Multi-scale assessments reveal changes in the distribution of the endangered seagrass Posidonia australis and the role of disturbances

Seagrasses are threatened globally by multiple anthropogenic disturbances, and management of these threats requires detailed information on where losses are occurring and why. Seagrass distribution is determined by processes operating at multiple scales, yet most assessments of change to seagrass extent are done at a single spatial scale. This study applied a multi-scale approach to quantify changes in the extent of the endangered seagrass Posidonia australis over the last 10–18 years using high-resolution mapping from 15 estuaries in New South Wales, Australia. Changes in P. australis extent and relationships with anthropogenic disturbances were examined at two spatial scales: across entire estuaries and at sub-estuary “local” scales within 50 × 50 m grids. Although increases in P. australis area were observed in ten estuaries, losses at local scales were prevalent in all estuaries. No disturbances correlated with seagrass change at the estuary scale; however, the greatest losses occurred in Botany Bay which is a highly modified estuary with a heavily urbanised catchment. At local scales, losses of P. australis were strongly associated with large areas of artificial structures and distance to the sea, and the greatest increases were observed in areas with marine reserves and no oyster aquaculture. These findings highlight the importance of quantifying changes in seagrass extent at multiple scales, as estuary-scale trends can mask localised losses if they are offset by increases in other parts of the estuary. Identifying hotspots of declines and the disturbances causing them are essential for applying focussed management actions to conserve seagrasses.