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In Situ Effects of Low pH and Elevated HCO3- on Juvenile Massive Porites spp. in Moorea, French Polynesia


Abstract and Figures

Juvenile colonies of massive Porites spp. were exposed to manipulated pH and bicarbonate ([HCO3(-)]) in situ to test the hypothesis that ocean acidification (OA) does not affect respiration and calcification. Incubations lasted 28 h and exposed corals to ambient temperature and light with ecologically relevant water motion. Three treatments were applied: (1) ambient conditions of pH 8.04 and 1751 μmol HCO3(-) kg(-1) (Treatment 1), (2) pCO2-induced ocean acidification of pH 7.73 and 2011 μmol HCO3(-) kg(-1) (Treatment 2), and (3) pCO2 and HCO3(-)-enriched seawater of pH 7.69 and 2730 μmol HCO3(-) kg(-1) (Treatment 3). The third treatment providing elevated [HCO3(-)] was used to test for stimulatory effects of dissolved inorganic carbon on calcification under low pH and low saturation of aragonite (Ωarag), but it does not reflect conditions expected to occur under CO2-driven OA. Calcification of juvenile massive Porites spp. was affected by treatments, with an 81% elevation in Treatment 3 versus Treatment 1, but no difference between Treatments 1 and 2; respiration and the metabolic expenditure concurrent with calcification remained unaffected. These findings indicate that juvenile massive Porites spp. are resistant to short exposures to OA in situ, and separately, that they can increase calcification at low pH and low Ωarag if [HCO3(-)] is elevated. Juvenile Porites spp. may therefore be limited by dissolved inorganic carbon under ambient pCO2 conditions.
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In Situ Effects of Low pH and Elevated HCO
Juvenile Massive Porites spp. in Moorea,
French Polynesia
Department of Biology, California State University Northridge, 18111 Nordhoff Street, Northridge,
California 91330-8303, and
College of Life Sciences, Santa Monica College, 1900 Pico Boulevard,
Santa Monica, California 90405-1628
Abstract. Juvenile colonies of massive Porites spp. were
exposed to manipulated pH and bicarbonate ([HCO
]) in
situ to test the hypothesis that ocean acidification (OA) does
not affect respiration and calcification. Incubations lasted
28 h and exposed corals to ambient temperature and light
with ecologically relevant water motion. Three treatments
were applied: (1) ambient conditions of pH 8.04 and 1751
mol HCO
(Treatment 1), (2) pCO
-induced ocean
acidification of pH 7.73 and 2011
mol HCO
(Treatment 2), and (3) pCO
and HCO
-enriched seawater
of pH 7.69 and 2730
mol HCO
(Treatment 3). The
third treatment providing elevated [HCO
] was used to test
for stimulatory effects of dissolved inorganic carbon on
calcification under low pH and low saturation of aragonite
), but it does not reflect conditions expected to occur
under CO
-driven OA. Calcification of juvenile massive
Porites spp. was affected by treatments, with an 81% ele-
vation in Treatment 3 versus Treatment 1, but no difference
between Treatments 1 and 2; respiration and the metabolic
expenditure concurrent with calcification remained unaf-
fected. These findings indicate that juvenile massive Porites
spp. are resistant to short exposures to OA in situ, and
separately, that they can increase calcification at low pH and
if [HCO
] is elevated. Juvenile Porites spp. may
therefore be limited by dissolved inorganic carbon under
ambient pCO
Coral reefs are threatened by natural and anthropogenic
factors, including global climate change and ocean acidifi-
cation (OA) (Hoegh-Guldberg et al., 2007; Wilkinson,
2008). OA poses a particularly severe threat, because if
corals do not first succumb to rising temperature (Hoegh-
Guldberg et al., 2007), OA challenges their capacity to
produce calcareous skeletons (Hofmann et al., 2010). OA
originates in the dissolution of atmospheric CO
in seawa-
ter, resulting in decreases in seawater pH, the concentration
of carbonate ([CO
]), and the saturation state of aragonite
), and an increase in the concentration of bicarbonate
]) (Kleypas et al., 1999). Together, these effects
impair the ability of corals to deposit CaCO
(Jury et al.,
2010; Hofmann et al., 2010; Edmunds et al., 2012; Comeau
et al., 2013b), and diminish their capacity to function as
ecosystem engineers (Wild et al., 2011).
Laboratory studies have contributed much to the under-
standing of the response of corals to OA (Andersson and
Gledhill, 2013), particularly with regard to the underlying
physiological mechanisms (Erez et al., 2011). However,
laboratory experiments typically are performed under con-
ditions that do not replicate the natural environment per-
fectly, notably by providing light at a constant low intensity
throughout the day, and with a spectral composition differ-
ing from ambient sunlight (Kinzie et al., 1984; Schlacher et
al., 2007). Light stimulates coral calcification (Gattuso et
al., 1999), fuels Symbiodinium photosynthesis, and facili-
tates autotrophic nutrition by the holobiont (Pearse and
Muscatine, 1971). Also of great importance to corals is the
hydrodynamic regime of reef environments (Monismith,
2007), which is difficult to recreate in tanks and yet is
Received 18 February 2013; accepted 5 September 2013.
* To whom correspondence should be addressed. E-mail: cbw0047@
, saturation state of aragonite; OA, ocean acidifica-
tion; PAR, photosynthetically active radiation; TA, total alkalinity.
Reference: Biol. Bull. 225: 92–101. (October 2013)
© 2013 Marine Biological Laboratory
critical for multiple aspects of coral physiology (Dennison
and Barnes, 1988; Patterson et al., 1991). Given the limi-
tations of laboratory experiments, there is a need to evaluate
the effects of OA on corals in the natural environment
(Langdon and Atkinson, 2005; Holcomb et al., 2010). In
situ experiments have been used to study the physiology of
corals and coral reefs for decades (Porter, 1980; Pearson et
al., 1984; Carpenter, 1985; Patterson et al., 1991; Ralph et
al., 1999; Levy et al., 2006), but to our knowledge, there
have been only two studies in which in situ approaches have
been used to study the effects of OA on corals (Chauvin et
al., 2011; Klein et al., 2012). Natural CO
seeps afford the
potential to study aspects of the effects of OA in the natural
environment (Hall-Spencer et al., 2008; Fabricius et al.,
2011), but the rarity of such locations and their suitability
mostly for mensurative analyses (sensu Hurlbert, 1984)
limits the general application of this approach.
The objective of this study was to utilize chambers placed
in situ to examine the effects of low pH and high [HCO
on juvenile massive Porites spp. Three hypotheses were
tested: (1) low pH and high [HCO
] have no effect on
respiration or calcification, (2) the energetic expenditure
concurrent with calcification is unaffected by OA, and (3)
low pH impairs calcification at high [HCO
] (2730
seawater). As rationales for these hypotheses, we
reasoned that OA typically reduces coral calcification
(Comeau et al., 2013b) and can increase the rate of respi-
ration (Crawley et al., 2010; but see Edmunds, 2012), with
the energetic cost of depositing CaCO
referred to as a
cause of stimulatory effects of OA on respiration (Fabry et
al., 2008; Erez et al., 2011). As elevated [HCO
] can
mitigate the negative effects of OA in vitro (Marubini et al.,
2008; Comeau et al., 2012), it was compelling to test for
similar effects in situ.
Materials and Methods
Experimental design
We used three seawater treatments of contrasting pH and
carbonate chemistry to examine the effects of OA and
elevated [HCO
] on juvenile massive Porites spp. Juvenile
massive Porites spp. in Moorea is a functional group con-
sisting of about 85% P. lutea (Edwards and Haime, 1860)
and about 15% P. lobata (Dana, 1846) (Edmunds, 2009),
which are difficult to distinguish from each other while
alive. Experiments were performed in three trials (begin-
ning on 4, 6, and 8 February 2012), with six corals in each
trial exposed to treatments (n2 corals treatment
) for
28 h in acrylic chambers at 1.5-m depth on a fringing reef
(Fig. 1). Seawater treatments contrasted (1) ambient pH and
ambient [HCO
] (Treatment 1, the control against which
the other treatments were gauged); (2) low pH and high
] (Treatment 2); and (3) low pH and very high
] (Treatment 3). Treatment 1 represented present
seawater conditions on the shallow reefs of Moorea (pH
8.00 and 1750
mol HCO
); Treatment 2 reflected
the seawater conditions projected to occur in 100 y (pH
7.70 and 2000
mol HCO
) under representative
concentration pathway (RCP) 6.0 (95 Pa pCO
) (van
Vuuren et al., 2011); and Treatment 3 (pH 7.70 and 2700
mol HCO
) tested the hypothesis that elevated
] stimulates calcification at reduced
and low
pH. Although Treatment 3 is not a feasible combination of
future seawater conditions attributable to OA, it allowed
further exploration of the role of HCO
in coral calcifica-
tion (sensu Comeau et al., 2012), as well as consideration of
the possibility that elevated [HCO
] alleviates the effects
of low pH (Comeau et al., 2012). We reasoned that the
results of Treatment 3 would therefore be valuable in de-
signing future experiments focused on the mechanism by
which corals can respond to OA.
Nine chambers (2.47 l) were made from custom-cast,
UV-transparent acrylic (Industrial Plastics, California),
which transmits UV A and B radiation (280 400 nm) and
phosynthetically active radiation (PAR; 400 –700 nm)
(Gleason et al., 2006). A test of the light transmission
qualities (for UV A and PAR) of the acrylic polymer was
performed using a cosine-corrected sensor attached to a
fiber optic cable and a spectrophotometer (USB-2000,
Ocean Optics, Dunedin, Florida) and operated using a PC
running the manufacturer’s software (Spectra Suite 2.0,
Ocean Optics). Light was measured with spectral resolution
between 350 and 1000 nm and corrected against a known
color temperature (°Kelvin). There was minimal attenuation
of UV A and PAR, with greater than 96% transmission
through the acrylic of light between 350 nm and 1000 nm
wavelength (Fig. 2); transmission of light at wavelengths
below 350 nm was not measured.
Three chambers were allocated to each treatment, with
two exposing corals to treatments (n1 coral chamber
and one filled with seawater alone as a control. All cham-
bers were filled with fresh seawater at the start of each trial
(n3), and stirred with a motor (Playmobil, Zirndorf,
Germany) that provided a mean flow speed of 15.8 1.3
cm s
(SE, n12) in the chamber. Flow speeds were
determined in a preliminary study in which hydrated Ar-
temia spp. eggs within the chambers were photographed
(after Sebens and Johnson, 1991). At the start of each trial,
chambers were filled with treatment seawater, and bubbles
were removed before the chambers were sealed and placed
on the reef. Chambers remained on the reef for 28 h, but
were retrieved at 4-h intervals to replenish treatment sea-
water and batteries in the stirring motors. This procedure
generally took less than 10 min, and corals were kept
submerged at all times. Light intensity (PAR) and seawater
temperature adjacent to the chambers were recorded at
10-min intervals using a 4spherical quantum sensor
(MkV-L, JFE Advantech Co., Kobe, Japan) and Hobo Wa-
ter Temp Pro v2 loggers (0.2 °C, Onset Computer,
Massachusetts), respectively.
Coral collection and carbonate chemistry
Juvenile Porites spp. (4.0-cm diameter, n18) were
collected from 2–3-m depth in the back reef of Moorea,
between 28 January 2012 and 1 February 2012. Corals were
transported to the Richard B. Gump South Pacific Research
Station, attached to plastic bases with epoxy (Z-Spar A788),
and placed in a flow-through tank (1000 l) to recover. This
tank was filled with filtered seawater (50
m) at about 27.5
°C and illuminated with two 75-W LED lamps (Sol White
LED Module, Aquaillumination, Iowa) that supplied light at
515 30
mol photons m
(mean SE, n75), as
measured underwater using a spherical quantum sensor (Li-
Cor LI-193). Colonies remained in the recovery tank until
used in the trials.
To create pH and [HCO
] treatments, the pCO
seawater was manipulated by bubbling ambient air or CO
enriched air into three 150-l reservoirs supplied with filtered
seawater (50
m). Gas mixtures were created using a sole-
noid-controlled system (Model A352, Qubit Systems, On-
Figure 1. Photo of (A) juvenile massive Porites spp., (B) fringing reef off the Richard B. Gump South
Pacific Research Station in Cook’s Bay, Moorea, and (C) the 2.47-l experimental chambers (23 x 23 x 8 cm) each
housing a single coral colony in situ.iLocation of chambers on the fringing reef at 1.5-m depth, ii juvenile
colony attached to mesh with epoxy, and iii submersible stirring motor. (D) Schematic illustrating the location
of coral and stirring motor within chambers. Photo credits (A) D. Liittschwager, used with permission; (B–D)
C. Wall.
tario, Canada), as detailed in Edmunds (2012). Seawater for
Treatment 1 was un-manipulated; seawater for Treatment 2
was equilibrated with pCO
at 98.9 Pa; seawater for Treat-
ment 3 was obtained by augmenting Treatment 2 seawater
with baking soda (0.9 mmol l
, Church and
Dwight Co., California) to increase [HCO
] to 2730
at pH
of 7.69. Seawater conditions within the three
reservoirs were monitored for salinity, pH on the total scale
), total alkalinity (TA,
mol kg
), and carbonate
chemistry for one week prior to the start of the experiment,
and daily during the experiment (4 –9 February). Analyses
of seawater were conducted according to standard operating
procedure (SOP) 3b of Dickson et al. (2007), as detailed in
Comeau et al. (2012), with final values for carbon chemistry
calculated using the seacarb script in R(R Foundations for
Statistical Computing; Lavigne and Gattuso, 2011). Titra-
tions of certified reference materials (Batch 105) provided
by A. G. Dickson (Scripps Institution of Oceanography)
revealed determination of TA to be within 0.3– 4.5
of certified TA values (i.e., 0.2%).
Calcification and respiration
After 24-h incubations, the response of the corals was
assessed as rates of calcification and respiration. Calcifica-
tion was measured in situ as the change in TA of seawater
over 4 h (after Smith and Key, 1975) from about 1000 to
1400 h, with the volume of the stirring motor and coral
subtracted from each seawater sample. Calcification was
adjusted for the change in TA in the control chambers, and
standardized by time and the surface area of the coral tissue
g CaCO
). Surface area of the corals was
determined using aluminum foil (Marsh, 1970) and was
completed after the respiration measurements.
After measurements of calcification (at 1500 h), corals
were transferred to respirometers (0.24 l) filled with filtered
seawater (50
m) representing the same conditions as the in
situ treatments, and kept in darkness for about 30 min to
reduce the effects of light on respiration (Edmunds and
Davies, 1988). After dark acclimation, respiration of corals
was measured at 28 °C with a stir bar used to create a flow
speed of 3.0 0.2 cm s
(mean SE, n16) determined
as described above. Changes in pO
in seawater were mea-
sured with optrodes (Foxy-R, 1.58-mm diameter, Ocean
Optics) calibrated against water-saturated air (100%) and a
zero solution (saturated sodium sulfite in 0.01 mol l
sodium tetraborate), and connected to spectrophotometers
(USB2000 and NeoFox, Ocean Optics) that logged O
uration onto a PC running Ocean Optics software (OOISen-
sors, ver. 1.00.08 and NeoFox Viewer ver. 2.3, Ocean
Optics). O
saturation was converted to
mol O
using tabulated values for O
solubility [N. Ramsing and J.
Gundersen at Unisense (Aarhus, Denmark), based on Garcia
and Gordon (1992)], yielding a saturated oxygen content of
mol O
at 28 °C, a salinity of 36, and 101.3
kPa atmospheric pressure. Respiration rates were deter-
mined by regressing O
concentration against time, correct-
ing for O
fluxes in control chambers, and standardizing
oxygen consumption to the surface area of the coral tissue
Respiration rates were used to estimate the metabolic
energy expenditure concurrent with calcification by con-
verting hourly O
consumption to energy units (joules)
using a conversion of 440 J mmol O
(Elliot and Davison,
1975), assuming lipids are the primary respiratory substrate
(Patton et al., 1977). Respiratory energy expenditure (J
) was normalized to the calcification rate (
) and expressed as J g
to eval-
uate the energy expenditure concurrent with calcification.
Statistical analyses
Calcification, respiration, energy expenditure concurrent
with calcification, and incubation conditions (pH
]) were compared among treatments using one-way
ANOVA. As consecutive trials were performed over 3 d
with potential differences in physical conditions among
days, a one-way ANOVA was used to test for differences in
PAR and temperature among trials. Post hoc analyses were
performed using Tukey’s Honestly Significant Difference
(HSD) test, and statistical assumptions of normality and
homoscedasticity required for ANOVA were tested by
graphical analysis of residuals. Analyses were completed
Figure 2. The spectral comparison of the relative irradiance of sun-
light measured from 350 to 1000 nm and corrected against a known color
temperature (°Kelvin) for natural sunlight (black) and sunlight transmitted
through UV-transmitting acrylic polymer (gray).
with Systat 11 software (Systat, Inc., Illinois) using a Win-
dows operating system. Power analysis was performed on
select results according to Zar (2010).
The chemical conditions of seawater used to fill the
chambers were maintained with a high degree of precision
(Table 1) and differed significantly among treatments in
terms of pH
44.84, P0.001) and [HCO
720.79, P0.001). Although the chemical con-
ditions were affected by the incubations, notably for oxygen
concentration (through the effects of respiration and photo-
synthesis) and TA (through the effects of calcification),
these effects were minimized by changing the seawater
every 4 h. Mean salinity throughout the treatments was
35.9 0.1 (SE, n16), and the mean temperature was
28.8 0.2 °C (n514, pooled among days). However,
temperature differed among trials (F
35.75, P
0.001) and was slightly warmer during the last trial (8
February, 29.4 0.2 °C) than the first trial (4 February,
28.2 0.2 °C) (both mean SE, n170 –171). The mean
light intensity during the daylight portion of the incubations
was 1072 51
mol photons m
(SE, n227
measurements over 3 trials), but sustained maximum inten-
sities reaching about 2300
mol photons m
. Inte-
grated each day, the light regime provided 52.3 8.5 mol
photons m
(SD, n3 days). Over the 4-h period
when calcification was measured (i.e., 1000 –1400 h), the
mean irradiance at 1.5-m depth was 1769 60
photons m
(SE, n86) and light did not differ
among trials (F
2.17, P0.121).
All corals appeared healthy and showed no visible signs
of stress or loss of pigmentation during the experiments;
however, the loss of two seawater samples in Treatment 2
reduced calcification measurements to four corals. The in
situ incubations proved successful in exposing corals to
seawater of contrasting carbonate chemistry under physical
conditions similar to those in the natural environment. Rates
of calcification in the light (mean SE) for control corals
(e.g., Treatment 1) were 67.92 14.48
g CaCO
and 52.19 8.76
g CaCO
and 123.04
g CaCO
in Treatment 2 and Treatment 3,
respectively. Hourly calcification rates differed significantly
among treatments (F
6.56, P0.011) (Fig. 3).Post
hoc analyses revealed that hourly calcification was in-
creased significantly in Treatment 3 (HCO
compared to Treatment 1 (ambient conditions) (P0.05),
but no other contrasts were significant (P0.50). Calcifi-
cation in Treatment 3 (HCO
-enriched) was significantly
higher than in other treatments and was elevated 82% and
113% relative to Treatment 1 and Treatment 2, respectively
(Fig. 3). Mean respiration (SE) ranged from 0.39 0.05
mol O
(Treatment 2) to 0.46 0.06
mol O
(Treatment 3), but it did not vary significantly
among treatments (F
0.37, P0.697) (Fig. 3),
although the power (1-
) of this test was low (0.30).
Converting respiratory O
consumption to joules revealed
a mean energy expenditure of 0.17 to 0.22 J cm
corals across treatments, which corresponded to mean en-
ergy expenditures per colony of 3.30 to 4.38 J h
, and
calcification rates of 52.19
g and 123.04
g CaCO
. Together these values revealed that the energy expen-
diture coincident with calcification ranged from (SE)
1804 260 to 3461 459Jg
in Treatment 3 and
Treatment 2, respectively (Fig. 3), although these values did
not differ among treatments (F
2.35, P0.135).
Laboratory-based mesocosms have dominated studies of
the effects of ocean acidification (OA) on corals, in part due
to the logistical challenges of manipulating seawater chem-
istry, but such approaches have limited utility in evaluating
how corals respond to similar conditions in situ. While
laboratory experiments are designed to create conditions
resembling those found in the natural environment at the
site where study corals are collected, many fall short of
attaining this objective. For instance, the common use of
low-intensity artificial lighting does not reflect conditions
Table 1
Summary of physical and chemical conditions for three treatments of Porites spp. colonies maintained in 2.47-l acrylic chambers in situ at 28.8 °C
between 4 and 9 February 2012
Treatment pH
mol kg
mol kg
mol kg
mol kg
1 8.04 0.01 2359 3 41.1 0.8 1751 6 246 2 2008 4 3.95 0.03
2 7.73 0.04 2358 2 98.9 9.5 2011 25 141 10 2177 17 2.27 0.15
3 7.69 0.03 3131 8 146.4 10.5 2730 21 171 8 2938 16 2.75 0.12
Values are mean SE (n6 for carbonate chemistry; TA total alkalinity; DIC dissolved inorganic carbon;
aragonite saturation state;
Treatment 1 ambient pH, ambient HCO
; Treatment 2 (e.g., end of century pCO
)low pH, high HCO
; Treatment 3 (e.g., pCO
and HCO
enriched) low pH, very high HCO
. Carbonate chemistry was calculated using the seacarb package in R.
on shallow reefs or simulate the intensity and spectral
composition of sunlight reaching the study depth (Kinzie et
al., 1984). As the quality and quantity of light has important
implications for photoautotrophic nutrition and calcification
in corals (Kinzie et al., 1984; Barnes and Chalker, 1990;
Grottoli and Wellington, 1999) and affects the response of
coral fragments (Suggett et al., 2013) and coral recruits
(Dufault et al., 2013) to pCO
, it is clear that light is one
physical parameter that would benefit from closer attention.
To address this limitation (and others) of laboratory-based
experiments, here we emphasize the utility of simple in situ
approaches (Chauvin et al., 2011). Our in situ approach has
the potential to support tests for short-term biological ef-
fects of OA under physical conditions closely resembling
those found on coral reefs, and may provide a means to
evaluate effects of various light treatments (both UV and
PAR) on coral calcification under ambient and elevated
The mean intensity of PAR adjacent to the incubation
chambers at 1.5-m depth (1072 51
mol photons m
) was 2–10 times higher than irradiances reported in OA
studies in which incubations have been conducted in labo-
ratory mesocosms. For instance, Edmunds (2012) reported
an intensity of 537
mol photons m
, Schneider and
Erez (2006) reported a light intensity of about 350
photons m
, Crawley et al. (2010) employed a maxi-
mum irradiance of about 110
mol photons m
, and
Albright et al. (2008) used a mean light intensity of less than
mol photons m
. However, a few OA experiments
have been performed under natural irradiances in outdoor
mesocosms on land (noon irradiances 700
mol photons m
; Anthony et al., 2008) and in flumes
(17 mol– 40 mol photons m
; Langdon and Atkinson,
2005). In our study, not only was the light intensity greater
than in most previous studies using corals from shallow
reefs, but our incubations also included UV radiation, which
is important for the rhythmicity of Symbiodinium photosyn-
thesis in hospite (Sorek and Levy, 2012), coral bleaching
(Lesser, 1997), and photooxidative stress in Symbiodinium
(Shick et al., 1995). Moreover, exposure to ecologically
relevant intensities of UV radiation may favor slight reduc-
tions in calcification rates (Jokiel and York, 1982; Roth et
al., 1982) that are relevant to corals under natural condi-
tions. While explicitly evaluating the role of UV radiation
on coral calcification was beyond the scope of this study, we
note that in situ chambers create the potential for such
analyses in the future using UV-transparent and UV-absorb-
ing polymers (Shick et al., 1995; Gleason et al., 2006). This
would not be possible in the laboratory using metal-halide
and LED lamps that do not emit significant radiation at less
than 400 nm (Schubert and Kim, 2005), and instead would
require specialized UV lights (Shick et al., 1999). We
suggest that our in situ approach could be used to examine
OA effects on corals using light conditions (PAR and UV)
of similar intensity and flux (i.e., diel oscillations) to those
experienced by corals on the reef. Striving for ecologically
relevant light treatments should be a priority of OA research
in the laboratory.
In addition to the advantages of being able to manipulate
UV light to test for interactive effects with OA on corals, in
situ chambers provide access to high intensities of PAR that
are important to symbiotic corals for photosynthesis (Da-
vies, 1991) and calcification (Barnes and Chalker, 1990;
Anthony et al., 2002; Allemand et al., 2004). Light intensity
also modulates the response of corals to OA (Marubini et
al., 2001; Suggett et al., 2013), with recent work character-
Figure 3. Calcification, respiration, and the metabolic expenditure
concurrent with calcification in juvenile massive Porites spp. exposed for
28 h to three treatments (ambient seawater [SW], ocean acidification
conditions for the year 2100, and HCO
-enriched/low pH treatment) in
chambers maintained in situ. (A) Calcification (left ordinate) and respira-
tion (right ordinate) normalized to surface area (cm
). (B) Metabolic
expenditure concurrent with calcification. Treatments significantly affected
calcification (P0.011), with letters indicating treatments that differed as
determined from post hoc analysis; there was no effect of treatment on
respiration (P0.697) or metabolic expenditure concurrent with calcifi-
cation (P0.135). Values displayed are mean SE.
izing a pCO
-dependent shift in the nonlinear relationship
between light and calcification in recruits of Pocillopora
damicornis (Dufault et al., 2013). For these recruits, light
intensity affected calcification rates with an effect at 50
versus 89 Pa pCO
at intermediate light intensities (70
mol photons m
), but reduced effects at lower and
higher light intensities (31
mol vs. 226
mol photons m
) (Dufault et al., 2013). It has been hypothesized that the
effect of light intensity on the response of corals to OA is
driven by a light-dependent utilization of HCO
that stim-
ulates calcification and reduces the negative implications of
decreasing [CO
] at low pH (Gattuso et al., 1999;
Comeau et al., 2012). The high rates of calcification in
Treatment 3 (HCO
-enriched) of the present study lend
support to this possibility, particularly since ecologically
relevant high light intensities were attained with our in situ
Few studies have reported hourly calcification rates for
massive Porites spp., and no study has attempted to measure
in situ calcification under pCO
enrichment and ambient
light intensities. As a result, there are no comparable data
with which our results can be contrasted for the purpose of
testing for generality. However, hourly calcification rates
recorded in the light during the present study are within the
range reported for Porites (Comeau et al., 2013a), and when
extrapolated to a 24-h day assuming calcification in the light
is 3-fold greater than in the dark (Gattuso et al., 1999) and
each day consists of 12-h light and 12-h dark, daily calci-
fication for juvenile Porites spp. is similar to that previously
reported for this taxon in Moorea (Edmunds, 2011, 2012).
Therefore, it is reasonable to interpret our results as eco-
logically relevant, with treatment effects that are insightful
for understanding how corals respond to similar conditions
on the reef.
The calcification rate of juvenile massive Porites spp. in
the light under ambient conditions (e.g., Treatment 1) did
not differ significantly from calcification under conditions
mimicking those expected by the end of the present century
(e.g., Treatment 2). The Treatment 2 result is inconsistent
with studies showing that OA reduces coral calcification
(Langdon and Atkinson, 2005; Schneider and Erez, 2006;
Comeau et al., 2013b), but is consistent with reports that
Porites is resistant to OA (Fabricius et al., 2011; Edmunds,
2012; Edmunds et al., 2012; Comeau et al., 2013a). How-
ever, interpreting Treatment 2 results relative to previous
studies is complicated by the measurement here of calcifi-
cation at high mid-day irradiances that probably would have
maximized both calcification and photosynthesis. It remains
possible that the present results might have differed during
longer incubations lasting weeks or months that would also
have captured the effects of reduced dark calcification and
skeletal dissolution at low saturation of aragonite (
In Treatment 3, HCO
-enrichment (2730
mol kg
increased calcification 81% relative to ambient conditions,
despite coinciding with reduced
and low pH. Stimu-
lation of calcification in corals through elevated [HCO
has been reported previously (Marubini and Thake, 1999;
Herfort et al., 2008; Comeau et al., 2012); furthermore,
under OA conditions, elevated [HCO
] increases coral
calcification (Marubini et al., 2008; Jury et al., 2010; de
Putron et al., 2011; Comeau et al., 2012). At constant pH
for example, calcification in Madracis auretenra declined
with reduced [HCO
] but was stimulated with increased
], yet it was unresponsive to pH
(7.6 – 8.1) and
(1.7– 4.3) (Jury et al., 2010). HCO
enrichment (2.0
mmol l
above ambient seawater) stimulated calcification
in Stylophora pistillata at pH
7.6, 8.0, and 8.2, suggesting
that calcification may be carbon-limited in this species
(Marubini et al., 2008). While the exposure period for the
treatments was short in the current study (28 h), short
incubations (3 h) have been useful in past experiments
examining the effects of nutrients and pCO
enrichment on
corals (Furla et al., 2000; Langdon and Atkinson, 2005;
Chauvin et al., 2011; Cumbo et al., 2013). While the im-
plications of the short incubations in the present study
deserve attention, our finding nevertheless suggests that
calcification in juvenile Porites spp. may not be DIC satu-
rated in situ at ambient [HCO
In tropical corals, calcification requires substantial
amounts of metabolic energy and may account for 13%–
30% of the entire metabolic costs (Allemand et al., 2011).
These costs are met by the supply of ATP from aerobic
respiration (with smaller amounts of ATP presumably from
glycolysis), and therefore aerobic respiration is likely to
reflect changes in the cost of calcification. The extent to
which shifts in costs of calcification can be detected through
measurements of aerobic respiration depends, however, on
the magnitude of the changes involved and the degree to
which the demands of other energy-requiring processes
covary. In the present study, treatments had no effect on
aerobic respiration, and we were unable to detect a change
in the total metabolic costs concurrent with calcification,
even though calcification increased 81% with elevated
]. The biological meaning of the null effect of OA
(Treatment 2) and HCO
enrichment (Treatment 3) on
coral respiration is uncertain, in part due to low statistical
power to detect an effect of the treatment on this dependent
Given the effect of [HCO
] on calcification in Treat-
ment 3 and a mean metabolic cost of calcification of 1804 J
, aerobic respiration should have increased from
to equal the metabolic costs
scaled proportionately from the control costs (i.e., Treat-
ment 1) associated with calcification (e.g., Jg
). In
contrast to this prediction, the mean aerobic respiratory
demand in Treatment 3 was indistinguishable from that of
corals in other treatments, indicating that elevated [HCO
stimulated calcification without affecting metabolic expen-
diture. Resolving the causes of this thermodynamic paradox
(i.e., the increase in a metabolically costly process without
an increased demand for metabolic energy) was beyond the
scope of this study, but we suspect it might reflect hyperoxic
stimulation of respiration (Shick, 1990) after maximum
photosynthesis in the control corals, which could conceal
the additional costs associated with enhanced calcification.
Alternatively, it is possible that respiration rates were mass-
transfer-limited in the small respiration chambers em-
ployed, where we were only able to create a flow speed of
3.0 cm s
, which is lower than the 15.8 cm s
in the in
situ chambers where calcification was measured. Notwith-
standing these limitations, our results for the respiration of
corals exposed to OA are at least inconsistent with the
notion that the metabolic costs of calcification are elevated
by high pCO
(Erez et al., 2011).
With the important caveats that our short incubations
were conducted in the natural environment where we cannot
separate the effects of incubation duration from the effects
of ecologically relevant physical conditions relative to pre-
vious studies, our results suggest that the respiration of
massive Porites spp. is resistant to short-term exposure to
OA conditions. Previously, using laboratory-based meso-
cosms, we have shown that respiration of juvenile Porites
spp. is depressed 36% after longer exposures (11 d) to 87 Pa
(but not to 77 Pa pCO
), although, as reported here,
the metabolic costs concurrent with calcification were un-
affected by pCO
(Edmunds, 2012). Analyses of the effects
of pCO
on the dark respiration of corals generally have
provided equivocal results, with most studies reporting no
effects at pCO
levels as high as 207 Pa (Reynaud et al.,
2003; Anthony et al., 2008; Godinot et al., 2011; S.
Comeau, California State University Northridge, pers.
comm.), one study reporting a trend for increased light-
enhanced respiration with high pCO
(Crawley et al., 2010),
and a few studies reporting metabolic depression at 77– 87
Pa pCO
(Edmunds, 2012; Edmunds et al., 2013). Clearly
there is scope for further research on the impacts of OA on
coral respiration, specifically to resolve the effects of expo-
sure duration, pCO
regime, and the assemblage of physical
and chemical conditions that collectively determine the
ecological relevancy of manipulative experiments. The
completion of the necessary experiments in situ rather than
in vitro will need to be an important part of this agenda and
will likely be critical to advance a mechanistic appreciation
of the effects of OA on marine organisms (Kelly and Hof-
mann, 2013). In situ chambers, such as the ones we use
here, can play an important part in this effort.
This research was funded by the U.S. National Science
Foundation through grants BIO-OCE 08-44785 (to PJE) and
OCE 10-26852 to the Moorea Coral Reef Long-Term Eco-
logical Research site, gifts from the Gordon and Betty
Moore foundation, and a Graduate Thesis Support grant
from California State University, Northridge (CSUN) (to
CBW). This work was submitted in partial fulfillment of the
MS degree for CBW. We thank S. Comeau, N. Spindel, E.
Rivest, B. Lenz, and V.R. Cumbo for field assistance; D.
Liittschwager for the use of his photograph; and two anon-
ymous reviewers who improved an earlier draft of this
paper. This is contribution number 199 of the CSUN Marine
Biology Program.
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... Since the analysis by Chan & Connolly (2013), other manipulative studies have demonstrated declines in coral calcification under acidified conditions (Iguchi et al. 2012, Crook et al. 2013, Drenkard et al. 2013, Okazaki et al. 2013, Comeau et al. 2014b, while Takahashi & Kurihara (2013) and Wall & Edmunds (2013) showed no such effect. Interestingly, both of the last-named studies were carried out under ambient light, whereas many laboratory exposures have been performed under low artificial light levels. ...
... In addition, workers are continually striving to introduce conditions that are more environmentally realistic into manipulative OA experiments. These include enclosing corals in chambers actually in situ on the reef (Okazaki et al. 2013, Wall & Edmunds 2013 or the construction of mesocosms around deconstructed reef communities in shallow reef areas (Dove et al. 2013). ...
The number of ecophysiological studies involving reef corals has increased markedly over the last 20 years, driven primarily by the concern over the potential effects of anthropogenic change on coral communities. In particular, the evaluation of the effects of global climate change has prompted major research efforts into understanding the consequences of both rising seawater temperatures and ocean acidification on the physiology of corals. In recent years the recognition that corals harbour not only symbiotic algae but also a diverse microbial consortium, which may both influence and be influenced by the physiology of the animal host, has added an extra layer of complexity to this biological system known collectively as the 'coral holobiont'. The present review draws together an extensive literature on ecophysiological responses of the coral holobiont to anthropo-genic change, with specific references to the latest molecular and genetic developments in the field. It also highlights gaps in our basic understanding of coral physiology and draws attention to the value of extreme physical habitats in elucidating the acclimatory and adaptive scope of reef corals to climate change.
... Most studies of the effects of high pCO 2 on corals have measured the outcome as perturbed rates of calcification attributed to low ⍀ without paying close attention to the underlying mechanism (Zeebe, 2012). Promising new lines of investigation addressing the mechanism by which coral calcification is depressed by high pCO 2 focus on at least four possibilities: the regulation of protons (H ϩ ) at the site of calcification (Jokiel, 2011), most likely with active transporters (Tambutté et al., 2011); perturbations in the activity of enzymes that modulate host-Symbiodinium interactions (Crawley et al., 2010); the role of nutrients (e.g., nitrogen, sulfur, and iron) in mediating changes in the quantity of biomass (i.e., heterotrophy [Edmunds, 2011]) and the rate of calcification (Holcomb et al., 2010); and the capacity of aerobic respiration to supply metabolic energy at high pCO 2 (Erez et al., 2011;Edmunds, 2012;Wall and Edmunds, 2013). The role of aerobic respiration in the response of corals to high pCO 2 is germane to a discussion of the effects of OA, because respiration plays a central role in metabolism (Prosser, 1991), and can be affected directly by high pCO 2 (Crawley et al., 2010;Edmunds, 2012). ...
... In March 2012 and June 2012 when the effects of pCO 2 were investigated, treatments were created by bubbling mixtures of pure CO 2 and air into seawater, and the seawater was assayed for pH and total alkalinity (TA), with the values used to calculate dissolved inorganic chemistry parameters. The techniques to manipulate pCO 2 and measure dissolved inorganic chemistry parameters have been applied in other experiments we have conducted, and the methodology is described elsewhere Wall and Edmunds, 2013). Both sets of experiments were designed to contrast ambient (control) pCO 2 of about 40 Pa with the elevated pCO 2 predicted for the end of the century (ϳ81 Pa; van Vurren et al., 2011), but actual pCO 2 values departed slightly from these targets (see Table 1). ...
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Early life stages of the coral Seriatopora cali-endrum were used to test the hypothesis that the depression of dark respiration in coral recruits by high pCO2 is caused by perturbed protein metabolism. First, the contribution of protein anabolism to respiratory costs under high pCO2 was evaluated by measuring the aerobic respiration of S. caliendrum recruits with and without the protein synthesis inhib-itor emetine following 1 to 4 days at 45 Pa versus 77 Pa pCO2 . Second, protein catabolism under high pCO2 was evaluated by measuring the flux of ammonium (NH4+) from juvenile colonies of S. caliendrum incubated in darkness at 47 Pa and 90 Pa pCO2 . Two days after settlement, respira-tion of recruits was affected by an interaction between emetine and pCO2 , with emetine reducing respiration 63% at 45 Pa pCO2 and 27% at 77 Pa pCO2 . The interaction disappeared 5 days after settlement, when respiration was reduced 27% by emetine under both pCO2 conditions. These findings suggest that protein anabolism accounted for a large proportion of metabolic costs in coral recruits and was affected by high pCO2 , with consequences detected in aerobic respiration. Juvenile S. caliendrum showed net up-take of NH 4 ϩ at 45 Pa pCO2 but net release of NH4 + at 90 Pa pCO2 , indicating that protein catabolism, NH4+ recycling, or both were affected by high pCO2 . Together, these results are consistent with the hypothesis that high pCO2 affects protein metabolism in corals.
... Considering that the calcification rates of P. australiensis and I. palifera decreased according to the increase of pCO 2 levels, these two species used in this study may be comparatively sensitive to acidified seawater. On the other hand, some studies have shown that massive Porites species was insensitive to acidified seawater (Fabricius et al., 2011;Edmunds et al., 2012;Wall and Edmunds, 2013), thus, differences of experimental conditions (e.g., light intensity as discussed above, experimental periods which affect acclimatization of corals; Marubini et al., 2008) should be also considered. ...
In this study, we report the acidification impact mimicking the pre-industrial, the present, and near-future oceans on calcification of two coral species (Porites australiensis, Isopora palifera) by using precise pCO2 control system which can produce acidified seawater under stable pCO2 values with low variations. In the analyses, we performed Bayesian modeling approaches incorporating the variations of pCO2 and compared the results between our modeling approach and classical statistical one. The results showed highest calcification rates in pre-industrial pCO2 level and gradual decreases of calcification in the near-future ocean acidification level, which suggests that ongoing and near-future ocean acidification would negatively impact coral calcification. In addition, it was expected that the variations of parameters of carbon chemistry may affect the inference of the best model on calcification responses to these parameters between Bayesian modeling approach and classical statistical one even under stable pCO2 values with low variations.
... The null effect of pCO 2 treatments on coral respiration agrees with studies, showing no effect of pCO 2 (up to 217 Pa) on the dark respiration of S. pistillata (Reynaud et al. 2003;Crawley et al. 2010;Godinot et al. 2011), Acropora intermedia or Porites lobata (120.1 Pa) (Anthony et al. 2008), A. eurystoma (56.0 Pa) (Schneider and Erez 2006), and Porites spp. at 76.6 Pa (Edmunds 2012) and 99 Pa (Wall and Edmunds 2013), but conflict with studies reporting OA reduces coral respiration (Edmunds 2012). In the present study, high flow rates may have stimulated coral respiration across treatments by reducing the diffusion boundary layer (DBL) across the corals tissue (Lesser et al. 1994), thereby reducing the ability to detect a treatment effect on respiration. ...
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The objective of this study was to test whether elevated pCO2 predicted for the year 2100 (85.1 Pa) affects bleaching in the coral Seriatopora caliendrum (Ehrenberg 1834) either independently or interactively with high temperature (30.5 °C). Response variables detected the sequence of events associated with the onset of bleaching: reduction in the photosynthetic performance of symbionts as measured by maximum photochemical efficiency (F v/F m) and effective photochemical efficiency (ΔF/F m′) of PSII, declines in net photosynthesis (P net) and photosynthetic efficiency (alpha, α), and finally, reduced chlorophyll a and symbiont concentrations. S. caliendrum was collected from Nanwan Bay, Taiwan, and subjected to combinations of temperature (27.7 vs. 30.5 °C) and pCO2 (45.1 vs. 85.1 Pa) for 14 days. High temperature reduced values of all dependent variables (i.e., bleaching occurred), but high pCO2 did not affect Symbiodinium photophysiology or productivity, and did not cause bleaching. These results suggest that short-term exposure to 81.5 Pa pCO2, alone and in combination with elevated temperature, does not cause or affect coral bleaching.
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Summary of the available experimental data examining the effect of ocean acidification conditions on coral calcification The table contains information on tropical reef-building corals only. Please note that for some studies level of response to seawater acidification was affected by an additional environmental factor such as nutrients, light intensity, etc. (detailed in ‘Notes’ column). Incubation time and incubation volume are only relevant for experiments that the TA method was used to derive net calcification rates. 0 value, calcification rates were not significantly different from ambient pH; TA, total alkalinity; BW, buoyant weight.
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As the oceans become less alkaline due to rising CO2 levels, deleterious consequences are expected for calcifying corals. Predicting how coral calcification will be affected by ongoing ocean acidification (OA) requires an accurate assessment of CaCO3 deposition and an understanding of the relative importance that decreasing calcification and/or increasing dissolution play for the overall calcification budget of individual corals. Here, we assessed the compatibility of the 45Ca-uptake and total alkalinity (TA) anomaly techniques as measures of gross and net calcification (GC, NC), respectively, to determine coral calcification at pHT 8.1 and 7.5. Considering the differing buffering capacity of seawater at both pH values, we were also interested in how strongly coral calcification alters the seawater carbonate chemistry under prolonged incubation in sealed chambers, potentially interfering with physiological functioning. Our data indicate that NC estimates by TA are erroneously ∼5% and ∼21% higher than GC estimates from 45Ca for ambient and reduced pH, respectively. Considering also previous data, we show that the consistent discrepancy between both techniques across studies is not constant, but largely depends on the absolute value of CaCO3 deposition. Deriving rates of coral dissolution from the difference between NC and GC was not possible and we advocate a more direct approach for the future by simultaneously measuring skeletal calcium influx and efflux. Substantial changes in carbonate system parameters for incubation times beyond two hours in our experiment demonstrate the necessity to test and optimize experimental incubation setups when measuring coral calcification in closed systems, especially under OA conditions.
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The sources and mechanisms of inorganic carbon transport for scleractinian coral calcification and photosynthesis were studied using a double labelling technique with H14 CO3 and 45 Ca. Clones of Stylophora pistillata that had developed into microcolonies were examined. Compartmental and pharmacological analyses of the distribution of 45 Ca and H14 CO3 in the coelenteron, tissues and skeleton were performed in dark or light conditions or in the presence of various seawater HCO3− concentrations. For calcification, irrespective of the lighting conditions, the major source of dissolved inorganic carbon (DIC) is metabolic CO2 (70–75 % of total CaCO3 deposition), while only 25–30 % originates from the external medium (seawater carbon pool). These results are in agreement with the observation that metabolic CO2 production in the light is at least six times greater than is required for calcification. This source is dependent on carbonic anhydrase activity because it is sensitive to ethoxyzolamide. Seawater DIC is transferred from the external medium to the coral skeleton by two different pathways: from sea water to the coelenteron, the passive paracellular pathway is largely sufficient, while a DIDS-sensitive transcellular pathway appears to mediate the flux across calicoblastic cells. Irrespective of the source, an anion exchanger performs the secretion of DIC at the site of calcification. Furthermore, a fourfold light-enhanced calcification of Stylophora pistillata microcolonies was measured. This stimulation was only effective after a lag of 10 min. These results are discussed in the context of light-enhanced calcification. Characterisation of the DIC supply for symbiotic dinoflagellate photosynthesis demonstrated the presence of a DIC pool within the tissues. The size of this pool was dependent on the lighting conditions, since it increased 39-fold after 3 h of illumination. Passive DIC equilibration through oral tissues between sea water and the coelenteric cavity is insufficient to supply this DIC pool, suggesting that there is an active transepithelial absorption of inorganic carbon sensitive to DIDS, ethoxyzolamide and iodide. These results confirm the presence of CO2-concentrating mechanisms in coral cells. The tissue pool is not, however, used as a source for calcification since no significant lag phase in the incorporation of external seawater DIC was measured.
seacarb calculates parameters of the seawater carbonate system and assists the design of ocean acidification perturbation experiments.
In aquatic invertebrates that form exoskeletons, the partitioning of energy between skeletal and tissue growth is an important tradeoff, especially under resource limitation or physiological stress. Here, we provide the first comparative analysis of energy investment into tissue and skeleton in corals. We develop a mathematical growth model based on colony geometry, tissue mass and quality (enthalpy), and predicted cost of calcification. For hemispherical colonies, the model predicts greater investment in tissue at small sizes, but a shift to skeletal-dominated growth at colony sizes greater than 5-14 cm radius, depending on tissue mass and quality. A similar transition occurs in branches, but is a function of radius and length. An experimental study to assess the impact of resource (light) limitation and physiological stress (sediment load) on energy partitioning in small hemispherical colonies (Goniastrea retiformis Lamarck) and branches (Porites cylindrica Dana) showed that tissue mass and quality varies greatly over small increments in colony or branch size. In particular, allocations to tissue growth varied tenfold (from positive to negative) more across sediment treatments than did allocations to skeletal growth. A model of energy acquisition versus loss (scope for growth) indicated that tissue growth is more responsive to resource variation and physiological stress than skeletal growth. These results suggest that (1) skeletal and tissue growth rates are weakly correlated across environmental conditions, and that (2) variation in tissue properties is a better proxy for coral health or stress than skeletal growth.
The water flow régimes on a coral reef change with depth, distance from shore, and orientation to incoming waves. Low flow conditions may limit the ability of corals to capture prey, because the rate of encounter with particles suspended in the water column depends on the transport of water past the corals’ feeding structures. High flow speeds, however, cause deformation or collapse of feeding structures, and prey capture success is likely to decrease under such conditions. The specific relationship between flow speed and particle capture by passive suspension feeders like anthozoans may be determined by tentacle size and shape, tentacle and polyp stiffness and colony morphology. The importance of particle capture, as well as the types (zooplankton, detritus) and the size range of particles captured by corals, are almost unknown. The only published information for any species to date is Porter’s (1974) report on coelenteron contents of Montastrea cavernosa. The relationship between water flow and particle (hydrated Artemia cysts) capture has been studied for two octocoral (Alcyonium) species by Patterson (1984) and McFadden (1986) and for one scleractinian coral (Meandrina meandrites, Johnson & Sebens, unpubl.) in laboratory flume studies. The present study is an attempt to relate prey capture to water flow under field conditions for two species (Meandrina meandrites, Madracis decactis) in Salt River Canyon, St Croix, U.S.V.I., using the underwater habitat ‘Aquarius’ in July 1988. Concurrent measurements of water flow and wave height were made at several positions on the reef between 7 and 45 m using recording electromagnetic current meters (Interocean S4) mounted on rigid posts 0.5 m off the substrate. During the same two-week period, particle release (hydrated Artemia cysts) experiments were carried out over colonies of Meandrina and Madracis in the field. Particles were released upstream of the corals for 3 min, after which corals were caused to retract their tentacles, and flow above the coral was measured using macrovideo photography of moving cysts in a slit beam of light parallel to the flow. The corals were then collected and preserved under water for coelenteron content analysis, including both captured cysts and natural prey (Zooplankton). Particle concentrations were determined from samples collected by pumping water from directly over the coral during the experiment, using intake heads that allowed omnidirectional lateral sampling.
Although it is well known that primary production is generally higher in near-shore waters than in adjacent oceanic regions, this inequality in tropical reefs, where nutrient concentrations are not higher than in the oceanic water that bathes them (1), is particularly difficult to explain. Progress in answering this fundamental question has been hampered by the tremendous diversity of reef habitats and photosynthetic organisms, the methodological difficulties of working on them (2), and inaccuracies and confusion in the interpretation of data obtained in early field studies (3). In this paper, I present information on annual surface light energy inputs to a coral reef; patterns of light utilization by one group of primary producers, the reef corals; and equations designed to allow interpretation of studies on the oxygen metabolism of symbiotic invertebrates.
The objective of this study was to investigate whether a tipping point exists in the calcification responses of coral reef calcifiers to CO2. We compared the effects of six partial pressures of CO2 (PCO2) from 28 Pa to 210 Pa on the net calcification of four corals (Acropora pulchra, Porites rus, Pocillopora damicornis, and Pavona cactus), and four calcified algae (Hydrolithon onkodes, Lithophyllum flavescens, Halimeda macroloba, and Halimeda minima). After 2 weeks of acclimation in a common environment, organisms were incubated in 12 aquaria for 2 weeks at the targeted PCO2 levels and net calcification was quantified. All eight species calcified at the highest PCO2 in which the calcium carbonate aragonite saturation state was ∼1. Calcification decreased linearly as a function of increasing partial PCO2 in three corals and three algae. Overall, the decrease in net calcification as a function of decreasing pH was ∼10% when ambient PCO2 (39 Pa) was doubled. The calcification responses of P. damicornis and H. macroloba were unaffected by increasing PCO2. These results are inconsistent with the notion that coral reefs will be affected by rising PCO2 in a response characterized by a tipping point. Instead, our findings combined among taxa suggest a gradual decline in calcification will occur, but this general response includes specific cases of complete resistance to rising PCO2. Together our results suggest that the overall response of coral reef communities to ocean acidification will be monotonic and inversely proportional to PCO2, with reef-wide responses dependent on the species composition of calcifying taxa.
Primary productivity of reef-building algae was studied by putting samples from the reef in a closed system and measuring oxygen exchange in the light and in the dark. Gross productivity determined for 32 samples in full sunlight had a mean value of 0.048 mg O"2 cm^@o^2 hr^@o^1. Photosynthesis was found to increase with the logarithm of light intensity up to 1,000 ft-c and was constant between 1,000 and 8,000 ft-c. Rates of gas exchange in flowing water showed no correlation with water velocity but were greater than rates in still water. Daily patterns of photosynthesis were calculated for populations of calcareous algae living on the submarine faces of the windward sides of atolls. During most of the daylight hours light is probably not a limiting factor for photosynthesis in these populations. Calculated productivity of various calcareous algal zones indicates that these do not contribute significantly to overall reef production on atolls of the northern Marshall Islands. Island reefs are less productive than previously studied inter-island reefs.