Silent Waters

The Swan-Canning River system, like other Australian estuaries, is dying by suffocation caused by nutrient overload and reduced rainfall
The health of Australia's rivers has never been more precarious, threatened by declining rainfall and man's relentless incursions. Professor Jorg Imberger, Director of the Centre for Water Research University of Western Australia presents a case study with alarming implications for all our estuarine systems.

Death Notice: Swan-Canning Estuary
Born: 10 Million Years before present
Died: 2011
Cause of Death: Suffocation
Burial Rites: By Flooding

According to geologists, the Swan-Canning River Estuary, in its present configuration, was formed around 10 million years ago and remained an ecologically functioning river estuary system until about 60,000 years ago when Aboriginal people started to settle the area. It is now recognised that their fire practices had some impact on the functioning of the river system. But by all accounts, when Captain James Stirling first explored the Swan-Canning River Estuary in 1827, it was still an ecologically healthy, brackish estuary. At that time, it was low in nutrients with extensive fringing vegetation, sandy substrate and with the ocean water being kept at bay by a limestone bar and sandy shoals at the mouth where Fremantle is now located. An extensive series of billabongs protected the aquatic freshwater biodiversity in the dry summer months, the high concentration of tannins from the native fringing vegetation protected life in the river against algal blooms and river flows were periodically high enough to completely flush the system.

The Swan-Canning is typical of all Australian south coast estuaries. All face similar low tidal variations, relatively energetic surface wave movements, low nutrient waters, both ocean and fresh, and extremely variable river flows. While all were leading a fragile existence before European settlers arrived, all are now battling increased nutrient loading, reduced river flows and, in most cases, severe alterations to the sand bars controlling the ocean entrances.

Their stories differ in detail, as do the origin of the stresses to which they are exposed. But, in essence, all are suffering as a result of the same transition from a water body with only weak salinity layering, low biological productivity and high water colour; the organisms in these systems evolved accordingly.

Now obese with nutrient, devoid of annual cleansing flushing as river flows are either depleted because of lower rainfalls or diverted for agricultural purposes, and robbed of the sun block lotions (the colour in the water from native vegetation tannins), they are dying of suffocation. These observations apply to the Murray Basin estuary, as well as to the Swan-Canning Estuary, indeed to most of our beloved estuaries, where we like to build our mansions hoping to look down of nature's beauty. Sadly, soon the waters in the estuary will flow silently as they do in the Coorong, the Swan, the Peel Harvey and Wilson Inlet - peaceful as a cemetery!

The visionary engineer, CY O'Connor, constructed the Fremantle inner harbour and, by the early 1900s, the estuary at Fremantle was kept permanently open to a depth of more than four metres. As seawater is heavier than freshwater, this allowed seawater to enter and fill the estuary from below. In the last 100 years, the estuary water has undergone a seasonal cycle where, in spring, as the river flow recedes, seawater progressively fills the estuary from Melville Water to up to Midland, leaving the estuary almost completely filled with seawater by autumn.

As the winter rains cause water to flow in the Avon and Canning Rivers, this freshwater enters the estuary. Being lighter than the resident seawater, it flows along the water surface in a thin layer towards Fremantle and out to sea. The extent and depth of the freshwater overflow depends on the force of the river flows. In a flood year, such as 1958, the strong freshwater flow would have pushed the seawater all the way out past Fremantle, whereas for a relatively low flow year such as 2011, the freshwater barely made it, as a thin surface layer, to Fremantle.

Whenever the estuary becomes layered in salinity as in 2011, the exchange between the surface waters and the deeper bottom waters is inhibited, due to seawater being heavier than freshwater. The surface and bottom waters will only mix when there is a stirring agent; in the estuary this can be a large tidal velocity or a strong surface wind. Energy is required to mix the heavier bottom waters with the lighter surface waters, like stirring the sugar in the bottom of a cup of coffee into the cup as a whole. When the stirring agents are weak, as in the Swan-Canning Estuary, the waters barely mix and the bottom waters become isolated from the free water surface.

This has many consequences, the most important being that the waters become oxygen depleted. In the waters from Point Walter to the Reid Bridge, life has been suffocated below about 2-4 meters and once dead all creatures can only return by migration. This appears the reason for people no longer finding prawns and crabs in the river, a common enjoyment 20 years ago. There seems to be a tendency, by government and from some members of the general public, to deny the severity of this situation with comments such as "this is a temporary situation". This is a well known psychological first response to death, but there is no such thing as "temporary death".

We are only just fully understanding the functioning of the estuary, but it appears that with lower rainfall, the river inflows have decreased and the freshwater inflows are now arrested around Point Walter. The time the water takes to travel from the upper reaches of the estuary to Point Walter is about one month, the time necessary for photosynthesis to drain the nutrients, originally introduced by fertiliser application in the catchment, out of the water column into the phytoplankton biomass in the surface waters. These algae then settle to the bottom where they decompose and, in the process, deplete the oxygen in the underlying water body.

This model would suggest that, year after year, nutrients that enter the estuary with the inflows are sequestered into the mud on the estuary bottom, where they are available for recycling into the water column whenever a mixing event takes place following a few months of low oxygen. Analysis has shown nutrient levels in the bottom water about three times higher than in the surface waters, ready to fuel a summer algal bloom. In simple language, the Swan-Canning Estuary has been suffocated by a surplus of food - death by obesity!

The only reason we are not seeing huge algal blooms throughout the estuary is that the tannins in the water - the brown colour - are robbing the algae of light so they cannot thrive continuously. But recent results indicate that certain types of algae do grow at deeper levels in very low light conditions. In moderate concentrations, they produce oxygen to breathe life back into the deeper waters by out competing the loss of oxygen due to microbial action breaking down the organic material in the water.

The Swan Estuary has thus changed from a predominantly freshwater/brackish system with low stratification and low nutrients being flushed annually by vigorous river flows, to a system held in a precarious balance between microbial activity feeding on the dead carcases of plankton and zooplankton, consuming oxygen in the process, and the algae photosynthesising thereby introducing oxygen into the water column.

Much has been made of the estuary health indicators not showing a decrease in ecosystem health over the last few years. But it must be remembered these indicators measure only fish diversity in the upper waters and then only at selected times. Fish are transitory and come and go through Fremantle at will, searching for food and habitat at a time when the ocean waters are becoming depleted of both. Suggesting that these indicators are a reflection of ecosystem health is a bit like counting visitors to a cemetery and then suggesting, when visitor numbers are up, the cemetery is a healthy place! For indicators to be meaningful they must include the abundance and diversity of all the bottom dwellers, yet little or no work has been done to include these in the Swan Estuary.

The danger now is that as the freshwater carrying the tannins recedes, the water will clear, the high nutrient levels will come to the surface, the water will warm further - all in all, the perfect cocktail for a massive algal bloom!

Unfortunately, it is not simply a matter of reducing the inflowing nutrient levels. As suggested above, there is now a store of probably close to 50 years of food supply locked in the sediments, available whenever the bottom waters become anoxic (lacking in dissolved oxygen). The long term solution lies in reducing the nutrient inflows. The short term fix, maybe a semi permanent one, is to prevent the salinity stratification from forming and, instead, maintaining a homogeneous water column that is readily mixed by the tide and the wind, preventing the bottom water from depleting in oxygen.

The other danger that awaits the Swan-Canning Estuary - and us in the future - is global sea level rise. Simulated flood results based on a hydrodynamic model of the inundation level referenced to the Australian Height Datum (AHD)superimposes the high tides experienced in July 2011, river flows of about flood values in 1983, an uncontentious 50 cm sea level rise due to global warming and a coastal wave surge of 50 cm, as recorded when Tropical Cyclone Alby passed along the coast in 1978. Not included is the known rate of subsidence observed in Fremantle.* This is a realistic scenario, and maybe compared to the levels reached many times in the past at Fremantle, most recently in 1988, 2003 and 2004. What these simulations show is that such events will become more commonplace with even quite small rises in sea level. So climate change sceptics please read and think before, yet again, going ballistic!

A Suggested Solution

Clearly, the flooding alone would have a devastating impact of some of the iconic areas of Perth. However, equally important is the realisation that much of the Swan-Canning Estuary shoreline would require protective stonewalls, changing the current gentle landscape to a much harsher outline.

Is there a solution to these two catastrophic realisations - the death of the estuary being an observed fact and the flooding being a conservative prediction for the next 30 or so years? The answer lies in preventing the sea from entering the estuary at Fremantle; this would prevent the estuary from developing anoxic bottom waters, while, the same time, offering water level control to mitigate flooding. Such a solution has been implemented elsewhere - in Singapore for the control of Marina Bay, in New Orleans, in Venice through the construction of the three gates of the Moses scheme, the world's largest insurance policy against global warming, and is being considered at numerous other coastal cities around the world.

Such a lock and barrage could easily be built somewhere between the two bridges in Fremantle at an estimated cost of around A$20M. We need to ask now what value do we place on the life of the river that has been the lifeblood of settlement in Perth?

* See Figure 2 in the article Silent Waters in our online magazine at
http://www.novaholisticjournal.com (Page 12)