We developed a benthic habitat map of coral reef ecosystems around Majuro, an atoll in the Republic of the Marshall Islands. The maps were created from satellite images and depict coral reefs, seagrass, sand, reef flats, and other ecosystem features in the shallow waters of the atoll. Benthic maps are foundational tools for monitoring, conservation, development planning, stock assessments and other management actions.
WHY WE CARE
Preservation and management of coral reef ecosystems are dependent on current, accurate, and consistent maps. With comprehensive maps and habitat assessments, coral reef managers can be more effective in designing and implementing a variety of conservation measures, including:
- Long-term monitoring programs with accurate baselines from which to track changes;
- Sustainable development of the coastal zone and place-based conservation measures, such as Marine Protected Areas; and
- Targeted research to better understand the oceanographic and ecological processes affecting coral reef ecosystem health.
WHAT WE DID
We generated a benthic habitat map of the shallow water reef ecosystems of the Majuro atoll.
Benthic habitats were visually interpreted from Digital Globe’s Quickbird II (QB2) commercial satellite imagery collected between 2004 and 2006. Benthic maps are created by drawing boundaries between different bottom types that are visible in remote sensing imagery on a computer monitor using a mouse or digitizing pad, a process known as “on-screen” digitizing. Bottom types were described using a habitat classification scheme customized for use at Majuro to capture local reef formations and was also simplified, compared to other schemes, to meet the constraints of the limited field work that was possible with this project.
WHAT WE FOUND
Reef zones (e.g. fore reef, reef flat) mapped at Majuro reflect typical atoll morphology in the Marshall Islands. All zones circled the entire atoll but were quite narrow, only 10 to a few hundred meters wide. Zones were somewhat wider on the east and west sides of the atoll. The lagoon was by far the largest zone, comprising nearly 80% of the atoll. Reef flat was the next largest zone and covered 7.5% of the atoll. Zones with the largest number of features were back reef (677 polygons) and pinnacles (432), with most of the shallow pinnacles occurring in the western half of the lagoon.
Major geomorphological structures in the classification scheme include coral reef/hard bottom and unconsolidated substrate. Of the total 73.9 km2 of coral reef ecosystem, 77% was coral reef and hard bottom, 8% was unconsolidated substrate, and 15% was other delineations such as “land”. Similarly, area was tabulated for Detailed Geomorphologic Structure. Of the total 73.9 km2 of coral reef ecosystem mapped, the largest proportion (29%) was “pavement” followed by “aggregate reef” (18%). Common combinations of zone and structure around the atoll included pavement on reef flats, spur and groove on fore reefs, aggregate reefs on back reefs, rubble in channels, and patch reefs on pinnacles.
BENEFITS OF OUR WORK
A total of 1829 features covering 366 km2 were mapped, including 11 km2 of land and 288 km2 of deep unknown areas in the central lagoon. The whole set of GIS map products are displayed comprehensively in a PDF atlas available via the links below. Maps produced in this project (GIS and PDF atlas), satellite imagery, videos, and pictures taken in the field are all available via the Biomapper website and can be used to support a variety of coastal management activities identified as important in Majuro.