Martin et al. 2023 Deep Meadows: Deep-Water Seagrass Habitats Revealed

Martin, Belinda C., Ana Giraldo-Ospina, Sahira Bell, Marion Cambridge, Matthew W. Fraser, Brooke Gibbons, Euan S. Harvey, Gary A. Kendrick, Tim Langlois, Claude Spencer, and Renae K. Hovey. n.d. “Deep Meadows: Deep-Water Seagrass Habitats Revealed.” Ecology n/a(n/a):e4150. doi: 10.1002/ecy.4150.

Seagrass meadows across the globe are under increasing pressure from human activities and climate change (Dunic et al., 2021; Waycott et al., 2009). A major challenge limiting effective seagrass conservation is the paucity of global seagrass distribution data. Global estimates of seagrass distribution range from 150,000 to 600,000 km2 (Green & Short, 2003), with the most recent estimate of 160,387 km2, which is greater than global estimates of mangrove, saltmarsh, and kelp habitats (Mckenzie et al., 2020). However, global distribution models often do not account for deep-water seagrasses that can grow in clear waters such as the Mediterranean Sea and the Indian Ocean (Carruthers et al., 2007; Gattuso et al., 2006).

Here we reveal the location and extent of deep-water (>30 m) seagrass meadows from a combination of autonomous underwater vehicle (AUV), baited remote underwater video (BRUV) and drop camera imagery. These deep-water meadows were primarily comprised of a perennial habitat-forming seagrass, Thalassodendron pachyrhizum den Hartog, which is endemic to western and southern Australia (Figure 1; Video S1). The maximum depth of T. pachyrhizum that could be confirmed from all available imagery was 63 m, which would make this the deepest reliable record for a habitat-forming perennial seagrass species to date.
Analysis of BRUV imagery revealed that the Southwest Corner Marine Park off Western Australia’s coast has an extensive area of deep-water T. pachyrhizum meadows extending out to 20 km from the coast (Figure 2). Deep-water seagrass meadows were also observed in drop camera imagery from elsewhere in Western Australia, including Jurien Bay (maximum depth 44 m and 32 km offshore) and by AUV at the Houtman Abrolhos (maximum depth 36 m) (Figure 2). However, due to the nature of AUV imagery that captures benthic habitats from above, we were not always able to distinguish morphological features to identify seagrass species or genera.

FIGURE 2 Map depicting the locations of deep seagrass records in Western Australia. Records from the Abrolhos Island and Jurien Bay were recorded from drop camera and autonomous underwater vehicle still images and have not been identified to the genus or species level. Records from the Southwest Corner Marine Park were all confirmed as Thalassodendron pachyrhizum observations recorded from drop camera and baited remote underwater video deployments.