ABSTRACT
Jamaican coral reefs are undergoing accelerating deterioration wherever human activity physically disturbs reefs, degrades water quality, or overharvests key species (Goreau 1992)� Many reefs no longer function as vital ecosystems: the coral-dominated wave-resistant upward-growing structures are turning into benthic ecosystems with a minor component of isolated corals� These ecosystems are coral communities rather than coral reefs, because biodiversity is severely degraded and the reef structure, being bioeroded faster than it grows, is less able to protect shorelines, keep up with rising sea level, or provide shelter and food for the many other organisms that live between corals� Degraded reefs have fleshy algae dominant over calcareous algae and can no longer provide beach sand to replenish that lost to erosion after damaged reef crests allow increased wave energy to reach the shore�
Several types of “artificial reefs” have been built as wave-resistant barriers and hiding places for fish (Goodwin and Cambers 1983)� They have a poor record, because structures built from steel, poured concrete, stone, concrete blocks, gabions (wire baskets containing rocks), sandbags, sunken ships, wrecked airplanes, or old automobiles unavoidably rust, corrode, and are broken by waves� Their fate is ultimate destruction by storms, requiring expensive and inevitably futile replacement� They turn into dangerous projectiles in hurricanes� After hurricane Andrew hit southern Florida,
CONTENTS
Introduction �������������������������������������������������������������������������������������������������������������������������������������� 35 Materials and Methods ��������������������������������������������������������������������������������������������������������������������36 Results ���������������������������������������������������������������������������������������������������������������������������������������������� 38 Discussion ���������������������������������������������������������������������������������������������������������������������������������������� 43 Acknowledgments ����������������������������������������������������������������������������������������������������������������������������44 References ���������������������������������������������������������������������������������������������������������������������������������������� 45
a survey of “artificial reefs” found that all had moved� Some had one or several fragments found, but many had vanished entirely� Although fish will hide behind any underwater obstacle, hard corals will not colonize them for a very long time, if ever, and they are mainly colonized by soft corals and sponges rather than reef-building corals� “Artificial reefs” made of automobiles (a popular excuse for creating marine junkyards) rust and break apart before corals will settle on them (Goodwin and Cambers 1983)� The failure of exotic materials to instigate natural hard coral reefs is caused by unsuitable surface chemistry and leaching of toxic hydrocarbons and metals from engines, paints, plastic fillers, concrete, and steel�
A novel technology, developed by architect W� Hilbertz in the 1970s, uses electrolysis of seawater to precipitate calcium and magnesium minerals to “grow” a crystalline coating over artificial structures to make construction materials (Hilbertz 1975, 1979)� The mineral accretion, largely aragonite (CaCO3) and brucite (Mg(OH)2), is very similar in chemical and physical properties to reef limestones (Hilbertz 1992), which are primarily the remains of the aragonite skeletons of corals and green calcareous algae� This chapter describes the results of work done in Jamaica since 1988 building and growing mineral-accretion artificial reefs for enhanced coral growth and reef restoration (Figure 4�1)�
MATERIALS AND METHODS
Electrolysis of seawater results in mineral deposition at the cathode� The physical properties of the material depend on mineralogy and crystal size, functions of deposition rate, and electrical current parameters� Higher current densities result in faster growth but weaker material dominated by brucite, while lower current densities produce slower deposition dominated by harder aragonite (Hilbertz 1992)� Mineral accretion materials have a mechanical strength comparable to, and often greater than, concrete (Hilbertz 1979)�
Deposition of minerals results from alkaline conditions created at the cathode by the reduction reaction:
2H O 2e H 2OH2 2+ = +− −
which precipitates calcium and magnesium minerals from seawater:
OH HCO Ca CaCO H O3 3 2− +++ + =− +
2OH Mg Mg(OH)2− +++ =
In contrast, the anode becomes acidic due to
2H O 4H O 4e2 2= + ++ −
and highly oxidizing conditions result in
2Cl Cl 2e2− −= +
The sum of the net reactions at both electrodes should be neutral with regard to hydrogen ion production, and hence with regard to CO2 generation through acid-base equilibrium and carbonic acid hydrolysis:
2HCO CO CO H O3 2 2 − −
= + +3 2
Cathodes and anodes can be made in any size and shape, with current flow dependent on their spacing and surface area� Typically, the cathode is built out of expanded steel mesh constructed as simple geometric forms such as cylinders, sheets, triangular prisms, or pyramids, but we have also molded complex forms using square mesh or chicken-wire mesh� An experimental reverse catenary was even built supported by floating spheres-that is, a buoyant metal chain structure fixed to a cathodic base plate� This structure was initially flexible and became rigid with progressing mineral accretion� Other new applications include molding shapes out of powdered sand or other materials, containing a cathode to enhance cementation by mineral accretion�
Pilot artificial reefs have ranged up to 3 m high and 10 m across, but there is no theoretical limit on their size, provided sufficient current density is applied� Anodes are typically much smaller than cathodes and shaped as sheets, rods, or mesh, depending on the materials used� Cathode materials are entirely protected from rusting by reducing conditions, whereas anodes are subjected to rapid oxidation unless resistant material is used� We have used a wide variety of anode materials, including lead, graphite, and steel, but had best results with specially coated titanium (Figure 4�2)�
Although any direct-current source will work, our preference is to use solar-and wind-generated power rather than alternating current generated from renewable fossil fuels that pollute the atmosphere with CO2 and acid rain� Current is applied across the terminals from a variety of power sources� We have empirically found it best to use lower voltages and higher currents� We have used transformers and battery chargers at both 12 and 6 V, photovoltaic panels in a direct-charge mode at a range of voltages between 3�8 and 17 V, and have plans to use windmill-generated current as well� Electricity consumption of each structure is equivalent to a single light bulb� These current levels are entirely safe to swimmers and divers, and it is possible to feel only a slight tingle when one directly short-circuits the current by touching both anode and cathode simultaneously with bare hands� Electrical currents are transmitted using insulated copper cables, either mono-or multistrand� Anode cable connectors are protected by clear silicone to detect the green color formed if salt water corrodes the electrical contact�
Small pieces of corals were transplanted onto the structures and attached with plastic ties, iron wire, or monofilament line, or simply allowed to sit on them� These corals largely consisted of fragments, which had been naturally broken by storms and damaged by anchors, divers, or spearfishermen, or corals whose bases were so bioeroded that they would be broken by storms, as well as
small pieces of branching corals from nearby “control” colonies� Most species of Caribbean corals have been tried (see Table 4�1 later in the chapter)� Only corals were attached, but these included some epifaunal sponges, calcareous algae, and other organisms on their undersides�
Artificial reef structures have been built in depths ranging from 0�5 to 7 m, in locations ranging from extremely protected back reef sites, open sites on the leeward western end of the island, to open shores fully exposed to the direct impact of winter northers� One structure, located in a depth of 1�5 m, continues to work despite being exposed to breaking waves that can reach up to 7 m high� They have been built on seagrass beds, limestone hard ground, white sand, and mud bottoms� We also built control structures receiving different current levels or no current at all and structures that were allowed to accrete for a period of time and were then turned off� In addition, we have connected corals growing in situ directly to current sources via wires leading to artificial reefs�
RESULTS
Crystal growth and hydrogen gas bubbling began as soon as rust on the steel had been reduced to iron� The surface changes from red to black to gray and then white as minerals grow on it� Minerals accreted to a thickness of up to 20 cm over three years� Iron and steel remain bright and shiny as long as sufficient electrical current flows to maintain cathodic protection� They are protected from corrosion by overlying mineral layers after the current is turned off, unless this coating is broken�
Structures on limestone hard ground became solidly cemented onto it, while those on sand and mud remain loosely attached and are vulnerable to being toppled in severe storms�
In almost all cases, transplanted corals healed quickly and were cemented solidly onto the mineral accretion within weeks (Figures 4�3 through 4�7)� They showed bright, healthy tissue pigmentation and prolific polyp feeding-tentacle extension� However, some Acropora cervicornis have been killed by bristle worm (Hermodice carunculata) and gastropod (Coralliophila) attack, and some were broken by severe storm waves� Transplanted corals grew skeletons at rates faster than the highest values measured in the field (Gladfelter et al� 1978), even though all sites had suboptimal water quality� Growth rates were determined by periodically measuring the diameter of colonies with a ruler or by measurements from sequential photographs or video images� Fragments of Porites porites grew from 5 to 30 cm across in two years� Acropora cervicornis branched prolifically and grew 5-8 cm in just 10 weeks� The tissue of corals attached to the structures via wires soon began to grow over the mineral accretion� Such corals were visibly brighter than adjacent corals of similar species, but became less colorful when the current was turned off for periods of up to two months and then regained bright pigmentation within days when the current was restored� Young corals colonized and grew on the mineral accretion� We found juvenile coral colonies up to 1 mm in diameter at densities of around 0�7 cm-2 on 3-year-old artificial reef substrate� One artificial reef was colonized by around a hundred young Agaricia agaricites and Favia fragum in two years, and these grew to a size of several centimeters across, in a polluted lagoon where little or no natural recruitment was observed�
Except for transplanted corals and a few small organisms encrusting their bases, all other species found on the artificial reef spontaneously settled on it or migrated to it� A highly diverse coral reef community (Table 4�1) established itself on the mineral accretion structures, including foraminifera, cyanobacteria, chlorophytes, rhodophytes, phaeophytes, porifera, bryozoans, cerianthids, coralliomorpharia, gorgonaceans, sabellid, serpulid, and nereid polychaetes, oysters, gastropods, octopods, squids, echinoids, holothurians, ophiuroids, crinoids, cleaning shrimp, crabs, hermit crabs, and spiny lobsters� A large variety of adult and juvenile fish became permanent or temporary residents, including morays, trumpetfish, squirrelfish, sea bass, fairy basslets, cardinalfish, grunts, drums, butterfly fish, angelfish, damselfish, wrasses, parrot fish, blennies, gobies, surgeonfish, filefish, and porcupine fish� The geometry of the structure appears to strongly affect the type of species recruited� Dolphins were observed swimming near the structures� No organism was observed to show aversive behavior�
The main difference between our artificial reefs and nearby natural reefs was the preponderance of fleshy algae, which were overgrowing corals on nearby reefs, while the artificial reefs had balanced coral and algal growth and the algae were predominantly sand-producing calcareous reds and greens with a much lower density of weedy algae than adjacent natural reefs� Large masses of calcareous Jania, Amphiroa, and Halimeda grew on the sides of the structures, generating sand� Mineral-accretion structures whose power was turned off subsequently had their calcareous algae
and corals overgrown by fleshy algae� In sharp contrast to electrified structures, the control structures that received no current rusted and fell apart within months, and the rusting fragments were not colonized by corals or other organisms�
DISCUSSION
Rapid coral growth and recruitment even in areas of known poor water quality (Goreau 1992) show that our method is able to partly counteract coral-reef eutrophication due to coastal zone nutrient fertilization, and so can contribute to restoring damaged reefs and creating new ones in even degraded areas� As the structures become stronger with age, they are also able to contribute more and more to shore protection from waves and to keep pace with rising sea level� Unlike “artificial reefs” made of exotic materials, Biorock reefs get constantly stronger with time� As long as current is applied, they are self-repairing, since any cracks and breaks of mineral accretion are rapidly and preferentially filled in by new material� While some structures have been damaged by storm waves or impacting objects, such damage is easily repaired by itself as long as the electrical current is applied�
The stimulation of calcareous organisms of all types on the artificial reef, and the relative paucity of noncalcifying organisms, is probably largely due to the boost the former receive from locally alkaline conditions, which allow them to grow their skeletons at lower energetic cost because they do not have to use metabolic energy to pump protons away from calcification sites to maintain internal pH homeostasis (Goreau 1977)� The bright colors of the colonies and their high degree of tentacle expansion may be due to the extra biochemical energy freed as a result� An alternative explanation could be due to the high density of electrons on the cathode, some of which may be trapped and
used to generate adenosine triphosphate, but if this were the major factor, non-calcareous organisms would also be stimulated� This was later discovered to be the case� The general stimulation of growth of marine organisms by both processes on mineral-accretion substrates is covered under pending US patent 08/374993, issued to Hilbertz and Goreau (1995)�
The view that mineral accretion is the most suitable substitute substrate for coral recruitment compared to natural limestone is supported by marine archaeology� Where only iron metal is found in shipwrecks, it rusts away and is not colonized by hard corals unless first covered by encrusting calcareous red algae� Where several dissimilar metals are found in wrecks as well, such as brass, bronze, copper, magnesium, or aluminum, the differing electromotive potentials of the metals results in electrolytic current flows that cause deposition of mineral accretion over the cathodic metals until the anodes are consumed, ending the reactions� Natural electrolysis is responsible for the preservation of most metal artifacts in shipwrecks dating as far back as the Bronze Age, which are found under thick concretions of limestone minerals� We have observed old iron anchors and chains completely covered with hard mineral accretion, allowing corals to settle and grow prolifically on them� This would probably not have happened without electrolytic mineral coatings and concurrent cathodic protection�
We believe that apart from protection of living reefs, mineral accretion is the best substitute for enhancing coral growth and restoring natural coral-reef ecosystems even under stressed conditions� Since the method is able to rely entirely on nonpolluting and renewable energy, it is suitable for remote areas� Laboratory experiments showed that 1�07 kg of mineral accretion was precipitated per kilowatt-hour of electricity� At Jamaican residential customer rates for imported fossil-fuelgenerated electricity (US$ 0�10 per kilowatt-hour), resulting materials are nearly an order of magnitude cheaper than the equivalent weight in concrete blocks�
Typical costs for seawalls and breakwaters using conventional techniques run around US$13,000 per meter, the amount it cost the Maldives to replace mined-out reefs with stacked precast concrete tetrapod breakwater structures to protect the shore from erosion and the aquifers from saltwater intrusion� Unlike concrete blocks, mineral-accretion structures can be built in any size and shape, contain internal steel reinforcement, and get stronger with age rather than weaker� Submerged seawalls could therefore be built that would eventually become much stronger than concrete structures, at a fraction of the cost�
We expect that mineral-accretion technology will eventually become the preferred form of reef restoration and shore protection, where reefs have been degraded due to anthropogenic or natural causes, especially if sea level continues to rise more rapidly than coral reefs grow upward� The global average sea-level rise measured by the TOPEX/Poseidon radar satellite has been 3-4 millimeters per year, as fast as most healthy reef structures are accumulating but faster than degraded reefs or bleached corals can grow (Goreau and Macfarlane 1990)�
ACKNOWLEDGMENTS
We are grateful for support from the European Union for the latest phase of artificial-reef construction under a grant to the Negril Coral Reef Preservation Society for the establishment of the Negril Marine Park� We thank Ursula Hilbertz-Rommerskirchen, Maya Goreau, Bill Wilson, Katy Thacker, Karen McCarthy, Martin Brinn, and Bert Bentley for assistance during artificial-reef construction and deployment� This work would not have been possible without permission from Richard Murray and David Cunninghame to use Tensing Pen property to house the power supply and their support for electricity bills� Anodes were donated by Heraeus Elektrochemie GmbH, Germany�
REFERENCES
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