August 8, 2018

A mackerel sky above the TV Bluefin in Boags Marine Park
A mackerel sky looms above the TV Bluefin. Boags Marine Park is in the Boags Bioregion which covers waters off northern Tasmania. An iconic brewery provided inspiration for the name, unlike most Tasmanian bioregions named after biogeographical features (Flinders, Freycinet, Bruny, Davey). Image: Neville Barrett

Five days of swath mapping in near gale-force conditions has rewarded scientists with intricate imagery of extensive, mobile dune fields rippling across the Boags Marine Park seabed north of Tasmania.

The remarkable swatch of Bass Strait – part of the South-east network of offshore Australian Marine Parks – was mapped in July by a Marine Biodiversity Hub team in partnership with Parks Australia.

Survey leader Neville Barrett of the Institute for Marine and Antarctic Studies says some of the dunes in the park were more than 12 metres high and very steep on the eastern side, possibly due to asymmetric current flows.

A map showing the seabed dune patterns in Boags Marine Park
The massive dune fields in Boags Marine Park are formed by strong tidal currents that appear to weaken gradually with depth. Even so they are sufficiently strong at 40 m depths for marked dune formation.
Detail of the seabed sand dunes at 40 metre depths
Intricate dune patterns revealed by the high-resolution bathymetry mapping.

‘The mobile dunes that cover this vast area of seabed are formed by a combination of relatively shallow depths (40–80 m) and the strong tidal currents that sweep Bass Strait,' Dr Barrett says.

‘Tasmania’s northern coastline has a tidal rise and fall of 2.5–3 m. This gain and loss of water is made up of water entering and exiting Bass Strait at the eastern and western entrances.

‘That huge volume of water creates currents in excess of two knots in places, strong enough to pick up sand grains and transport them, just like at a windy beach or in the desert.

‘Eventually this process erodes the steep faces of dunes and deposits material in the more sheltered hind-dune area.'

CSIRO Marine National Facility hydrographers Phil Vandenbossche and Craig Davey in front of the TV Bluefin
CSIRO Marine National Facility hydrographers Phil Vandenbossche and Craig Davey were part of the seabed mapping team. Image: Neville Barrett

Dr Barrett says the dunes are unlikely to support complex invertebrate cover such as the sponge gardens seen in the Beagle Marine Park to the north-east of Boags.

Apart from the crustaceans, polychaete worms and molluscs that live in and on the sediments, the Park possibly hosts benthic fishes such as flathead, skates, rays and latchets.

This can be explored with follow-up biological sampling now that mapping during the Marine Biodiversity Hub survey has revealed the primary habitat features.

The clear differences in habitat and biodiversity in adjacent Australian Marine Parks highlights the need to survey more of them: each will have its own particular features, varying with depth.

A more detailed picture of the different plants, animals and habitats provides valuable information for Parks Australia to help manage these special places.

Sunset over Three Hummock Island
Sunset over Three Hummock Island. Boags Marine Park lies north of seabird breeding colonies on the Hunter group of islands (Three Hummock Island, Hunter Island, Steep Island, Bird Island, Stack Island and Penguin Islet) and is an important foraging area for a variety of seabirds. Image: Neville Barrett