Mar Biol (2007) 152:405–414
DOI 10.1007/s00227-007-0700-6
RESEARCH ARTICLE
Chlorophyll fluorescence measures of seagrasses Halophila ovalis
and Zostera capricorni reveal differences in response
to experimental shading
Juanita S. Bité Æ Stuart J. Campbell Æ
Len. J. McKenzie Æ Robert G. Coles
Received: 5 May 2006 / Accepted: 3 April 2007 / Published online: 28 April 2007
Springer-Verlag 2007
Abstract In coastal waters and estuaries, seagrass
meadows are often subject to light deprivation over short
time scales (days to weeks) in response to increased
turbidity from anthropogenic disturbances. Seagrasses may
exhibit negative physiological responses to light deprivation and suffer stress, or tolerate such stresses through
photo-adaptation of physiological processes allowing more
efficient use of low light. Pulse Amplitude Modulated
(PAM) fluorometery has been used to rapidly assess
changes in photosynthetic responses along in situ gradients
in light. In this study, however, light is experimentally
manipulated in the field to examine the photosynthesis
of Halophila ovalis and Zostera capricorni. We aimed to
evaluate the tolerance of these seagrasses to short-term
light reductions. The seagrasses were subject to four light
treatments, 0, 5, 60, and 90% shading, for a period of
14 days. In both species, as shading increased the photosynthetic variables significantly (P < 0.05) decreased by
up to 40% for maximum electron transport rates (ETRmax)
and 70% for saturating irradiances (Ek). Photosynthetic
efficiencies (a) and effective quantum yields (DF/Fm¢)
increased significantly (P < 0.05), in both species, for 90%
shaded plants compared with 0% shaded plants. H. ovalis
was more sensitive to 90% shading than Z. capricorni,
showing greater reductions in ETRmax, indicative of a reduced photosynthetic capacity. An increase in Ek, Fm¢ and
DF/Fm¢ for H. ovalis and Z. capricorni under 90% shading
suggested an increase in photochemical efficiency and a
more efficient use of low-photon flux, consistent with
photo-acclimation to shading. Similar responses were
found along a depth gradient from 0 to10 m, where depth
related changes in ETRmax and Ek in H. ovalis implied a
strong difference of irradiance history between depths of
0 and 5–10 m. The results suggest that H. ovalis is more
vulnerable to light deprivation than Z. capricorni and that
H. ovalis, at depths of 5–10 m, would be more vulnerable
to light deprivation than intertidal populations. Both species showed a strong degree of photo-adaptation to light
manipulation that may enable them to tolerate and adapt to
short-term reductions in light. These consistent responses
to changes in light suggest that photosynthetic variables
can be used to rapidly assess the status of seagrasses when
subjected to sudden and prolonged periods of reduced light.
Communicated by G.F. Humphrey.
J. S. Bité (&)
School of Tropical Environment Studies and Geography,
James Cook University, Townsville, QLD 4810, Australia
e-mail: Juanita.Bite@jcu.edu.au
S. J. Campbell Len. J. McKenzie R. G. Coles
Queensland Fisheries Service, Department of Primary Industries
and Fisheries, Northern Fisheries Centre, PO Box 5396,
Cairns, QLD 4870, Australia
S. J. Campbell Len. J. McKenzie
CRC Reef Research Centre, P.O. Box 772,
Townsville, QLD 4810, Australia
Introduction
Coastal waters and estuaries are highly productive and
ecologically valuable ecosystems. These systems are under
increasing stress from anthropogenic disturbances due to
sediment dredging, catchment runoff and urbanisation.
These disturbances and the increasing frequencies of natural disturbances (e.g. flooding and cyclones) (Preen et al.
1995; Campbell and McKenzie 2004) directly reduce the
distribution of ecologically important primary producers
123
406
and inhibits the maintenance of healthy marine ecosystems
(Dennison 1987; Duarte 1991; Alcoverro et al. 2001).
Seagrasses are often the dominant primary producers in
coastal ecosystems and contribute to maintaining water
quality by increasing the stability of sediments, biogeochemical cycling and trophic dynamics. The distribution of
seagrasses is widely accepted as a barometer of coastal
water quality (Duarte 1991; Gallegos and Kenworthy 1996;
Biber et al. 2005). However, loss of seagrass biomass and
suboptimal seagrass growth are usually detected when
coastal ecosystems are already degraded and their poor
condition is unable to be reversed by management actions.
Consequently, for seagrass ecosystems, the development of
a set of rapid and predictable sublethal stress indicators that
respond to low-light fluxes have been the focus of intense
research efforts in recent times (Kraemer and Hanisak
2000; Biber et al. 2005).
In response to the need for measures of sublethal stress
thresholds for seagrasses, there have been many efforts to
quantify the physiological responses of seagrasses in response to light deprivation over short time scales (days to
weeks) commensurate with timescales of turbidity events
in coastal waters (Longstaff et al. 1999; Ibarra-Obando
et al. 2004; Biber et al. 2005). In situ and rapid assessments
of sublethal stress have generally been restricted to morphological and structural photo-adaptive responses of
seagrasses to light reduction, including measures of canopy
height, shoot densities and leaf area indices (Longstaff and
Dennison 1999).
Chlorophyll fluorescence techniques, such as Pulse
Amplitude Modulated (PAM) fluorometery, offer a rapid
in situ tool to develop sublethal physiological indicators of
stress, detecting a response in seagrasses to light reduction
within days. Chlorophyll fluorescence (fluorescence yield)
can be measured and used to calculate the proportion of
photons absorbed by the photosynthetic PSII reaction
centres (i.e. the quantum yield), since fluorescence is
inversely correlated to photosynthetic efficiency. In lightadapted plants, these photons are used for photosynthetic
electron transport, described as the effective quantum yield
(DF/Fm¢). The effective quantum yield of a plant’s photosystem II (PSII) is DF=Fm0 ¼ F Fm0 =Fm0 ; where the
minimum fluorescence (F), as measured immediately before the saturating pulse, and subtracted from the maximal
fluorescence (Fm¢), measured immediately after a saturating pulse of light and then divided by Fm¢. F – Fm¢ is
described as the variable fluorescence (DF) (Beer et al.
2001; Ralph and Gademann 2005).
Research has focused on maximum quantum yield in
dark adapted plants, but this has not been found to be a
sensitive indicator of plant stress (Longstaff and Dennison
1999; Biber et al. 2005). In naturally fluctuating light
climates there are predictable changes in maximal electron
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Mar Biol (2007) 152:405–414
transport rates (ETRs), photosynthetic efficiencies and
saturating irradiances (Beer et al. 1998; Ralph et al. 1998;
Campbell et al. 2003). Such measures can be simply derived from rapid light curves (RLCs), using PAM fluorescence, and offer a more effective means of evaluating the
photosynthetic performance of seagrasses as affected by
their previous light history (Beer et al. 2001; Ralph and
Gademann 2005). Nonetheless, there is an absence of
manipulative studies that have investigated photosynthetic
derivatives of chlorophyll fluorescence (e.g.ETRmax, photosynthetic efficiency and saturating irradiance), as physiological indicators, that respond to changes in light
availability over relatively short timescales of days to
weeks, before seagrass mortality.
In recent times, seagrasses in Hervey Bay have fluctuated considerably in response to anthropogenic disturbance
(Preen et al. 1995; Campbell and McKenzie 2004). Hervey
Bay is a subtropical embayment containing ~2,000 km2 of
fast growing seagrass species including Halophila ovalis,
Halodule uninervis and Zostera capricorni. These species
have relatively small rhizomes and roots with low carbohydrate storage capacity (Abal et al. 1994), making them
susceptible to light reduction over short timescales (i.e.
days to weeks) (Longstaff and Dennison 1999).
Currently there are no rapid methods to detect stress, in
seagrasses, in response to light reduction before morphological changes or mortality occurs. In this study, we
examined differences in the photosynthetic responses of
two co-existing species of seagrass to 2-week light
reduction, a period that seagrass meadows are often subject to severe depletion of light caused by anthropogenic and natural disturbances. Photo-adaptation was also
investigated as a mechanism potentially employed by both
species to survive severe light depletion. Chlorophyll
fluorescence and, in particular, RLCs were used to rapidly assess if photo-adaptation could be detected under
different short-term shading manipulations and before
seagrass mortality.
Methods
Shading experiment
Site location
Two, 0 m depth, intertidal sites were chosen in Hervey
Bay. The sites were located at Urangan (2518.249¢
15254.394¢), and Burrum Heads (2511.349¢ 15237.559¢)
(Fig. 1). Both sites consisted of mixed meadows of
H. ovalis and Z. capricorni and were exposed at low tide
for 2–3 h during a tidal cycle. The maximum tidal height
ranges from 2 to 3 m.
Mar Biol (2007) 152:405–414
407
Fig. 1 Location of
experimental shading
experiments in Hervey Bay,
Australia
At both sites three 20 · 20 cm2 replicate plots were
exposed to three different shade treatments. Three replicate
control plots with no shade treatment were also marked,
giving a total of 12 plots. The shade treatments consisted of
100 · 100 cm2 mesh cloths suspended on an aluminium
structure 15 cm above the sediment. The Z. capricorni
canopy reached a maximum of 10 cm at Urangan and 6 cm
at Burrum Heads, while H. ovalis plants had a maximum
length of 3 cm. The shade cloths were positioned so that
plants received attenuated light during daylight. A
25 cm · 25 cm square was cut into the centre of each
shaded plot, 15 cm into the sediment, ensuring all rhizomes
were cut. Cutting prevented the shaded seagrass receiving
photosynthates translocated, via the rhizomes, from nonshaded parts of the plant (Longstaff and Dennison 1999).
The cuts were repeated several times during the duration of
shading. The three treatments consisted of 90, 60 and 5%
shading. Control plots of seagrass, without shade cloth,
were also marked. The shade cloths were deployed for a
total of 14 days, being changed every 3 days to reduce the
effect of algal fouling.
At day 0, between 11:00 and 13:00 h during low tide
and under sunny conditions, measures of RLCs, using a
diving PAM, were made on light adapted leaves of
H. ovalis and Z. capricorni from the centre of nine replicate
plots to be treated and each of three control plots. Healthy
green leaves were placed in a plastic ‘leaf clip’ in situ
(Walz, Effeltrich, Germany) to record RLCs. The PAM
light probe was attached to the leaf clip at a fixed distance
from the leaf. The diving PAM automatically generated
RLCs using an incremental sequence of 10 s actinic illumination periods, with light intensities increasing in the
eight steps 0, 50, 150, 340, 580, 850, 1,180 and 1,760 lmol
quanta m–2 s–1 photosynthetically active radiation (PAR).
For each illumination period initial fluorescence (F) and
following a saturating pulse of white light (800 ms of
8,000 lmol quanta m–2 s–1 PAR), maximum fluorescence
(Fm¢), were measured. The same procedure was followed
after 14 days (14 days) of continuous shading.
Depth gradient experiment
Site location
Two sites were chosen at each depth of interest (0, 5 and
10 m) for analyses of photosynthetic variables of H. ovalis
along a depth gradient.
Using the diving PAM and the procedure described
above, the photosynthetic performance of H. ovalis was
examined at three depths (0, 5 and 10 m), with two replicate sites for each depth (total of six sites). At each site,
RLCs were measured, as described above, for ten replicate
light adapted H. ovalis plants, between 11:00 and 13:00 h,
under sunny conditions.
In situ underwater light measurements
Ambient underwater light was measured as photon flux
using the diving PAM, which was calibrated underwater
with a Li-189 light meter (LiCoR, Lincoln, NE, USA). Five
123
408
photon flux measurements were made under every shade
cloth at days 0 and 14 during PAM measurements (n = 75).
Three to five measurements were made at each depth
during PAM measures along the depth gradient. All light
measurements were made under sunny conditions, avoiding cloudy periods. Mean values of light measures are
presented.
Season
The shade experiment and depth gradient photosynthetic
measures were completed within the month of November
2003, immediately prior to the onset of the wet season. The
water temperature was 23C for the duration of the shade
experiment and the depth gradient photosynthetic measures.
Calculation of photosynthetic variables
Apparent photosynthetic ETRs were calculated as the
product of effective quantum yield (F Fm0 =Fm0 ¼
DF=Fm0 ; where F is initial fluorescence, Fm¢, is maximum fluorescence and DF is variable fluorescence), the
incubation irradiance (I) and the absorbance factor (AF),
i.e. the fraction of light absorbed by the leaf. This product
was further multiplied by 0.5 because it was assumed that
half the photons required for the movement of electrons
along the photosystem pathways are absorbed by PSII
(Schreibers et al. 1995). Effective quantum yield (DF/Fm¢)
was calculated from the RLC for the first of the
eight actinic illuminations. AF values for H. ovalis
and Z. capricorni were derived by measuring the proportion of light absorbed by single leaves according to the
method described by Beer et al. (2001). The maximum
ETR (ETRmax) and photosynthetic efficiency (a) were
calculated by fitting the RLC data to an exponential
function; ETR ¼ ETRmax ð1 exp½aðIÞ=ETRÞ (Jassby
and Platt 1976) where ETR = electron transport rate and
I = irradiance. The onset of light saturation (Ek) was
calculated as ETRmax/a. In the few cases where a downregulation drop in ETR was recorded, measures from high
photon flux (e.g. 1,760 lmol quanta m–2 s–1 PAR) were
removed before fitting data to the exponential model.
Data analysis
Shading experiments
One-way ANOVAs were used to determine the effects of
plots prior to shading treatments on ETRmax, photosynthetic efficiency (a), minimum irradiance at which the plant
is photosynthetically saturated (Ek), initial fluorescence
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Mar Biol (2007) 152:405–414
yield (F), maximum fluorescence yield (Fm¢), effective
quantum yield (DF/Fm¢) and AFs. Effective quantum yield
and AF data from Urangan and Burrum Heads were arcsin
square-root transformed prior to analysis.
Three-way ANOVAs were used to determine the effects
of species, shade treatment and site on ETRmax, a, Ek, F,
Fm¢, DF/Fm¢ and AF. Photosynthetic efficiency, DF/Fm¢
and AF data from Urangan and Burrum Heads were arcsin
square-root transformed prior to analysis. ETRmax and Fm¢
data from Burrum Heads were loge transformed.
Depth gradient and shading
ANOVAs were used to test for significant differences in
mean photosynthetic variables in Halophila ovalis (ETRmax,
a, Ek, F, Fm¢, DF/Fm¢ and AF) (n = 3–10) among both sites
(Urangan and Burrum Heads) and depths (0, 5 and 10 m),
and among all shade treatments (control, 5, 60 and 90%) at
the two sites, at 14 days (n = 14). Post hoc Bonferroni tests
were used to test for significant differences among means.
ETRmax and Ek, data were loge (x + 1) transformed prior to
analysis. All analyses were performed using SYSTAT
(Version 10.2).
Results
Before shading
Immediately prior to the shading manipulations at day 0,
there were no significant differences (P < 0.05) for photosynthetic variables among each of the treatment plots at
either site, Urangan and Burrum Heads.
Shading manipulations
Three-way ANOVA revealed a significant interaction between site, species and shade treatment for ETRmax and
saturating light intensity (Ek) (Table 1). Post hoc, Bonferroni, analysis showed the interaction was due to a significantly higher mean (±SE) ETRmax (178.24 ± 17.53) for nonshaded (control) Z. capricorni at Burrum Heads compared
with the means of non-shaded Z. capricorni (96.87 ± 20.61)
and Halophila ovalis plants (81.21 ± 5.32) at the Urangan
site (Fig. 2). At Burrum Heads the mean ETRmax for Z.
capricorni non-shaded plants (178.24 ± 17.53) was also
significantly higher than 60% (94.62 ± 23.69) and 90%
(83.78 ± 7.64) shaded plants (Fig. 2). At Urangan, means of
ETRmax for 5% (103.95 ± 12.16) and 60% (91.26 ± 10.00)
shaded Z. capricorni were higher than the mean of 90%
shaded Z. capricorni (74.49 ± 2.29) plants (Fig. 2). Mean
ETRmax of 90% shaded H. ovalis at Burrum Heads
Mar Biol (2007) 152:405–414
409
Table 1 Three-way ANOVA of the effect of site (Urangan and
Burrum Heads), species (Halophila ovalis and Zostera capricorni)
and shade treatment (0, 5, 60 and 90% light reduction) on photosynthetic variables in Hervey Bay, n = 3
Photosynthetic variable
df
MS
F
P
Table 1 continued
Photosynthetic variable
df
MS
F
P
0.001
DF/Fm¢
Site
1
0.116
24.348
ETRmax
Species
1
0.052
10.812
0.002
Site
1
1.68
34.258
0.001
Shading
3
0.040
8.322
0.001
Species
Shading
1
3
0.873
0.622
17.811
12.691
0.002
0.001
Site · species
1
0.007
1.446
0.238
Site · shade
3
0.024
5.028
0.006
Site · species
1
0.027
0.555
0.462
Species · shade
3
0.015
3.163
0.038
Site · shade
3
0.169
3.452
0.028
Site · species · shade
2.239
0.103
3
0.011
32
0.005
Site
1
0.001
0.297
0.590
Species
1
0.004
1.777
0.192
Species · shade
3
0.062
1.274
0.300
Error
Site · species · shade
3
0.176
3.591
0.024
Absorbance factor
32
0.049
Error
A
Site
1
0.034
18.911
0.001
Shading
3
0.008
3.524
0.026
Site · species
1
0.002
0.983
0.329
Species
1
0.003
1.633
0.211
Shading
3
0.021
11.491
0.001
Site · shade
3
0.019
8.639
0.001
3
3
0.004
0.003
1.568
1.165
0.216
0.338
32
0.002
Site · shade
3
0.006
3.409
0.029
Species · shade
Site · species · shade
Species · shade
3
0.005
2.519
0.076
Error
Site · species · shade
3
0.001
0.249
0.861
Error
Ek
32
0.002
Site
1
Species
1
Shading
3
Site · species
1
0.002
0.881
0.355
603,386.68
84,445.329
274,982.58
42.883
0.001
6.002
0.020
19.543
0.001
Site · species
1
1,220.384
0.087
0.770
Site · shade
3
38,327.461
2.724
0.061
Species · shade
3
66,161.828
4.702
0.008
Site · species · shade
3
50,470.265
3.587
0.024
32
14,070.504
Error
F
Site
1
Species
1
59,854.687
Shading
3
8,406.854
2.654
0.065
Site · species
1
623.521
0.197
0.660
Site · shade
Species · shade
3
3
750.021
3,945.91
0.237
1.246
0.870
0.309
3
6,087.854
1.922
0.146
32
3,167.854
42.883
0.001
Site · species · shade
Error
155,838.02
49.19
18.89
0.001
0.001
Fm¢
Site
1
Species
1
Shading
3
603,386.68
84,445.329
274,982.58
6.002
0.020
19.543
0.001
Site · species
1
1,220.384
0.087
0.770
Site · shade
3
38,327.461
2.724
0.061
Species · shade
3
66,161.828
4.702
0.008
3
50,470.265
3.587
0.024
32
14,070.504
Site · species · shade
Error
(78.02 ± 14.86) and Urangan (48.61 ± 1.92) was significantly lower compared with non-shaded and 5% shaded
H. ovalis at both sites.
For Ek the significant three-way interaction (Table 1)
was best explained by significantly higher mean Ek of nonshaded Z. capricorni (733.07 ± 92.40) and H. ovalis
(573.23 ± 51.51) at Burrum Heads compared with nonshaded Z. capricorni (449.21 ± 113.64) and H. ovalis plants
(365.06 ± 50.58) at Urangan (Fig. 2). Species interactions
were best explained by higher mean Ek for 60%
(294.73 ± 54.93) and 90% (209.70 ± 9.07) shaded Z. capricorni compared with mean Ek for 60% (209.70 ± 21.32)
and 90% (120.07 ± 3.88) shaded H. ovalis, respectively, at
Urangan, but no such differences were found at Burrum
Heads (Fig. 2). At Burrum Heads there was no difference
between mean Ek values in 60% shaded (342.86 + 92.70)
and 90% shaded Z. capricorni (331.03 ± 72.68), but at
Urangan mean Ek of 60% shaded Z. capricorni (294.73 ±
54.93) was significantly higher than the mean Ek of 90%
shaded plants (209.70 ± 9.07) (Fig. 2). At Burrum Heads
the mean Ek of 5% shaded Z. capricorni (534.65 ± 69.62)
was significantly higher than 60% shaded (342.86 + 92.70)
Z. capricorni, whereas at Urangan no difference was found
between mean Ek of 5% shaded (328.66 ± 10.49) and 60%
shaded (294.73 ± 54.93) Z. capricorni (Fig. 2).
A shading effect was evident for all parameters
measured, except F. At both sites, 90% shade treated
H. ovalis (1,100.67 ± 69.20 at Urangan; 230.33 ± 12.35
at Burrum Heads) showed higher mean Fm¢ compared
123
Mar Biol (2007) 152:405–414
60%
90%
0d
Initial Fluorescence (F )
600
400
200
5%
60%
600
BH
800
Contr ol
90%
400
200
0
60%
90%
2000
BH
1600
1200
800
400
0
0d
Contr ol
5%
60%
0d
5%
60%
BH
0.8
0.6
0.4
0.2
0d
Contr ol
5%
60%
90%
0d
600
400
200
0.4
0.2
5%
60%
90%
UG
400
200
0
0d
Contr ol
5%
60%
2000
UG
1200
800
400
0d
Contr ol
5%
60%
0d
90%
1600
Contr ol
5%
60%
1.0
90%
UG
0.8
0.6
0.4
0.2
0.0
90%
0d
1.0
Absorbance factor (AF)
0.6
Contr ol
UG
BH
0.8
0.2
600
0
0.0
90%
0.4
90%
800
90%
1.0
1.0
Absorbance factor (AF)
Contr ol
Maximum Fluorescence (Fm')
5%
Effective Quantum Yield (DF/Fm')
Contr ol
60%
5%
0
0
0d
Contr ol
1000
BH
UG
0.0
0d
Effective Quantum Yield (DF/Fm' )
5%
0.6
α (µmol e-/µmol photons)
0.2
UG
200
180
160
140
120
100
80
60
40
20
0
Initial Fluorescence (F)
0.4
E k (µmol photons m-2 s -1)
Contr ol
1000
Maximum Fluorence (Fm')
BH
0.0
0d
E k (µmol photons m-2 s -1)
0.6
BH
ETR max (µmol e- m-2 s -1)
200
180
160
140
120
100
80
60
40
20
0
α (µmol e-/µmol photons)
ETR max (µmol e- m-2 s -1)
410
Contr ol
5%
60%
90%
UG
0.8
0.6
0.4
0.2
0d
Contr ol
5%
60%
90%
0d
Contr ol
5%
60%
90%
Fig. 2 Effect of shading on photosynthetic variables ETRmax, a, Ek,
F, Fm¢, DF/Fm¢ and AF, at Urangan (UG) and Burrum Heads (BH)
sites. Values are shown at 0 day and after 14 days shading for control,
5, 60 and 90% shade treated plots for H. ovalis (white bars) and
Z. capricorni (shaded bars) (mean ± SE, n = 3)
with control H. ovalis (47.33 ± 113.87 at Urangan; 187.33 ±
42.29 at Burrum Heads) plants (Fig. 2, Table 1). At both
sites, 90% shaded H. ovalis had significantly higher mean
a(0.2813 ± 0.0003 at Burrum Heads; 0.4047 ± 0.0045 at
Urangan) and mean DF/Fm¢ (0.6943 ± 0.0068 at Burrum
Heads; 0.7553 ± 0.0180 at Urangan) compared with nonshaded (a – 0.2120 ± 0.0189 at Burrum Heads and
0.2267 ± 0.0156 at Urangan; DF/Fm¢ – 0.5143 ± 0.0088
at Burrum Heads and 0.6070 ± 0.0255 at Urangan) and
5% shaded (a – 0.2010 ± 0.0492 at Burrum Heads and
0.2650 ± 0.0420 at Urangan; DF/Fm¢ – 0.4647 ± 0.0799 at
Burrum Heads and 0.7423 ± 0.0234 at Urangan) H. ovalis
plants, at respective sites. At Urangan, 90% shaded Z. capricorni had significantly higher mean a (0.3557 ± 0.0062)
compared with non-shaded (0.2220 ± 0.0140), 5% (0.3147 ±
0.0278) and 60% (0.3207 ± 0.0296) shaded Z. capricorni. At
Urangan, DF/Fm¢ was significantly higher for 90% shaded
Z. capricorni (0.7683 ± 0.0143) compared with non-shaded
Z. capricorni (0.6940 ± 0.0381). For ETRmax and saturating light intensity (Ek), the opposite was generally observed for both species at both sites, where 90% shaded
plants had significantly lower values than control and 5%
shaded plants, especially in H. ovalis (Fig. 2). At Urangan
90% shaded Z. capricorni (0.6368 ± 0.0154) and H. ovalis
(0.6632 ± 0.0023) had higher AFs than respective 5%
shaded Z. capricorni (0.6120 ± 0.0297) and H. ovalis
(0.5060 ± 0.0481) plants (Fig. 2).
123
Halophila ovalis: depth gradient and shading
experiment
Plots of ETRmax, photosynthetic efficiency (a), saturating
irradiance (Ek), effective quantum yield (DF/Fm¢) and AF
against depth and shading treatments revealed a decline in
Mar Biol (2007) 152:405–414
411
0.5
a
a
120
100
b
b
b
80
-2
a
a
b
b
bc
60
c
c
40
c
20
0
ab
0.3
ab
a
b
a
ab
a
a
a
a
a
0.2
0.1
0.8
90%
5%
60%
90%
5%
60%
10m
Cont rol
5m
10m
0m
5m
Cont rol
d
bc
c
ac
a
a
90%
5%
60%
90%
Control
5%
60%
0.4
90%
60%
5%
Cont rol
90%
5%
60%
10m
Cont rol
5m
10m
5m
0m
0m
a
0.5
10m
c
b
0.6
Control
b
b
c
c
5m
c
b
10m
200
b
b
c
b
0.7
5m
b
b
0m
a
ab
0.8
0m
ab
400
b
0.9
0m
600
1.0
0m
a
a
Effective Quantum Yield (DF/Fm' )
a
a
0m
90%
5%
60%
Cont rol
90%
5%
60%
10m
Cont rol
5m
10m
0m
5m
0m
-2
-1
PFD (µmol photons m s )
1200
200
0
60%
90%
60%
5%
90%
Control
5%
60%
10m
Control
10m
5m
5m
0m
0m
0.4
400
90%
b
0.5
600
5%
abc abc
a
Cont rol
ab
a
800
90%
a
abc
60%
a
5%
a
Cont rol
a
10m
a
a
1000
10m
c
5m
0.7
5m
-2
-1
E k (µmol photons m s )
800
0
Absorbance factor
b
b
0
1000
0.6
d
0.4
-
-
-2
-1
ETR max (µmol e m s )
-1
a
140
α (µmol e /µmol photons m s )
160
Fig. 3 Effect of depth (0, 5 and 10 m) on photosynthetic variables
(ETRmax, a, Ek, F, Fm¢, DF/Fm¢ and AF) for H. ovalis. Each graph
includes the effect of shading treatments at 14 days (control, 5, 60 and
90% shade plots) at Urangan (grey bars) and Burrum Heads (white
bars). Photon flux density at each depth and shade treatment are also
shown (mean ± SE, n = 3). Different letters above bars indicate
significant differences (Bonferoni) between treatment means at
P < 0.05
ETRmax and Ek and a general increase in a and DF/Fm¢
with increasing depth and shade (Fig. 3). Significant
differences among means of each variable were found
(Table 2). Mean ETRmax of H. ovalis in non-shaded and
5% shaded plots at Urangan were significantly lower than
plants at 0 m but not significantly different than ETRmax of
plants at 5 m (Fig. 3). In contrast ETR max and Ek of nonshaded plants at Burrum Heads were no different to plants
at 0 m, while 90% shaded H. ovalis had mean ETRmax and
Ek values that were significantly lower than plants at 0 m
but not significantly different compared with plants growing at 5 and 10 m (Fig. 3). Mean a and DF/Fm¢ values in
control and 5% shaded plots were not significantly different compared with mean values at 0 m. Similarly mean a
and DF/Fm¢ recorded for 60% shaded H. ovalis were not
significantly different when compared with mean a and DF/
Fm¢ of H. ovalis at 5 m (Fig. 3). Mean a and DF/Fm¢
values in 90% shaded H. ovalis were significantly lower
than plants at 0 m but not significantly different compared
with H. ovalis at 5 and 10 m.
Discussion and conclusions
Light is a key factor controlling the photosynthetic performance of seagrasses. Both long-term and short-term
light deprivation directly influence seagrass photosynthesis
and survival (Longstaff et al. 1999; Ruiz and Romero 2001;
123
412
Mar Biol (2007) 152:405–414
Table 2 Halophila ovalis: ANOVA of the effect of shade plots
(control, 5, 60 and 90% at two sites) and depths (0, 5 and 10 m) in
Hervey Bay
Photosynthetic variable
df
MS
F
P
F
Treatment
13
26,290.1
Error
69
2,956.8
Treatment
13
810,173.8
Error
69
41,911.4
8.891
0.001
19.331
0.001
44.212
0.001
16.129
0.001
59.406
0.001
44.357
0.001
2.281
0.022
Fm¢
ETRmax
Treatment
13
2.172
Error
69
0.049
a
Treatment
13
0.021
Error
69
0.001
Treatment
13
3.119
Error
69
0.052
Ek
DF/Fm¢
Treatment
13
0.097
Error
69
0.002
Absorbance factor
Treatment
13
0.007
Error
41
0.003
ETRmax and Ek values were loge transformed prior to analysis
(n = 3–10)
Peralta et al. 2002). We found short-term changes in the
physiology of seagrasses by manipulating light during
experimental shading. Of all the photosynthetic variables
tested (ETRmax, Ek, a, Fm¢, DF/Fm¢ and AF) ETRmax and
Ek showed the most consistent responses among shade
treatments and depth gradients, characterised by reductions
in ETRmax and increases in Ek, with light depletion. Both
Halophila ovalis and Z. capricorni exhibited typical responses to shading yet H. ovalis had a more conspicuous
response, suggesting that this species is more sensitive to
reduced transient light than Z. capricorni. The results
suggest a tolerance and acclimation to light deprivation in
both species of seagrass between 5 and 60% of surface
irradiance and are consistent with findings that H. ovalis
has a limited tolerance to light deprivation when compared
with morphologically larger species of seagrass (Longstaff
et al. 1999).
There was a consistent trend among species and sites, as
shown by the decreasing ETRmax and Ek and increasing
a and DF/Fm¢ with light depletion, and a strong separation
of photosynthetic responses between high and low light
treatments. These photosynthetic responses are evidence
that photosynthetic parameters derived from RLCs (Beer
123
et al. 2001) are a useful tool that can be used to evaluate
short-term responses of seagrasses between highly differentiated light climates. The parameters sensitive to 90%
shading in H. ovalis were reduced ETRmax, increased a,
reduced saturating irradiance (Ek) and increased effective
quantum yield (DF/Fm¢). While a reduction in both ETRmax
and Ek suggests a limited photosynthetic capacity such
reduction indicates photo-acclimation and improved photon capture for conversion to chemical energy. Similarly,
the increase in both a and DF/Fm¢ in response to shading
indicates a greater proportion of photons were used in
photosynthesis and a more efficient use of light (Beer et al.
2001). These photo-adaptive responses to irradiance or
photon flux deprivation are analogous with other photoadaptive responses such as chlorophyll increases under low
light conditions (Dennison and Alberte 1986; Longstaff
and Dennison 1999). These findings are consistent with
reports of reductions in photosynthetic performance and
mortality in seagrasses in response to shading (Longstaff
and Dennison 1999; Longstaff et al. 1999) and depth
(Schwarz and Hellblom 2002; Durako et al. 2003).
The differences in photosynthetic performance of both
seagrass species among shade treatments at different sites
are easily explained by site features influencing light fluxes
and the species responses to these differences. At low tide
there was little obvious difference in light climates between
sites, yet during tidal inundation high amounts of suspended fine ‘silty’ sediments at Urangan were evident more
so than at Burrum Heads, where the sediments have a high
sand composition and are less inclined to be re-suspended
during tidal flux. These high silt loads at Urangan resulted
in lower light availability and more pronounced shade-type
responses (e.g. lower Ek) in both seagrass species at
Urangan, compared with seagrasses at Burrum Heads. In
addition, the fouling of shade screens appeared heavier at
the Urangan site, despite regular cleaning, and may also
have contributed to lower photon fluxes under the shade
screens. At both sites Halophila ovalis was more sensitive
to changes in light availability than Zostera capricorni and
thus exhibited the greatest reduction in ETRmax and Ek and
the highest increases in photosynthetic efficiencies in response to shading and in situ light depletion at Urangan.
A likely explanation is that H. ovalis is less structurally
complex and faster growing than Z. capricorni, with low
storage capacity for carbohydrates that can be used for
growth during periods of low light (Longstaff et al. 1999).
The increase in effective quantum yield (DF/Fm¢) of
both seagrass species under 90% shading, relative to
controls, was due to elevated Fm¢ values. Elevation of
Fm¢ suggests an increase in photochemical efficiency, a
response that is consistent with photo-acclimation of
plants to shading. This contrasts with Longstaff et al.
(1999) who found that quantum yield in dark adapted
Mar Biol (2007) 152:405–414
H. ovalis remained unchanged during complete light
deprivation for 24 days. However, experimental shading
units deployed by Longstaff and Dennison (1999) and
Longstaff et al. (1999) showed photo-adaptive responses
of reduced chlorophyll a and sugar concentration in
H. ovalis leaves after 15 days of shading that are consistent with findings of reduced ETRmax, increased a and
increased Ek in the present study. In the present study no
change in DF/Fm¢ was found for Z. capricorni. Biber
et al. (2005) concluded that the maximum quantum yield
of Z. marina in response to light deprivation was not a
sensitive indicator of chronic stress as Z. marina was able
to acclimate to ambient light conditions. A close examination of Biber et al. (2005) shows a clear stress response
in healthy leaves, with quantum yield values of 0.5 at
4 weeks, 1 week prior to an observed decline in shoot
numbers and leaf area. Similarly, they show that quantum
yield values in Halodule wrightii, in dark conditions,
declined to 0.6 after 3 weeks and 0.2 after 4 weeks, at
least a week prior to noticeable declines in shoot numbers. In the context of the clear photosynthetic responses
to light reduction found in the present study, this result
demonstrates the importance of using a suite of photosynthetic variables, such as ETRmax, a, Ek and DF/Fm¢,
derived from RLCs to assess short-term responses of
seagrasses to light deprivation, not measures of maximum
quantum yield alone.
The influence of depth on ETRmax and Ek implies a
strong separation of irradiance history between depths of
0 and 5–10 m. After 2 weeks of exposure to 90% reduced
light availability, H. ovalis exhibited shade-type physiological responses, i.e. lower ETRmax and Ek, similar to
plants growing at 5–10 m. The similarity in the light climates between 90% shaded plants and ambient light at 5–
10 m is the likely explanation. The 40% lower ETRmax and
67% lower Ek values in 90% shaded H. ovalis compared
with controls, was generally lower than depth related
reductions of ETRmax and Ek from 0 to 5 m and from 0 to
10 m. Intertidal plants may be more susceptible to light
reduction than subtidal plants and exhibit greater photoadaptive responses in order to optimise light acquisition
and maximise the use of low-light climates. In addition,
fouling of shade cloths in between cleaning may have
lowered available photon flux density (PFD) below recorded levels and contributed to a heightened photoadaptive response. Comparative lowering of ETRmax and
Ek values has been recorded for Halophila stipulacea
(Schwarz and Hellblom 2002) and H. ovalis (Ralph 1996).
In order to optimise light acquisition and maximise their
use of low-light climates, these plants can diurnally regulate their saturating irradiance as detected in a number of
seagrass species (Ralph et al. 1998; Campbell et al. 2003).
We have also measured a three to fourfold increase in light
413
saturating irradiance, between 0600 and 1200 hours, in
deepwater H. ovalis (unpublished data).
Absorbance factors have been shown to vary considerably with geographic location (Silva and Santos 2003),
depth (Schwarz and Hellblom 2002), species (Beer et al.
2001; Campbell et al. 2003; Lan et al. 2005) and leaf age
(Enrı́quez et al. 2002). In Hervey Bay there was no change
in AF with depth, but the high AFs of dark shaded (90%)
H. ovalis at Urangan were indicative of a photo-adaptive
response but were unlikely alone to be a reliable indicator
of light reduction. The general lack of change in AFs with
depth and shading suggests that factors (e.g. time of year,
nutrient status) other than light attenuation may be influencing AFs over time scales of weeks to months.
The stronger responses to shading suggest that H. ovalis
was more vulnerable to light deprivation than Z. capricorni
and that H. ovalis at depths of 5–10 m would be more
vulnerable to light deprivation than intertidal populations.
Intertidal H. ovalis appears to be more vulnerable to
reductions in irradiance than intertidal Z. capricorni,
implicating local light conditions as important influences
on species specific photosynthetic performance. The lower
photosynthetic responses of H. ovalis at depths greater than
5 m compared with intertidal plants, also suggest a greater
vulnerability to light deprivation with increased depth.
Compared with large seagrasses, the more pronounced
responses of H. ovalis to light reduction may be due to
differences in the capacity of small seagrasses to store
carbohydrates and maintain photosynthetic performance
(Czerny and Dunton 1995; Kraemer and Alberte 1995;
Longstaff et al. 1999). The short-term response of seagrasses to in situ reductions in light availability is likely to
result in reduced growth rates and therefore can provide a
rapid and precise indicator of sub-lethal stress. The photoadaptive responses from experimental light reduction and
the measures of light attenuation and saturating irradiances
along depth gradients, particularly at lower depth limits of
survival, could also be used to predict reductions in photosynthetic capacity before mortality eventuates.
Both species showed a photo-adaptive response to light
depletion that may help them tolerate short-term yet severe
reductions in light commonly encountered in Hervey Bay.
Responses of ETRmax, saturating irradiance (Ek) and photosynthetic efficiency (a) to light reduction were the most
consistent and are therefore likely to provide the best
indicators of photo-adaptation and possible seagrass stress.
Acknowledgements This work was conducted as part of a Ph.D.
degree in the School of Tropical Environment Studies and Geography, James Cook University. The work was supported by an Australian Postgraduate Award (Industry) from the Australian Research
Council and Industry partners, Queensland Department of Primary
Industries and Fisheries, Queensland, Parks and Wildlife Service
(Environment Protection Authority) and World Wide Fund for Nature
123
414
(WWF). The appropriate permit, required to remove seagrasses in
Queensland, was obtained from the Queensland Department of
Industries and Fisheries. The authors would like to acknowledge S.
Kerville and D. Foster for their technical and field assistance.
References
Abal EG, Loneragan NR, Bowen P, Perry CJ, Udy JW, Dennison WC
(1994) Physiological and morphological responses of the
seagrass Zostera capricorni Aschers. to light intensity. J Exp
Mar Biol Ecol 178:113–129
Alcoverro T, Cerbian E, Ballesteros E (2001) The photosynthetic
capacity of the seagrass Posidonia oceanica: influence of
nitrogen and light. J Exp Mar Biol Ecol 261:107–120
Beer S, Vilenkin B, Weil A, Veste M, Susel L, Eshel A (1998)
Measuring photosynthetic rates in seagrasses by pulse amplitude
modulated (PAM) fluorometry. Mar Ecol Prog Ser 174:293–300
Biber PD, Paerl HW, Gallegos CL, Kenworthy WJ (2005) Evaluating
indicators of seagrass stress to light. In: Bortone SA (ed)
Estuarine indicators. CRC, Boca Raton, FL, pp 193–210
Campbell SJ, McKenzie LJ (2004) Flood related loss and recovery of
intertidal seagrass meadows in southern Queensland, Australia.
Estuarine Coast Shelf Sci 60:477–490
Campbell SJ, Miller C, Steven A, Stephens A (2003) Photosynthetic
responses of two temperate seagrasses across a water quality
gradient using chlorophyll flourescence. J Exp Mar Biol Ecol
291:57–78
Czerny AB, Dunton KH (1995) The effects of in situ light reduction
on the growth of two subtropical seagrasses, Thalassia testudinum and Halodule wrightii. Estuarties 18:418–427
Dennison WC (1987) Effects of light on seagrass photosynthesis,
growth and depth distribution. Aquat Bot 27:15–26
Dennison WC, Alberte RS (1986) Photoadaptation and growth of
Zostera marina L. (eelgrass) transplants along a depth gradient.
J Exp Mar Biol Ecol 98:265–383
Duarte CM (1991) Seagrass depth limits. Aquat Bot 40:363–377
Durako MJ, Kunzelman JI, Kenworthy J, Hammerstrom KK (2003)
Depth-related variability in the photobiology of two populations
of Halophila johnsonii and Halophila decipiens. Mar Biol
142:1219–1228
Enrı́quez S, Merino M, Iglesias-Prieto R (2002) Variations in the
photosynthetic performance along the leaves of the tropical
seagrass Thalassia testudinum. Mar Biol 140:891–900
Gallegos CL, Kenworthy WJ (1996) Seagrass Depth limits in the
Indian River Lagoon (Florida, USA): application of an optical
water quality model. Estuarine, Coast Shelf Sci 42:267–288
Ibarra-Obando SE, Heck KL, Spitzer PM (2004) Effects of simultaneous changes in light, nutrients, and herbivory levels, on the
structure and function of a subtropical turtlegrass meadow. J Exp
Mar Biol Ecol 301:193–224
123
Mar Biol (2007) 152:405–414
Jassby AT, Platt T (1976) Mathematical formulation of the relationship between photosynthesis and light for phytoplankton.
Oceanography 21:540–547
Kraemer GP, Alberte RS (1995) Impact of daily photosynthetic
period on protein synthesis and carbohydrate stores in Zostera
marina L. (eelgrass) roots: implications for survival in lightlimited environments. J Exp Mar Biol Ecol 185:191–202
Kraemer GP, Hanisak MD (2000) Physiological and growth responses
of Thalassia testudinum to environmentally-relevant periods of
low irradiance. Aquat Bot 67:287–300
Lan C-Y, Kao W-Y, Lin H-J, Shao K-T (2005) Measurement of
chlorophyll fluorescence reveals mechanisms for habitat niche
separation of the intertidal seagrasses Thalassia hemprichii and
Halodule uninervis. Mar Biol 148:25–34
Longstaff BJ, Dennison WC (1999) Seagrass survival during pulsed
turbidity events: the effects of light deprivation on the seagrasses
Halodule pinifolia and Halophila ovalis. Aquat Bot 65:105–121
Longstaff BJ, Loneragan NR, O’Donohue MJ, Dennison WC (1999)
Effects of light deprivation on the survival and recovery of the
seagrass Halophila ovalis (R.Br.) Hook. J Exp Mar Biol Ecol
234:1–27
Peralta G, Pérez-Lloréns JL, Hernández I, Vergara JJ (2002) Effects
of light availability on growth, architecture and nutrient content
of the seagrass Zostera noltii Hornem. J Exp Mar Biol Ecol
269:9–26
Preen AR, Long WJL, Coles RG (1995) Flood and cyclone related
loss, and partial recovery, of more than 1000 km2 of seagrass in
Hervey Bay, Queensland, Australia. Aquat Bot 52:3–17
Ralph PJ (1996) Diurnal photosynthetic patterns of Halophila ovalis
(R.Br.) Hook f. In: Kuo J, Phillips RC, Walker DI, Kirkman H
(eds) Seagrass biology: proceedings of an international workshop, Rottnest Island, Western Australia, pp 197–202
Ralph PJ, Gademann R (2005) Rapid light curves: a powerful tool to
assess photosynthetic activity. Aquat Bot 82:222–237
Ralph PJ, Gademann R, Dennison WC (1998) In situ seagrass
photosynthesis measured using a submersible, pulse-amplitude
modulated fluorometer. Mar Biol 132:367–373
Ruiz JM, Romero J (2001) Effects of in situ experimental shading on
the Mediterranean seagrass Posidonia oceanica. Mar Ecol Prog
Ser 215:107–120
Schreibers U, Hormann H, Neubauer C, Klughammer C (1995)
Assessment of photosystem II photochemical quantum yield by
chlorophyll fluoroscence quenching analysis. Aust J Plant
Physiol 22:209–220
Schwarz A-M, Hellblom F (2002) The photosynthetic light response
of Halophila stipulacea growing along a depth gradient in the
Gulf of Aqaba, the Red Sea. Aquat Bot 74:263–272
Silva J, Santos R (2003) Daily variation patterns in seagrass
photosynthesis along a vertical gradient. Mar Ecol Prog Ser
257:37–44