Emerging Sensor Platforms Allow for Seagrass Extent Mapping in a Turbid Estuary and from the Meadow to Ecosystem Scale

Johannes R Krause,Andrew B Gray,Elizabeth Burke Watson,Alejandro Hinojosa-Corona

doi:10.3390/rs13183681

Abstract

Seagrass meadows are globally important habitats, protecting shorelines, providing nursery areas for fish, and sequestering carbon. However, both anthropogenic and natural environmental stressors have led to a worldwide reduction seagrass habitats. For purposes of management and restoration, it is essential to produce accurate maps of seagrass meadows over a variety of spatial scales, resolutions, and at temporal frequencies ranging from months to years. Satellite remote sensing has been successfully employed to produce maps of seagrass in the past, but turbid waters and difficulty in obtaining low-tide scenes pose persistent challenges. This study builds on an increased availability of affordable high temporal frequency imaging platforms, using seasonal unmanned aerial vehicle (UAV) surveys of seagrass extent at the meadow scale, to inform machine learning classifications of satellite imagery of a 40 km2 bay. We find that object-based image analysis is suitable to detect seasonal trends in seagrass extent from UAV imagery and find that trends vary between individual meadows at our study site Bahía de San Quintín, Baja California, México, during our study period in 2019. We further suggest that compositing multiple satellite imagery classifications into a seagrass probability map allows for an estimation of seagrass extent in turbid waters and report that in 2019, seagrass covered 2324 ha of Bahía de San Quintín, indicating a recovery from losses reported for previous decades.

Highlights

Seagrass meadows provide important ecosystem services such as shoreline protection, constitute habitat for commercially important fish, and potentially sequester significant volumes of carbon [1,2,3]. Both anthropogenic and natural environmental stressors have led to a worldwide reduction in coastal seagrass extent, with rates of decline increasing from 0.9% per year before 1940 to 7% per year since 1990 [4]
The area covered by eelgrass at the three focus sites as estimated from unmanned aerial vehicle (UAV) surveys differed by site and month (Figure 3)
At the focus site ‘GR’, no statistically significant trend could be detected; the area classified as eelgrass was 0.5 ha higher in July and October compared to January and March

Summary

Introduction

Seagrass meadows provide important ecosystem services such as shoreline protection, constitute habitat for commercially important fish, and potentially sequester significant volumes of carbon [1,2,3]. Both anthropogenic and natural environmental stressors have led to a worldwide reduction in coastal seagrass extent, with rates of decline increasing from 0.9% per year before 1940 to 7% per year since 1990 [4]. In recognition of their potential to offset carbon emissions, seagrass meadows are being evaluated as National Determined. These are related to poor performance of remote sensing techniques under adverse atmospheric and hydrologic conditions characterized by high particle concentrations that scatter and absorb electromagnetic radiation

Methods

Results

Conclusion