The reproducibility of remotely piloted aircraft systems to monitor seasonal variation in submerged seagrass and estuarine habitats

Abstract

Seasonal variation in seagrass growth and senescence affects the provision of ecosystem services and restoration efforts, requiring seasonal monitoring. Remotely piloted aircraft systems (RPAS) enable frequent high-resolution surveys at full-meadow scales. However, the reproducibility of RPAS surveys is challenged by varying environmental conditions, which are common in temperate estuarine systems. We surveyed three eelgrass ( Zostera marina) meadows in Newfoundland, Canada, using an RPAS equipped with a three-color band (red, green, blue [RGB]) camera, to evaluate the seasonal reproducibility of RPAS surveys and assess the effects of flight altitude (30–115 m) on classification accuracy. Habitat percent cover was estimated using supervised image classification and compared to corresponding estimates from snorkel quadrat surveys. Our results revealed inconsistent misclassification due to environmental variability and low spectral separability between habitats. This rendered differentiating between model misclassification versus actual changes in seagrass cover infeasible. Conflicting estimates in seagrass and macroalgae percent cover compared to snorkel estimates could not be corrected by decreasing the RPAS altitude. Instead, higher altitude surveys may be worth the trade-off of lower image resolution to avoid environmental conditions shifting mid-survey. We conclude that RPAS surveys using RGB imagery alone may be insufficient to discriminate seasonal changes in estuarine subtidal vegetated habitats.

The Role of Boreal Seagrass Meadows in the Coastal Filter

“…Additionally, parts of the shallow coastal waters freeze in the winter exposing the eelgrass to ice scouring. Eelgrass in Placentia Bay reaches peak density in late‐summer/early fall (Murphy et al., 2021; Prystay, Adams, et al., 2023).…”

“…RPAS survey methods, analyses, and results are described in Prystay, Adams, et al. (2023). Non‐eelgrass habitats were removed from the classified maps, and the eelgrass rasters were converted into polygons.…”

“…Eelgrass, sediment, and water quality data supporting this research are available on Seanoe Sea Scientific Open Data Publication platform (Prystay, Sipler, et al., 2023).…”

“…Lastly, eelgrass meadow aerial extents were estimated using a DJI Matrice remotely piloted aircraft system (RPAS) and supervised image classification using random forest analyses. RPAS survey methods, analyses, and results are described in Prystay, Adams, et al (2023). Non-eelgrass habitats were removed from the classified maps, and the eelgrass rasters were converted into polygons.…”

Mapping and Characterizing Eelgrass Meadows Using UAV Imagery in Placentia Bay and Trinity Bay, Newfoundland and Labrador, Canada

“…In contrast, eelgrass in nearby coastal areas of the island of Newfoundland not colonized by green crabs has been reported to be expanding [25]. Previous eelgrass mapping efforts on the island of Newfoundland, however, consisted mostly of point observations indicating where eelgrass is present or is likely to occur [26], but provided limited value in terms of baseline data to monitor temporal changes in eelgrass extent (but see [27]).…”