What Are Mesophotic and Deep Benthic Habitats?
Deep below the surface of the ocean, where the sunlight begins to fade, lies a rich network of habitats home to a diverse community of fish, corals, and other invertebrates. These mesophotic and deep benthic habitats, or areas of the seafloor that see little to no sunlight, are hard to reach and hard to study–meaning we are just beginning to understand them.
Although Flower Garden Banks National Marine Sanctuary is known for its shallow, colorful coral reefs, these areas make up just a small part of the sanctuary. Most of the sanctuary stretches 130-725 feet below surface–deep into the mesophotic zone. Mesophotic means “middle light,” and this zone extends below the sunlit zone and only receives at most 1-10% of the light from the surface. In the sanctuary, this area is home to black coral and octocoral forests that support shrimp, brittle stars, squat lobsters, and even reef fish.
Threats Facing Mesophotic and Deep Benthic Habitats
In the Gulf of Mexico, mesophotic and deep benthic habitats faced an unprecedented threat in 2010, when the Deepwater Horizon (DWH) oil spill resulted in more than 134 million gallons of oil spilled over an 87-day span. More than 770 square miles of deep-sea habitat were injured, an area larger than the city of Houston. After the DWH oil spill, federal and state agencies formed the Deepwater Horizon Natural Resource Damage Assessment Trustee Council (DWH Trustees) to assess the impacts and identify and implement actions to restore injured habitats, species, and the services they provide.
In 2019, the Open Ocean Trustees selected four Mesophotic and Deep Benthic Communities (MDBC) restoration projects in the northern Gulf of Mexico through 2023. The projects are focused on:
- mapping and habitat modeling
- coral propagation
- habitat assessment
- active management
The restoration goals of these projects are to improve understanding of mesophotic and deep benthic communities (MDBCs) to inform management and ensure resiliency, restore injured MDBC species, actively manage and protect valuable MDBC habitats, and provide a framework for monitoring, education, and outreach.
MDBC Restoration in the Sanctuary
Not all benthic areas in the Gulf were injured by the oil spill. Dozens of sampling trips by the DWH Trustees documented no evidence of exposure to DWH oil, dispersants, or disruptive responses within Flower Garden Banks National Marine Sanctuary. However, long-term monitoring studies within the sanctuary have provided crucial data to support MDBC restoration throughout the Gulf. By developing a better understanding of healthy, unimpacted habitat, experts can more effectively implement restoration and evaluate progress in impacted areas. Recent efforts to install deep mooring buoys, remove marine debris, and remove invasive species also provide additional protection to these habitats within the sanctuary.
Spotlight on: MDBC Mission in the Sanctuary
MDBC projects involve extensive field work throughout the northern Gulf of Mexico, including many days at sea each year to inform and conduct restoration activities. One of the most ambitious expeditions to date took place in the sanctuary in July 2024, and brought together NOAA, United States Geological Survey (USGS), the US Navy, and other academic, governmental, non-governmental organizations, and private partners. The expedition used a diverse collection of technology—from saturation divers to remotely operated vehicles (ROVs, or "underwater robots")—to significantly advance restoration efforts in the Gulf and further our understanding of mesophotic and deep benthic communities in and around the sanctuary.
The 19-day mission on the M/V Island Intervention deployed highly trained saturation divers from the US Navy Experimental Dive Unit (NEDU), used advanced ROVs, and employed other top-side technology like large cranes to achieve many restoration objectives. These tools allowed experts to plant coral fragments, remove dozens of lionfish, collect almost one ton of marine debris, deploy benthic landers to collect environmental data, and install six new mooring buoys. All of this work occurred in and around sanctuary boundaries–including in areas recently added to the sanctuary. The mission not only helped protect mesophotic communities within the sanctuary, but expanded our knowledge of these habitats in ways that will help restore them throughout the Gulf of Mexico.
