Increasing confidence in eelgrass maps for project review and ocean planning

Funded by a 2021 NOAA Project of Special Merit Grant.

MassBays, CZM and several agency and nonprofit partners are teaming up on a study that will assess different methods of mapping eelgrass. Eelgrass (Zostera marina) is a marine meadow-forming plant that provides critical fisheries habitat, improves water quality, stores carbon, and protects the shoreline from storms.

To properly manage and protect eelgrass, accurate mapping information is needed to track the location and size of meadows. Several remote sensing methods such as satellites, airplanes, drones, and underwater side scan sonar provide imagery that can be analyzed for eelgrass, but little is known about the accuracy and limitations of maps generated from these sources, especially when it comes to the meadow’s edge.

This 18-month study, funded through the National Oceanic and Atmospheric Administration’s (NOAA) Project of Special Merit grant program, will compare each remote sensing method against SCUBA diver surveys to determine how well each method detects the eelgrass meadow’s edge and at what eelgrass densities each method performs best. Field work will take place May-June 2022 and will include diver transects, boat-based underwater photo ground truthing, drone and airplane flights, and sonar surveys at eelgrass sites in Gloucester, Beverly, Swampscott, Nahant, and Cohasset in Massachusetts.

Table of Contents

The Problem

Scientists and coastal resource managers rely heavily on eelgrass maps derived from remote sensing methods when making management, planning, and construction project siting decisions. Remote sensing methods often use imagery to detect and delineate habitats, however, their accuracy and detection limitations are not well known, and confidence is especially low at the meadow’s edge.

Diving is considered the most reliable method for determining the boundaries of eelgrass meadows, but it is also the most time- and resource-intensive.

Since 1995, MassDEP has conducted aerial surveys to observe and track the spatial extent of eelgrass beds in coastal Massachusetts waters, obtaining data for a given area every 5 years. More recently, advances in technology are making more survey methods available, including drones, boat-based side-scan sonar, and satellites. All of these methods have some limitations on the data quality gathered however, and if portions of eelgrass meadows are underrepresented in the maps, there is greater risk of inadequate resource protection.

Study design and purpose

eelgrass

Download the detailed MassBays Eelgrass Mapping Quality Assurance Project Plan 2022.

In October 2021, MassBays and CZM kicked off their new eelgrass mapping study. The project will compare meadow edge-detection capabilities across several remote sensing methods and compare their accuracies against diver surveys. We will look at data from side scan sonar, drones, fixed-wing manned airplanes, and satellites to assess each method’s ability to detect the edge and low-density areas of five eelgrass meadows in northern Massachusetts.

The goal is to better understand if and by how much each method underestimates the meadow, and in terms of management, if there are appropriate buffers that can be applied to delineations from each method for better resource protection. Ultimately, we hope to also develop a process by which the data generated from different methods can be more reliably integrated. We are working with many partners, both agency and nonprofit, in an effort to build capacity and fluency for this type of work.

Eelgrass SS image

Eelgrass as captured by side scan sonar (Photo: MA Division of Marine Fisheries)

Eelgrass by drone

Eelgrass captured by drone (Photo: J. Carr)

Stay tuned

Coming soon: Eelgrass Mapping Project Story Map. Check back so you can follow along with project progress.

Project contact: Jill Carr, MassBays Coastal Data Scientist. 

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