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1. Geological age and environments shape reef habitat structure

   https://doi.org/10.1111/geb.13691  

   Asbury, Mollie ; Schiettekatte, Nina M. D. ; Couch, Courtney S. ; Oliver, Thomas ; Burns, John H. R. ; Madin, Joshua S. ( April 2023 , Global Ecology and Biogeography)

   Habitat complexity plays an important role in the structure and function of ecosystems worldwide. On coral reefs, habitat complexity influences ecosystem services such as harvestable fish biomass and attenuation of wave energy. Here, we test how three descriptors of surface complexity—rugosity, fractal dimension, and height range—trend with the geological age of reefs (0.2–5.1 million years old), depth (1–25 m), wave exposure (1–306 kW/m), coral cover (0–80%), and three habitat types (aggregated reef, rock and boulder, and pavement).

   We surveyed across 234 sites and 4 degrees of latitude in the eight main Hawaiian Islands.

   April 2019 – July 2019.

   Reef building corals.

   We estimate three surface descriptors (rugosity, fractal dimension and height range) using structure‐from‐motion photogrammetry. We evaluate hypothesized relationships between these descriptors and geological reef age, depth, wave exposure, coral cover and reef habitat type using generalized linear models that account for survey design.

   The rugosity of reef habitats decreased with geological reef age; fractal dimension (and coral cover) decreased with wave exposure; and height range decreased with depth. Variations in these patterns were explained by the different habitat types and the way they are formed over time. Nonetheless, the three surface descriptors were geometrically constrained across all habitat types, and so habitats occupied distinctly different regions of habitat complexity space.

   This study showed how broad environmental characteristics influence the structural complexity of habitats, and therefore geodiversity, which is an important first step toward understanding the communities supported by these habitats and their ecosystem services.

    

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2. Particle‐Based Lagrangian Filtering for Locating Wave‐Generated Thermal Refugia for Coral Reefs

   https://doi.org/10.1029/2020JC016106  

   Bachman, Scott D. ; Shakespeare, Callum J. ; Kleypas, Joan ; Castruccio, Frederic S. ; Curchitser, Enrique ( July 2020 , Journal of Geophysical Research: Oceans)

   Tides and tidally generated waves play a key role in modulating the heat budget of the nearshore environment, which can significantly affect the ecology of the coastal and reef zones. The high spatial resolution required to resolve such waves in numerical models means that diagnosing wave‐generated heat fluxes (WHFs) has so far only been possible over very limited areas. Because many marine ecosystems that are affected by WHFs, such as coral reefs, are patchily distributed within domains of hundreds of kilometers, an alternative to fully wave‐resolving models is needed to identify thermal refugia and guide conservation efforts over larger regions. In this study, a one‐way nested series of regional ocean simulations has been conducted to study the role of waves in driving high‐frequency temperature variations in the Coral Triangle and to develop a method for identifying WHF in coarser‐resolution models. A filtering method using Lagrangian particles is used to separate the wave component of the flow, which is used to diagnose the wave energy and to identify locations experiencing large, high‐frequency temperature variability. These locations are shown to possess three key characteristics: large time‐mean wave kinetic energy, shallow depth, and a steep bathymetric gradient that gives access to a nearby source of cold, subthermocline water. A function of the wave kinetic energy and bathymetric slope is found to closely correlate with areas of maximal temperature variance, suggesting its use as a convenient and readily calculable metric to locate thermal refugia in observations or numerical experiments over large spatial domains.

    

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3. Instrumentation for direct measurements of wave‐driven flow over a fringing reef crest

   https://doi.org/10.1002/lom3.10337  

   Hefner, Benjamin B. ; Rogers, Justin S. ; Maticka, Samantha A. ; Monismith, Stephen G. ; Woodson, C. Brock ( October 2019 , Limnology and Oceanography: Methods)

   Shallow dynamic flows are very important processes in environmental systems, yet they are notoriously difficult to measure. For example, on coral reefs, shallow flows over the reef crest typically range from 0 to 1 m depth and velocity up to 1–2 m s⁻¹including oscillatory motion of waves. To directly measure depth and velocity in this challenging environment, we retrofitted a vertical acoustic Doppler current profiler (ADCP, Teledyne, RDI) designed for streams into a new shallow water acoustic Doppler current profiler system (SW‐ADCP) to expand memory and power limitations and allow for deployment in wave‐dominated environments. We captured variations in shallow reef crest depth and flow over a period of 3 weeks in the reef/lagoon system of Ofu Island, American Samoa. The new SW‐ADCPs recorded water depth and three‐dimensional velocity in 3 cm bins every 3 s. We then used velocity profiles to estimate volumetric flow and drag coefficients and verified these estimates using boundary layer theory. We observe that the mean velocity profile is well approximated by a log‐layer formulation withz₀of 3.5 cm, despite the shallow depths, strong flows, and breaking waves. Our observations validated the use of SW‐ADCPs as a tool for measuring flows in shallow (0.1–1 m), dynamic coastal marine environments.

    

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4. Place‐based management can reduce human impacts on coral reefs in a changing climate

   https://doi.org/10.1002/eap.1891  

   Delevaux, Jade M. S. ; Stamoulis, Kostantinos A. ; Whittier, Robert ; Jupiter, Stacy D. ; Bremer, Leah L. ; Friedlander, Alan ; Kurashima, Natalie ; Giddens, Jonatha ; Winter, Kawika B. ; Blaich‐Vaughan, Mehana ; et al ( April 2019 , Ecological Applications)

   Declining natural resources have contributed to a cultural renaissance across the Pacific that seeks to revive customary ridge‐to‐reef management approaches to protect freshwater and restore abundant coral reef fisheries. We applied a linked land–sea modeling framework based on remote sensing and empirical data, which couples groundwater nutrient export and coral reef models at fine spatial resolution. This spatially explicit (60 × 60 m) framework simultaneously tracks changes in multiple benthic and fish indicators as a function of community‐led marine closures, land‐use and climate change scenarios. We applied this framework in Hā‘ena and Ka‘ūpūlehu, located at opposite ends of the Hawaiian Archipelago to investigate the effects of coastal development and marine closures on coral reefs in the face of climate change. Our results indicated that projected coastal development and bleaching can result in a significant decrease in benthic habitat quality and community‐led marine closures can result in a significant increase in fish biomass. In general, Ka‘ūpūlehu is more vulnerable to land‐based nutrients and coral bleaching than Hā‘ena due to high coral cover and limited dilution and mixing from low rainfall and wave power, except for the shallow and wave‐sheltered back‐reef areas of Hā‘ena, which support high coral cover and act as nursery habitat for fishes. By coupling spatially explicit land–sea models with scenario planning, we identified priority areas on land where upgrading cesspools can reduce human impacts on coral reefs in the face of projected climate change impacts.

    

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5. Landscape‐scale patterns of nutrient enrichment in a coral reef ecosystem: implications for coral to algae phase shifts

   https://doi.org/10.1002/eap.2227  

   Adam, Thomas C. ; Burkepile, Deron E. ; Holbrook, Sally J. ; Carpenter, Robert C. ; Claudet, Joachim ; Loiseau, Charles ; Thiault, Lauric ; Brooks, Andrew J. ; Washburn, Libe ; Schmitt, Russell J. ( October 2020 , Ecological Applications)

   Nutrient pollution is altering coastal ecosystems worldwide. On coral reefs, excess nutrients can favor the production of algae at the expense of reef‐building corals, yet the role of nutrients in driving community changes such as shifts from coral to macroalgae is not well understood. Here we investigate the potential role of anthropogenic nutrient loading in driving recent coral‐to‐macroalgae phase shifts on reefs in the lagoons surrounding the Pacific island of Moorea, French Polynesia. We use nitrogen (N) tissue content and stable isotopes (δ¹⁵N) in an abundant macroalga (Turbinaria ornata) together with empirical models of nutrient discharge to describe spatial and temporal patterns of nutrient enrichment in the lagoons. We then employ time series data to test whether recent increases in macroalgae are associated with nutrients. Our results revealed that patterns of N enrichment were linked to several factors, including rainfall, wave‐driven circulation, and distance from anthropogenic nutrient sources, especially human sewage. Reefs near large watersheds, where inputs of N from sewage and agriculture are high, have been consistently enriched in N for at least the last decade. In many of these areas, corals have decreased and macroalgae have increased, while reefs with lower levels of N input have maintained high cover of coral and low cover of macroalgae. Importantly, these patchy phase shifts to macroalgae have occurred despite substantial island‐wide increases in the density and biomass of herbivorous fishes over the time period. Together, these results indicate that nutrient loading may be an important driver of coral‐to‐macroalgae phase shifts in the lagoons of Moorea even though the reefs harbor an abundant and diverse herbivore assemblage. These results emphasize the important role that bottom‐up factors can play in driving coral‐to‐macroalgae phase shifts and underscore the critical importance of watershed management for reducing inputs of nutrients and other land‐based pollutants to coral reef ecosystems.

    

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