Spatial scale and the detection of impacts on the seagrass Posidonia australis following pier construction in an embayment in southeastern Australia

Coastal development can impact on ecologically significant seagrasses. We sought to determine whether the construction of a pier and associated boat launching facilities would impact on Posidonia australis within Jervis Bay Marine Park, New South Wales, Australia. Using a modified ‘beyond BACI’ analysis we tested for impacts at three spatial scales; directly beneath the pier deck, within metres of the pier and approximately 50 m from the construction zone. We compared shoot density, leaf length and epiphytic cover prior to the commencement of construction to 7 months and 46 months later. Impacts were highly localised; we detected significant seagrass loss directly below the open-mesh deck of the pier, but not at the scale of metres and tens of metres relative to two non-impacted reference locations. Our data also supported the use of the trigger point approach to initiate management intervention; shoot density beneath of the deck of the pier fell below the 30th percentile, ending 73% lower than the mean after 46 months. If similar developments occur in waterways supporting seagrass species of high conservation significance, then management agencies need to weigh the inevitable loss of a small area of meadow with the advantages the development will provide.