Impact of ship-way channel dredging on a seagrass community in the Gulf of Carpentaria, Australia

Abstract

Port construction at McArthur River Mine in the western Gulf of Carpentaria, Australia, required the dredging of an access channel through a prolific seagrass community. A 4 km by 60 m channel dredged to 5 m depth removed 18.95ha of intertidal and shallow subtidal seagrass. Dredge spoil was pumped onshore to settle in multiple ponds before the decant water returned to the gulf. Quantitative surveys of the seagrass community surrounding the channel using a Before After Control Impact Repeated measures (BACIR) experiment were conducted to investigate the effect of the channel construction on the seagrass community.Nine seagrass species were found during the surveys, but not all species were found in each of the impact or control sectors at all times. Syringodium isoetifolium (Ascherson) Dandy and Cymodocea serrulata (R.Brown) Ascherson and Magnus were the most common and extensive seagrasses (occurring mid- and offshore), while Halophila ovalis (R.Brown) Hook and Halodule uninervis (Forsskal) Ascherson were also abundant (occurring inshore). Twenty-five months after the completion of dredging no significant impact of channel dredging was identified: neither a retreat of seagrasses from the channel, a decline in seagrass cover, nor a reduction in biomass or shoot density. The sediment particle size distribution in the impact sector did not change. Changes in seagrass abundance were detected, but they were consistent in both the impact and control sectors (5–24 km distant from the dredged channel). During both pre- and all post-dredging surveys, seagrasses were found at about 80% of sites (range 79%–84%) in the impact sector (>300 sites). Dredge spoil was pumped onshore, then contained and settled in bunded ponds. Decant water was free of sediment. Sedimentation, elevated turbidity and light attenuation in the vicinity of the operating dredge was minimal and had no deleterious effect on the seagrass community.