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At least 361 records · Page 20Linked to original sources

USGS2013

The influence of sea level rise and changes in fringing reef morphology on gradients in alongshore sediment transport

Climate‐change‐induced alterations to coral reef ecosystems, in combination with sea level rise, have the potential to significantly alter wave dissipation across reefs, leading to shifts in alongshore sediment transport gradients and alterations to tropical coastlines. We used Delft3D to model schematized profiles of two reef flat widths based on the south Molokai, Hawaii coast. Simulated anthropogenic modifications include incremental degradation of the reef structure as well as sea level rise. Our findings indicate that sea level rise has a greater relative effect on wave energy flux and alongshore sediment transport over a wide flat, whereas both reef degradation and sea level rise exert similar influence over a narrow flat. These results suggest reefs that vary in width alongshore are more likely to experience changes in alongshore sediment transport gradients, and therefore shifts in shoreline erosion and accretion patterns, than more uniform reef systems.

Hawai'i↗

USGS2008

Utility of shallow-water ATRIS images in defining biogeologic processes and self-similarity in skeletal scleractinia, Florida reefs

A recently developed remote-sensing instrument acquires high-quality digital photographs in shallow-marine settings within water depths of 15 m. The technology, known as the Along-Track Reef-Imaging System, provides remarkably clear, georeferenced imagery that allows visual interpretation of benthic class (substrates, organisms) for mapping coral reef habitats, as intended. Unforeseen, however, are functions new to the initial technologic purpose: interpr??table evidence for real-time biogeologic processes and for perception of scaled-up skeletal self-similarity of scleractinian microstructure. Florida reef sea trials lacked the grid structure required to map contiguous habitat and submarine topography. Thus, only general observations could be made relative to times and sites of imagery. Degradation of corals was nearly universal; absence of reef fish was profound. However, ???1% of more than 23,600 sea-trial images examined provided visual evidence for local environs and processes. Clarity in many images was so exceptional that small tracks left by organisms traversing fine-grained carbonate sand were visible. Other images revealed a compelling sense, not yet fully understood, of the microscopic wall structure characteristic of scleractinian corals. Conclusions drawn from classifiable images are that demersal marine animals, where imaged, are oblivious to the equipment and that the technology has strong capabilities beyond mapping habitat. Imagery acquired along predetermined transects that cross a variety of geomorphic features within depth limits will ( 1) facilitate construction of accurate contour maps of habitat and bathymetry without need for ground-truthing, (2) contain a strong geologic component of interpreted real-time processes as they relate to imaged topography and regional geomorphology, and (3) allow cost-effective monitoring of regional- and local-scale changes in an ecosystem by use of existing-image global-positioning system coordinates to re-image areas. Details revealed in the modern setting have taphonomic implications for what is often found in the geologic record.

Conference Paper↗

USGS2008

Phase shift from a coral to a corallimorph-dominated reef associated with a shipwreck on Palmyra atoll

Coral reefs can undergo relatively rapid changes in the dominant biota, a phenomenon referred to as phase shift. Various reasons have been proposed to explain this phenomenon including increased human disturbance, pollution, or changes in coral reef biota that serve a major ecological function such as depletion of grazers. However, pinpointing the actual factors potentially responsible can be problematic. Here we show a phase shift from coral to the corallimorpharian Rhodactis howesii associated with a long line vessel that wrecked in 1991 on an isolated atoll (Palmyra) in the central Pacific Ocean. We documented high densities of R. howesii near the ship that progressively decreased with distance from the ship whereas R. howesii were rare to absent in other parts of the atoll. We also confirmed high densities of R. howesii around several buoys recently installed on the atoll in 2001. This is the first time that a phase shift on a coral leef has been unambiguously associated with man-made structures. This association was made, in part, because of the remoteness of Palmyra and its recent history of minimal human habitation or impact. Phase shifts can have long-term negative ramification for coral reefs, and eradication of organisms responsible for phase shifts in marine ecosystems can be difficult, particularly if such organisms cover a large area. The extensive R. howesii invasion and subsequent loss of coral reef habitat at Palmyra also highlights the importance of rapid removal of shipwrecks on corals reefs to mitigate the potential of reef overgrowth by invasives.

Palmyra Atoll↗

USGS2024

Modern coral range expansion off southeast Florida falls short of Late Holocene baseline

As thermal stress and disease outbreaks decimate coral reefs throughout the tropics, there is growing evidence that higher latitude marine environments may provide crucial refuges for many at-risk, temperature-sensitive coral species. However, our understanding of how coral populations expand into new areas and sustain themselves over time is constrained by the limited scope of modern observations. Here, we provide geological insights into coral range expansions by reconstructing the composition of a Late Holocene-aged subfossil coral death assemblage on the southeast Florida reef tract and comparing it to modern reefs throughout the region. Our findings show that the Late Holocene coral assemblages were dominated by now critically endangered Acropora species between ~3500 and 1800 years before present, mirroring classic zonation patterns characteristic of healthy pre-1970s Caribbean reefs. In contrast, the modern reefs off southeast Florida are becoming increasingly dominated by stress-tolerant species like Porites astreoides and Siderastrea siderea despite modest expansions of Acropora cervicornis over the past several decades. Our results suggest that ongoing anthropogenic stressors, not present during the Late Holocene, are likely limiting the ability of modern higher latitude reefs in Florida to function as long-term climate refugia.

Florida↗

USGS2021

Viral-like particles are associated with endosymbiont pathology in Florida corals affected by stony coral tissue loss disease

Stony coral tissue loss disease (SCTLD) was first documented in 2014 near the Port of Miami, Florida, and has since spread north and south along Florida’s Coral Reef, killing large numbers of more than 20 species of coral and leading to the functional extinction of at least one species, Dendrogyra cylindrus . SCTLD is assumed to be caused by bacteria based on presence of different molecular assemblages of bacteria in lesioned compared to apparently healthy tissues, its apparent spread among colonies, and cessation of spread of lesions in individual colonies treated with antibiotics. However, light microscopic examination of tissues of corals affected with SCTLD has not shown bacteria associated with tissue death. Rather, microscopy shows dead and dying coral cells and symbiotic dinoflagellates (endosymbionts) indicating a breakdown of host cell and endosymbiont symbiosis. It is unclear whether host cells die first leading to death of endosymbionts or vice versa. Based on microscopy, hypotheses as to possible causes of SCTLD include infectious agents not visible at the light microscopy level or toxicosis, perhaps originating from endosymbionts. To clarify this, we examined corals affected with SCTLD and apparently healthy corals using transmission electron microscopy. Endosymbionts in SCTLD-affected and apparently healthy corals consistently had varying degrees of pathology associated with elongated particles compatible in morphology with filamentous positive single-stranded RNA viruses of plants termed anisometric viral-like particles (AVLP). There was apparent progression from early to late replication of AVLP in the cytoplasm of endosymbionts adjacent to or at times within chloroplasts, with morphologic changes in chloroplasts consistent with those seen in plant cells infected by viruses. Coral host cell pathology appeared limited to massive proliferation and lysis of mucus cells. Based on these findings, we hypothesize that SCTLD is a viral disease of endosymbionts leading to coral host death. Efforts to confirm the presence of a virus associated with SCTLD through other means would be appropriate. These include showing the presence of a virus through molecular assays such as deep sequencing, attempts to grow this virus in the laboratory through culture of endosymbionts, localization of virus in tissue sections using immunohistochemistry or in situ hybridization, and experimental infection of known-virus-negative corals to replicate disease at the gross and microscopic level.

Florida↗

USGS1994

Hurricanes and anchors: preliminary results from the National Park Service regional reef assessment program

The U .S . National Park Service NPS began a Regional Assessment Program for coral reefs in the U.S. Virgin Islands and Florida in 1988. Scientists from NPS and six other institutions have now established longterm monitoring sites at Virgin Islands National Park St. John, USVI, Buck Island Reef National Monument St. Croix, USVI, Biscayne National Park Florida and Fort Jefferson National Monument Florida. Hurricane Hugo passed through the USVI in 1989, causing severe destruction in some reef areas while leaving others untouched. Patchy damage to reefs in Florida was also noted after Hurricane Andrew; damage from this August 1992 storm is still being assessed. Fort Jefferson National Monument escaped the onslaught of Andrew. No significant recovery in live coral cover has been evident at the Buck Island or Virgin Islands National Park VINP study sites 3.5 years after Hurricane Hugo. Similarly, no recovery was evident at another site in St. John which was destroyed by a large anchor 4.5 years ago.

Conference Paper↗

USGS2010

Terrigenous sediment provenance from geochemical tracers, south Molokai reef flat, Hawaii

Land-derived runoff is one of the greatest threats to coral-reef health. Identification of runoff sources is an important step in erosion mitigation efforts. A geochemical sediment provenance study was done in uplands and across the adjacent fringing reef on the southeast shore of Molokai, Hawaii, to determine whether sediment runoff originated from hillsides or gulches. Source-region identification was based on geochemical differences between alkalic basalt, which outcrops on hillsides, and tholeiitic basalt, which outcrops in gulches. In Kawela watershed, copper to iron ratios (Cu/Fe) were distinct in hillside soil versus gulch sediment and suggest that hillside erosion is the predominant mechanism of sediment delivery to the nearshore. This suggests that runoff-mitigation efforts should take steps to reduce hillside erosion. Cadmium to thorium ratios (Cd/Th) in nearshore sediment suggest that there is a high-Cd source of runoff east of Kamalo Gulch. This compositional difference is consistent with the predominance of tholeiitic basalt on the eastern end of Molokai.

Open-File Report↗

USGS1986

Age of -360-m reef terrace, Hawaii, and the rate of late Pleistocene subsidence of the island

Observations from a manned submersible vehicle indicate that the −360-m reef terrace northwest of the island of Hawaii is a drowned coral reef. The preferred uranium-series age of coralline algae collected from the reef face is 120 ± 5 ka. This age agrees with the notion that the reef was drowned during the sea-level rise following the major lowstand of the sea that occurred at 145 ka (oxygen isotope stage 6). This drowning pattern is similar to the previously determined radiocarbon age of 13 ka for drowning of the −150-m reef off west Hawaii, which drowned during the sea-level rise following the last major lowstand of the sea at 18 ka (oxygen isotope stage 2). Estimated average subsidence of the reef site off northwest Hawaii is 2.7 mm/yr since final drowning of the −360-m reef.

Hawaii↗

USGS2022

Smoothed Particle Hydrodynamics simulations of reef surf zone processes driven by plunging irregular waves

As waves interact with the slopes of coral reefs and other steep bathymetry profiles, plunging breaking usually occurs where the free surface overturns and violent water motion is triggered. Resolving these surf zone processes pose significant challenges for conventional mesh-based hydrodynamic models, due to the rapidly-deforming nature of the free surface and associated flows. Yet the accurate prediction of these surf zone hydrodynamics is critical for predicting a wide range of nearshore processes driven by wave breaking (e.g., wave dissipation and energy transfers; mean water levels and currents; and wave runup). In this study we assess the ability of the mesh-free, Lagrangian particle-based numerical modelling approach Smoothed Particle Hydrodynamics (SPH) based on DualSPHysics, to simulate the fine-scale hydrodynamic processes driven by irregular wave transformation over a fringing reef profile, by comparing results against detailed experimental observations from a physical modelling study. To greatly improve the computational efficiency, the SPH model was coupled to the mesh-based multi-layer nonhydrostatic wave-flow model SWASH. With this coupled approach, SWASH was used to efficiently simulate the evolution of non-breaking waves from the wavemaker up to the fore reef slope, with the SPH model then used to simulate the detailed hydrodynamic processes over the reef from just offshore of the breakpoint to the shoreline . The SPH model was able to accurately reproduce the complex free surface deformations during plunging breaking, the spectral evolution of waves across the reef flat (including nonlinear wave shape), the mean water levels and currents, and wave runup at the shoreline. Using the long duration simulations (>400 wave periods), the model was able to reproduce the full range of wave motions over the reef (from sea-swell to infragravity frequencies), including the increasing dominance of low frequency waves towards the shoreline and the large cross-reef standing wave motions excited by the reef geometry.

Ocean Modelling↗

USGS2025

Benthic habitat map of Olowalu Reef, Maui, Hawaii—Geomorphological structure, biological cover, and geologic zonation determined with spectral, lidar, and acoustic data

The fringing coral reef off Olowalu, Maui, Hawaii, has been identified as a local conservation priority site. In 2007, the National Oceanic and Atmospheric Administration (NOAA) produced a benthic habitat map of the Hawaiian Islands that was used as a foundation for this study. To support place-based management of the reef in the future, the U.S. Geological Survey (USGS) mapped the geologic zone, major and dominant geomorphological structure, biological cover type, and percent of biological cover for 11 square kilometers (km 2 ) of Olowalu Reef at a minimum mapping unit (MMU) of 100 square meters (m 2 ) to create a benthic habitat map. Heads-up digitization was employed on 0.50-meter (m) natural color satellite orthoimagery with ancillary 1-m acoustic backscatter imagery from single-scan sonar (sound navigation and ranging). A 1-m, 4-m, and 8-m digital bathymetric model (DBM) was interpolated from bathymetric lidar (light detection and ranging), and various geomorphometric layers derived from the DBMs were used for habitat interpretation. Still-frame imagery of the seafloor extracted from vessel-towed underwater video transects on Olowalu Reef served as ground validation points ( n =870) during active mapping and accuracy assessment points ( n =216) for thematic accuracy assessment. Thematic accuracy was cross-validated by the Hawai‘i Department of Land and Natural Resources Division of Aquatic Resources. Final thematic accuracy was 88.8 percent for major structure, 85.6 percent for dominant structure, 86.0 percent for major biological cover, and 78.6 percent for type and percent of major biological cover. Reef and hardbottom constituted 52 percent of the total mapped habitat, comprising mostly aggregate reef (31 percent) and pavement (11 percent), with large swaths of spur-and-groove (9 percent). Of this hardbottom, 17 percent was covered with moderate (10 to <50 percent) coral and 27 percent with high coral cover (50 to <90 percent). High (50 to <90 percent) macroalgae cover dominated the continuous sand sheets in offshore bank/shelf zones. The map created in this study supplements the NOAA 2007 map and expands on the observations made by USGS sampling of the reef. The NOAA 2007 map and our map differed in total areal extent by a negligible 6 m 2 and were in general thematic agreement. Our map is intended to serve as a baseline for public access, general research, local-level management, and reef change for future studies.

Hawaii↗

USGS2025

Land-based nutrient flux to a fringing reef: Insights from Ofu Island, American Samoa

Submarine groundwater discharge (SGD) is a critical driver of nutrient transport in coral reef ecosystems, shaping water quality, primary productivity, and overall reef health. This study quantifies SGD fluxes and associated nutrient dynamics in two reef flat pools within the Ofu Unit of the National Park of American Samoa: Papaloloa and Fatuana. A multi-method approach integrating unoccupied aerial system-based thermal infrared (UAS-TIR) surveys, radon-based SGD measurements, multichannel electrical resistivity tomography (ERT), and discrete water sampling was used to assess SGD rates and nutrient contributions. UAS-TIR imagery revealed cooler sea surface temperatures in both pools, indicative of SGD, with the higher fluxes observed in Papaloloa. Radon measurements revealed a strong inverse correlation between SGD rates and tidal stage, with a more immediate SGD response at Papaloloa due to its highly permeable calcareous sand and gravel substrate. In contrast, a 2–3-hour lag in SGD response at Fatuana suggests discharge from a more inland aquifer that has lower diffusivity. Nutrient concentrations correlated with temperature and salinity, confirming SGD as the dominant nutrient transport mechanism, whereas isotopic analyses indicated inputs from both groundwater and potential anthropogenic sources. Despite lower SGD flux at Fatuana, higher algal cover suggests additional factors influencing algal proliferation, including substrate availability and hydrodynamic conditions. Excess nutrient inputs from SGD may contribute to algal overgrowth, which threatens Ofu’s thermally tolerant corals by increasing competition for space and light. These findings underscore the complexity of SGD-mediated nutrient dynamics in reef environments and emphasize the need for integrated hydrological and ecological assessments to support effective reef conservation and management strategies.

American Samoa, Ofu Island, Olosega Island↗

USGS2024

Stony coral tissue loss disease indirectly alters reef communities

Many Caribbean coral reefs are near collapse due to various threats. An emerging threat, stony coral tissue loss disease (SCTLD), is spreading across the Western Atlantic and Caribbean. Data from the U.S. Virgin Islands reveal how SCTLD spread has reduced the abundance of susceptible coral and crustose coralline algae and increased cyanobacteria, fire coral, and macroalgae. A Caribbean-wide structural equation model demonstrates versatility in reef fish and associations with rugosity independent of live coral. Model projections suggest that some reef fishes will decline due to SCTLD, with the largest changes on reefs that lose the most susceptible corals and rugosity. Mapping these projected declines in space indicates how the indirect effects of SCTLD range from undetectable to devastating.

Virgin Islands↗

USGS2021

Global tropical reef fish richness could decline by around half if corals are lost

Reef fishes are a treasured part of marine biodiversity, and also provide needed protein for many millions of people. Although most reef fishes might survive projected increases in ocean temperatures, corals are less tolerant. A few fish species strictly depend on corals for food and shelter, suggesting that coral extinctions could lead to some secondary fish extinctions. However, secondary extinctions could extend far beyond those few coral-dependent species. Furthermore, it is yet unknown how such fish declines might vary around the world. Current coral mass mortalities led us to ask how fish communities would respond to coral loss within and across oceans. We mapped 6964 coral-reef-fish species and 119 coral genera, and then regressed reef-fish species richness against coral generic richness at the 1° scale (after controlling for biogeographic factors that drive species diversification). Consistent with small-scale studies, statistical extrapolations suggested that local fish richness across the globe would be around half its current value in a hypothetical world without coral, leading to more areas with low or intermediate fish species richness and fewer fish diversity hotspots.

Proceedings of the Royal Society B↗

USGS2017

Sediment transport in the presence of large reef bottom roughness

The presence of large bottom roughness, such as that formed by benthic organisms on coral reef flats, has important implications for the size, concentration, and transport of suspended sediment in coastal environments. A 3 week field study was conducted in approximately 1.5 m water depth on the reef flat at Ningaloo Reef, Western Australia, to quantify the cross-reef hydrodynamics and suspended sediment dynamics over the large bottom roughness (∼20–40 cm) at the site. A logarithmic mean current profile consistently developed above the height of the roughness; however, the flow was substantially reduced below the height of the roughness (canopy region). Shear velocities inferred from the logarithmic profile and Reynolds stresses measured at the top of the roughness, which are traditionally used in predictive sediment transport formulations, were similar but much larger than that required to suspend the relatively coarse sediment present at the bed. Importantly, these stresses did not represent the stresses imparted on the sediment measured in suspension and are therefore not relevant to the description of suspended sediment transport in systems with large bottom roughness. Estimates of the bed shear stresses that accounted for the reduced near-bed flow in the presence of large roughness vastly improved the relationship between the predicted and observed grain sizes that were in suspension. Thus, the impact of roughness, not only on the overlying flow but also on bed stresses, must be accounted for to accurately estimate suspended sediment transport in regions with large bottom roughness, a common feature of many shallow coastal ecosystems.

Journal of Geophysical Research C: Oceans↗

USGS1989

Artificial reef observations from a manned submersible off southeast Florida

Examination of 16 artificial reef structures in depths ranging from 30-120m indicated that the highest numbers of fish are found around reefs in water shallower than 46m. Fewer fish, especially those with tropical coral reef affinities, <46m was probably caused by a thermocline. Algae and reef community encrusters, abundant on shallower structures, were absent below 46m. Structures that penetrated above the thermocline, such as upright oil rigs and a hopper barge, were also effective reefs. The open structure and high profile of the rigs enhance their use as artificial reefs by providing a range of well-aerated habitats. Greatest diversity and numbers of fish were observed at the Miami sewer outfall. -from Authors

Bulletin of Marine Science↗

USGS2016

Disease prevalence and snail predation associated with swell-generated damage on the threatened coral, Acropora palmata (Lamarck)

Disturbances such as tropical storms cause coral mortality and reduce coral cover as a direct result of physical damage. Storms can be one of the most important disturbances in coral reef ecosystems, and it is crucial to understand their long-term impacts on coral populations. The primary objective of this study was to determine trends in disease prevalence and snail predation on damaged and undamaged colonies of the threatened coral species, Acropora palmata, following an episode of heavy ocean swells in the US Virgin Islands (USVI). At three sites on St. Thomas and St. John, colonies of A. palmata were surveyed monthly over 1 year following a series of large swells in March 2008 that fragmented 30–93% of colonies on monitored reefs. Post-disturbance surveys conducted from April 2008 through March 2009 showed that swell-generated damage to A. palmata caused negative indirect effects that compounded the initial direct effects of physical disturbance. During the 12 months after the swell event, white pox disease prevalence was 41% higher for colonies that sustained damage from the swells than for undamaged colonies (df = 207, p = 0.01) with greatest differences in disease prevalence occurring during warm water months. In addition, the corallivorous snail, Coralliophila abbreviata, was 46% more abundant on damaged corals than undamaged corals during the 12 months after the swell event (df = 207, p = 0.006).

Frontiers in Marine Science↗

USGS2025

Acute heat stress and the extirpation of a threatened coral species from a remote, subtropical reef system

The ecological significance of the reef-building elkhorn coral, Acropora palmata , is threatened by heat-stress-induced mortality. The intensity and duration of the ocean heatwave affecting Dry Tortugas National Park in the summer of 2023 was historically unprecedented in its early timing and maximum temperatures reached and resulted in 100% A. palmata mortality. To understand the lethality of this event, we examined temperature data measured by in situ data loggers and estimated from satellites (National Oceanic and Atmospheric Administration’s Coral Reef Watch) to investigate the timing of peak ocean temperatures and coral mortality. The in situ dataset revealed warmer water temperatures during the heatwave than those reported from the satellites, and the difference between the datasets was significantly pronounced during the summer. Our results support that, at least for this subtropical population, A. palmata may have an upper-threshold temperature that caused lethality faster than expected from the gradual accumulation of heat stress.

Florida↗

USGS2022

Biofilms as potential reservoirs of stony coral tissue loss disease

Since 2014, corals throughout Florida’s Coral Reef have been plagued by an epizootic of unknown etiology, colloquially termed stony coral tissue loss disease (SCTLD). Although in Florida the movement of this waterborne coral disease has been consistent with natural transport via water currents, outbreaks in the Caribbean have been more sporadic, with infections occurring in locations inconsistent with spread via natural means. Often Caribbean outbreaks have been clustered near ports, potentially implicating ships as mediators of SCTLD into new regions. Biofilms attached to ship hulls, ballast tank walls, or other surfaces could represent a possible vector for the disease. We investigated whether bacteria shed by healthy and SCTLD-diseased corals would form distinct biofilms, and whether a SCTLD signal would be detectable within biofilm bacterial communities. Stainless steel plates serving as proxies for ship hulls, ballast tank walls, and other colonizable surfaces were incubated for three days in filtered seawater mesocosms containing healthy or SCTLD-infected corals. Resulting biofilm bacterial communities were characterized through sequencing of the V4 region of the 16S rRNA gene. We determined that bacteria shed by healthy and diseased corals formed significantly different biofilms consisting of highly diverse taxa. Comparison with 16S data from previous SCTLD investigations spanning different coral species, collection locations, years, and source material revealed the presence of numerous genetically identical sequences within the biofilm bacterial communities formed during exposure to SCTLD-infected corals, including several previously identified as possible SCTLD bioindicators. These results suggest ship-associated biofilms may have the potential to be vectors for the transmission of SCTLD into new regions.

Frontiers in Marine Science↗