Landsat historical records reveal large-scale dynamics and enduring recovery of seagrasses in an impacted seascape.

Seagrasses are considered indicators of anthropogenic impact but surprisingly little is known about their temporal and spatial dynamics in impacted seascapes. In this study, we used three decades of Landsat imagery (1988-2018) off the coast of Adelaide, South Australia, to investigate how seagrass cover over 501 km responds to changes in land-based inputs, including breakpoints in system trajectory and associated timelags, and the identification of vulnerable meadows. Field data was used to help train benthic classification of summer imagery and define its accuracy. Temporal dynamics of seagrass cover were investigated in relation to annual and multi-year nitrogen and suspended solids loads. Spatial dynamics were inferred from maps of benthic cover persistence and trajectory for each decade. The region experienced a net regrowth of some 11,000 ha of seagrasses since the early 2000s, with the initial large-scale recruitment visible in the imagery 6 years after the closure of sludge outfalls. Seagrass expansion occurred primarily in deeper waters (>10 m) of the central coast and at the seaward edge of the distribution. Recovery continued until 2011 assisted by a window of opportunity created by a decade-long drought and further reductions in nitrogen loads from wastewater treatment plants and industry. Localized seagrass losses however continued to be observed as a result of either permanent or transient increases in suspended solids loads. Seagrass area in the central coast was well correlated (r = 0.88) with 5-year running averages of nitrogen and suspended solids loads. Meadows particularly vulnerable to changes in land-based discharges were located at the edges of the distribution, along erosional scarps and at depths >10 m south of the Torrens River. These areas were identified as useful indicators of seagrass status. Overall, seagrass persistence expanded from 48 to 69% of the mapped area, with the region now mostly covered by stable seagrasses.