Sponge below tannin waters
by Heidi Dungey
Ascidian
Ninepin Point has an unusual aquatic environment. Cold, nutrient-rich sea water from the southern ocean is overlaid with tannin-rich freshwater flowing from the Huon River. Tannins are leached from the decaying organic matter in the Huon River catchment. The resulting tea-coloured water reduces the light levels on the reef. This allows the growth of a fascinating array of invertebrates, fish and red seaweeds, normally found only in much deeper water on Tasmania's east coast. Over 100 species of seaweed have been recorded in the reserve.
The tannin layer varies throughout the year. It may be almost absent during the summer, but can extend down 12 metres or more after heavy rain and run off from the Huon River. Ninepin Point is probably not worth diving if the Huon River is in flood, or if there is a strong wind blowing up the D'Entrecasteaux Channel mixing the tannin layer with the seawater. When these conditions occur, the visibility can drop to almost zero. Diving at Ninepin Point is best during summer or autumn, or when rainfall is low and flow from the Huon River is at a minimum. The best way to see this reserve is using scuba, but a snorkel in the shallows when visibility is good reveals a diverse and beautiful flora. Red and green algae are particularly prominent. Usually the murky tannin layer overlies relatively clear seawater, but it can be dark, especially if the tannin layer extends down more than six metres. A torch is useful.
The low light means that the kelp plants that normally cover rocky reefs soon disappear and are replaced by sponges and other invertebrate animals. Amongst the sponges you will see colourful seastars and pencil urchins or red rock cod or common gurnard perch. Southern hula fish are common across the reef, and along the reef edge you might see a school of jackass morwong. A small forest of giant kelp can often be found on the top of the reef.
The reserve is jointly managed by the Parks and Wildlife Service and Marine Resources.