ABSTRACT
Introduction and History of the Eastern Oyster in NY/NJ Harbor ������������������������������������������������ 141 The Study: Background ����������������������������������������������������������������������������������������������������������������� 143 Materials and Methods ������������������������������������������������������������������������������������������������������������������ 143 Results and Discussion������������������������������������������������������������������������������������������������������������������� 145
Growth Rates ���������������������������������������������������������������������������������������������������������������������������� 145 Growth Comparison ������������������������������������������������������������������������������������������������������������������ 146 Mortality ����������������������������������������������������������������������������������������������������������������������������������� 147 Accretion ����������������������������������������������������������������������������������������������������������������������������������� 148 Chemistry Involved in Accretion Process to Biorock Structure ����������������������������������������������� 148
Conclusions ������������������������������������������������������������������������������������������������������������������������������������ 149 References �������������������������������������������������������������������������������������������������������������������������������������� 149
and increased water quality� Oysters do not cleanse the water in the sense that they remove harmful pollution permanently, but they trap impurities and expel them in larger particles that sink to the bottom rather than make the water turbid (Comi and Willner 2006)� One adult oyster can filter up to 50 gallons of water per day (Comi and Willner 2006), thereby improving water purity and clarity as they remove suspended sediments and microalgae (Newell 1988)� Increased water clarity encourages more biological activity at greater depths and results in higher dissolved oxygen levels (Baird 2008)� The whole population of oysters in the HRE in the past centuries was counted in billions, filtering the entire estuary in a few days (Coen, Luckenbach, and Breitburg 1999)� In addition, about 75% of all commercial fish and shellfish depend on estuaries at some point in their lives (Coen, Luckenbach, and Breitburg 1999), and over 30 fish species were found to be associated with the oyster reefs of the Mid-Atlantic Bight, including juvenile striped bass, tautog, black sea bass, adult black drum, and even the American eel, whose populations have been in decline for decades (Southworth and Mann 1998)�
Aiding in oyster reef formation could have large-scale regional benefits� Commercial and recreational fisheries could benefit from the reintroduction of the keystone species of the past, and increased habitat as a result of reef restoration can exhibit a 10-fold increase in species abundance (Coen, Luckenbach, and Breitburg 1999)�
THE STUDY: BACKGROUND
Different projects have been initiated to create oyster reefs, with mixed results� The Oyster Growth Study described in this chapter is examining one possible way to help jump-start a reef by growing larger oysters in a shorter amount of time� The method we examined, known as Biorock technology, has been shown to increase coral growth in numerous coral-reef restoration projects� The technology works by using electrically mediated calcium carbonate deposition on submerged metal structures (Figure 12�1)� The accreted minerals may be more bioavailable to the oyster on the metal structure and, therefore, possibly promote oyster shell growth� The Biorock technology may also counteract threats to reefs due to increased ocean acidification (Baird 2008)� This and other threats are probably due to increased CO2 partial pressure in the atmosphere� Reefs are also a low cost and self-sustaining armor for vulnerable coasts, which is especially important with an assumed increase in sea-level rise and hurricanes due to global climate change�
One goal of this study was to determine whether this method can be used to aid in oyster reef restoration� Our project also served as an educational tool in collaboration with The River Project (TRP); a nonprofit environmental organization on Pier 40 in Manhattan that houses a wet lab with a natural flow-through estuarine aquarium system� TRP also works closely with students and volunteers from throughout New York City and several high school and undergraduate volunteers aided in data collection and maintenance of this project throughout its duration� Our experimental setup
was similar to one previously conducted by Kaitlin Baird-then a master’s student at Columbia University-in the intertidal zone of the East River at College Point, New York� At TRP, we were interested in repeating the study in a more controlled environment, since Baird’s results were partially impacted by the large amount of oysters lost due to crab predation and other disturbances such as tidal and other environmental factors, which made data collection a challenge�
MATERIALS AND METHODS
Oysters are called natural bioengineers since they create three-dimensional structures (reefs) as they grow on the backs of older oysters (Dalton, Stringer, and Lock 2003)� Oysters cannot naturally sustain themselves without having old shell (cultch) or another hard substrate to attach to� In our study, the source of minerals was artificially produced using the Biorock technology� Oysters, which can grow intertidally, rid themselves of many predators by being able to survive out of the water for long periods of time� This characteristic was a great advantage to us in our study, since we could lift the metal reef structures out of the tanks to attach oysters and subsequently measure them conveniently in the wet lab (see Figure 12�2)�
The Oyster Growth Study was conducted over two consecutive years-in the summer/fall seasons of 2007 and 2008-at the facilities of TRP�
We tested if oyster reef formation can be supported through the use of Biorock technology, which possibly promotes faster growth, creating larger and stronger shell as well as reduced mortality in oysters� It was found that shell size is positively correlated with reduced mortality due to the ability to avoid predation (Galtsoff 1964; Arnold et al� 1996)� We were able to reduce predation by conducting the study in large tanks using a flow-through system of Hudson river water with screens that stop any organisms larger than a quarter of an inch from getting into the tanks�
The experiment examined the use of Biorock to determine
The mechanism of the Biorock technology, also called Mineral Accretion method, Seament, or Seacrete, is based on creating a difference in pH across the metal reef structure� The cathode is connected to the reef structure, reducing it and thereby attracting positively charged ions like calcium and magnesium� The resulting deposition of CaCO3 through this application of an electric potential may make these important minerals more bioavailable to the oysters due to close proximity (Hilbertz and Goreau 1996; Kurlansky 2006)�
The study was conducted in a flow-through system with two identical round fiberglass tanks of approximately 300 gallons each� Tanks 1 and 2 each contained three replicate metal “reefs,” housing an equal number of oysters (200 per reef = 600 per tank)� Tank 1 used the Biorock technology, and a low-voltage (6-9 V) current was added to the reefs to promote mineral accretion (Experiment), and tank 2 acted as replicate control with no electric current� A random subset of 30 oysters per reef were chosen using a random number generator and marked with nail polish for the duration of the study (see Figure 12�3)� The marked 90 oysters per tank were measured twice per month for growth (their “height” as defined by Cardoso et al� 2007) for approximately three months in two consecutive years� Mineral accretion and water chemistry (salinity, temperature, and pH) were also recorded to ensure they were in a normal range, equal to the estuarine water chemistry, and identical in both tank environments�
RESULTS AND DISCUSSION
The results of the Oyster Growth Study showed that oysters in the experimental tank grew significantly larger and faster than the oysters in the control tanks that were not connected to an electric current� In addition, we found statistically significant differences between the total mortality, with fewer deaths in the experimental than control tanks� The results were consistent for both years we conducted the study�
Growth Rates
Figure 12�4a and b show the average growth rates for all the reefs� The top three lines (circle, square, diamond) follow the growth of the experimental tank oysters, and the bottom three lines (triangles) represent the reefs in the control tank� For both years, we observed that all reefs in the experimental tank grew at a faster rate than the reefs in the control tank and were therefore larger at the end of the study�
Growth Comparison
The histograms (Figure 12�5a and b) show how much the oysters grew in each tank from August 15 until November 15, 2007 (Figure 12�5a), and from June 20 until September 15, 2008 (Figure 12�5b)� On the left is the control tank (C) showing oyster’s final height minus the initial height, and on the right is the experimental (E) tank showing oyster’s final height minus initial height� In 2007, the control group grew an average of 2�36 mm in three months and the experimental group grew 6�49 mm (Figure 12�5a)� In 2008, the control group grew an average of 13�13 mm in three months, and the experimental group grew 21�28 mm (Figure 12�5b)� To statistically validate these increases in growth, we conducted a two-tailed t-test to compare the means of the experimental and control groups in both years to test the null and alternative hypotheses:
The calculated results for both years can been seen in Table 12�1�
Mortality
We also compared total mortality in both trials� Figure 12�6 depicts a comparison of total mortality for 2007 and 2008� For both years, the experimental tanks showed a lower mortality than the control tanks� We conducted a t-test to determine if the differences were significant between the control and the experimental reefs to test the null and alternative hypotheses:
The calculated results for both years can been seen in Table 12�2� For both total mortality and mean growth rates, we found significant differences across trial
years�
Accretion
We also examined the amount of mineral accretion on the experimental reefs� The diameter of the metal bars of the reef structure used in 2007 and reused in 2008 was about 0�8 cm� Over the course of the study, the metal structure rusted at places but remained about the same size except for the experimental tank’s structures� All three reefs in the experimental tank, which were connected to about 9 V of an electric current for the entire study time, showed significant accretion of minerals� Reef A was the first in line and received more of a current than reef B, and reef C received a little less than reef B� The difference was visible in terms of the amounts of accretion of minerals (aragonite and brucite), and reef A seemed to be covered with the most minerals (in terms of overall mineral coating and thickness), reef B with a little less, and reef C with the least� The accretion was thickest on all reefs close to where they were connected to the wire running the current and showed patches of accretion throughout the structures (see Figure 12�7)� The most prominent accretion on reef A, close to the wire connection, measured 1�8 cm, which is more than twice the original diameter�
Chemistry Involved in Accretion Process to Biorock Structure
In marine environments, the pH is determined by reactions among dissolved carbon dioxide (CO2), carbonate ions (CO32−), and bicarbonate ions (HCO3−):
The increase in pH at the reef structure is caused by the establishment of an electric potential, which promotes the deposition of CaCO3 (Hilbertz 1992):
We found a clear increase in mineral accretion across experimental reefs in both years�
CONCLUSIONS
The initial oyster samples were close in their average size and distributed normally, a factor important as to have a meaningful comparison of the two sample populations at the end of the study� Over the course of approximately three months, a subset of 90 oysters in each of the control and the experimental tank, both containing 600 oysters, were measured from the hinge to the farthest point (height) approximately every two weeks� Oysters in the experimental tank grew statistically significantly faster, 2�75 and 1�62 times faster than oysters in the control tank during the course of both studies (2007 and 2008)� Mortality was significantly higher for control oysters than Biorock groups in both years, by 2�08 and 1�36 times (2007 and 2008)� Accretion of minerals onto the reef structure was evident and most prominent closest to the attachment point of the wire that ran the electric current� Overall, by increasing growth rate and survival of oysters, Biorock technology may be a viable resource to restore oyster reefs in estuaries where they were once abundant�
REFERENCES
