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Oyster Recruitment and Growth on an Electrified Artificial Reef Structure in Grand Isle, Louisiana

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BULLETIN OF MARINE SCIENC E, 84(1): 59–66, 2009
59
Bulletin of Marine Science
© 2009 Rosenstiel School of Marine and Atmospheric Science
of the University of Miami
OYSTER RECRUITMENT AND GROWTH ON AN ELECTRIFIED
ARTIFICIAL REEF STRUCTURE IN GRAND ISLE, LOUISIANA
Bryan P. Piazza, Mason K. Piehler, Bryan P. Gossman,
Megan K. La Peyre, and Jerome F. La Peyre
Coastal protection remains a global priority as rising sea levels, development,
and tropical storms threaten coastal habitat. A common tool used to combat shore-
line erosion involves armoring the land/water interface (Yohe and Neumann, 1997;
Mimura and Nunn, 1998; Klein et al., 2001). While typical armoring is done with
heavy, often non-native materials, recent shoreline protection projects are moving
towards promoting the use of native, living materials. One promising method that
has been used to restore degraded reef systems and protect shorelines in southeast
Asia, is mineral accretion technology which involves the electrochemical deposition
of minerals from seawater (Hilbertz, 1979; Hilbertz and Goreau, 1996; Sabater and
Yap, 2002, 2004). Essentially, the method involves creating reef units consisting of a
rebar structure and passing a weak electrical current through the structure. ese
artificial reef units use low-voltage direct current that, in the right conditions, results
in the precipitation of dissolved minerals in seawater to create a reef structure ten
times stronger than concrete (Hilbertz, 1979). In addition to rapidly building mineral
substrate, electrified reef structures may also confer survival and growth benefits
to coral and mollusks that become attached to the structure (Hilbertz and Goreau,
1996; Sabater and Yap, 2002, 2004). Enhanced growth is suggested to result from the
effects of electrolysis of seawater which raises the pH on the mineral precipitate, thus
reducing the metabolic energy requirements needed for growth (Goreau and Hil-
bertz, 2005). It is hypothesized that the electrolysis of seawater creates the high pH
conditions for the organisms, hence providing an energy subsidy to the organisms,
allowing them to put more energy in growth, reproduction, and disease resistance.
e goal of this project was to explicitly test the effects of this mineral accretion
technology (1) in waters off coastal Louisiana, and (2) on growth and recruitment
of the native eastern oyster, Crassostrea virginica (Gmelin, 1971). Not only might
three-dimensional oyster reefs protect shorelines (Piazza et al., 2005), but they can
also provide critical ecosystem functions such as nekton habitat and water quality
services (e.g., Breitburg, 1999). Specific objectives of this study were to (1) determine
the rate of calcium carbonate (CaCO3) accretion of electrified reefs located off the
coast of Louisiana, (2) compare oyster spat recruitment and growth of electrified
reefs (high, medium, and low DC current) to a control, non-electrified reef and on
shell spat collectors, and (3) evaluate juvenile oyster growth on electrified reefs.
M  M
S S
e study was conducted at the Louisiana Sea Grant oyster hatchery in Grand Isle, Louisi-
ana from April 2006–June 2007. During this time period, daily mean (SD) water temperature
was 23.6 (6.4) °C (range 7.28–32.7 °C) and daily mean (SD) for salinity was 22.1 (4.1) (range
11.7–31.1) (USGS continuous data recorder 073802515 Barataria Bay Pass E of Grand Isle,
NOTE
BULLETIN OF MARINE SCIENC E, VOL. 84, NO. 1, 2009
60
LA). ese temperature and salinity conditions provide ideal conditions for oyster growth
(Galtsoff, 1964) and adequate conditions for the precipitation of calcium carbonate on electri-
fied reef structures (T. Goreau, Global Coral Reef Alliance, pers. comm. April 9, 2006)
E R U
In April 2006, eight experimental units were created. Each unit consisted of two “staples”
made with two pieces of 12-foot (3.7 m) PVC attached using a 36" (0.9 m) horizontal piece of
PVC and two T-joints. e two staples for each unit were then pushed into the mud bottom
parallel to one another, approximately 24" (0.6 m) apart. Five pieces of 30" (0.8 m) rebar were
then measured [diameter (mm)] with a caliper (Scienceware, Bel-Art Products, Pequannock,
NJ) and secured between the two staples using ties (Fig. 1). e units were organized in two
rows of four, parallel to the shoreline. Using a DC power source located on-shore, electrical
current was supplied to each individual rebar piece; replicates of a control (no electrical cur-
rent), low, medium, and high electrical current reefs were established based on distance from
the power source (Ohm’s Law). Electricity was supplied to each individual rebar piece using
Romex 12/2 UF-B electrical cable. Power was supplied to the cable with a Hoefer Scientific
Instruments PS 500X DC power supply (500 V, 400 mA, 200 W) that was connected to an
Figure 1. (A) Diagram and (B and C) photos of experimental set up of reefs. Reefs were suspended
approximately 0.3 m off the substrate by four PVC poles (reef legs). Letters refer to electrical
current level treatments: H = high, M = medium, L = low, C = control (no current). (B) Two of
the experimental reefs raised above the water surface. (C) Close-up of reefs with adult oysters
cemented onto bars.
NOTES 61
onshore AC power source (120 V). e DC power supply was able to maintain a constant low
current (~2 A) necessary for optimum precipitation of CaCO3 on the electrified bars (van
Treeck and Schuhmacher, 1997). is set-up uses the BiorockTM technique developed for res-
toration of coral reefs (U.S. Patent # 5,543,043). Traditional oyster shell spat collectors, con-
sisting of 10 oyster shells affi xed on a wire hanger, were placed adjacent to each experimental
unit to monitor spat availability.
S D
Experiment 1: Spat Recruitment and Bar Accretion.—Experimental units were sampled ev-
ery 3 wks from June–November 2006, (June 23, July 12, August 2, August 23, September 13,
October 6, November 1, and November 12). One rebar piece from each experimental unit was
sampled with replacement of a clean bar every 3 wks and a second one every 6 wks (Fig. 1).
e remaining three bars were sampled once on the final day of the experiment. ree oyster
shell spat collectors were also collected and replaced with clean spat collectors every 3 wks, 6
wks and at the end of the experiment. Sampled bars and spat collectors were transported to
the laboratory (School of Renewable Natural Resources, LSU AgCenter) for processing. In the
lab, the diameter of each sampled piece of rebar was measured at five locations. Spat number
and spat size (mm) were measured on each bar and on spat collectors using a dissecting mi-
croscope.
Experiment 2: Oyster Growth.—From November 12, 2006 through May 2007, oyster growth
on the experimental units was measured. Four double (two oysters with attached shells) oys-
ters (3.9 ± 0.03 cm; mean size and SD) were cemented along each piece of rebar with Quikrete
Hydraulic Water-Stop Cement (Fig. 1C). e rebar were “saddled” with the double oysters to
facilitate attachment without affecting shell hinges or openings of the animals. Oyster size
(shell height measured at largest hinge-lip distance, mm) was measured using calipers at the
initiation of the experiment, and monthly throughout the experiment (November 2006–May
2007). All oysters for this experiment were obtained from the Louisiana Sea Grant Oyster
Hatchery, Grand Isle, Louisiana.
S A
Statistical analyses were performed with SAS software (version 9.1; SAS Institute) and re-
sults were considered significant at α = 0.05. All data (mineral accretion, number and size of
oyster spat, and oyster growth) were tested for normality, by examining model residuals, and
homogeneity of variance. Logarithmic [log(x + 1)] transformation was performed for spat
number and spat size to satisf y model assumptions.
Reef bars and shell collectors were analyzed separately, and results from the shell collec-
tors were used for comparison purposes only. Two-way analysis of variance (ANOVA, Proc
MIXED) was used to examine mineral accretion [electrical current (H, M, L, C), and time in-
terval (3 wks, 6 wks, 22 wks)], and three-way ANOVA was used to test the influence of factors
on number and size of oyster spat [factors = electrical current, time interval, date]. Analysis of
number and size of spat on traditional shell collectors was conducted with two-way ANOVA
[factors = time interval, date]. One-way ANOVA was used for analysis of oyster growth [factor
= electrical current]. Least-square means with a Tukey adjustment was used following signifi-
cant ANOVA results (P < 0.05) to examine the differences among treatments.
R
E : S R  B A.—Accretion of mineral
precipitate resulted in increases in bar diameter ranging from 0–6.95 mm (Table 1).
Accretion differed only by current level (ANOVA: F11,92 = 19.8, P < 0.001). Control
bars had the lowest accretion, followed by low and medium current bars, with high
electrical current bars having highest accretion (LSMeans with Tukey adjustment: P
< 0.05; Table 1; Fig. 2).
BULLETIN OF MARINE SCIENC E, VOL. 84, NO. 1, 2009
62
Spat number differed by treatment (F3, 93 = 10.73, P < 0.0001, N = 104), with control
bars having approximately twice as many spat as compared to any of the electrified
bars (LSMeans with Tukey adjustment: P < 0.05; Table 1). Number of spat was also
positively affected by time (F2, 93 = 30.18, P < 0.0001, N = 104), with the 22-wk samples
containing significantly greater numbers of spat than either the 3- or 6-wk samples
(LSMeans with Tukey adjustment: P < 0.05; Fig. 3). Number of spat on both bars and
shell collectors was significantly affected by date (bars: F5, 93 = 9.94, P < 0.0001; N =
104), driven by higher spat recruitment in October and November (LSMeans with
Tukey adjustment: P < 0.05).
Spat size on the experimental units did not differ by treatment (Table 1) or sam-
pling time interval. Mean spat size on bars and shell collectors was significantly dif-
ferent by date (F6, 49 = 5.33, P = 0.0003, N = 61), and this effect was driven by the
higher mean spat sizes (mm) in August and September, that were almost double the
Table 1. Characteristics of mineral accretion and oyster recruitment on experimental reef structures
that were exposed to different electrical current levels. Data are mean ± SD (range); N = 26
sampled rebar pieces except for oyster growth where N refers to the number of attached live
oysters remaining in March 2007. Electric current levels are high, medium, low, and control (no
electrical current).
Electrical current level
Response variable High Medium Low Control
Mineral accretion (mm) 4.4 ± 1.6 3.4 ± 1.0 2.8 ± 0.9 0.4 ± 0.3
(0.9–6.9) (1.2–5.4) (0.9–4.5) (0–1.0)
Number of oyster spat 3.2 ± 7.1 5.2 ± 12.8 4.6 ± 8.7 10.3 ± 13.8
(0–30) (0–53) (0–28) (0–54)
Oyster spat size (mm) 4.1 ± 1.9 4.3 ± 1.4 5.0 ± 3.0 5.6 ± 2.0
(2.0–7.6) (2.0–5.6) (2.0–12.5) (2.5–11.5)
Oyster growth (mm mo–1) 1.5 ± 0.9 1.4 ± 0.8 1.5 ± 0.7 1.3 ± 1.1
(0.2–3.8) (0.3–2.9) (0.3–3.6) (0.02–3.3)
N = 27 N = 23 N = 20 N = 11
Figure 2. Accretion of mineral precipitate on rebar by electrical current supplied and amount of
time electricity was applied to bars. Mean rebar size at initiation of experiment was 13.01 ± 0.36
mm (SD). The 22 wk bars had signicantly lower accretion as compared to the bars sampled at 3
and 6 wk time periods. All bars receiving some level of electrical current accreted signicantly
higher levels than the control bars.
NOTES 63
size of spat in June and July. Mean spat size (mm) did not differ significantly between
traditional shell collectors or experimental bars over the entire time period (Fig. 3).
E : O G.—Mean oyster growth did not vary significant-
ly among treatments (Table 1). Mean oyster growth was low (1.4 ± 8.6 mm mo–1; N
= 81). e experiment was considerably shorter in duration (4 mo) and sample size
was considerably lower than desired as a result of oyster loss from drill [(Stramonita
haemastoma (Linnaeus, 1758)] predation.
D
While the use of electricity to precipitate minerals from seawater to create strong
reef structures and enhance coral reef growth has been shown to be effective in areas
around the world (Hilbertz and Goreau, 1996; van Treeck and Schuhmacher, 1997;
Figure 3. (A) Number (SD) and (B) size of oyster spat recruited to experimental reef structure
by electrical current level or shell, and amount of time electricity was supplied to the bars. Shell
spat counts were divided by ten. Control bars recruited higher number of spat than bars receiving
electric current for 3 and 6 wk samples (LSMeans with Tukey adjustment: P < 0.05), but had simi-
lar recruitment to bars receiving low, medium and high electrical current at 22 wk spat counts.
Spat size was not signicantly affected by time or electrical current level, but did increase in size
towards the end of the summer (i.e., August and September).
BULLETIN OF MARINE SCIENC E, VOL. 84, NO. 1, 2009
64
Sabater and Yap, 2004), the use of electrical current to enhance bivalve recruitment
and growth had not previously been tested in the Gulf of Mexico. We found that
electrolysis in seawater off Grand Isle, LA induced cathodic accretion of minerals
on rebar, however, no significant positive effects were detected on spat recruitment
or size, or oyster growth during this experiment. While the development of a robust
reef-like structure using electrolysis of seawater could prove useful in shoreline sta-
bilization and erosion control, this technique does not appear to confer any growth
or recruitment advantages on oysters in this region.
Accretion of mineral precipitate occurred on all of the bars, however, it appeared
that after 3 wks, accretion slowed significantly. is negative asymptotic relationship
has been described for these types of structures, as the growing precipitate decreases
the electrical field by insulation (van Treeck and Schuhmacher, 1997). Overgrowth
of coral nubbins has been observed in other electrified reef systems (van Treeck and
Schuhmacher, 1997), and while we did not observe overgrowth, given that the tradi-
tional shell collectors consistently had high spat recruitment, it is possible that the
rapid initial accretion found in our study contributed to the lower spat recruitment
found on the 3- and 6-wk electrified bars.
At extremely low current levels only, one past study found successful spontane-
ous settlement of benthic organisms on reef structures exposed to electric current
(Schuhmacher and Schillak, 1994). us, development of an oyster reef using elec-
trolysis in seawater may involve initially applying electrical current for a set period
of time to develop a strong reef framework of precipitated minerals, and then sepa-
rating the reef from power for a period of time to allow spat to settle and grow to a
size where they would not be smothered before reapplying power. is technique has
been shown to result in high survival of transplanted coral nubbins in the northern
Gulf of Aqaba in the Red Sea (van Treeck and Schuhmacher, 1997), and empirical
studies agree that long term survival of organisms is high on these electrified reefs
(Schuhmacher and Schillak, 1994; van Treeck and Schuhmacher, 1997; Sabater and
Yap, 2002, 2004).
We tested another approach for solving the potential spat overgrowth problem
by directly attaching juvenile oysters to the reef backbone. Previous studies with
transplanted coral and mollusks have shown that electrified reef structures acceler-
ate the growth of transplanted animals (Sabater and Yap, 2002, 2004). Our results
showed no significant growth enhancement for transplanted juvenile oysters, and
lower growth rates (1.4 ± 8.6 mm mo–1), compared to other oysters maintained at
the Grand Isle hatchery during the same time period (2.2 ± 2.0 mm mo–1). Although
localized effects are known to affect oyster growth rates and variable oyster growth
rates have been noted around the Grand Isle oyster hatchery previously (J. La Peyre,
pers. obs.), it is possible that the use of cement as an attachment medium affected our
results. Specifically, it is not clear if the electricity was still transferred to the organ-
isms or whether the cement acted as an insulator that kept current from reaching
the oysters. Interestingly, while increased pH in seawater is widely held to increase
calcification by marine organisms that make their shells and skeletons from calcium
carbonate (Kleypas et al., 1999; Riebesell et al., 2000; Zondervan et al., 2001), recent
evidence suggests that this may not always be the case. Calcification of the cocco-
lithophore species Emiliania huxleyi (Lohman) Hay and Mohler increased with high
CO2 partial pressures (Iglesias-Rodriguez et al., 2008); increased CO2 partial pres-
sures in seawater results in formation of carbonic acid, which causes a reduction in
NOTES 65
pH of the ocean water suggesting that this long-held belief may need to be examined
on a more species specific level.
e establishment of oyster reefs using electrolysis of seawater to create a strong
framework to support reef development is particularly attractive for shoreline stabi-
lization. Reefs created using this approach have been found to be effective in reduc-
ing wave energies and protecting shorelines (Goreau et al., 2000, 2004). e dual
spawning seasons in Louisiana ensure that high concentrations of spat are available
for settlement (Supan and Wilson, 2001). Our results demonstrate that along the
Louisiana coast, the use of electrolysis of seawater to induce cathodic accretion of
minerals has the potential for mineral precipitation and growth in high salinity wa-
ters. While the data failed to demonstrate enhanced oyster recruitment or growth
on the experimental reef structures, there may be ways to manage structures initially
developed through electrolysis of seawater, to support long-term development of oys-
ter reefs. Furthermore, as predation by oyster drill may be an issue in high salinity
waters, it may be worth investigating the lower salinity ranges and the potential for
electrolysis of seawater in areas that either experience only short periods of high
salinity, or are maintained at lower salinities such that predation does not decimate
the oyster population. In a region such as coastal Louisiana where the landscape is
dominated by soft sediments, and there is a need for hard, clean substrate, further
investigation of the use of this approach to create a substrate for development and
future growth of oyster reefs could be beneficial.
A
is work was supported by a grant to M. Piehler from the Louisiana Sea Grant Undergrad-
uate Research Opportunity Program. We thank T. Goreau for help in experimental reef set-up
and for helpful discussions regarding past work and observations related to the BiorockTM sys -
tem. We thank M. Miller for technical assistance and help in locating an appropriate power
source. We thank J. Supan for supplying juvenile oysters. We thank W. Gayle for help in data
entry and graphical presentation of the data. Use of BioRockTM technique for creation of the
experimental units does not imply endorsement of the process by the U.S. Geological Survey,
Department of Interior. LSU AgCenter (contribution number 2008-242-2095).
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D S: 25 June, 2008.
D A: 2 October, 2008.
A O: 11 November, 2008.
A: (B.P.P., M.K.L.P., B.P.G.) School of Renewable Natural Resources, Louisiana State
University Agricultural Center, Baton Rouge, Louisiana 70803. (M . K. L. P.) USGS, Louisiana
Fish and Wildlife Cooperative Research Unit, School of Renewable Natural Resources, Louisi-
ana State University Agricultural Center, Baton Rouge, Louisiana 70803. ( J. F. L. P.) Department
of Veterinary Science, Louisiana State University Agricultural Center, Baton Rouge, Louisiana
70803. C A: (B.P.L.P.) E-mail: <ml apey@lsu.edu>.
... Calcium ions Ca 2þ from seawater combine with dissolved bicarbonate HCO 3 À to precipitate as aragonite CaCO 3 and magnesium ions Mg þ with hydroxide ions to precipitate as brucite Mg(OH) 2 . Several experiments have been conducted to study the effect of this mineral accretion method on survival and growth rate of marine calcifying organisms, such as corals and oysters (Borell et al., 2010;Piazza et al., 2009;Yap, 2002, 2004;van Treeck and Schuhmacher, 1997). Results vary considerably, since some studies on the effect of the mineral accretion method report increased survival rate of coral transplants (van Treeck and Schuhmacher, 1997;Sabater and Yap, 2002) and enhanced coral growth rate (Sabater and Yap, 2004) whereas other studies show lower growth rates for juvenile oysters (Piazza et al., 2009) and no effect or a negative effect on coral survival (Borell et al., 2010). ...
... Several experiments have been conducted to study the effect of this mineral accretion method on survival and growth rate of marine calcifying organisms, such as corals and oysters (Borell et al., 2010;Piazza et al., 2009;Yap, 2002, 2004;van Treeck and Schuhmacher, 1997). Results vary considerably, since some studies on the effect of the mineral accretion method report increased survival rate of coral transplants (van Treeck and Schuhmacher, 1997;Sabater and Yap, 2002) and enhanced coral growth rate (Sabater and Yap, 2004) whereas other studies show lower growth rates for juvenile oysters (Piazza et al., 2009) and no effect or a negative effect on coral survival (Borell et al., 2010). Surprisingly, studies on the effect of electrolysis on mollusk and coral biomineralization have only focused on biometric analysis of calcifying tissues. ...
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