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This article briefly explains the value of estuaries and the threats they are facing.

Estuaries – A Primer

Estuaries – once the world’s powerhouse of productivity – are now predominantly the habitat of humans.  Yes, the world’s most productive ecosystems are where tide meets freshwater, a mixing zone of biodiversity and food – of diatoms, phytoplankton, birds, fish and prawns.

Unfortunately globally, these are also now the most trashed systems.  Humans love to live near water, and here in Australia the vast majority of us crowd the coast and fill, drain, divert, dam and occupy what were once our estuaries.

I am defining estuaries here in a broad functional and management sense – that area of our coasts where fresh river water meets our oceans, where tide goes to and fro and where well over 80% of the fish and virtually all of the shellfish and prawns we eat spend much of their lifecycle.

So what are the functional components of estuaries?  How have they changed? And how can we restore some of their productivity?  After all we like to eat seafood and we appreciate the spectacle of pelicans gliding across the water or migratory waders swooping down to land and feed on a sand spit.

Broadly speaking, there are seven functional components to estuaries. Three physical features – catchment hydrology, tidal flows and connectivity drive the ecological features.  The ecological features include fresh to brackish wetlands, intertidal salt marshes, sand spits and mangroves and sub-tidally, seagrasses, sandy / mud bottoms and shellfish reefs. Collectively we can call these ecological components seascapes.

Catchment hydrology

Catchment function starts back in hills and feeds freshwater into watercourses.  Originally rain mostly ran down the boles of trees, seeped into the ground through leaf and bark litter, filled up the soil and then percolated down slope through creek and river towards the ocean.

Indeed I still see this process on my farm where a forested catchment of less than a hectare comprising trees I have planted ensures that the stream continues to run right through North Queensland’s deepest of droughts.

Most catchments in east coast Queensland are now cleared.  Hard-hooved cows and sheep plus roads and houses have compacted and hardened soils, markedly increasing runoff so that when it rains it floods.  Once our rivers featured a dampened, long-period, increasing-and decreasing flow – a sine wave shaped hydrograph. This ensured a prolonged and extensive highly productive mixing zone between fresh and ocean.  Now it’s high flow or no flow off our catchments.  The mixing zone is much diminished, as is net primary productivity.  More rapid runoff also means increased erosive power, forming gullies and washing muds down to choke our estuaries.

A great example of the triple whammy of changed catchments, more erosion, muds and loss of seascapes is the Great Barrier Reef.  Net primary productivity in the estuarine mixing zone is reduced. More muds have meant mangroves have increased in area.  More muds also mean turbid water, so that bottom-dwelling fauna and flora such as seagrass are smothered.

How much seagrass is lost – we don’t know.  We do have indicators. Dugong feed on seagrass:

  • Much of the Torres Strait area, comparatively undisturbed – over 110,000 dugongs.
  • Cape York through to Mossman, extensive cattle grazing, – only about 8000 dugongs.
  • then for all the inshore Reef lagoon areas, south to Hervey Bay fewer than 2000.

Seagrass is not just food for dugongs – it’s also shelter for a host of juvenile fish and one of our culinary favourites – tiger prawns.  Repairing hydrology – re-vegetation, plantation forestry, smart pasture management and excluding stock from watercourses will have multiple benefits to our estuaries and their productivity.

Tidal flows

Tides ensure interchange of nutrients in all forms that drive estuary productivity and connect salt marshes and other wetlands to the estuary waterway. Think detritus, benthos, algae, diatoms, phytoplankton and zooplankton.  So what happens when we constrain tidal flows?  I like to start the conversation about tides by recounting the detail from Mathew Flinders journals.  When he passed the Clarence, the biggest estuary in New South Wales, Flinders could not detect any particular entrance from the maze of sand spits.  We will never know the role of these sand spits in fish and prawn spawning with immediate larval recruitment on the next flood tide back into the estuary, but we do know tidal flows were unobstructed and the mass of water that was interchanged between the estuary and ocean was not constrained.

Over many years a series of training walls, bridges and causeways were built on the Clarence and indeed on most Australian estuaries.  For the Clarence our research on school prawns has shown that certain parts of the lower estuary, where tidal ventilation was better, were far more productive.  In comparison, Lake Wooloweyah on a side arm of the Clarence, with its tidal interchange heavily constrained by bridges and causeways, though with all other habitats equal, is far less productive – think over 50% less. That work also showed that if we reconnect salt marshes to the estuary then up to $16,000 per hectare of school prawns are produced, each and every year.  Far more than the purchase price of these lands ….and we are not counting the other species that our carbon signature work has shown also benefit – crabs, flathead, bream.

Connectivity

Talking about tides and the mixing of fresh and ocean water leads to the third important physical component that drives our estuaries – connectivity.  Tidal water needs to flow and mix with fresh water, to connect seascapes and their biota with the main channels of an estuary.  Stop that connectivity and so too, the productivity is lost.  Right around Australia we have drained wetlands and then constructed barrages to exclude tidal water.

Some examples:

  • over 1500 barrages to fish and prawn passage on the Burdekin floodplain;
  • in excess of over 1000 kilometres of river channel no longer accessible to Barramundi and Mangrove Jack on the Fitzroy; and
  • the once biggest estuary in Australia, joining our biggest river to the ocean well, it’s no more.

Yes, the Murray estuary comprising the Coorong and the Lower Lakes, Alexandrina and Albert is totally constrained by barrages. Prior to the 1930s when the construction of these barrages started, porpoises were often seen as far up the Murray as Wellington, cruising and feeding. Where there are schools of porpoises of course there are also fish.  Indeed in the period between the two World Wars there were over 100 fishing enterprises on the Murray estuary – feeding Adelaide and its growing population.

Mulloway dominated the catch.  Mulloway is southern Australia’s equivalent to northern Australia’s Barramundi.  Like Barramundi, Mulloway spawn near the entrance or just outside the estuary mouth with juveniles then recruiting back upstream to fresh to brackish waterways and wetlands.  Mulloway, like Barramundi grow to a big size and again, like Barramundi, are a target of recreational fishing because of their fighting prowess.  To be a predator and to live, feed and grow to sizes of around 1 metre in Australia’s estuaries requires strength and food!

Mulloway right throughout their range are now comparatively rare.  Certainly it’s about loss of connectivity. Most importantly one of the key ecological elements that ensures estuary productivity is now missing, our fresh to brackish wetlands.

Unfortunately some of our political leaders are either given poor advice or perhaps act in haste to secure votes.  Port Phillip is a great example. Denying commercial fishers the opportunity to fish in Port Phillip because of a much larger voting and vocal recreational fishing lobby makes good short-term politics but alas is no long term solution.  For long-term solutions, repairing habitat is the answer.

For Port Phillip, we will never know the full suite of tropic relationships and spawning behaviours that once made a bountiful estuary.  There are a few things we do know. Port Phillip was once dominated by masses of shellfish reefs.  These reefs with their multi-dimensional structures and crevices played host to many species.  They filtered the water, taking out nutrients, creating biomass and forming colloids from fine sediments that then dropped to the bottom and were no longer part of the water column.  This ensured a clean, light-penetrating Bay so other important habitat components like seagrasses and King George Whiting could thrive.  We also know Snapper, that made the Port Phillip fishery, famous were mostly caught near these shellfish reefs.  Did the Snapper also spawn near the reefs – well that is possible but as yet we don’t know.

Remedies

So what can we do about it?  Fortunately, there is a growing momentum to repair Australia’s estuaries.  Recreational and commercial fishers plus conservation groups and government agencies are getting together – one simple goal – more biodiversity, fish and prawns forever.

For Western Australia, Victoria and NSW specific funds are each year set aside investing monies collected from fishing licences into habitat repair.

More is needed. Our calculations show that a one-off Australia-wide investment of about $350M in estuary repair would be returned through additional product in our fish markets in less than 5 years.  Nature repairs quickly given the chance.  Our calculations were extremely conservative and involved only a few indicator species.  There are many multipliers like the benefits of more jobs, food forever and biodiversity that would make the return on investment even more attractive.

In Port Phillip The Nature Conservancy is working collaboratively with government and community to repair the shellfish reefs.  Already proven in Chesapeake Bay, USA as an effective way to bring back fish and water quality, re-creating shellfish reefs will provide an excellent foundation for a more productive Bay, more food and better amenity for the people of Melbourne. Similar activities are commencing in the South Australian Gulfs, in Oyster Harbour, Albany and in Lane Cove, Sydney.

In Queensland, a trial restoration of shellfish reefs is under way in Pumicestone Passage.

In NSW for estuaries such as the Manning and Wallis Lake, local governments have led the charge towards repairing wetlands and re-establishing connectivity.  For the Clarence we hope to soon embark on a multi-objective plan – better flood management and tidal interchange, repairing fisheries habitat and improving floodplain agriculture.

For the Reef, a great example is the 46 km long earthen wall and extensive wetland / ponded pasture in grazing land being added to the Shoalwater Defence facility.  Imagine the increased productivity in Barramundi or Banana Prawns and the improved Reef water quality – achievable at minimal cost by a day or two of a traxcavator, breaching the bund and reshaping the landforms into tidal creeks and salt marshes.

For the Murray – well I wonder if we will ever get back what was once Australia’s largest estuary?  Restoration could require something as simple as a ring main for irrigation water supply and related works.

Much is yet to be done but I am optimistic that we are close to the null point in our productivity baseline….that we will through multiple actions repair what we practically can of our estuaries – biodiversity and food forever.

Colin Creighton

Aired on ABC Rural in January 2017.

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Col Creighton on Estuaries

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