Visitor's Guide to Port Davey - Bathurst Harbour Marine Reserve

Introduction

Bathurst Narrows
(Photo copyright Matthew Newton)

The marine area of Port Davey - Bathurst Harbour comprises 17,000 ha of the Southwest National Park and the Tasmanian Wilderness World Heritage Area. 

Port Davey, Bathurst Channel and Bathurst Harbour together form a large estuarine and coastal embayment on the southwest coast of Tasmania. Between Macquarie Harbour on Tasmania’s west coast and Recherche Bay on the southeast coast, a distance of 250 km of coastline, Bathurst Harbour is the only sheltered inlet.

Because of the remote location and often harsh climatic conditions, very little was known about the marine ecology of Port Davey - Bathurst Harbour until 1984. Since then, various surveys and research studies have identified a truly unique and fragile marine environment. Just as the terrestrial environment contains relict fauna from 80 million years ago (following the split of the ancient supercontinent of Gondwana), so too does the aquatic environment.

A layer of tannin stained water and low nutrient levels create a unique circumstance that allows scientists and recreational divers to visit habitats that have much in common with those in much deeper water out in the Indian Ocean.

The tannin layer varies in thickness according to rainfall, and may almost disappear after a dry spell. Usually it is several metres thick in the Bathurst Narrows and Bathurst Harbour and forms a layer where visibility is often less than 2 metres. Underneath, the water is usually much clearer, although light levels can be very low. Even during the day a torch is often needed to see details on the seabed or read instruments. But a dive here provides the opportunity to see giant sea pens on the soft bottom, and on the reef there are colourful invertebrate assemblages with deepwater bramble corals and gorgonians. Divers visiting this area should be familiar with the Diving Code of Practice and need to take special care not to damage fragile marine life with careless fin kicks or stirred up sediment.  

Bathurst Harbour is a large basin, seven kilometres long and five kilometres wide, believed to have once been a large button grass plain, now under water and surrounded by hills. Bathurst Harbour is one of only three large estuaries in Australia where dark, tannin-stained, brackish waters overlie clear marine water, a situation known as stratification. The harbour floor is relatively flat, with depths ranging from seven metres in the eastern areas to six metres near the western outlet. Bathurst Harbour is linked to the sea by Bathurst Channel. The Old and the North Rivers drain into Bathurst Harbour generating a major corridor for transporting fresh water to the sea, from Bathurst Harbour through Bathurst Channel to Port Davey.

Bathurst Harbour has a unique marine environment because of the very low nutrient levels.  The low nutrient levels come about due to:

• little nutrient input from surrounding streams – because of the inert rocks and blanket bogs
• no sources of nutrients from fertilisers, sewage, dams or nutrient rich bottom waters
• high degree of stratification with no major upwelling
• high densities of phytoplankton able to absorb nutrients and ultimately transport them from surface waters to the sediments.

Bathurst Channel is a deep and narrow drowned river valley, 12 km long and 1 km wide, connecting Bathurst Harbour with Port Davey. At one stage the channel was thought to be a fiord, but is now known to be a drowned river valley with water depths varying from 15 to 40 metres. Fresh water flows from Bathurst Harbour through Bathurst Channel, while the Spring River flows into Bathurst Channel. At the western end of Bathurst Channel a shallow ledge (about 13 metres deep) separates the Channel from the deeper waters of Port Davey and the ocean.

Port Davey contains a wide variety of habitats, including cliffs, rocky shores, sandy beaches, sheltered bays and inlets harbouring seagrass beds and mudflats. The waters of Port Davey are quite deep, however there is a shallow sill where it joins Bathurst Channel. Port Davey differs from other drowned river valleys throughout Australia because of the flow of acidic, tannin-stained waters. West of the Breaksea Islands there is very little evidence of the unique estuarine influences, as the ocean currents and wave action associated with Port Davey disperse the waters flowing from Bathurst Channel and Bathurst Harbour.