Wetlands are some of the most important, but most threatened ecosystems in the world. February 2 is World Wetlands Day and an opportunity to celebrate local wetlands and raise awareness of their importance. Can we rehabilitate wetlands without creating more opportunities for mosquitoes?
What is World Wetlands Day?
World Wetlands Day is celebrated each year on 2 February and marks the anniversary of the signing of the Convention on Wetlands of International Importance in Ramsar, Iran, on 2 February 1971.
As described by the Australian Department of the Environment:
“World Wetlands Day was first celebrated in 1997. Since then government agencies, non-government organisations and community groups have celebrated World Wetlands Day by undertaking actions to raise public awareness of wetland values and benefits and promote the conservation and wise use of wetlands.”
The 2014 theme for World Wetlands Day is “Wetlands and Agriculture” and may, at first, seem a little unusual. However, wetlands play a critical role in supporting many agricultural pursuits by improving water quality and assisting water conservation, providing a buffer against flood and storm events, assisting nutrient removal from runoff and supporting populations of predators of agricultural pests.
Wetlands, although important in many ways to local communities and economies, have not always had a healthy relationship with agriculture in Australia. Draining, filling and polluting of wetlands have all resulted from agricultural development in both coastal and inland regions of Australia. Debate regarding the allocation of inland river flows to environmental and agricultural uses continues today. However, there is an increasing awareness of the need to balance the interests of agriculture and wetland conservation in many regions and an integrated approach is required. Fortunately, many land owners are now actively engaged in sustainable land management practices and efforts are made to limit impacts to onsite and offsite wetlands.
Looking ahead, there are new opportunities too. There is much potential in carbon farming or “blue carbon” that may see an increase in wetland areas. At the least, this may provide a valuable opportunity to assign an economic value to our wetlands, something that is often difficult to achieve but may eventually help with their conservation.
Historically, coastal floodplains have been significantly impacted by flood mitigation, agriculture and urban development. Most importantly, the restriction of tidal flows through the construction of sea walls, levees, roads, railways and floodgates have substantially altered the hydrology of these environments leading to changes in flora and fauna. In many instances, the “reclaimed” land has been used for agriculture, particularly grazing livestock. The decision to install floodgates may have been driven by concerns regarding potential flooding and to increase available area for grazing, limited consideration appears to have been given to the potential impact on the environment. Over 4000 structures have been identified in NSW alone that influence tidal flows into coastal wetlands.
Coastal wetlands in Australia are under threat on many fronts so where there are opportunities to rehabilitate degraded habitats, strategies should be implemented to improve environmental health. So, don’t you just open or remove the floodgates?
With tidal flows restricted from these degraded habitats for decades, there have been substantial changes in these local environments. Understanding how re-establishing tidal flows may impact the existing flora and fauna across the local estuary is critical.
One of the largest estuarine wetland rehabilitation projects in the southern hemisphere is the Hexham Swamp Rehabilitation Project near Newcastle, NSW. The wetlands cover approximately 2,000 hectares and rehabilitation plans have been underway for over 30 years. Floodgates were installed in 1971 and have resulted in significant changes to the local vegetation. While remnant areas of saltmarsh and mangrove remained, the exclusion of tidal water, as well as the accumulation of rainfall runoff, shifted the vegetation to a freshwater dominated system. In particular, the wetlands became dominated by extensive stands of Phragmites australis. While these dense stands of vegetation provided habitat for a range of birds, frogs and snakes, with regard to locally important wading birds, the quality of the habitat wasn’t up to scratch. With the loss of coastal saltmarsh a major concern for local authorities, a strategy was developed to reestablish tidal flows to Hexham Swamp.
Extensive hydrological model was undertaken to inform the strategy of floodgate openings. However, despite some complex modeling to predict how tidal waters would move into and out of the system, it really wasn’t until the floodgates were opened that the wetland hydrology could be measured. A staged opening of floodgates over a number of years was undertaken with all gates open in August 2013.
There has already been a dramatic change in the local environment. In areas where tides are now penetrating and increasing salinity, there has been a substantial die off in freshwater vegetation. This is now steadily being replaced with estuarine plants such as saltmarsh. There has been a big shift in the birds visiting the site too with an increase in waders in many areas. There are more fish and an increase in prawn populations in the local estuary has been credited to the openings of the flood gates.
It will come as no surprise that concerns regarding potential increases in mosquito populations were expressed at early stages of this rehabilitation plan. There were some key site-specific issues to consider here. Firstly, the Hunter estuary contains extensive existing and productive mosquito habitats. Hexham Swamp, while significant, is surrounded by a number of other extensive saltmarsh and mangrove environments that are well documented as productive habitats for the saltmarsh mosquito, Aedes vigilax.
The impacts of these saltmarsh mosquitoes as important vectors of Ross River virus, prompted the formation of the “Living with Mosquitoes” group. This group contains five adjacent local governments, as well as a number of other stakeholders, with oversight from the local health district. The group assists local authorities develop coordinated surveillance and community education programs to raise awareness of mosquito risk. Even without changes in the mosquito populations produced from Hexham Swamp, the region would still have an ongoing mosquito issue to address.
Tracking the mosquito populations and changes in their diversity and abundance with the reintroduction of tidal flows, a couple of key observations were made. Firstly, there was a dramatic decline in the abundance of “freshwater” mosquito populations. This was expected and it was hoping that any declines in freshwater mosquitoes would offset any increases in estuarine mosquitoes. Secondly, although an initial “first flush” of mosquitoes were produced following the introduction of tides, populations appear to have stabilsed (perhaps even fallen) in line with other habitats in the local estuary.
The factors contributing to the “first flush” are not completely understood but the most likely scenario is that an accumulation of mosquito eggs over previous years were laying in wait for those first big tides to flood in. Although some of these eggs would have hatched following major rainfall events, there would rarely have been the volume of water in the wetland following rainfall compared to the major tidal flooding events now the gates are open.
Our experiences with the Hexham Swamp rehabilitation program show that it is possible to “bring back” estuarine wetlands that may have almost disappeared through the restriction of tidal flows and damaged caused by agriculture (particularly cattle grazing). It is also possible to improve the health of these wetlands without creating additional or increased mosquito problems. Surveillance and planning (and perhaps a little mosquito control) will be required but it appears that if the health of the wetland improves, mosquitoes can be maintained at reasonable levels. It is important to remember that mosquitoes are a natural part of Australia’s wetlands and eradication should never be an objective of management.
Have fun during World Wetlands Day 2014! Don’t forget to check out the free eBook on managing urban wetlands produced by the Sydney Olympic Park Authority.