With the entrance work now complete and the mouth open to the sea, the amount of water moving into and out of the lake has increased. Over recent weeks, residents would have noticed that the lake level has been fluctuating in response to the tidal phases. For example, during neap tides the lake level has dropped and during spring tides the lake has risen. During the neap tide phase the average lake level has dropped by up to 100mm, said the Chairman of the Lake Illawarra Authority Doug Prosser.

When the lake level drops unusual areas of mud flats and seagrass areas are exposed. In particular, along the Back Channel up stream of the Windang Bridge, along the Windang peninsula and around shallow bays. This usually results in an odour around foreshore areas which can affect local residents at Primbee, Windang, Warrawong, Koonawarra, Lake Heights and Oak Flats.

Lake Illawarra Authority member, Prof John Morrison from the University of Wollongong advises that nutrient cycling processes within the lake can cause the lake to emit smelly gases. These gases (including hydrogen sulfide – rotten egg gas and other sulphur and nitrogen species) are produced by interactions between nutrients and organic matter on the lake floor. This is a natural process and only becomes problematic when water from the bottom of the lake is mixed and comes to the surface, or when the sediment is exposed.

Another cause of foul odours arises from rotting seagrass leaves and algae washed on to the shoreline. Tidal currents and waves push detached seagrass and organic matter toward the shoreline where it builds up and decomposes. This process is also natural and occurs throughout the year not just in Lake Illawarra but in estuaries around Australia and throughout the world.

Dr Trevor Daly, Fisheries Conservation Manager with the Department of Primary Industries, also a member of the Authority advises that the decaying seagrass and organic matter is extremely important in the ecological functioning of the lake as it forms part of the food chain for fish and a range of other aquatic organisms. Also, seagrasses provide a nursery for juvenile fish in the lake and about 20-25% of the lake bed is covered by seagrasses mainly Zostera or eelgrass. Without seagrasses the lake would produce far less fish, including those species sought after by recreational and commercial fishers. Along with seagrass, the distribution of other aquatic vegetation such as salt marsh, an endangered ecological community, will also be affected by fluctuations in lake water level. Salt marshes occur along the fringes of the lake which are inundated and drained by the tides. Salt marsh areas are also an important component of the ecosystem of the lake providing a food source and habitat for many organisms, limiting sediment transport and regulating chemical cycles.

Unfortunately, there is no permanent solution to removing the odour problem which can occur at Lake Illawarra as natural processes are responsible. However, to reduce the impact of odour on lake residents and users the Authority from time to time removes large quantities of decomposing seagrass and algae from shoreline areas using specialised machinery. The cost of the specialised machinery for these clean ups is often $10,000 per week.

Material removed from the lake is taken to the Authority’s green waste depot at Primbee and recycled as mulch for use on landscaping projects around the lake.

Each year the Authority spends around $100,000 removing organic seagrass wrack and algae from the shallows of the lake and foreshores.