trying to tell us something.
The next slide is a map of Gunnamatta Bay in 1930 and in 1999. We can see the distribution of Posidonia seagrass, Zostera seagrass and a 'mixed' community. What we can note is that in the southern area of Gunnamatta Bay, the mixed seagrass which in 1930 consisted of a consistent and large bed, has basically become much smaller in area and highly fragmented. We also notice that the Posidonia bed in the southern section, in the Salmon Hall Bay area, is much reduced in area. In terms of what's happened in the northern section of Gunnamatta Bay, well you can see that the Zostera communities have declined, probably due to some foreshore development and the creation of a sports oval. There's also been some damage to the Posidonia , due to boat moorings. This is particularly important because Posidonia grows so slowly and it may be years, if not decades, before some of those holes close over.
The second case study is Cabbage Tree Basin (CTB). CTB has the honour of having the greatest proportional loss of seagrass in all of Port Hacking: about 92% or 11 hectares between 1942 and 1999. Once again you can see there was some stability prior to 1951, then we have a massive drop all the way down to 1975 and then thereafter, there has been some recovery and some variability. But basically, most of the seagrass has now gone from CTB. It's unlikely that storms have caused this impact. What is more likely is that a lot of the sand that has been dredged from the channels of Port Hacking which was then dumped into Simpsons Bay just outside CTB, has basically migrated back in and smothered a lot of the seagrass. In 1942, seagrass communities were distributed right throughout 'The Channel' and mangrove communities just consisted of a few isolated stands. We go to 1999 we can see that Deeban Spit is now in its present form and CTB has been closed off somewhat. Seagrasses have more-or-less disappeared from this channel and mangroves have expanded. In other words, all this sand has moved in to CTB, smothered the seagrass and put down more sand for the mangroves to colonise onto.
We're now going to go right up to the other end of Port Hacking and look into the Hacking River. Once again a decline in seagrass by about 83% or 15 hectares between 1951 and 1999 has been evident. Unfortunately, we weren't able to view aerial coverage prior to 1951 so we have no idea about what was happening - whether the seagrass was stable, or whether it was decreasing or even increasing. However, the reason for this change is probably related to sedimentation possibly due to the housing development that has occurred on the northern shore of the Hacking River (Grays Point housing development). Bushfires, and some changes to the catchments may have also deposited a lot of sediment, which has basically been dumped on the seagrass and has destroyed it. The next slide shows the Hacking River in 1951. Zostera seagrass is distributed throughout the Channel in very large beds. When we go to 1999, most of it has disappeared. There are a few remaining beds and they are in reasonably good condition. But the area is much reduced. And its worth pointing out that just as in CTB, there was a decrease in seagrass and an increase in mangroves. The same thing has happened in the Hacking River - the mangroves have come in and colonised probably a lot of this new sediment that has come from development.
OK that's the bad news. The next question I think that arises is "Is there any chance that the seagrasses will grow back?" It's a little difficult to predict these things and I don't really want to have a prediction come back to haunt me but I think in some locations it probably will grow back. An example is in Gunnamatta Bay where the seagrasses seem to expand and contract according to what the storm levels are doing. So we might expect that in a period of relative calm that the seagrasses may expand. However in other areas where they were historically present, it's unlikely that they will return. For example CTB is now far too shallow to support extensive areas of seagrasses unless extensive dredging was to occur. Another point worth noting is that the regrowth is going to vary considerably between the different seagrass species. I've already mentioned that Posidonia grows back slowly. Zostera might be expected to grow back a little faster and has been shown to do so. However the complicating factor is Caulerpa taxifolia . It has already grown-over considerable areas of seagrasses in Port Hacking. It is difficult to predict whether the seagrass will grow back if this invasive weed firstly takes over the remaining seagrass, and secondly expands its range to some of these areas that had previously supported extensive seagrass communities.
What can we do and what can learn from this study? I think the first thing to point out is that there are natural changes in seagrass distribution. This is particularly true in the case of Port Hacking with respect to the entrance, which is quite exposed and experiences storm damage on an irregular basis. The second thing to recognise however, is that human impacts have caused far greater seagrass change and are probably responsible for most of the decline in seagrass over the last 50-70 years. It's important I suppose to discriminate between ongoing impacts and isolated impacts. One example of an isolated impact is shell grit mining which did occur in Port Hacking and did cause a significant loss in seagrass. However, such an impact is unlikely to occur again, for a variety of reasons. Other ongoing impacts though, are still present, such as sedimentation and general urban runoff that can impact on seagrasses.
This leads us on to "What can we do about this?" Well we need to ensure that development is environmentally sustainable. I know that's a fairly vague term, but I think, more specifically, that it would be useful to know the impacts of dredging, particularly channel dredging - not just in terms of direct impacts on the seagrass beds, but also indirectly in terms of what it means for the changes of distribution of sand throughout Port Hacking. Another thing to consider is urban runoff, stormwater etc. There is a need to perhaps put some buffer zones around any new residential development to ensure that adjacent seagrass communities are not impacted by it.
One of the most important things, I think, is the need to monitor the distribution of seagrass. We have undertaken an historical study and we've learnt quite a lot from it, but we really need to be a bit more proactive and monitor the distribution of seagrass on a regular basis. If we do detect small changes, we can actually do something about it and actively manage them. But we also need to consider the distribution of other communities such as mangrove and saltmarsh. They're important in their own right, and changes in those communities can also help in interpreting changes in the seagrass beds.
We obviously need to consider establishing some protected areas around parts of Port Hacking. Ron West has already mentioned Cabbage Tree Basin, but there are certainly large areas of seagrass in South West Arm Creek which are worth preserving, along with other areas adjacent to the National Park. We also might want to give some thought to transplanting seagrass, particularly Posidonia australis , to some of these damaged areas. In the case of Posidonia , it's particularly important since it has such a slow growth rate and won't recover naturally in less than about 10 or 20 years. Another factor to consider is the need to control the spread of Caulerpa taxifolia . I believe NSW Fisheries are currently working on some eradication programs that are having some degree of success and hopefully they will do some in Port Hacking in the near future.
So in conclusion, seagrass area in Port Hacking is about half of what it was in the mid 1900s. The seagrass distribution does have some degree of natural variability, but most of the loss in seagrass is related to human impacts and I think it's time not only to conserve the remaining seagrass in Port Hacking but to think about rehabilitating some of these areas so that we can repair some of the damage.
Just before I go, I'd like to acknowledge that this research was funded by the Coast and Clean Seas Program and would like to thank Graham Boller of Sutherland Shire Council who provided some data to help us to rectify the aerial photography.
Thank you for your time. |
