We present results of monitoring studies on emergent coral reefs and submerged shoals, two potentially sensitive seabed habitats found within range of the modeled hydrocarbon plume from the 2009 Montara uncontrolled release in the Timor Sea.

Divers conducted reef surveys 6 and 16 months after the release was stopped. Hydrocarbons were detected in surface carbonate sediments at very low levels and declined between the two surveys in both frequency of occurrence and concentration. While hydrocarbon degradation precluded source matching, some samples were consistent with a Montara type oil, but there was also evidence for multiple sources of hydrocarbons in the region. Coral and fish communities were in good condition and potential indicators of disturbance in some elements, for example moderate levels of coral bleaching observed in 2010, were related to unusually warm sea surface temperatures rather than distance from the well head platform or plume.

The submerged shoals component targeted a series of nine discrete shoals ~30–150 km from Montara well head platform. The shoals have abrupt bathymetric profiles rising from 100–200 m depths to within 15–36 m of the sea surface. Sufficient light reaches these plateau environments to support benthic habitats for primary producers, including algae, corals and seagrass. Sampling used remotely deployed cameras and grabs.Benthic and fish communities were diverse and shared many species with shallow coral reefs. Hydrocarbon contamination was measured around the base of the shoals. While there was no conclusive evidence of a impact from the spill, spatial patterns in a subset of the fish data showed a reduction in abundance and diversity at shoals closest to the well head. Similarly a marked reduction in seagrass was noted on one shoal closest to the well head platform in the period between surveys, 6–16 months after the release was stopped. These observations may reflect an influence from the hydrocarbon release but could equally be the result of natural spatial patterns and disturbance events in the region.

Overall, the lack of sufficient prior data characterizing the region, especially for the shoals, constrained insights into any effect or otherwise of the spill and reinforces the need for regional scale baseline data. These surveys make a significant contribution and an excellent starting point for baseline characterization of the broader suite of emergent reefs and submerged shoal habitats in the Browse Basin.

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