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Abstract:

This report is one in a series of eco-narrative documents that synthesise our existing knowledge of individual Australian Marine Parks. This series is a product of the National Environmental Science Program Marine Biodiversity Hub Project D1, which seeks to collate, synthesise and visualise biophysical data within the parks. These documents are intended to enable managers and practitioners to rapidly ascertain the ecological characteristics of each park, and to highlight knowledge gaps for future research focus.

Joseph Bonaparte Gulf Marine Park is representative of the broad inner continental shelf of tropical northern Australia that is influenced by seasonal river inflow, strong tidal currents and regular cyclones. This combination produces a dynamic oceanographic setting within the marine park, characterised by high turbidity, rich nutrient levels and active sediment transport. The seabed within the park incorporates incised valleys and channels, tidal sand bars, gently sloping plains and small areas of localised reef. The valleys provide bathymetric relief of up to 40 m and define the ancestral pathways of the large rivers that drain into Joseph Bonaparte Gulf, including the Ord, Victoria and Daly Rivers. These offshore valleys were last active as rivers during the last ice age (ca. 18,000 – 10,000 years ago) when global sea level was tens of meters lower than present. Today, these valleys provide a conduit for the tidal transport of terrestrial sediments, mixed with carbonate marine sands. Tidal currents also shape the sand bars, which rise up to 15 m above the seabed and extend up to 20 km offshore. Reefs are restricted to small patch reefs in the northern part of the park, where they rise to within 2-3 m of the sea surface.

Our knowledge of biological communities associated with these features is limited. By inference from other studies further offshore in the Gulf, the patch reefs are likely to support sparse sponge and soft coral communities. Similar sessile assemblages are also expected to occur along the steeper side of incised river valleys. The park intersects the known distribution area for a range of pelagic megafauna, including a variety of dolphins and turtles. However, the park is not expected to be a major area for dugong, given the lack of seagrass, and the area is too shallow for whales. With proximity to coastal estuaries, the park overlaps the known range for the endangered Northern River shark, sawfish, stingray and catfish; plus a range of demersal tropical fish species.

The park is not without environmental pressures, with evidence for a consistent trend of warming surface waters and marine heat waves which can affect the health of the ecosystem at upper trophic levels. The region is also impacted by recreational and commercial fishing activities. The former are concentrated offshore from the Ord River mouth (Cambridge Gulf), whereas the latter range across the Joseph Bonaparte Gulf targeting prawns, barramundi and threadfin salmon. All these fishing activities result in some degree of direct and indirect impacts, including bycatch of a range of fish, invertebrates, sea snakes and sea birds.

The information in this eco-narrative forms an initial characterisation of Joseph Bonaparte Marine Park.


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