In a new study, scientists at AOML and the University of Miami’s Cooperative Institute for Marine and Atmospheric Studies (CIMAS) demonstrate how photogrammetry-based techniques enable their team to monitor the persistence and structural complexity of coral reef habitats at an unprecedented scale.
Led by John Morris, Ph.D., a CIMAS Postdoctoral Associate with AOML’s Coral Program, the study introduces a novel method that integrates high-resolution two-dimensional (2D) and three-dimensional (3D) models of reef communities to quantify reef habitat growth or loss over time.
This innovative approach advances the monitoring of reef persistence through the use of large sampling areas, simple field protocols, and streamlined analysis that can support highly-detailed and spatially-extensive assessments of coral reef structure and resilience.
Photomosaic Fly-Through of Cheeca Rocks, a shallow coral reef and key environmental monitoring site in the Florida Keys
Coral reefs stretch hundreds of miles of U.S. coastline, hosting rich biodiversity, sustaining key fisheries and providing billions in flood risk reduction along U.S. coastlines annually due to their hard, highly complex calcium carbonate skeleton.
However, this calcium carbonate skeleton that produces entire coral reef frameworks experiences both natural growth and erosional processes. And this is what scientists are trying to study.
Hard corals and other calcifiers pull carbonate ions from the seawater to produce new layers of calcium carbonate skeleton – similar to the rings on a tree – while changes in seawater chemistry, wave action and marine organisms, such as parrotfish and sea urchins (i.e., bioeroders), remove reef skeleton.
Monitoring the balance between carbonate production (i.e., growth) and erosion (i.e., degradation) of a reef’s calcium carbonate skeleton is what scientists refer to as “the carbonate budget.”
“Carbonate budgets are an incredibly useful metric to understand how coral reef communities and structural frameworks are changing over time,” explains John Morris, Ph.D. “Our new photogrammetry-based carbonate budget method can improve the spatial scope of carbonate budget assessments and can be particularly valuable at deep or remote reef sites where traditional census-based survey methods are less feasible.”
With this new study, Morris compared methods historically used to quantify a reef’s carbonate budget to one developed by AOML’s Coral Program.
Coral ecologists and researchers have primarily relied on divers performing field-based surveys of coral coverage, the diversity of species, and other conditions along designated transects to monitor a coral reef’s carbonate budget – essentially a census-based approach.
Large-area imagery (LAI) has been introduced as a valuable alternative to traditional reef monitoring techniques, and in the case of carbonate budget assessments, has only recently been applied at reefs in the Pacific.
John Morris, Ph.D. preparing a transect line to perform a census-based carbonate budget survey of coral reef within the Dry Tortugas National Park.
However, with both the census-based methods and Pacific LAI approach, scientists only monitor changes to a reef’s carbonate budget, coral species composition and coverage – and therefore the persistence of a reef’s structure – along designated transects, rather than capturing the entire site.
By employing the photogrammetry-based carbonate budget (PBCB), a technique that involves divers swimming over the reef in a lawn-mower-like pattern with a camera, AOML’s Coral Program collects thousands of photos that are ultimately stitched together by an advanced software to produce a 2D and 3D model of the entire reef. Coral and other benthic taxa are then annotated directly from these models to quantify the reef’s carbonate budget.
“One of the more important advantages of this approach is that the data products are archivable, visual records of coral reefs,” says Morris, Ph.D. “This allows scientists to measure long-term trends in reef structure and identify hot spots in reef growth or persistence.”
Morris demonstrates this photogrammetry-based method more effectively captures the diversity of coral species present at a site while significantly expanding the sampling area they can monitor – a scale that is unprecedented globally.
Under natural conditions, healthy coral reefs produce new layers of skeleton at a rate that outpaces their erosion – and therefore, experience net growth.
However, a 2022 study – also led by Morris, Ph.D. and scientists at AOML – determined that an estimated 70 percent of Florida’s coral reefs are experiencing net erosion – a fact only made worse by the exacerbated impacts of ocean acidification threatening crucial reefs throughout the Florida Keys.
Ocean acidification – a stressor made worse by the ocean’s increased accumulation of anthropogenic carbon – dramatically heightens erosion of coral reefs as it leads to a depletion of carbonate ions available in seawater for corals to produce new layers.
Research led by AOML’s Coral Program and CIMAS has also proven that under more acidic conditions, microorganisms accelerate the rate of bioerosion of coral reef structure – compounding the impacts of ocean acidification and the potential loss of essential habitat.
These findings underscore the need for effectively monitoring the complex structure comprising coral reefs – and to do this, accurately assess changes in their carbonate budget on a scale not seen before.