ABSTRACT
The American oyster (Crassostrea virginica) is considered a keystone species, because it builds reefs, provides shelter for other estuarine organisms, improves water quality by filtering particulate matter, and protects shorelines by reducing bank erosion� A keystone species is defined as the one that has a disproportionate impact (relative to its numbers or biomass) on the organization of a biological community� Loss of a keystone species may have far-reaching consequences for the community (Primack 2008)�
Oysters are not able to move, and their distribution depends on where the larvae set� The oyster larvae find a solid place to set themselves two to three weeks after spawning� The oyster attaches
CONTENTS
Introduction: Oyster Biology ��������������������������������������������������������������������������������������������������������� 151 Causes and Consequences of Oyster Decline �������������������������������������������������������������������������������� 152 Oyster Restoration Strategies ��������������������������������������������������������������������������������������������������������� 153 Biorock Oyster Restoration ������������������������������������������������������������������������������������������������������������ 153 Site of Project ��������������������������������������������������������������������������������������������������������������������������������� 154 Materials and Methods ������������������������������������������������������������������������������������������������������������������ 154 Results �������������������������������������������������������������������������������������������������������������������������������������������� 155
2010 Growth Rate with and without Electrical Stimulation ����������������������������������������������������� 155 2011 Growth Rate with and without Electrical Stimulation ����������������������������������������������������� 156 Winter 2010-2011 Survival Data ���������������������������������������������������������������������������������������������� 156
Discussion �������������������������������������������������������������������������������������������������������������������������������������� 158 Conclusions ������������������������������������������������������������������������������������������������������������������������������������ 158 References �������������������������������������������������������������������������������������������������������������������������������������� 159
itself with a drop of liquid cement from its foot and is now called spat (newly attached oysters)� Adult oysters filter large amounts of brackish water (water that has a salinity between fresh water and salt water) and remove flagellates and phytoplankton (Stanley and Sellers 1986)� Large populations of oysters and other suspension-feeding bivalves filter plankton out of the water so efficiently that they control blooms of phytoplankton and prevent symptoms of eutrophication (Jackson 2001)� Oysters most effectively filter particles in the 3-4 μm size range (Stanley and Sellers 1986)�
CAUSES AND CONSEQUENCES OF OYSTER DECLINE
There are many reasons for the decline of American oyster reefs (Eastern Biological Review Team 2007)� Many scientists attribute the decline of oysters to overharvesting (Newell 1988)� Oysters are harvested in a few ways: handpicking of clumps from reefs at low tide, harvesting by hand and by use of tongs from boats, and dragging and dredging from boats (Stanley and Sellers 1986)� These harvesting methods degraded the oyster habitat and depleted the oyster beds to a level that inhibited their net reproduction� Chesapeake Bay was once rich with oyster reefs� The oyster reefs before 1870 would have filtered the entire water column every three days� Even though aboriginal and early colonial people harvested these oysters over many millennia, the real problems did not start until the 1870s� This is when mechanical harvesting with dredges was invented, and deep channel reefs were affected (Kirby 2004; Mackenzie 2007)� Now, the oysters filter the water column of Chesapeake Bay every 325 days (Newell 1988)�
Overfishing could be one of the causes of eutrophication and outbreaks of disease� Eutrophication of Chesapeake Bay got worse in the 1930s� This was about two centuries after land clearing for agriculture, which greatly increased runoff of sediment and nutrients into the estuary� The large quantity of oysters had been able to remove the increase in phytoplankton, but mechanical harvesting progressively destroyed most of the oyster beds from the 1870s to the 1920s (Rothschild et al� 1994; Kirby 2004; Mackenzie 2007)� The oysters would have been major consumers of the spring phytoplankton bloom (Newell 1988)� The result of eliminating the water filtration by oysters has rippled throughout the ecosystem� When oysters were abundant, the water was clear, light reached the bottom, and algae grew there, providing the basis of the food chain that led to abundant blue crabs, which were also a major traditional fishery in Chesapeake Bay� With the loss of filtration and increased eutrophication from sewage from humans, cattle, pigs, and chickens, as well as agricultural fertilizer runoff, the waters are now turbid and so little light reaches the bottom that the benthic algae have disappeared, causing near total collapse of the crab fishery�
Ocean acidification as a result of CO2 from fossil fuels is another problem facing all calcareous marine organisms (Gazeau et al� 2007)� Carbon dioxide lowers ocean pH and will continue to lower it as humans continue to emit large amounts of CO2 into the atmosphere� The pH of the oceans has already declined by 0�1 units compared to preindustrial values and is expected to decrease by another 0�4 units by the end of the century� The Intergovernmental Panel on Climate Change predicts atmospheric partial pressure of CO2 to range from 490 to 1250 ppmv by 2100� Ocean acidification leads to a shift in inorganic carbon balance toward higher CO2 and lower carbonate ion (CO3
2− ) concentrations� This carbonate ion is one of the main building blocks in calcium carbonate (CaCO3)�
CO Ca CaCO3 2 2
Any change in carbonate ion concentration can hinder calcareous organisms from precipitating calcium carbonate (Gazeau et al� 2007)� The calcification rates of oysters and mussels have been shown to decrease with decreasing pH (Gazeau et al� 2007)� The larval phases of oysters are especially vulnerable to acidification due to their small size and high surface area� As a result of increasing acidity of seawater in the northeast Pacific, there has been a near total failure of oyster spat settlement in Oregon, Washington, and British Columbia in recent years (Barton et al� 2012)�
OYSTER RESTORATION STRATEGIES
Rothschild et al� (1994) proposed a very comprehensive recovery plan for the Eastern American oyster in the Chesapeake Bay in Maryland� They proposed a four-point strategy involving fishery management, repletion, habitat replacement, and broodstock sanctuaries� The first fishery management should be regulated by a scientific size-specified fishery mortality, recognizing that oyster larvae or spat sometimes settle on larger oysters that should not be fished out� The repletion strategy, placing oyster shells on already existing substrate, should be modified to areas that are known to increase oyster growth and survival� The habitat replacement strategy should be implemented to create additional oyster habitat� This new substrate will allow spat to attach and grow new oyster reefs� One of the main problems is that the oysters have no habitat on which to attach� The broodstock sanctuaries would implement a no-fishing zone where engineered reefs were created and where known areas of high spat settlements are known� This broodstock sanctuary idea will help reinforce the first three strategies (Rothschild et al� 1994)� If this plan were used throughout all estuaries, then perhaps there would be an increase in all bivalve populations, and cleaner waters would result�
There is a widespread recognition of the need for oyster restoration on a large scale to restore ecosystems and water quality (Beck et al� 2011; Leonard and Macfarlane 2011)� Large sums of federal and state funding have been spent to restore the lost oyster reefs of Northeastern US estuaries, not only to revive the oyster industry but also to improve water quality in places where oysters should not be eaten because of toxic contamination� Much of this has been based on building artificial reefs of old oyster shells, fossil shells, concrete, and other exotic materials and attaching oyster spat to them or hoping oysters will naturally settle on them� By and large these have been failures due to poor water quality (Schulte et al� 2009)�
It is clear that to restore oyster populations and water quality, a quantum leap in technology is needed, using methods that greatly increase oyster settlement, growth, survival, and resistance to environmental stress� This chapter describes the first field tests of such new approaches that do so�
BIOROCK OYSTER RESTORATION
Restoring oyster beds or reefs is important to protect shorelines from erosion, restore water quality, and supply the human demand for harvest� The first stage of restoring oyster reefs is to supply a hard layer where the oyster larvae can attach (SCORE)� A unique strategy to do so is to grow limestone minerals from seawater, the natural material that makes up oyster shell and the preferred substrate for larvae to settle and become spat� The Biorock process does this precisely by utilizing low-voltage electrical currents�
The oyster must pump protons and calcium ions across its membrane to generate a high pH that enables the secretion of shell material� Biorock technology uses a low electrical current that results in electrolysis and thus the precipitation of calcium carbonate and magnesium hydroxide directly to the Biorock structure� Oyster species are thought to grow at accelerated rates because they are utilizing the availability of dissolved calcium and carbonate ions, to increase efficiency of metabolic processes (Goreau and Hilbertz 2005)� Using energy supplied by photovoltaic panels, oysters should be able to increase their growth rates and have higher survival�
Electrodeposition of minerals in seawater is known as “mineral accretion” or “Biorock” technology� This technology can be used as building materials for a wide variety of purposes, including artificial reefs (Hilbertz 1979)� Using this technology, calcium carbonate, magnesium hydroxide, and hydrogen are created at the cathode, and oxygen and chlorine are created at the anode� At the cathode, water is broken down to create hydrogen gas and hydroxyl ions so the cathode is reducing and alkaline (Hilbertz 1992)� At the anode, water is broken down to create oxygen and hydrogen ions, making it oxidizing and acidic (Hilbertz 1992)� Calcium carbonate is created and deposited at the cathode when an electric current circuit is created�
The metal structure can be bent or welded to any size or shape, which can create different levels of structure where oysters can attach� This will also increase their filtering efficiency and have positive effects on the water quality (Goreau and Hilbertz 2005)� The resulting accretion has similar chemical and physical properties to reef limestone, which will give marine organisms an artificial reef to grow and live on, in, and around� The objective of this project is to increase the size and habitat of the American oyster in the East River at the College Point site by testing cutting-edge Biorock technology for field restoration�
SITE OF PROJECT
The East River was historically one of the best areas for harvest of American oysters� In the 1800s, this part of the East River between Queens and the Bronx was a prime location for harvesting oysters� The East river connects Upper New York Bay in the south with Long Island Sound in the north� The source of freshwater is from the Hudson River and brought to the East River via the Harlem River� The study location was at McNeil Park, Block 3914-Lot No�1�
The site is located within the intertidal zone and consists of mixed mud, sand, and industrial rubble� C. virginica are scarce at this site, presumably because of poor environmental conditions� The site lies next to a former Superfund toxic waste site that was formerly a Navy shipyard used for building ships during the Second World War� After the war, the shipyard was closed, and all the electroplating wastes were left on site in old, rusting, steel drums� The site was then used as an illegal waste dump for about 40 years� The remains of the demolished buildings from the 1958 New York World’s Fair, and other industrial debris were dumped here during this period�
While salt marsh, mussels, oysters, crabs, and horseshoe crabs have been observed to recover farther away from the former waste site, our projects were placed right next to the edge of the waste site, in an area affected by leaching drainage from the former dump, where no natural ecosystem recovery had occurred� It is thought that this lack of recovery may be due to the effects of the large number of trace metals, polychlorobiphenyls, polycyclic aromatic hydrocarbons, and other toxic materials leaching from the site� In other words, this was a worst-case restoration situation�
The experiments were initiated in September 2009 and started to precipitate calcium carbonate immediately� Measurements and photographs were taken periodically to see how fast they grew� In some places, more than 3 mm of mineral accretion had grown on top of the steel� The first stage of the experiment was to make sure the electric current was working and to see how much mineral accretion would grow over a few months�
MATERIALS AND METHODS
About 600 oysters were used in the spring for growth experiments in the 2010 growing season, another batch in the 2011 growing season, and a third larger-sized batch for survival measurements overwinter 2010-2011� The oysters were donated by Frank M� Flowers and Sons, a major oyster hatchery in Oyster Bay, Long Island, with some of the cleanest water in Long Island Sound�
Solar photovoltaic panels were used to grow artificial reefs where oysters and other benthic organisms could attach and grow� Two of the three solar panels were Sun Electronics model number HS-A-205-fa2, which supplied 205 W, 18�4 V, and 11�15 A at peak power� The third panel was Kyocera model number KC130TM, which supplied 130 W, 17�6 V, and 7�39 A at peak power� The solar panels were used in a direct power mode, with no storage batteries� Because the structures were located in the intertidal zone, they received power only when the sun was shining and the tide was high, probably no more than about a quarter of the time�
At the College Point site, we had four different steel helix-shaped oyster reef structures� These are vertical structures that reach from near the lower intertidal to above the high-tide mark� Oyster bags
were attached to the lower portions� The open helical structure allows the free flow of water through the bags from all directions� Three helices acted as the cathodes of solar panels, where the mineral accretion takes place� One of these helixes had no electric current passing through it and served as our control� Close to each cathode there was an anode encased in PVC pipe for protection� The seawater acts as a conductor between the anode and cathode� We attached bags of oysters onto the metal helixes�
We measured oyster size periodically to compare growth rates of oysters on the electrified helices and the control helix with no electric power during the 2010 and 2011 growing seasons� The bags were emptied into a tray, sorting live from dead oysters, and photographed with a scale� The dead oysters were removed and the live ones returned to the bag� Measurements were made from the images using the Photoshop measuring tool�
Another set of measurements were made to assess overwinter survival� Measurements were taken on September 5, 2010, and then again at the end of winter� Crassotrea virginica oysters were placed in bags and hung from the metal helices on September 5, 2010� They were checked periodically to make sure that they were fixed to the helices securely and not suffocating due to mud clogging holes in the bags� They were then removed from the water and measured on April 27, 2011�
RESULTS
2010 Growth Rate with and without Electrical Stimulation
Oysters were attached in late fall 2009 and monitored for one year� The oysters in the control structure getting no power decreased in size, and all had died by midsummer 2010� The structures getting low current showed very little growth, but the structure getting the highest power showed strong growth (Figure 13�1)� However, some of the electrical cables connecting the projects to the solar panels were broken several times during the year by heavy storm-wave action, and we could not be certain how long they had been without power� Because of these problems, the structures were rewired with stronger cables the following year�
2011 Growth Rate with and without Electrical Stimulation
In the 2011 growing season, there were no problems with cable breaks, and power was reliably maintained� The control oysters showed only slight growth of 4�86 mm, the medium-power oysters grew 5�82 times faster, and the higher-power oysters grew 9�30 times faster than the controls (Figure 13�2; Tables 13�1, 13�2, and 13�3)�
Winter 2010-2011 Survival Data
Zero day oyster measurements (mm) in random sample of 75/250 oysters� Measurements were made with a digital caliper on September 5, 2010; on April 27, 2011, measurements were made from photographs� Data are presented in Figures 13�3 and 13�4, and Table 13�4�
Over the winter period, the controls showed more than 91% mortality and a clear decrease in average length� In contrast, the electrified oysters all had high survival, from 66 to 100%� The lowpower oysters showed no growth, but those getting higher power had dramatic overwinter growth, even though the winter would normally be the dormant season�
The electrically treated oysters all had high survival, with no mortality at all in the high current treatment� The higher the electrical current, the faster the oyster growth and the greater the survival, indicating that the electrical current greatly increased oyster health and ability to resist winter physical stresses�
DISCUSSION
The results from the overwinter survival study showed that the Biorock treatment had markedly increased growth rates and survival� Young oysters grown in electrical fields under various conditions had very high overwinter survival (mean 79�8%, range 66�0-100�0%) and had shiny white shells� In contrast, control oysters in the same habitat had almost complete mortality (survival 8�5%), and the shells had a red-yellow color and appeared eroded� Spat placed in the fall were 30�9 ± 2�71 mm long� Electric oysters had clear growth over the normally dormant winter season (mean length 37�4 ± 5�0 and 39�4 ± 4�0 mm in two separate experiments)� In contrast, control oysters shrank in length over the winter to 27�4 ± 2�71 mm� The shrinkage of the controls was presumably due to acid water, since carbon dioxide is more soluble in cold water� The electricity appears to reverse the effects of acidity and physical stress�
The growth rate experiment also showed positive results for the Biorock experimental plots� There was an increase in size over time with more electricity� Figures 13�1 and 13�2 show that the oysters with the most power grew the largest compared to controls� Survival rate for helix 1 was about 43%� Survival rate for helix 2 was about 67%, and the survival rate for helix 3 was about 75%� Mortality increased as electricity decreased� The cause of oyster death is uncertain but can possibly be a result of water pollution, weather conditions, diseases, or predation�
CONCLUSIONS
Results obtained from the experiments that were performed at the College Point Biorock site showed that the electrical field had positive effects, increasing both growth rate and survival of C. virginica oysters� While all oyster groups that received electrical stimulation responded with increased growth and survival, the oysters given the highest amount of electrical stimulation had the fastest growth and highest survival rates� These findings indicate that electrical currents have induced positive growth in C. virginica oysters, even under the most severe pollution conditions when no growth would be expected�
Estuary sites that have been polluted by runoff and sewer drainage present ideal locations for future Biorock projects, particularly if these locations once flourished with oyster and other benthic populations� The benefits of these artificial oyster nurseries would be to create a habitat for oyster growth where they otherwise would not be able to survive in natural conditions� Increasing the oyster population in heavily polluted estuaries would create a natural filter and improve water quality in these areas�
“Restoring an abundant population of filter feeding oysters to the Chesapeake Bay and its tributaries might really help lend new meaning to the phrase clean up the bay” (Newell 1988)� Using this idea and applying it to the Hudson River and New York harbor, we can hopefully make a difference in the water quality surrounding the city� Because our site is a worst-case scenario, in which control oysters steadily shrink and die, if electrical stimulation can increase growth and survival under such severe conditions, then better results would be expected in cleaner waters, and it should be possible to restore vanished oyster populations even in places where no other restoration method would work� Large-scale application of this method should be started wherever oyster populations need to be restored� In particular, the method may be of particular use in places like the Pacific Northwest, where oyster growth is failing due to ocean acidification (Barton et al� 2012)�
REFERENCES
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