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Revista de Biología Tropical
On-line version ISSN 0034-7744Print version ISSN 0034-7744
Rev. biol. trop vol.58 suppl.3 San José Oct. 2010
Effect of severe hurricanes on Biorock Coral Reef Restoration Projects in Grand Turk, Turks and Caicos Islands
Lucy Wells1, Fernando Perez1, Marlon Hibbert1, Luc Clerveaux1, Jodi Johnson1 & Thomas J. Goreau2
1. Department of Environment and Coastal Resources (DECR), Grand Turk, Turks and Caicos Islands; lucydwells@gmail.com, FDPerez@gov.tc, MHibbert@gov.tc, lclerveaux@gmail.com, JJohnson@gov.tc
2. Global Coral Reef Alliance, Cambridge, Massachusetts, USA; goreau@bestweb.net
Abstract
Key words: Biorock, reef restoration, hurricanes, Turks and Caicos Islands.
Resumen
Con frecuencia no se favorece la creación de arrecifes artificiales en aguas someras debido a que se estima que las tormentas pueden producir daños en las estructuras y en el hábitat circundante. En las aguas de Grand Turk, a unos 5m de profundidad, se iniciaron proyectos de restauración de arrecifes coralinos en Oasis (octubre 2006) y en Governor´s Beach (noviembre 2007). Se utilizaron módulos de acero semicilíndricos, 4 en Oasis y 6 en Governor´s Beach. A cada proyecto se han trasplantado más de 1200 corales desde sitios con un elevado daño por sedimentación y se monitorean regularmente para evaluar crecimiento y mortalidad de los corales, así como la población de peces. Los corales muestran un crecimiento inmediato sobre los alambres utilizados para asegurar los corales. Este crecimiento se ha medido usando fotografías y un programa de computación y es más rápido en Governor´s Beach. Después de los huracanes Hanna e Ike (setiembre 2008), las estructuras en Governor´s Beach se mantuvieron erectas debido a que las olas pasaron a través de las mismas sin afectarlas, una de las estructuras en Oasis colapsó debido a que todas se amarraron con alambre en vez de soldarse (desde entonces se sustituyó con una nueva, soldada esta vez). El huracán Ike ha sido el más fuerte de los históricamente registrados que ha impactado Grand Turk. La mayor pare de los cables fueron reemplazados debido a daños causados por la acción de detritus y de las grandes olas. Los proyectos perdieron alrededor de un tercio de los corales debido a los huracanes. La mayoría de los corales perdidos habían sido amarrados a las estructuras unos días antes de los huracanes y consecuentemente no se habían adosado firmemente a las mismas. Estos proyectos han regenerado poblaciones de corales y peces en áreas inhóspitas de arena o roca y constituyen ahora una atracción para el buceo superficial. La alta tasa de supervivencia de corales y el bajo daño a las estructuras después de los huracanes indican que la tecnología Biorock para restauración de arrecifes coralinos es efectiva en áreas impactadas por tormentas.Palabras clave: Biorock, restauración de arrecifes coralinos, huracanes, Islas Turcas y Caicos
After storms artificial reefs have been broken or moved long distances from where they were deployed, up to more than a km (Turpin & Bortone 2002). A 155m long ship sunk at 40m depth, 10km offshore, was flipped upright and did damage to nearby reefs under only moderate wave conditions (Lukens & Selberg 2004). After Hurricane Andrew (1992) most south Florida artificial reefs, even those over 100km from the eye and up to 55m below the surface, suffered severe damage (Blair et al. 1994). For some one, two, or many pieces were found, often far from where they had been sunk, for many no trace was ever seen.
Massive concrete artificial reefs do not move as much as steel ship, airplane, car, or rubber tire reefs (Turpin & Bortone 2002), but they increase sediment scour and erosion by channelling currents around them. Scouring extends for a horizontal distance about equal to the height of the structure, and a vertical depth of half the height (Shyue & Yang 2002). These experiences suggest a strong need for artificial reefs in shallow water that are stable in strong wave forces and which do not accelerate erosion of sediments.
Need for reef restoration in TCI: The Turks and Caicos Islands have some of the best coral reefs in the Caribbean, and the strongest management and protection of them, with 17% of fringing barrier reefs protected (Carleton & Hambrey 2006). There is minimal pollution due to lack of industries, strong requirements that all developers treat their sewage and recycle all their wastewater as irrigation on their own property, and lack of rivers due to dry climate. Nevertheless the reefs have undergone decline, largely due to external factors like global warming and new diseases (Goreau et al. 2008).
The first phase of the implementation of Oasis pilot project (approximately June 2007) demonstrated that it was feasible in the local conditions of Grand Turk to restore coral reefs using Biorock technology. Consequently DECR budgeted funds to install a second project at Governor´s Beach, and the structures were built in late October 2007 and installed in November 2007.
The Biorock method uses low voltage, low amperage and direct currents to cause electrolysis of seawater. High pH is generated on cathodic structures of any size or shape, causing precipitation and growth of minerals dissolved in seawater. When growth is less than 2cm per year, minerals are primarily aragonitic limestone, with up to three times the compressive load bearing strength of concrete made from ordinary Portland cement. Under faster growth the material is predominantly softer magnesium hydroxide minerals, which convert to aragonite with age (Hilbertz 1979, 1992). Low current charging rate and slow mineral growth on the Grand Turk projects results in slow growth of hard materials, but may not accelerate coral growth as much as higher charging rates. Caribbean and IndoPacific corals typically show Biorock coral growth rates 2-6 times faster than controls (Goreau & Hilbertz 2005, 2008, 2009). Corals on Biorock in the Maldives had 16-50 times higher survival from bleaching in 1998 than corals on nearby reefs (Goreau et al. 2000), Very high spontaneous coral settlement is observed on slowly growing Biorock structures, but on fast growing structures they are overgrown by mineral growth. Therefore naturally broken, but still live, corals are transplanted onto Biorock to speed restoration. They are normally badly damaged by abrasion and sedimentation when transplanted, but heal very rapidly without releasing mucus, unlike controls. Visible skeletal growth is often visible within a day, and Biorock corals are more brightly colored and more densely branched than genetically identical controls, and feed more frequently during the daytime. Because Biorock is the only restoration method that greatly increases coral growth, healing from physical damage, settlement, and survival from high temperature and sedimentation stresses, it has unique advantages over any other method for keeping corals alive at locations affected by global warming and sedimentation, and restoring reefs in a few years in places where there is little or no recovery (Goreau & Hilbertz 2005, 2007, 2008, 2009).
Materials and methods
2) Governor´s Beach: Six modules were the same as Oasis but re-bars were welded instead of tie-wired. Modules are approximately 4.5m depth and 170m from the power supplies. Average current ranges from 8 to 24Amps, or 0.13 to 0.4A/m2. These current densities are only 3% to 25% of those recommended for maximum growth rate of strong structural material, so they are growing much more slowly than typical Biorock projects.
wave action. Coral growth and survival is documented photographically, and analyzed by a photograph area measurement program. Results of these long-term measurements will be presented elsewhere: this paper is to focuses on short-term hurricane responses. Galleries of photos of these projects are posted at:
http://www.flickr.com/photos/22251472@N04/sets/72157605703509121/
http://www.flickr.com/photos/22251472@N04/sets/72157605689429965/
http://www.flickr.com/photos/22251472@N04/sets/72157605689429965/
http://www.flickr.com/photos/22251472@N04/sets/72157606328793504/
Results
Physical Hurricane impacts on Grand Turk, maps of hurricane tracks: In 2008 Grand Turk suffered the worst hurricane damage on record. Hurricane Hanna hit August 31 to September 5, with the eye turning in a circle over TCI and causing prolonged wave damage from all directions. Two days later (September 7) Grand Turk was hit by the eye of Hurricane Ike, a category 4 hurricane, the strongest on record in TCI. Ike moved fast and straight, and although more severe in wind strength, the prolonged impact of Hanna may have caused more wave damage. Detailed tracks of these hurricanes and their strength are available at: http://www.nhc.noaa.gov/2008atlan.shtmlPhysical damage on land: Grand Turk suffered the worst hurricane damage in its history. Most wind damage came from Ike, but most wave damage on western Grand Turk was likely due to Hanna. At least 80% of all buildings on Grand Turk were destroyed or damaged (UNEP/OCHA 2008).
Several dives were made on Biorock projects in the weeks after the hurricanes. Sedimentation was not high as expected. Although local divers reported turbidity levels were elevated and persistent in some areas of Providenciales, West Caicos, French Cay, and West Sand Spit, dives in Grand Turk after the Hurricane Ike did not show a high level of turbidity (Fig. 1).
Three out of four power supplies were fully functioning after the hurricanes. Due to extensive cable damage both projects were without power whilst the cables were repaired. Most cables were replaced following the hurricanes due to damage from debris and high wave action. Despite the collapsed structure at Oasis, the anodes were still in place in the centre of the structure. At Governor´s Beach major substrate shifting was observed and cinder blocks used to hold the anode in place were buried in sand, covering the anode. Otherwise very little damage was sustained to the anode. Damage to the cables was more pronounced further inshore where greater wave action was observed, and cables were replaced where sand blasting by hurricane waves had stripped off the insulation. Details on repairs and on monitoring procedures can be found at the Turks and Caicos Biorock Maintenance Manual (Wells et al. 2009) at: http://www.environment.tc/content/root/ files/20090414105717-BiorockGuidelines- GrandTurk.pdf
After the hurricanes the Governor´s Beach structure was fully erect since the waves passed straight through with little damage. The Oasis structures, tie-wired rather than welded, had one module sag open in place (since replaced with a new, welded structure). However this structure was still intact, and when rewired, the corals continued to grow on it (Fig. 3). The projects lost about 30% to 40% of corals due to the hurricanes. Most of those lost had only been wired a few days before and had not yet attached themselves firmly. However, high coral survival and low structural damage after hurricanes indicate that Biorock reef restoration can be effective in storm-impacted areas. Photographs of both projects before and after the hurricanes are shown in Fig. 4 and Fig. 5.
Discussion
Only a minority of corals (about 30-40%) were lost. The vast majority of corals had been transplanted onto the structure only a few weeks before the hurricanes, and many were very loosely attached and had not yet had time to be solidly cemented on. The low level of loss and damage is remarkable given the poor history of survival of artificial reefs in storms and hurricanes. On the other hand the relatively high level of disease seen on the Governor´s Beach structure may have been caused by physical injury to coral tissues from sand blasting abrasion, allowing the entry of pathogenic bacteria. The Oasis structures, deeper and further from shore, may have much less disease because of less tissue injury from sand, or alternatively, a locally lower abundance of pathogenic bacteria. Because Biorock corals show exceptionally rapid healing of tissue injury, it is hard to distinguish these alternatives, since corals looked completely healed on the structures by the first opportunity DECR staff had to get into the water after the hurricane.
The exceptional survival and low damage of these structures despite their shallow location near shore, and not anchored to the bottom in any way, is in marked contrast to heavy damage and breakage seen in conventional artificial reefs even in deep waters. To our knowledge this is the first report of high survival of corals in a shallow artificial reef following a severe hurricane. The minimal damage to the structure and the corals is certainly due to the open framework that allowed waves to pass through them. The drag equation states that the surface drag force on an object in the direction of the flow, Fd=0.5CdDAV2 where Cd is the drag coefficient, D is the density of the fluid, A is the cross sectional area perpendicular to the flow vector, and V is the velocity. The drag coefficient is very low for thin cylinders oriented at right angles to the flow, but is very high for solid planar objects facing the flow. A flat object oriented perpendicular to the flow has a drag coefficient about a thousand times greater than one oriented parallel to the flow. That is why sunken ships, and closed steel or massive concrete artificial reefs, which have a high solid area cross section are so badly ripped apart, scoured, and moved in even moderate storms well below hurricane forces, and huge coral boulders, meters across, can be tossed on top of cliffs by hurricanes. Because the volume of the Biorock structure is more than 99% water, the actual solid material area cross section is very small compared to a solid object with the same dimensions. By careful attention to design that minimizes drag forces, our results show that viable artificial reefs can be constructed in shallow water and survive severe forces that would destroy conventional materials. This opens up many new possibilities for coral reef and fisheries habitat restoration structures in shallow habitats where they were previously impossible.
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Fernando Perez. Department of Environment and Coastal Resources (DECR), Grand Turk, Turks and Caicos Islands; FDPerez@gov.tc,
Marlon Hibbert. Department of Environment and Coastal Resources (DECR), Grand Turk, Turks and Caicos Islands; MHibbert@gov.tc
Luc Clerveaux. Department of Environment and Coastal Resources (DECR), Grand Turk, Turks and Caicos Islands; lclerveaux@gmail.com
Jodi Johnson. Department of Environment and Coastal Resources (DECR), Grand Turk, Turks and Caicos Islands; JJohnson@gov.tc
Thomas J. Goreau. Global Coral Reef Alliance, Cambridge, Massachusetts, USA;goreau@bestweb.net
Received 15-VII -2009. Corrected 19-VI-2010. Accepted 12-VII -2010.