Water masses move over reefs, seagrass beds, and sandbanks – and as they do, the seawater chemistry changes.
In the Florida Keys, changes in coral reef carbonate chemistry are driven by benthic metabolism, the origin of the water mass, and the connectivity of habitats. A new study from NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) shows how we can use existing monitoring data to better understand the combined influence of these factors on local reef water chemistry.
Dr. Heidi Hirsh, an Assistant Scientist with the AOML Coral Program, demonstrates how integrating the source water, or “endmember”, chemistry conditions, the benthic habitat, and the flow of water between habitats can be used to predict the nearshore carbonate chemistry on a specific coral reef.
Benthic communities (i.e. seagrass, coral), source water (“endmember”) chemistry and the complex flow of water (hydrodynamics) between habitats all influence the local carbonate chemistry of a coral reef. Derived from: Hirsh, et al., 2025
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As part of the four-year Florida Regional Ecosystems Stressors Collaborative Assessment (FRESCA), a collaboration co-led by NOAA’s Atlantic and Meteorological Laboratory (AOML) and the University of Miami, Hirsh has developed a statistical model to predict nearshore coral reef carbonate chemistry based on modeled trajectories of currents and the interconnection between relevant sourcewater and habitats.
This approach takes into account where the water came from and the influence of marine ecosystems (i.e. benthic community metabolism) on a water mass before it arrives on a reef in a specific area.
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To build the model, Hirsh used the discrete sampling points along ten of these cross-shelf transects in the Florida Keys, covering an estimated 250 kilometers (155 miles) of the Florida Coral Reef, and the carbonate chemistry measurements collected between 2015 and 2021 at each point.
The relevant starting chemistry (i.e. “endmember”) of the water sampled at these points and the habitats the water was exposed to along the path to the sampling station also shape the observed reef chemistry.
To effectively capture these changes, Hirsh needed to recreate the pathways of waterflow to each reef station. Collaborators from Université Catholique de Louvain (UC Louvain) – Dr. Emmanuel Hanert and Dr. Thomas Dobbelaere -simulated water particle trajectories using a hydrodynamic model: the second-generation Louvain-la-Neuve Ice-ocean Model (SLIM).
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“This spatial approach to understanding carbonate chemistry on coral reefs demonstrates how we can leverage existing datasets and models to make high-resolution predictions for reefs of interest. By utilizing data already at our disposal, we can be more strategic about designing future sampling strategies to fill the knowledge gaps and increase the utility of new data, ” says Hirsh, Ph.D.
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By developing and validating the model to capture these complex processes now, scientists at AOML aim to apply it to future predictions of how exacerbated ocean acidification could impact carbonate chemistry across the only barrier reef in the continental United States and third largest in the world.
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“One takeaway of our new modeling study is the importance of identifying the appropriate upstream endmember so that we understand the magnitude of change that is taking place across these reefs,” explains Hirsh, Ph.D.
Ultimately, this study builds on the goal of the larger project – FRESCA – to investigate the spatial variability of environmental stressors across South Florida’s ecosystems – and how the combined impacts of these stressors on key ecosystems will be exacerbated or mitigated over time.
Chris Malanuk, NOAA Atlantic Oceanographic & Meteorological Laboratory, 29 May 2025. Press release.
