Collier, Catherine. J., Sven Uthicke, and Michelle Waycott

Limnol. Oceanogr., 56(6), 2011, 2200–2210
2011, by the Association for the Sciences of Limnology and Oceanography, Inc.
doi:10.4319/lo.2011.56.6.2200
E
Thermal tolerance of two seagrass species at contrasting light levels: Implications for
future distribution in the Great Barrier Reef
Catherine J. Collier,a,* Sven Uthicke,b and Michelle Waycotta
a School
of Marine and Tropical Biology, James Cook University, Townsville, Queensland, Australia
Institute of Marine Science, Townsville, Queensland, Australia
b Australian
Abstract
This study assessed metabolism, growth, and survival of two seagrass species at three different seawater
temperatures (27uC, 30uC, and 33uC) under saturating (400 mmol photons m22 s21) and limiting (40 mmol photons
m22 s21) light over 1 month. Halodule uninervis grown at 33uC was within its physiological optimum temperature
range, exhibiting 2.33 higher photosynthetic rates than at 27uC, and increased net shoot carbon (C) production
(up to 103 higher) at saturating light levels. In contrast, 33uC exceeded the optimum temperature threshold for
Zostera muelleri, resulting in critical metabolic imbalances with large reductions in photosynthesis and increases
in leaf respiration. This led to substantially lower growth rates (0–2% of those at 27uC) and lower final biomass
(only 10% of that at 27uC) in the 33uC treatment after 1 month. This decline at higher temperatures occurred at
both light levels, but it was more severe in limiting light, where the C balance went into deficit. H. uninervis in the
Great Barrier Reef (GBR) exists well within its optimal temperature range and should continue to thrive at
projected future temperatures, at least under saturating light levels. In contrast, Z. muelleri currently exists near its
upper thermal threshold, and future temperature increases of the magnitude investigated here would likely lead to
the contraction of the range of this species from the northern GBR—potentially by more than 1000 km. This
could have ecologically significant ramifications, because Z. muelleri is often the only GBR species that currently
inhabits muddy estuarine areas, which are critical fisheries habitats.
Seagrasses are marine flowering plants, and they are
globally distributed and ecologically valued for their high
rates of productivity, coastal nutrient cycling, and as a
habitat that supports fisheries species and as a direct food
source for obligate seagrass feeders such as dugongs (Orth
et al. 2006; Heck et al. 2008; Unsworth and Cullen 2010). In
tropical regions, including the Indo-Pacific, where coastal
marine resources provide up to 100% of daily protein needs
for the communities living along the coast, seagrasses are a
vital fisheries habitat (Unsworth and Cullen 2010).
Seagrass-dominated ecosystems also support productivity
and biodiversity of adjacent habitats, particularly mangroves and coral reefs (Heck et al. 2008). Global seagrass
loss and the factors contributing to it are therefore of
critical concern for the sustainability of these coastal
ecosystems and the communities and industries supported
by them (Orth et al. 2006; Waycott et al. 2009).
Plant metabolism is responsive to temperature in a
largely predictable manner, whereby the balance between
carbon uptake (photosynthesis) and consumption (respiration) is affected. The photosynthetic processes of higher
plants are highly sensitive to temperature, with increases in
photosynthesis occurring up to a physiological optimum,
followed by sharp reductions in photosynthetic efficiency
after temperatures exceed specific thresholds (Berry and
Bjorkman 1980; Bulthius 1987; McDonald 2003). Respiration rates can continue to rise with increasing temperatures,
even after photosynthesis starts to decline. When this
happens, the availability of fixed carbon through photosynthesis no longer balances C requirements as temperatures rise, and the plants go into deficit (i.e., the ratio of
* Corresponding author: Catherine.collier@jcu.edu.au
photosynthesis to respiration, or P : R, , 1). Such
metabolic imbalances in plants are generally associated
with the remobilization of storage reserves, as well as
adjustment of morphology and productivity as a means to
reduce respiratory requirements (Chapin et al. 1987; Ralph
et al. 2007; Collier et al. 2009), but the consequences of
temperature-induced metabolic imbalances for seagrasses
are largely untested. The optimum temperatures for
photosynthesis and for maximizing P : R ratios are also
not known for many—particularly tropical—seagrass
species (Bulthius 1987; Campbell et al. 2006; Rasheed and
Unsworth 2011).
Many environmental factors influence plant growth and
physiology, but there is typically one factor that principally
limits growth (Schulze et al. 2002). Seagrasses have
relatively high minimum light requirements, yet their
predominance in coastal habitats exposes them to conditions of low and highly variable light (Dennison 1987;
Waycott et al. 2009). Natural variability in the light
environment occurs through daily and lunar cycles as
incident light and water depth change, and runoff and
resuspension of bottom sediments can cause chronic and
acute reductions in light (Ralph et al. 2007). This variable
light environment is a particularly strong feature of the
local environment in northern Australia, including the
Great Barrier Reef (Carruthers et al. 2002; De’ath and
Fabricius 2010). Declining water quality has led to
conditions in which light levels have become the primary
limiting factor in many seagrass meadows, and although
this tends to have local effects, it is a globally significant
problem (Waycott et al. 2009). Seagrasses have adapted to
their highly variable light environment primarily by
meeting respiratory and growth requirements with a
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Seagrass thermal tolerance
2201
Fig. 1. (A) Annual maximum monthly sea-surface temperature in the Indo-Pacific region (2002–2009, http://www.bio-oracle.ugent.
be) and the distribution of Halodule uninervis and Zostera muelleri (Waycott et al. 2004). (B) Sea-surface temperature in northeast
Australia showing the Great Barrier Reef and the study site.
combination of photosynthetic C fixation and reallocation
of reserves (Ralph et al. 2007). There are a number of
different processes and scales at which this balance can be
achieved. For example, the efficiency of light capture and
photosynthesis can be increased to boost photosynthetic
rates (Enrı́quez 2005), while at the same time, the demands
for fixed C can be lowered by reducing growth rates and the
amount of biomass retained by the plant (Fourqurean and
Zieman 1991; Collier et al. 2009). Plant-scale responses that
affect meadow structure, density, and productivity are
significant at a broader ecological level because they are
critical to the habitat and food value of seagrass meadows.
Halodule uninervis is a tropical seagrass species common
throughout the Indo-Pacific and east African coast,
whereas Zostera muelleri (syn. Zostera capricornii) is a
tropical to temperate species occurring in Australia and
New Zealand only (Fig. 1) (Lee Long et al. 2000; Waycott
et al. 2004). The two species only overlap in distribution in
northeastern Australia and the Great Barrier Reef (GBR).
In the GBR, the mean annual temperature is 25.8uC
(Lough 2007), and in the hottest month (usually around
February), mean monthly temperatures range from 27.5uC
in the southern GBR to 31.5uC in the north (Fig. 1). Seasurface temperature in the GBR is projected to increase
between 1uC and 3uC by 2100 under the Intergovernmental
Panel on Climate Change (IPCC) A2 scenario (Lough
2007). At the higher end of this range, summer water
temperatures would exceed 33uC. Although temperatures
can reach 33uC in seagrass meadows under extreme
conditions in the GBR (e.g., low tide), they do not persist,
and therefore long-term summer temperatures of 33uC will
be well above temperatures that occur within the distributional range of Z. muelleri.
The objectives of this work were to: (1) test the thermal
tolerance of two seagrass species with different geographical distributions to water temperatures at the lower and
higher ends of the range projected to occur in the GBR
within the current century, (2) identify the effects of light
limitation on plant responses to these temperature increases, and (3) infer, using the observed thermal tolerances,
likely changes to the future distributions of these species
due to climate change. We hypothesized that H. uninervis
would have a higher temperature threshold than Z. muelleri
and that exceedance of the optimum metabolic temperature
thresholds would be expressed at a plant scale.
Methods
Two seagrass species, Halodule uninervis Ascherson
(Cymodoceaceae) and Zostera muelleri Irmisch ex Ascher-
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Collier et al.
son (Zosteraceae), were tested for the combined effects of
increased water temperature and reduced light intensity.
The two species were collected from intertidal meadows at
Cockle Bay and Picnic Bay (19u10.889S, 146u50.639E),
Magnetic Island, northern Great Barrier Reef, in September 2009. At this site, daily mean water temperature ranges
from 19uC to 31.5uC throughout the year (C. Collier
unpubl. data), maximum monthly temperature is 29.6uC to
30.5uC (Fig. 1), and light intensity is 15.2 mol m22 d21
(annual average; C. Collier unpubl. data).
The experiments were run for 1 month to align with
maximum monthly temperature data (current and future
predicted) used to establish the treatment levels (Fig. 1).
Three temperature treatments were applied in a large-scale
aquarium experiment: 27uC, 30uC, and 33uC. The light
treatments were ‘‘high light’’ (400 mmol quanta m22 s21)
and ‘‘low light’’ (40 mmol quanta m22 s21), set on a 12-h
night and day cycle. The half-saturation constant (Ek) was
derived from rapid light curves measured with a pulse
amplitude modulated fluorometer at time 1, 2, and 3 and
was on average 102 mmol quanta m22 s21 and 40 mmol
quanta m22 s21 in high and low light for H. uninervis and
was 138 mmol quanta m22 s21 and 90 mmol quanta m22 s21
for Z. muelleri. Therefore, the high-light treatment was well
above saturating irradiance, and the low-light treatment
was below saturating irradiance.
Plugs of seagrass were collected using a 10-cm (internal
diameter) polyvinyl chloride (PVC) corer, which was pushed
into the sediment to a depth of 10 cm, and the intact plug of
seagrass and sediment was placed in a plastic pot and lined
with a plastic bag, which was pulled up and secured over the
seagrass to retain humidity during transport to the aquaria.
The seagrass was kept in an outdoor flow-through aquarium
for 2 weeks prior to the experiment. Two weeks of
‘‘acclimation’’ were provided to allow recovery from any
possible ‘‘shock’’ associated with harvesting of plants,
including severing of rhizomes and possible disruption to
roots in the sediment. The experiment was conducted in an
indoor flow-through aquarium system at the Australian
Institute of Marine Sciences. For each temperature treatment, there was a single header tank in which the water was
heated and then dispersed. A computer-controlled thermometer regulated temperature to within 0.5uC of the target
temperature. Water was supplied by a nearby coastal intake
as a continuous through-flow (i.e., not recycled), and flow
was maintained at a rate of , 9 mL s21 with complete
exchange of the water in the treatment aquaria every 45 min.
Light was supplied from a single halogen lamp mounted
over the top of each tank, and the low-light treatment
involved placing light-reducing black shade-cloth between
the light source and the water surface.
There were four replicate 50-liter tanks for each
temperature treatment. Each tank was divided into a
high-light and a low-light treatment using Perspex dividers,
which were randomly assigned within the tanks. A separate
inflow tap led to each side of the Perspex divider, and for
outflow, the water from one section passed through a small
mesh window (1-mm mesh) at the top of the divider into
the other section. In each light and temperature treatment,
there were two subreplicate pots of each species and the
data from these subreplicates were averaged (mean) for
later analysis.
The experiment was run for a total of 35 d from 10
October 2009. Ambient temperature was , 27uC, and the
temperature treatments were initiated on 10 October at 11:00
h, with the target temperatures of 27uC, 30uC, and 33uC
reached at 05:00 h on 11 October. During the experiment,
measurements were made in four blocks of time: days 26 to
21 (time 0), days 1–6 (time 1), days 17–23 (time 2), and days
28–34 (time 3). Growth was measured at each time according
to Short and Duarte (2001). Ten shoots from each pot were
marked at the top of the sheath with a needle. The length of
growth (mm) was measured after 5 to 7 d on the shoots
without removing them from the pots.
Photosynthetic rates and respiration rates were measured
at time 1 and time 3 on the upper 20–40 mm of the youngest
mature leaf of a random shoot from each subreplicate pot
using optical oxygen sensors (‘‘optodes,’’ PreSens, Sensor
spots-Pst3) and a PreSens Oxy 4 four-channel fiber-optic
oxygen meter. Small custom-made glass chambers (6.62 mL)
were set in an array of four (i.e., four separate chambers
allowing four parallel measures) and incubated at the
treatment water temperature (27uC, 30uC, 33uC) using a
flow-through water system connected to a water bath
(Lauda, Ecoline RE 106). Each chamber was stirred with a
glass-coated magnetic stirrer bar, and in each chamber, there
was a perforated plastic shelf separating the stirrer from the
seagrass material. The leaves were placed on top of the shelf
in a U-shape, which resulted in minimal contact of the leaf
with the glass chamber or plastic shelf. Oxygen consumption
(dark respiration) was measured over a 20- to 30-min period
in the dark. Photosynthetic rates were then measured on the
same leaf fragment in the light (treatment light intensity,
either 40 or 400 mmol quanta m22 s21) over 20 to 30 min.
Oxygen concentration data in the chambers were logged
every 30 s, and respective respiration and production rates
were calculated by fitting a linear regression to the data.
Regressions omitted the initial period of incubation (, 5 to
15 min) until rates had stabilized. Respiration rates of
belowground rhizomes were also measured, but at time 3
only, because removal of the rhizome is destructive. A small
piece of rhizome (5–10 mm) and associated roots were
removed from the sediment, rinsed at treatment temperature, and then transferred to incubation chambers.
Each optode was calibrated prior to initial measurements
using a two-point method (100% O2 in air: using a wet
sponge in the respiration chamber to saturate air; 0% using a
1% Na2SO3 solution). Individual chambers were cleaned
with ethanol between incubations to prevent biofilm buildup,
and at least 1 blank chamber was run on each measuring day
to test for blank respiration. At the end of the experiment, the
remaining biomass was collected, separated into above- and
belowground biomass, dried at 80uC, and weighed.
Metabolic rates were calculated according to the
following:
Lf PhsG mmol O2 g{1 dry weightðdry wtÞh{1
~Lf PhsN {Lf Resp
ð1Þ
where Lf PhsG is the gross photosynthetic rate of leaves,
Seagrass thermal tolerance
i.e., with loss from respiration removed, Lf PhsN (mmol O2
g21 dry wt h21) is the net rate of O2 production of the
leaf material (measured value, with respiration included),
Lf Resp (mmol O2 g21 dry wt h21) is the respiration rate in
the leaves, and dry wt is the mass of leaf material in dry
weight.
Total C budget of the shoot was calculated for time 3
only when biomass measures were also made. Firstly,
biomass per shoot (WtShoot, g shoot21) was calculated from
the sum of:
WtLf ~
biomassA=Gr
shoots
ð2Þ
biomassB=Gr
shoots
ð3Þ
and
Wtrhiz ~
where leaf weight (WtLf, g shoot21) is the weight of leaf
material in each shoot, biomassA/Gr is the biomass of
aboveground material (leaves and live sheath, g dry wt
pot21), rhizome weight (Wtrhiz, g shoot21) is the weight of
rhizome in each shoot, biomassB/Gr is the biomass of
belowground parts (rhizome and roots, g dry wt pot21),
and shoots is the number of shoots per pot (shoots pot21).
Total leaf photosynthetic O2 production for a 24-h
period was then calculated:
Prod24 ~ðLf PhsG |12hÞ{ðLf Resp|24Þ
ð4Þ
where Prod24 is the rate of photosynthetic O2 production of
the leaves after respiration rates of the leave are subtracted
for 24 h (mmol O2 g dry wt21 d21).
The respiration rate of the rhizome for a 24-h period was
then calculated:
RespRhiz24 (mmol O2 g dry wt{1 d{1 )~RespRhiz |24
ð5Þ
where RespRhiz24 is respiration rate in the rhizome over
24 h, and RespRhiz is the hourly rate of respiration in the
rhizome (mmol O2 g dry wt21 h21).
Finally, the total production of carbon for the entire
shoot (shoot, leaf + rhizome) over a 24-h period (C ProdN
mg C shoot21 d21) was calculated from:
C ProdN (mg C shoot{1 d{1 )~½ðProd24 {Resp24 Þ=P:Q:
|(Mol WtC =1000)|Wtshoot
ð6Þ
where P.Q. is the photochemical quotient of tropical
seagrasses (P.Q. 5 1; Pollard and Greenway 1993) and
Mol WtC is the molecular weight of carbon (12).
Statistical analysis—Metabolic data for time 1 and Time
3, and biomass data for time 3 were analyzed separately for
each species using a two-way analysis of variance (ANOVA) testing for the fixed effects of temperature (three
levels) and light (two levels). Repeated measures ANOVA
(RM ANOVA) was used for growth rates, with temperature and light as fixed effects (between-subjects effects) over
time (within-subjects effects). The variance–covariance
2203
matrices were tested using Mauchly’s test of sphericity,
and if the assumption was not met, the Greenhouse-Geisser
epsilon adjustment was applied to the degrees of freedom.
Temperature effects were interpreted using Tukey’s posthoc analysis with light levels combined if no temperature
and light interaction was observed or at each separate light
level if an interaction occurred. Data were checked for
homogeneity of variance using Levene’s test, and if they
failed (p . 0.05), data were square-root or log transformed.
If the transformation was not successful at improving the
variance in the data, the ANOVA was performed but with
significant p values set to 0.01 to minimize the risk of a type
1 error (Underwood 1997). Transformations and p values
are shown for all significant results.
Results
Metabolic rates and whole plant carbon budgets—For the
tropical species H. uninervis, the effects of light and
temperature on leaf photosynthesis (Lf PhsG mmol O2 g21
dry wt h21; Eq. 1) and leaf respiration (Lf Resp mmol O2
g21 dry wt h21) were similar after 5 d (time 1) and 30 d
(time 3). There was no effect of temperature or light on Lf
Resp at time 1 or time 3 (Fig. 2A,B). For Lf PhsN,
however, there was a significant temperature and light
interaction (time 1 temperature 3 light p , 0.05; time 3
temperature 3 light p , 0.05; Table 1). Post-hoc analysis
indicated that: in high light, Lf PhsG was significantly
higher at 33uC than 27uC (time 1 Tukey’s p , 0.05; time 3 p
, 0.01), but there was no difference between temperatures
in low light, and Lf PhsG was faster in high light than low
light at all temperatures at time 1 but only at 30uC (p ,
0.05) and 33uC (p , 0.01) at time 3 (Fig. 2A,B). In
summary, the photosynthesis to respiration ratio (P : R) of
leaves only of H. uninervis was 5.6, 8.8, and 11.2 at 27uC,
30uC, and 33uC in high light, but it showed less variation in
low light at 4.9, 6.5, and 7.0 at time 3.
Lf Resp in Z. muelleri was affected by temperature at
both time 1 and time 3 (two-way ANOVA time 1,
temperature p , 0.05; time 3 temperature p , 0.01;
Table 1) with Tukey’s post-hoc analysis revealing higher Lf
Resp at 33uC compared to the 30uC at time 1 (p , 0.05) and
at time 3 compared to both 27uC (p , 0.01) and 30uC (p ,
0.05) (Fig. 2C,D). Lf PhsG was significantly affected by
light at time 1 (two-way ANOVA p , 0.001) and was less
than half the rate in low light compared to high light for all
temperatures (Fig. 2C). At time 3, there was a significant
interaction between temperature and light on Lf PhsG (twoway ANOVA p , 0.01): Tukey’s post-hoc analysis
indicated that Lf PhsG was reduced at 33uC compared to
27uC (p , 0.01), but only in high light, and Lf PhsG was
significantly faster in high light than low light at 27uC (p ,
0.01) and 30uC (p , 0.01), but at 33uC, there was no
difference between light treatments. In summary, the P : R
ratios of Z. muelleri decreased with temperature: 14.1, 8.5,
and 2.1 at 27uC, 30uC, and 33uC in high light and 8.3, 3.8,
and 3.7 in low light at time 3.
Respiration rates in the rhizomes (RespRhiz) were
measured at time 3 only, and were, on average, 22 and
19 mmol O2 g21 dry wt h21 in H. uninervis and Z. muelleri,
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Collier et al.
Fig. 2. Gross photosynthetic rate of leaves (Lf PhsG) and respiration rate of leaves (Lf Resp) for H. uninervis at (A) time 1 (5 d) and
(B) time 3 (30 d) and Z. muelleri at (C) time 1 and (D) time 3 in high light and low light at 27uC (light gray), 30uC (medium gray), and
33uC (dark gray). Letters indicate significant differences between temperature treatments based on Tukey’s post-hoc analysis; asterisks
indicate significant differences between light treatments based on ANOVA results where there was no interaction between temperature
and light or based on Tukey’s post-hoc analysis performed at each temperature if an interaction occurred (** p , 0.01, *** p , 0.001); n 5 4 6 SE.
respectively (Fig. 3). This was substantially lower than the
respiration rate of leaves, which were, on average, three
times and two times faster at 71 and 41 mmol O2 g21 dry
wt h21 in H. uninervis and Z. muelleri, respectively.
Respiration was highly variable, but it was not significantly
affected by temperature or light for either species (Table 1).
Net C production (C ProdN, mg C shoot21 d21; Eq. 6) of
H. uninervis was affected by a temperature and light
interaction (two-way ANOVA temperature 3 light p ,
0.001; Table 1). Tukey’s post-hoc analyses reveal that in
high light only, C ProdN was lower at 27uC than at 30uC
(53 lower; p , 0.05) or 33uC (103 lower; p , 0.001), and C
ProdN was greater in high light than low light at 30uC (p ,
0.001) and 33uC (p , 0.01; Fig. 4A). For Z muelleri, there
was a significant effect of temperature on C ProdN (twoway ANOVA temperature p , 0.001), with Tukey’s posthoc analysis indicating that C ProdN was lower at 33uC
than at 27uC (p , 0.01) or 30uC (p , 0.01). There was also
an effect of light (two-way ANOVA light p , 0.001) on C
ProdN, which was reduced in low light compared to high
light (Fig. 4B).
Growth and biomass—There was a significant effect of
light on growth rates of H. uninervis, but it depended on
Seagrass thermal tolerance
2205
Table 1. Results of two-way ANOVA testing for the fixed effects of temperature (27uC, 30uC, 33uC) and light level (high,
400 mmol m22 s21 and low, 40 mmol m22 s21) on gross leaf photosynthesis (Lf PhsG), leaf respiration (Lf Resp), rhizome respiration
(RespRhiz), and net carbon production (C ProdN) of the seagrasses Halodule uninervis and Zostera muelleri. Transformations are indicated
where applied. MS, mean square. * p , 0.05, ** p , 0.01, *** p , 0.001; n 5 4 6 SE.
H. uninervis
Factor
df
Temperature
Light
Temp3light
2
1
3
Temperature
Light
Temp3light
2
1
3
Temperature
Light
Temp3light
2
1
3
Lf Resp (mmol O2
g21dry wt h21)
Temperature
Light
Temp3light
2
1
3
RespRhiz (mmol O2
g21dry wt h21)
Temperature
Light
Temp3light
2
1
3
C ProdN (mg C
shoot21 d21)
Temperature
Light
Temp3light
2
1
3
Time 1
Lf PhsG (mmol O2
g21dry wt h21)
Lf Resp (mmol O2
g21dry wt h21)
Time 3
Lf PhsG (mmol O2
g21dry wt h21)
MS
F
Z. muelleri
p
31,016.933
4.405
550,349.090
78.159
31,930.389
4.535
No transformation
p,0.05
1371.941
1.534
2307.545
2.580
116.242
1.534
No transformation
p,0.05
*
***
*
182,586.281
10.204
570,818.231
31.902
77,885.366
4.353
No transformation
p,0.05
202.423
0.181
336.002
0.300
342.615
0.306
No transformation
p,0.05
0.397
1.761
0.025
0.111
0.712
3.159
Ln transformation
p,0.05
0.258
17.439
0.813
54.843
0.212
14.314
Ln transformation
p,0.01
**
***
*
***
***
***
MS
F
4151.520
1.032
457,310.877
1.137
53.487
0.013
No transformation
p,0.05
1230.272
4.398
217.999
0.779
953.263
3.408
No transformation
p,0.05
36,961.325
6.636
101,572.708
18.237
35,136.168
6.309
No transformation
p,0.05
1360.479
7.867
566.516
3.276
246.063
1.423
No transformation
p,0.05
0.690
1.855
0.035
0.095
0.067
0.181
Ln transformation
p,0.05
0.258
10.953
0.492
20.897
0.029
1.230
No transformation
p,0.05
p
***
*
**
***
**
**
***
***
Fig. 3. Respiration rates in the rhizomes in high light (left-hand bars) and low light (right-hand bars) for (A) Halodule uninervis and
(B) Zostera muelleri at 27uC (light gray), 30uC (medium gray), and 33uC (dark gray); n 5 4 6 SE.
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Collier et al.
Fig. 4. Net shoot production (C ProdN mg C shoot21 d21) for (A) Halodule uninervis and (B) Zostera muelleri in high light and low
light at time 3 (30 d) at 27uC (light gray), 30uC (medium gray), and 33uC (dark gray). Letters indicate significant differences between
temperature treatments based on Tukey’s post-hoc analysis; asterisks indicate significant differences between light treatments based on
ANOVA results where no interaction occurred between temperature and light or based on Tukey’s post-hoc analysis if an interaction
occurred (** p , 0.01, *** p , 0.001); n 5 4 6 SE.
time (RM ANOVA time 3 light p , 0.001; Table 2).
Tukey’s post-hoc analysis indicated that there was no
difference between light treatments at time 0, but at times 1
(p , 0.05), 2 (p , 0.001), and 3 (p , 0.001), growth rate
was significantly faster in high light compared to low light
(Fig. 5A).
For Z. muelleri, there was a highly significant effect of
temperature on growth rates that was affected by time (RM
ANOVA time 3 temperature p , 0.001; Table 2), and
post-hoc analyses indicated no significant difference
between temperature treatments at time 0, but on all other
days, growth in the 33uC treatments was slower than all
other treatments (Tukey’s p , 0.001). There was also a
significant effect of light in the between-subjects test (RM
ANOVA light p , 0.001; Table 2), where leaf growth was
slower in the low-light compared to the high-light
treatments.
Biomass was measured only at the conclusion of the
experiment (time 3). For H. uninervis, there was a
significant effect of the light treatment (two-way ANOVA
light p , 0.001; Table 3) on aboveground biomass, with
less biomass in low light than in high light for all
temperatures (Fig. 6A). There was no significant effect of
temperature or light on the belowground biomass of H.
uninervis. In Z. muelleri, there was a significant effect of
temperature (two-way ANOVA temperature p , 0.001;
Table 3) on the aboveground biomass, with Tukey’s posthoc analysis indicating that biomass was reduced at 33uC
compared to both 27uC and 30uC (Tukey’s post-hoc p ,
0.001; Fig. 6B). There was also a significant effect of light
on the aboveground biomass (two-way ANOVA light p ,
0.001; Table 3) and on the belowground biomass (two-way
ANOVA light p , 0.01; Table 3), with reduced biomass in
low light compared to high light. The belowground
biomass was considerably larger than the aboveground
biomass, ranging from 3.73 (33uC high light) to 253 more
(30uC low light) in H. uninervis and from 2.13 (27uC high
light) to 213 (33uC low light) more in Z. muelleri
Table 2. Results of two-way RM ANOVA testing for the fixed effects of temperature (27uC, 30uC, 33uC) and light level (high,
400 mmol m22 s21 and low, 40 mmol m22 s21) on leaf growth (mm d21) of the seagrasses Halodule uninervis and Zostera muelleri.
Transformations are indicated where applied. MS, mean square; Sqrt, square root. *** p , 0.001.
H. uninervis
Test
Effects
df
Within-subjects effects
Time
Time3temperature
Time3light
Time3temperature3light
Temperature
Light
Temperature3light
3
6
3
6
2
1
2
Between-subjects effects
MS
F4
Z. muelleri
p
1.280
31.897
***
0.094
2.344
1.194
29.748
***
0.057
1.414
0.051
1.548
6.714
204.977
***
0.091
2.772
Ln transformation
p,0.01
MS
5.572
1.913
0.120
0.075
5.538
0.845
0.021
F4
p
90.058
***
30.916
***
1.945
1.209
121.594
***
18.551
***
0.461
Sqrt transformation
p,0.05
Seagrass thermal tolerance
2207
Fig. 5. Leaf growth rate (mm d21) of (A) Halodule uninervis and (B) Zostera muelleri at 27uC (circles), 30uC (square), and 33uC
(triangle) in high light (white fill) and low light (dark fill) at time 0 (T0, 0 d) through to time 3 (T3, 32 d) . Significant (p , 0.05) results of
RM ANOVA and post-hoc analyses are shown. Asterisks indicate significant differences between light treatments (* p , 0.05, ** p ,
0.01, *** p , 0.001); n 5 4 6 SE.
(Fig. 6A,B). Changes to morphological characteristics of
the shoots, including a reduction in leaves per shoot and
leaf length, were large contributors to the reduction in
aboveground biomass; there was little difference in shoot
density among treatments (C. Collier unpubl.).
Discussion
This study has demonstrated that H. uninervis and Z.
muelleri, although overlapping in their distribution
throughout the GBR, have distinctly different temperature
thresholds, which could affect their distributions and
relative dominance if sea temperatures increase according
to projections.
H. uninervis can be considered a truly tropical species: its
photosynthetic rates, overall net carbon production, and
consequently growth and biomass increased with temperature from 27uC to 33uC. These responses are consistent
with those of other higher plants following increases in
temperature that fall within their physiological optimum
temperature range (Berry and Bjorkman 1980; McDonald
2003). However, photosynthetic sensitivity to high temperature is greatest at saturating light levels (Berry and
Bjorkman 1980; Bulthius 1987), as demonstrated in this
study by the response of H. uninervis. In contrast, at low
and limiting light levels, photosynthetic rates of H.
uninervis were drastically reduced, and temperature had
little effect on net photosynthesis. As a result, in limiting
light, there was no difference in overall carbon (C)
production with increasing temperature and no temperature effect on growth and biomass, which were both
reduced compared to the high-light treatments. In low and
limiting light, photosynthesis and respiration rates are
expected to have a lower threshold temperature than in
high light (Berry and Bjorkman 1980), but this threshold
temperature was not reached in these experiments. Habitat
conditions that provide saturating light levels to H.
uninervis enable faster rates of photosynthesis, growth,
Table 3. Results of a two-way ANOVA testing for the fixed effects of temperature (27uC, 30uC, 33uC) and light level (high,
400 mmol m22 s21 and low, 40 mmol m22 s21) on aboveground and belowground biomass of the seagrasses Halodule uninervis and
Zostera muelleri. Transformations are indicated where applied. MS, mean square; Sqrt, square root. ** p , 0.01, *** p , 0.001.
H. uninervis
Factor
df
pot21)
Light
Temperature
Light3temperature
1
2
2
Belowground biomass (g pot21)
Light
Temperature
Light3temperature
1
2
2
Aboveground biomass (g
MS
F
Z. muelleri
p
0.150
64.730
***
0.003
1.202
0.003
1.330
Sqrt transformation
p,0.05
0.118
4.327
0.069
2.530
0.004
0.135
No transformation
p,0.05
MS
F
p
0.055
11.253
**
0.143
29.045
***
0.003
0.545
Sqrt transformation
p,0.01
0.076
11.869
**
0.007
1.171
0.005
0.797
No transformation
p,0.05
2208
Collier et al.
Fig. 6. Aboveground biomass (top) and belowground biomass (bottom) in high light (lefthand bars) and low light (right-hand bars) for (A) Halodule uninervis and (B) Zostera muelleri at
27uC (light gray), 30uC (medium gray), and 33uC (dark gray). Letters indicate significant
differences between temperature treatments based on Tukey’s post-hoc analysis; asterisks indicate
significant differences between light treatments based on ANOVA results where no interaction
occurred between temperature and light or based on Tukey’s post-hoc analysis if an interaction
occurred (** p , 0.01, *** p , 0.001); n 5 4 6 SE.
and biomass production and should enable it to not only
survive future increases in water temperature up to 33uC,
but may also result in an increased abundance of this
species in the GBR.
Sea-surface temperature throughout the range of H.
uninervis reaches 33–34uC (Fig. 1). The threshold temperature for H. uninervis is evidently greater than 33uC, since
the upper threshold was not reached in this study. This
would imply that the upper thermal limit of this species is
not currently exceeded, and future temperature increases
will lead to an increase in growth and biomass throughout
its distributional range until its thermal limit is reached.
However, in shallow nearshore habitats, water-temperature
increases of more than 6uC have been observed, leading to
temperatures in seagrass habitats over 40uC caused by high
air temperatures and the high conductivity of water (Campbell et al. 2006; Anthony and Kerswell 2007; Massa et al.
2009). These temperature spikes typically only last a few
hours at low tide, but the effect can be significant;
photosynthetic impairment and ‘‘burning’’ of leaves can
ensue for species that are not adapted to it. Increases in the
severity and frequency of high-temperature extremes are
likely to occur as the climate changes (IPCC 2007; Lough
2007), thus increasing the frequency with which physiological optima are exceeded in shallow-water habitats. Therefore, predictions of future seagrass distribution based on
Seagrass thermal tolerance
SST will be complex, and responses in shallow habitats will
deviate from those predicted in less dynamic water bodies.
At the sampling site, water temperature can reach 30–
30.5uC in summer (December–February). A maximum
increase of 3uC is projected by 2100 under the A2 scenario,
taking mean water temperatures to 33uC (Lough 2007), and,
therefore, H. uninervis should remain within its physiological
optimum temperature range within the GBR. We might
expect this species to colonize areas further south along the
east Australian coast as warmer temperatures extend to the
south, expanding its distribution to central New South
Wales; however, this assumes that its southern distribution is
not constrained by other habitat conditions, including
appropriate substrate, light, salinity, nutrient availability,
and suitable disturbance regimes.
In contrast to the responses of H. uninervis, Z. muelleri
was severely affected by the 33uC temperature treatment; its
photosynthetic rates plummeted and respiration rates
increased at 33uC compared to 30uC. Plants can withstand
negative carbon (C) balances for short durations by
drawing on storage reserves, but Z. muelleri also made
growth and structural modifications at 33uC, which
reduced the energetic costs of maintenance of the plant
(Fourqurean and Zieman 1991). These responses are
consistent with other higher plants when physiological
optimum temperatures have been exceeded (Berry and
Bjorkman 1980). The photosynthetic mechanisms became
impaired to such a degree at 33uC that Z. muelleri could not
benefit from the saturating light levels of the high-light
treatment. This change in photosynthetic rate was not
immediate, however; within the first 5 d (time 1), this 30–
33uC threshold was absent for photosynthesis, and only a
small reduction in respiration was observed. The Z.
muelleri plants used in these experiments were collected a
considerable distance from their northern distributional
limit; however, summer maximum sea-surface temperature
varies only slightly (0.5–1uC) over this distance (Fig. 1).
The region where samples were collected occurs towards
the northern, tropical limit of this species distribution, and
thus they are less likely to have broad phenotypic
tolerances compared with that of populations situated
within the center of the species distribution. The genetic
diversity within the region is likely to be reduced due to the
central-marginal theory, where edge of range populations
often exhibit reduced genetic diversity and narrower
phenotypic tolerances to environmental stressors (Vucetich
and Waite 2003). However, specific testing of plants from
extreme northern populations (i.e., those exposed to higher
water temperature) is needed to evaluate if they are better
adapted to higher temperatures.
The ecological roles of H. uninervis and Z. muelleri are
not interchangeable (Carruthers et al. 2002), and displacement of Zostera from northern seagrass ecosystems would
affect the overall functioning of the seagrass communities it
occupies. Projected increases in water temperature for the
GBR (up to 3uC by 2100 under A2 scenario; Lough 2007)
could see maximum monthly water temperature in the
GBR from the Torres Strait to Mackay increase to well
over 33uC (Fig. 1). This could potentially result in a
contraction of the distribution of Z. muelleri by more than
2209
1000 km and result in local extinction in the northern GBR.
Such a loss could have ecologically significant ramifications
because this species tends to occupy habitats that few other
species can inhabit—it is typically found in shallow, muddy
estuarine waters (Lee Long et al. 1993; Carruthers et al.
2002). In these habitats, there may be no equivalent species
that can act as a functional replacement amongst the
tropical species because it often forms monospecific
meadows, tolerating conditions that are not suitable for
other seagrass species. The environmental factors that
currently limit the distribution of the tropical estuarine and
coastal species (e.g., Enhalus acroides) are not known,
making it difficult to predict their responses to future
environmental change and their potential to act as a
functional replacement for Z. muelleri.
The two species studied here—Halodule uninervis, which
is predominantly tropical, and Zostera muelleri, which is
predominantly temperate—exhibited contrasting responses
to increasing water temperatures. They also represent
members of different seagrass families, possessing different
evolutionary histories. Their responses may reflect these
different histories, and different physiological strategies for
survival in the marine environment, which are as yet
unresolved for these taxa. In addition, light levels were also
critical to their response to increasing temperature: Saturating light was required for H. uninervis to respond positively
to increasing temperature, whereas Z. muelleri responded by
going into carbon deficit at limiting light levels, although it
maintained a positive balance under saturating light. This
has important implications for the management of these
species into the future, as maintaining and improving water
quality—and thus maximizing light levels—will enhance
resilience of both species to elevated temperatures.
Acknowledgments
We thank A. Negri and R. Berkelmans for assistance running
the aquaria systems, and A. Giraldo Ospina and D. Tracey for
assistance with seagrass collection and seagrass measurement
throughout. We are grateful to S. Kininmonth for his assistance in
preparing temperature Geographical Information Systems maps
based on the data provided at http://www.oracle.ugent.be/. We
also thank the reviewers of this manuscript for their feedback.
This work was funded by the Marine and Tropical Sciences
Research Facility, Department of Environment, Heritage and the
Arts (Australian government).
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Associate editor: Anthony W. D. Larkum
Received: 07 May 2011
Accepted: 04 August 2011
Amended: 09 August 2011