photosynthetic responses of seven tropical seagrasses to … · photosynthetic responses of seven...

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Journal of Experimental Marine Biolog

Photosynthetic responses of seven tropical seagrasses to elevated

seawater temperature

Stuart J. Campbell a,b, Len J. McKenzie a,b,*, Simon P. Kerville a

a Northern Fisheries Centre, Department of Primary Industries and Fisheries, PO Box 5396, Cairns, Qld, 4870, Australiab CRC Reef Research Centre, PO Box 772, Townsville, Qld, 4810, Australia

Received 8 December 2004; received in revised form 24 April 2005; accepted 15 September 2005

Abstract

This study uses chlorophyll a fluorescence to examine the effect of environmentally relevant (14 h) exposures of thermal stress

(3545 8C) on seagrass photosynthetic yield in seven tropical species of seagrasses. Acute response of each tropical seagrassspecies to thermal stress was characterised, and the capacity of each species to tolerate and recover from thermal stress was

assessed. Two fundamental characteristics of heat stress were observed. The first effect was a decrease in photosynthetic yield (Fv /

Fm) characterised by reductions in F and FmV. The dramatic decline in Fv /Fm ratio, due to chronic inhibition of photosynthesis,

indicates an intolerance of Halophila ovalis, Zostera capricorni and Syringodium isoetifolium to ecologically relevant exposures of

thermal stress and structural alterations to the PhotoSystem II (PSII) reaction centres. The decline in FmV represents heat-induced

photoinhibition related to closure of PSII reaction centres and chloroplast dysfunction. The key finding was that Cymodocea

rotundata, Cymodocea serrulata, Halodule uninervis and Thalassia hemprichii were more tolerant to thermal stress than H. ovalis,

Z. capricorni and S. isoetifolium. After 3 days of 4 h temperature treatments ranging from 25 to 40 8C, C. rotundata, C. serrulataand H. uninervis demonstrated a wide tolerance to temperature with no detrimental effect on Fv /FmV qN or qP responses. These

three species are restricted to subtropical and tropical waters and their tolerance to seawater temperatures up to 40 8C is likely to bean adaptive response to high temperatures commonly occurring at low tides and peak solar irradiance. The results of temperature

experiments suggest that the photosynthetic condition of all seagrass species tested are likely to suffer irreparable effects from

short-term or episodic changes in seawater temperatures as high as 4045 8C. Acute stress responses of seagrasses to elevatedseawater temperatures are consistent with observed reductions in above-ground biomass during a recent El Nino event.

Crown Copyright D 2005 Published by Elsevier B.V. All rights reserved.

Keywords: Seagrass; Elevated temperature; Global warming; El Nino; Tropical; Australia

1. Introduction

Shallow water tropical seagrasses are exposed to a

range of environmental stresses, including high solar

radiation, ultra-violet radiation, desiccation and temper-

0022-0981/$ - see front matter. Crown Copyright D 2005 Published by Els

doi:10.1016/j.jembe.2005.09.017

* Corresponding author. Northern Fisheries Centre, Department of

Primary Industries and Fisheries, PO Box 5396, Cairns, Qld, 4870,

Australia. Tel.: +61 7 4035 0131; fax: +61 7 4035 4774.

E-mail address: [email protected] (L.J. McKenzie).

ature fluctuations. Fluctuations in seasonal seawater

temperatures in tropical habitats range from 19.8 to

41 8C (McKenzie, 1994; McKenzie and Campbell,2004). Critical thermal stress has been reported in

temperate seagrasses at temperatures exceeding 35 8C(Bulthuis, 1983; Ralph, 1998), but few studies have

examined temperature responses of tropical seagrass

species (Fong and Harwell, 1994; Terrados and Ros,

1995). Tropical species of seagrasses generally increase

their photosynthesis at elevated temperatures (Perez and

y and Ecology 330 (2006) 455468

evier B.V. All rights reserved.

S.J. Campbell et al. / Journal of Experimental Marine Biology and Ecology 330 (2006) 455468456

Romero, 1992; Terrados and Ros, 1995). However,

temperatures rising above the normal upper limit of

35 8C can inhibit carbon production in plants becausehigh temperatures bring about increased respiration

(Bulthuis, 1983; Ralph, 1998) and photosynthetic en-

zyme breakdown (Bruggeman et al., 1992; Ralph,

1998). Light requirements for carbon production are

also greater at higher temperatures because of increased

compensation irradiance (Bulthuis, 1987).

Thermal stress has a significant impact on the bio-

geographical distribution and condition of seagrass

meadows (Bulthuis, 1983; Hillman et al., 1989; McMil-

lan, 1984; Ralph, 1998). Climatic factors influencing

the ambient water temperature of seagrass meadows

include global temperature oscillations (e.g. El Nino),

global warming, seasonal and diurnal fluctuations

(Short and Neckles, 1999). Anthropogenic causes of

seawater temperature increases include the release of

heated effluents into shallow embayments where sea-

grasses proliferate (Thorhaug et al., 1978). Seawater

and air temperatures are factors that limit the upper

distributional limits of seagrass meadows. Seagrasses

in shallow pools of water are not only exposed to

elevated temperatures during midday sun exposure,

but they are also subject to desiccation during low

tidal periods and high PAR and UV radiation (Dawson

and Dennison, 1996; Durako and Kunzelman, 2002).

Mean sea temperature increases of up to 2 8C may havea severe impact on species of seagrass that survive at

the upper limit of their thermal tolerance (Ralph, 1998).

Increases in seawater temperatures of up to 10 8C abovethe seasonal average can occur in tropical shallow water

pools especially during spring low tides and during

midday solar exposure (McKenzie and Campbell,

2004;www.seagrasswatch.org). During these events

seagrasses may be exposed to elevated seawater tem-

peratures for periods of 3 to 4 h. High seawater tem-

peratures and desiccation have negatively affected

seagrass meadows in a number of areas worldwide

(Seddon and Cheshire, 2000; Walker and Cambridge,

1995; Erftemeijer and Herman, 1994) with one episode

of seagrass loss linked to an El Nino climatic pattern

(Seddon and Cheshire, 2000).

The effects of thermal stress on photosynthesis,

productivity and morphology of seagrasses have been

examined mostly in temperate habitats (Drew, 1979;

McMillan and Phillips, 1979; McMillan, 1983;

Bulthuis, 1987). These studies suggest that changes in

seawater temperature on seagrass habitats are depen-

dent on a number of interacting factors including: du-

ration of exposure, acclimatisation (thermal history),

light regime and leaf maturity (Bulthuis, 1987; Seddon

and Cheshire, 2000). Fong and Harwell (1994) suggest

that the productivity of tropical seagrass species starts

to decline above 30 8C but few studies have examinedthe thermal tolerance of tropical seagrass species to

changes in seawater temperature. Thorhaug et al.

(1978) reported that at temperatures elevated 34 8Cabove ambient, Thalassia testudinum showed evidence

of reduced standing crop and productivity, and that

tropical plants were more tolerant than subtropical

plants to elevated temperature. However, some species

(e.g. Halophila ovalis) with a wide geographical toler-

ance have a broad temperature tolerance (Ralph, 1998)

but tolerance of tropical seagrass species to periods of

high temperature exposure is less studied.

We used measures of effective photosynthetic quan-

tum yield to examine the effect of 14 h exposures of

thermal stress (3545 8C) on seagrass photosyntheticyield over a 3-day period. A pulse amplitude modulated

(PAM) fluorometer was used to measure effects of

thermal stress on chlorophyll a fluorescence within

PhotoSystem II (PSII), the most temperature sensitive

component of the photosynthetic apparatus (Ralph,

1998). The objectives of the present study were to:

characterise the acute response of 7 species of tropical

seagrass to thermal stress; assess each species capacity

to tolerate thermal stress and to assess the capacity of

tropical seagrasses to recover from thermal stress.

2. Methods

Six species of seagrass (Cymodocea rotundata,

Cymodocea serrulata, Halodule uninervis, Halophila

ovalis, Syringodium isoetifolium, Thalassia hempri-

chii) were collected at Green Island in northern

Queensland, Australia (38815V S 145821V E). Zosteracapricorni was collected from Cairns Harbour, Queens-

land, Australia (16853V S, 145846V E) (Fig. 1). Allplants were collected in the afternoon prior to the

start of the experiment and acclimated overnight in

oxygenated seawater maintained at 26 8C.Three temperature treatments were conducted over

three 5-day periods in May 2003. Each experiment

consisted of a control temperature treatment at 26 8C(n =8), representing the ambient temperature of seawa-

ter at Green Island in May, and three elevated temper-

ature treatments (each n =8) consisting of plants

exposed to 35, 40 or 45 8C. Plants were held in separate2 l containers with oxygenated re-circulating seawater.

All containers were immersed within a larger aquarium

(60 l), where they were acclimated overnight (26 8C).On each experimental day, the 2 l containers of

plants were directly transferred to the re-circulating

http:www.seagrasswatch.org

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Fig. 1. Location of Green Island and Cairns Harbour on the north-eastern coast of Queensland, Australia.

S.J. Campbell et al. / Journal of Experimental Marine Biology and Ecology 330 (2006) 455468 457

treatment seawater system at 1100 h, which was then

elevated to the treatment temperature over a period of

1530 min. Plants were exposed to treatment tempera-

tures (35, 40 or 45 8C) for 4 h (from 1100 to 1500 h).

The photochemical efficiency of PhotoSystem II (PSII)

and the effective quantum yield of photochemistry

(DF /FmV) were evaluated by subjecting light adaptedleaves to saturating pulses of light using a pulse ampli-


tude modulated (PAM) fluorometer (Diving-PAM)

(Walz, Germany). For each seagrass plant the basic

parameters of chlorophyll fluorescence (F, FmV) were

measured on the leaf shoot adjacent to the meristem, on

8 replicate leaves of each seagrass species, every hour

from 1100 to 1500 h. The mid-portion of each leaf (3

cm from meristem) was held in a leaf clip (Walz,

DIVING LC) and fluorescence measurements were

made underwater with the light probe joined to the

leaf clip. A weak pulsed red light (b1 Amol quantam2 s1) was applied to determine F in an illuminated

state. A saturating pulse (800 ms of 8000 Amol quantam2 s1 PAR) was then applied to FmV in an illumi-

nated state. The change in fluorescence (Fv or DF)caused by the saturating pulse (FmVF) in relationto the maximal fluorescence (FmV) is a measure of

quantum yield (Genty et al., 1989). The effective quan-

tum yield is given by DF /FmV (Eq. (1)). The separationof photochemical quenching (qP) and non-photochem-

ical quenching (qN) of chlorophyll fluorescence pro-

vides insight to the regulatory processes occurring

within the photosynthetic apparatus (Bolhar-Norden-

kampf et al., 1989). The quenching characteristics at

each time interval were calculated from Fv, F, and FmV(Bolhar-Nordenkampf et al., 1989).

Y FmV FV =FmV Fv=FmV 1

where

Y = effective quantum yield

FmV = maximal fluorescence in light adapted leaves

FV = initial fluorescence in light adapted leavesFv = (FmVFV)

At 1500 h all replicate plants were returned to

acclimation conditions at 26 8C overnight. On days 2and 3, the experiment was repeated. On days 4 and 5 all

plants were retained at 26 8C to determine recoveryrates following thermal stress. Parameters of chloro-

phyll fluorescence (F, FmV), photochemical quenching

Table 1

Absorbance factors, ecological niche, substrate and location of 7 species of

Species Absorbance factor

(F95% confidence interval)Ecological

Cymodocea rotundata 0.72F0.02 Mid-intertiCymodocea serrulata 0.79F0.02 Shallow suHalodule uninervis 0.70F0.03 Mid-intertiHalophila ovalis 0.56F0.04 Upper inteThalassia hemprichii 0.78F0.01 Mid-intertiZostera capricorni 0.48F0.03 Upper inteSyringodium isoetifolium 0.73F0.04 Shallow-de

(qP) and non-photochemical quenching (qN) were

made on 8 replicate leaves of each species at 1100

h on each day for control and treatment plants.

The proportion of incident irradiance absorbed by

leaves was determined for 5 replicate leaves for each

species. Each leaf was placed over the light sensor of

the Diving-PAM and incident saturating PAR recorded

with and without leaf according to the method de-

scribed by Schwarz and Hellblom (2002). The light

path distance was 3 mm, as that gave consistent optimal

readings of quantum yield of 0.700.80 for dark

adapted leaves healthy leaves. The measures were

made to determine if there were differences in leaf

absorbance that may explain responses to seawater

temperature exposure.

Mean quantum yield values at 50 h (end of temper-

ature exposure period) and 96 h (end of recovery

period) were analysed relative to mean control values

to provide a summary of plant responses at the 3

temperature exposure treatments. A three way

ANOVA (SYSTAT vers. 10.2) was used to test for

the effect of species, experiment (35, 40 and 45 8C)and treatment (control, temperature exposed). Data

were arcsin square-root transformed prior to analysis.

3. Results

3.1. Absorbance factors

The proportion of incident irradiance absorbed by

leaves ranged from 0.70 to 0.79 for C. serrulata, C.

rotundata, H. uninervis, T. hemprichii, S. isoetifolium

and less than 0.70 for H. ovalis and Z. capricorni

(Table 1). Habitat characteristics of all species are

also presented (Table 1).

3.2. Quantum yield relative to controls

The percentage of effective quantum yield of plants

exposed to temperature treatments relative to control

tropical seagrasses

niche Substrate Location

dalshallow subtidal Sand Green Island reef-flat

btidal Sand Green Island lagoon


rtidaldeep subtidal Sand Green Island reef-flat


rtidalshallow subtidal Mud Cairns Harbour mud-bank

ep subtidal Sand Green Island lagoon


treatments was calculated at 50 and 96 h. At 50 and 96

h the exposure/control percentage of effective quantum

yield of C. rotundata, C. serrulata, H. uninervis and T.

hemprichi, at 35 and 40 8C, ranged from 86% to 105%;for H. ovalis and S. isoetifolium these values ranged

from 43% to 69% and 61% to 81%, respectively (Fig.

2). At 45 8C (50 and 96 h) all species had effectivequantum yield values 537% of controls (Fig. 2). Re-

sponse was significantly different between control and

exposed for each treatment and for all species at both

50 (Table 2) and 96 h (Table 3) treatments.

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Fig. 2. Percentage (exposed/control) of effective quantum yield

(DF /FmV) for each seagrass species (o = Halophila ovalis, x =Halodule uninervis, 5 = Syringodium isoetifolium, E = Zosteracapricorni, w = Cymodocea rotundata, D = Cymodocea serrulata,. = Thalassia hemprichii) (4 h at 35, 40 and 45 8C) exposed toseawater temperatures, at (a) 50 h (day 3 of exposure) and (b) 96

h (end of recovery period).

3.3. Fluorescence values ( F and FmV)

For light adapted C. rotundata, C. serrulata and H.

uninervis the initial minimum fluorescence (F) was

relatively steady when exposed to 35 8C, 40 8C andrespective control treatments during the 3-day 4 h expo-

sures (Fig. 3). A few times during the 5-day experi-

ment lower F values were evident for S. isoetifolium

and T. hemprichi at 35 8C compared with controls, andfor H. ovalis, S. isoetifolium and T. hemprichi at 40 8Ccompared to controls (Fig. 3). At 45 8C the decline inF, compared with control treatments, occurred after 24

h in all species and persisted during the 2-day recovery

(25 8C) period.No difference in maximum fluorescence (FmV) could

be detected between control (26 8C) and thermal treat-ments (35, 40 8C) for C. rotundata, C. serrulata and H.uninervis during the 3-day exposure and 2-day recovery

(25 8C) period (Fig. 4). Similarly no difference betweencontrol and 35 8C treatments was found for Z. capricorniand H. ovalis. In contrast lower FmV values relative to

controls were found for T. hemprichi and S. isoetifolium

at 35 8C and for T. hemprichi, S. isoetifolium and H.ovalis at 40 8C during the 3-day thermal exposure and 2-day recovery period. For Z. capricorni lower FmV values

at 40 8C were found relative to controls during the finalday of thermal treatment and day 1 of recovery (Fig. 4).

For all species FmV declined by greater than 50% in the

first 2 h of 45 8C exposure, and during day 2 of 45 8Cexposure FmV declined to less than 80% of the initial

value in all species (Fig. 4).

3.4. Effective quantum yield (DF /FmV)

For C. rotundata, C. serrulata , H. uninervis and Z.

capricorni the effective quantum yield was relatively

steady when exposed to 35 8C, 40 8C and respectivecontrol (26 8C) treatments during the 3-day 4 h expo-sures (Fig. 5). At 40 8C, DF /FmV values for S. iso-etifolium, Z. capricorni and H. ovalis were 1035%

lower than control plants (Fig. 5). A decline in H. ovalis

DF /FmV to 50% of initial values occurred during the 2-day post-thermal stress recovery period (Fig. 5). At 45

8C a rapid decline in DF /FmV after 1 h was observedfor all species and persisted during the 3-day exposure

and 2-day recovery period.

A significant three way interaction between species,

experiment and treatment at 50 h was explained by

similar yield values of all species at 35 8C relative tocontrols, lower yield values for H. ovalis and S. iso-

tetifolium at 40 8C compared to all other species andcontrols and lower values for all species at 45 8C

Table 2

Three way ANOVA of the effects of temperature (control vs exposed), treatment (35, 40, 45 8C) and species on effective quantum yield in 8 seagrassspecies at 50 h after exposure to temperature treatments for 4 h over a 3-day period (n =328)

Source Sum-of-squares df Mean-square F-ratio P

Temperature 7.862 2 3.931 302.238 0.000

Treatment 5.064 1 5.064 389.344 0.000

Species 0.203 6 0.034 2.601 0.018

Temperature treatment 7.089 2 3.544 272.531 0.000Temperature species 0.550 12 0.046 3.527 0.000Treatment species 0.137 6 0.023 1.758 0.108Temperature treatment species 0.286 12 0.024 1.830 0.043Error 3.720 286 0.013

Data were arcsin square-root transformed prior to analysis.


compared to controls. A similar pattern was evident at

96 h, except that S. isotetifolium at 40 8C was not lowerthan controls suggesting recovery of this species after

exposure to 25 8C.

3.5. Quenching coefficients (qP and qN)

There was no apparent difference in qP between

control plants and those exposed to 35 8C. Similarlyfor plants of C. rotundata, C. serrulata, H. uninervis, S.

isoetifolium, T. hemprichii and Z. capricorni exposed

to 40 8C, photochemical quenching (qP) values did notdiffer from respective control (26 8C) treatments duringthe 3-day 4 h exposures (Fig. 6). For H. ovalis and qP

values at 40 8C were up to 20% lower compared withcontrols. A decline in H. ovalis qP to 75% of initial

values occurred during the 2-day post-thermal stress

recovery period (Fig. 6). At 45 8C a rapid decline inqP after 1 h was observed for all species and persisted

during the 3-day exposure and 2-day recovery period.

Non-photochemical quenching remained near 0 for

most species at 35 8C, 40 8C and respective control (268C) treatments (Fig. 7). At 40 8C non-photochemicalquenching (qN) for H. ovalis ranged between 0.2 and

0.6 on days 1 and 3 and increased to 0.8 during the

recovery period. By day 2 at 45 8C non-photochemical

Table 3

Three way ANOVA of the effects of temperature (control vs exposed), treatm

species at 96 h after exposure to temperature treatments for 4 h over a 3-da

Source Sum-of-squares

Temperature 8.819

Treatment 6.968

Species 0.671

Temperature treatment 9.600Temperature species 1.636Treatment species 0.220Temperature treatment species 0.507Error 3.533

Data were arcsin square-root transformed prior to analysis.

quenching (qN) was 1012 fold higher than respective

control (25 8C) treatments.

4. Discussion

The key finding of this study was that C. rotundata,

C. serrulata , H. uninervis and T. hemprichi were more

tolerant to thermal stress than S. isoetifoilum, Z. capri-

corni and H. ovalis. After 3 days of 4 h temperature

treatments ranging from 26 to 40 8C, C. rotundata, C.serrulata and H. uninervis demonstrated a wide toler-

ance to temperature with no effect on DF /FmV, qN orqP responses. These three species are restricted to

subtropical and tropical waters and their tolerance to

seawater temperatures of 40 8C is likely to be anadaptive response to high temperatures commonly

occurring at low tides and peak solar irradiance. Both

C. rotundata and C. serrulata were also more tolerant

for the initial 4 h treatment of 45 8C compared with allother species. Experimental responses in the present

study occurred at less than 100 Amol m2 s1, belowsaturation for these species (unpubl data), and the sen-

sitivity of seagrass species is likely to be greater at high

in situ irradiances during peak solar intensities. Indeed

Ralph (1999) found that elevated light and osmotic

stress increased the sensitivity of H. ovalis to thermal

ent (35, 40, 45 8C) and species on effective quantum yield in 8 seagrassy period (n =328)

df Mean-square F-ratio P

2 4.410 368.227 0.000

1 6.968 581.836 0.000

6 0.112 9.342 0.000

2 4.800 400.813 0.000

12 0.136 11.385 0.000

6 0.037 3.068 0.006

12 0.042 3.526 0.000

295 0.012

C. rotundata (45 degrees)

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C. serrulata (45 degrees)

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1000

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Fig. 3. Minimum fluorescence ( F) for each seagrass species exposed to control (o) (26 8C) and treatment (n) (4 h at 35, 40 and 45 8C) seawatertemperatures, for each treatment day and subsequent recovery period. The break in the data-line represents the end of each exposure period.



0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')


0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Max

imu

m f

lou

resc

ence

(F

m')

Fig. 4. Maximum fluorescence ( FmV) for each seagrass species exposed to control (o) (26 8C) and treatment (n) (4 h at 35, 40 and 45 8C) seawatertemperatures, for each treatment day and subsequent recovery period. The break in the data-line represents the end of each exposure period.



0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ldH. ovalis (40 degrees)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

effe

ctiv

e q

uan

tum

yie

ld

Fig. 5. Effective quantum yield ( Y) for each seagrass species exposed to control (o) (26 8C) and treatment (n) (4 h at 35, 40 and 45 8C) seawatertemperatures, for each treatment day and subsequent recovery period. The break in the data-line represents the end of each exposure period.



0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

1

2

3

4

5

6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

1

2

3

4

5

6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

Ph

oto

chem

ical

qu

ench

ing

(q

P)

Fig. 6. Photochemical quenching (qP) for each seagrass species exposed to control (o) (26 8C) and treatment (n) (4 h at 35, 40 and 45 8C) seawatertemperatures, for each treatment day and subsequent recovery period. The break in the data-line represents the end of each exposure period.



0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 24 25 26 27 28 48 49 50 51 52 72 96

No

n-p

ho

toch

emic

al q

uen

chin

g (

qN

)

Fig. 7. Non-photochemical quenching (qN) for each seagrass species exposed to control (o) (26 8C) and treatment (n) (4 h at 35, 40 and 45 8C)seawater temperatures, for each treatment day and subsequent recovery period. The break in the data-line represents the end of each exposure

period.



stress, suggesting that seagrass species may be even

less tolerant to high seawater temperatures during mid-

day saturating light exposure.

Symptoms of thermal stress found for H. ovalis, S.

isoetifolium and Z. capricorni at 40 8C suggest alower thermal tolerance and a susceptibility to eco-

physiological damage when climatic conditions result

in atypically high seawater temperatures. All are eu-

rythermal species with a broad thermal tolerance (10

35 8C) found growing in both tropical (McKenzie,1994) and temperate waters (Ralph, 1998; Campbell

et al., 2003a,b). Their low tolerance to high tempera-

tures (N35 8C) suggests that their photosynthetic con-dition would suffer detrimental effects from episodic

increases in water temperatures greater than 35 8C.Ralph (1998) also found that H. ovalis was intolerant

to temperatures above 37 8C over a 5 h period. Thequantity of light absorbed by leaves of H. ovalis is

lower than other species examined (Table 1) suggest-

ing it has lower leaf thickness and chlorophyll pig-

mentation. These characteristics may in part explain its

susceptibility to temperature relative to other species.

McMillan (1983) also found that H. ovalisminor

complex was susceptible to temperature with no sur-

vival of small leaved cultures after 18 h exposure to

39 8C and similarly no survival of larger leaf culturesafter 48 h exposure to 39 8C.

The susceptibility of S. isoetifolium to temperature

may be associated with its cylindrical morphology

(high surface area to volume ratio), the presence of

a thin leaf cuticle and thin vascular-bundle cell walls

(Kuo, 1993). Such leaf anatomy and ultrastructure is

advantageous for nutrient uptake and gaseous ex-

change and this species has been found to have a

relatively high production rate (0.92 g DW m2

d1) compared with other species used in this exper-

iment (0.010.50 g DW m2 d1) (Duarte and Chis-

cano, 1999). The morphology, however, would also

expose a proportionally high area of leaf tissue per

volume to stress from increased seawater temperature.

It is a species that previously has been restricted to

subtidal waters because of an intolerance to high

temperatures (i.e. 40 8C) in intertidal reef habitats(McMillan, 1983; Bridges and McMillan, 1986).

McMillan (1983) reported that both S. isoetifolium

and S. filiforme cultures were least tolerant to tem-

peratures of 39 8C relative to 7 other tropical genera.This susceptibility was attributed to its lack of distri-

bution in shallow waters that are exposed to high air

and seawater temperatures. At elevated temperatures

damage to photosynthetic tissue is likely to arise from

a loss of specific enzyme activity and changes in

transport of molecules and to membrane function

(Ralph, 1998). Damaged cells are characterised by a

brownish-black discoloration of cell wall and contents

(Biebl and McRoy, 1981; Walker and Cambridge,

1995).

We observed two fundamental characteristics of heat

stress to seagrasses using saturation PAM fluorometry.

The first effect was a decrease in photosynthetic yield

characterised by reductions in F and FmV. The dramatic

decline in Fv /FmV ratio, due to chronic inhibition of

photosynthesis, indicates an intolerance of H. ovalis, S.

isoetifolium and Z. capricorni to ecologically relevant

exposures of thermal stress. The decline in F and FmV is

indicative of structural alterations to the PSII reaction

centres that can take several days to recover (Ralph,

1998; Macinnis-Ng and Ralph, 2004). The decline in

FmV represents heat-induced photoinhibition related to

closure of PSII reaction centres and chloroplast dys-

function (Havaux, 1994). Such a response is likely due

to a combination of factors including increased respi-

ration, photoinhibition and enzyme breakdown. For

both Z. capricorni and H. ovalis the inability to recover

following 3 days of elevated temperature exposure (40

and 45 8C) suggests irreparable damage to chloroplaststructure and PSII function. The resulting burden to the

carbon balance of the plant would limit the survival

potential of seagrass leaves at temperatures greater than

40 8C.The second effect was an increase in non-photo-

chemical quenching, coupled with a decrease in pho-

tochemical quenching. Non-photochemical quenching

is considered to reflect a mechanism for photo-protec-

tion which is designed to protect the over-reduction of

the photosynthetic electron transport chain by dissipa-

tion of excess absorbed light energy in the PSII an-

tenna system as heat (Demmig-Adams, 1990).

Increases in non-photochemical quenching in H. ovalis

after 96 h exposure to temperatures greater than 30 8Chave been reported (Ralph, 1998), and the current

study shows that non-photochemical quenching rapidly

increased at temperatures of 40 8C and above. Rapidand complete decline of qP in three species at 40 8Cand in all 7 species at 45 8C suggests chloroplastmembrane damage eliminating electron transport and/

or temperature-dependent enzyme inactivation (Brug-

geman et al., 1992). As thermal stress was observed in

all 7 seagrass species after 1 h at 45 8C, heat inducedphotoinhibition and closure of PSII reaction centres

occurred rapidly and was irreparable. Damage to

PSII function in heat stress has been reported for 1

species of seagrass (Ralph, 1999) and some corals

(Jones et al., 1998). The lack of recovery by all species


at 45 8C suggests that photosynthetic enzymes havebeen inactivated or damaged (Bruggeman et al., 1992)

and acclimation to these temperatures for short (4 h)

exposure periods is unlikely. The resultant reduced

consumption of ATP can lead to an accumulation of

stored energy in the thykaloid membrane and an in-

crease in the proton gradient within PSII (Ralph,

1998). Excess absorbed light energy unable to be

used in the photosynthetic process is dissipated with

the resulting increase in non-photochemical quenching

(qN) and decrease in photochemical quenching (qP),

as demonstrated in this study. The recovery of Z.

capricorni photosynthetic yield after exposure to

high temperatures indicates a protective role of PSII

down-regulation consistent with that found previously

in H. ovalis (Ralph, 1998).

Results from the present study show that relatively

short exposures (b4 h) of elevated seawater tempera-

tures can damage seagrass physiology. Such conditions

are most likely to occur from November to May when

high solar irradiance and high seawater temperatures

combine to inhibit photosynthesis. Such findings are

consistent with declines in seagrass abundance during

the recent climatic anomalies in 2001/2002 in northern

Queensland (Roelofs et al., 2003; Campbell et al.,

2003a,b). Average seawater temperatures during the

austral summer of 2001/2002 increased by 12 8C(Berkelmans et al., 2004; Jones et al., 2004) and ele-

vated seawater temperatures (N35 8C) were recorded inshallow pools at low tide (McKenzie and Campbell,

2004). Meteorological data suggests that negative tides

were infrequent during solar noon periods in the austral

summer of 2001/2002, suggesting that exposure and

desiccation of seagrass leaves at low tide could not

fully explain seagrass dieback.

The results of temperature experiments here sug-

gest that the photosynthetic condition of all seagrass

species tested is likely to suffer irreparable effects

from short-term or episodic changes in seawater

temperatures as high as 4045 8C. Species differin their thermal tolerance making some (e.g. H.

ovalis, S. isoetifolium, Z. capricorni) more suscepti-

ble to thermal stress than others. Tolerance to tem-

peratures up to 40 8C in C. rotundata, C. serrulata,H. uninervis and T. hemprichii demonstrates an

adaptive tolerance of PSII reaction centres and chlor-

oplasts to temperature increases. H. ovalis, S. iso-

etifolium and Z. capricorni would appear least

thermo-tolerant of all species examined and that

may explain in part their biogeographical distribution

and recent loss in some tropical habitats in north

Queensland (unpubl data).

It may at first appear unusual that both H. ovalis and

Z. capricorni were the species least tolerant to tempera-

tures of z40 8C, as species appear well adapted to growin upper intertidal areas where shallow pools of water

can exceed 40 8C. The results of temperature experi-ments here suggest that they cannot tolerate extended

periods (N4 h) of temperatures z40 8C. Z. capricorni isnear the northern limit of its distribution in north Queens-

land, suggesting it may be intolerant of seawater tem-

peratures at latitudes above 108 S. In contrast H. ovalisand S. isoetifoilium are widespread throughout tropical

latitudes and morphological characteristics would ap-

pear to limit their tolerance to high seawater tempera-

tures. The ability of H. ovalis leaves to grow and

turnover rapidly may in part compensate for shoot loss

during excessive seawater temperature rises. The small

leaves of H. ovalis also flatten on the sediment surface

during low tide, a feature that keeps them moist and

protects them from desiccation (Bjork et al., 1999). On

the other hand S. isoetifolium is restricted to shallow

subtidal waters and its ecological niche suggests a ge-

netic intolerance to excessive temperatures, although

other ecological factors are likely to determine the

niche of seagrasses within habitats experiencing extreme

changes in environmental conditions.

An understanding of the thermal tolerance of tropical

species of seagrass is important to understand the stress

symptoms of seagrass ecosystems to climatic changes

that may lead to changes in seagrass species composition

and seagrass decline. Implications of the loss of species

and change in species dominance have ramifications for

herbivorous marine animals that target seagrass for food

and animals that use seagrass areas for habitat.

Acknowledgments

We thank Rudi Yoshida and Kate Pritchard for their

assistance in conducting laboratory experiments and

Rob Coles for providing review comments. This work

was supported by the Australian Cooperative Research

Centre Program through the CRC Reef Research Cen-

tre, and the Department of Primary Industries and

Fisheries, Queensland. [SS]

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Photosynthetic responses of seven tropical seagrasses to elevated seawater temperatureIntroductionMethodsResultsAbsorbance factorsQuantum yield relative to controlsFluorescence values (F and Fm)Effective quantum yield (DeltaF/Quenching coefficients (qP and qN)

DiscussionAcknowledgmentsReferences

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