200 J
.E. Kaldy, K.H. Dunton J. Exp. Mar. Biol. Ecol. 240 1999 193 –212
vs. I measurements was determined using the MBTH 3-methyl-2-benothiazolinone hydrazone hydrochloride method as outlined by Lee and Dunton 1996. Briefly,
ground tissue samples were hydrolyzed with dilute HCl, neutralized with NaOH and reduced to alditols with KBH . The alditol was oxidized with periodic acid to form 2
4
mol of formaldehyde per mole of monosaccharide and the aldehyde content was determined spectrophotometrically with MBTH. Absorbances were compared with a
21
glucose standard and converted to equivalent carbon with units of mg NSCC gdw Lee and Dunton, 1996.
2.7. Statistical analysis Statistical analysis was performed using a general linear model procedure SigmaStat,
Jandel Scientific, San Rafael, CA, USA. One-way ANOVA was used to test for differences in photosynthetic parameters, biomass and NSCC reserves between seedlings
of different ages. ANOVA assumptions were tested, when assumptions were not satisfied data were transformed. In all cases, ANOVA on transformed and untransformed data
exhibited identical results. Because transformation and detransformation alters the calculated means and associated variances, the presented statistical analyses are based on
untransformed data. When a significant difference was detected, the means were analyzed using Student–Newman–Keuls multiple comparisons tests to determine where
the differences occurred.
3. Results
3.1. Fruit seed buoyancy and propagule dispersal Laboratory experiments conducted during summer 1995 and 1996 indicated that fruits
were positively buoyant for ,1 to 10 days. Although some seeds were buoyant for 3 days, over 90 of the seeds lost buoyancy in ,1 day. Current meter data from the
Blucher platform located over the seagrass beds indicates that during August and
21
September 1996 there was a net current movement of 1.5 km d to the north-east.
Estimates of propagule dispersal based on representative values for buoyancy duration and measured current transport predict that potential dispersal ranges between 0.06 and 3
km for seeds and ,1 to 15 km for fruits.
3.2. Seedling survival
22
During September 1996, naturally settled seedling density was 66614 seeds m of
bare area at the survival site. Laboratory experiments indicate that 96 of the seeds cultured survived to 6 months of age. Field experiments conducted during the same time
period showed that seedling survival in bare areas was about 22 after 6 months with only 11 surviving to 1 year of age. Field seedlings exhibited a Deevey type III
survivorship curve Fig. 2.
J .E. Kaldy, K.H. Dunton J. Exp. Mar. Biol. Ecol. 240 1999 193 –212
201
Fig. 2. Deevey survivorship curve for Thalassia testudinum seedlings grown in the field between September 1995 and September 1996.
3.3. Biomass allocation Total seedling biomass doubled from 0.1 to 0.2 gdw between 0.25 months 1 week
and 9 months and doubled again to about 0.5 gdw between 9 and 15 months Table 1. Pairwise comparisons indicated that there was no difference in total biomass among
plants in the younger 9 months or less age groups P . 0.05, but that plants at 15
Table 1 Changes in Thalassia testudinum seedling biomass, biomass allocation and non-structural carbohydrates
associated with increasing plant age. All values are mean6SE; n, sample size
b
Age n
Shoot Rhiz1seed
Root Total
R:R1S:L B A ratio Seed
a c
months biomass
biomass biomass
biomass gdw ratio NSCC
gdw gdw
gdw 0.25
5 0.00760.002 0.08860.010 0
0.09660.010 0:11.3:1
13.162.6 23.963.0
2 5
0.02260.003 0.05860.006 0.02360.003 0.10360.011 1:2.7:1
3.860.3 8.562.0
6 6
0.04360.008 0.04460.008 0.02460.005 0.11060.020 1:1.9:1.9
1.660.1 3.260.5
9 5
0.06960.013 0.05960.007 0.05660.014 0.18560.033 1:1.2:1.3
1.760.2 3.060.5
15 4
0.09960.019 0.31960.113 0.0960.013 0.48660.112
1:2.8:1.1 4.461.2
55.9610.9
a
Root:rhizome1seed:leaf ratio.
b
Below- to above-ground biomass ratio.
c 21
Units are mg non-structural carbohydrate carbon plant .
202 J
.E. Kaldy, K.H. Dunton J. Exp. Mar. Biol. Ecol. 240 1999 193 –212
months were 3–4-fold larger P , 0.05 than plants at any other age. Seedlings at age 15 months had developed 3–5 short shoots with 10–15 cm of horizontal rhizome and were
nearly indistinguishable from adult plants in the field. Biomass allocation changed with increasing plant age. Root:rhizome1seed:leaf
R:R1S:L ratio varied from 0:11:1 at 0.25 months to ca. 1:3:1 by 15 months Table 1. Dynamics of the R:R1S:L ratio were the result of changes in all three biomass
compartments associated with seedling development Table 1. The below- to above- ground biomass ratio B A ratio also changed during the experiment, decreasing from
13:1 at 0.25 months to 2:1 at 6 and 9 months and increasing to 4.4:1 at 15 months Table 1.
The NSCC reserves of the plants decreased by 87 P , 0.05 from 24 mg NSCC
21 21
plant at 0.25 months to 3 mg NSCC plant
by 6 months with no significant P . 0.05 change between 6 and 9 months of age Table 1. Older plants 15 months
had NSCC levels that were 3–20 fold higher than plants at any other age P , 0.05.
3.4. Photosynthetic performance Gross P
exhibited a significant P , 0.0001 three-fold increase from 78 mmol O
max 2
21 21
21 21
gdw sht h
at 0.25 months of age to ca. 220 mmol O gdw sht h
at 6 months
2
Table 2. Pairwise comparisons indicate that there were no significant differences P . 0.05 between 0.25 and 2 month old plants and there were no differences among 6
and 15 month old plants. However, plants at 6, 9 and 15 months had gross P values
max
four-fold higher P , 0.05 than plants at 0.25 and 2 months Table 2. Respiration values exhibited a significant P 5 0.0041 three-fold decrease from 170 mmol O gdw
2 21
21 21
21
sht h
at 0.25 months to 50 mmol O gdw sht h
at 2 months Table 2. Pairwise
2
comparisons indicated that there were no significant differences P . 0.05 for plants of any age between 2 and 15 months; however, respiration rates for 1 week old 0.25
months plants were three-fold higher P , 0.05 than any other age group. Conse- quently, net photosynthetic production in all 1 week old plants was negative Fig. 3, but
was slightly positive for 2 month old plants. All plants older than 6 months exhibited substantial positive net photosynthesis.
In general, shoot tissues had respiration rates that were 25 to 50 higher than the
Table 2 Summary of photosynthetic parameters measured in the lab for whole Thalassia testudinum seedlings of
different ages. Values are mean6SE. NC, not calculated; n, sample size Age
n Gross P
Respiration Alpha mmol O
I I
max 2
c k
21 2 1
months mmol O gdw
mmol O gdw gdw sht
h mmol
mmol
2 2
21 21
21 21
22 21 21
22 2 1
22 21
sht h
sht h
mmol m s
m s
m s
0.25 5
77.6625.2 2167.8638.0
1.6260.30 NC
49.3610.0 2
5 58.164.7
247.364.5 0.6760.08
73.067.6 94.6619.6
6 6
222.969.4 262.9610.9
2.0560.25 30.064.3
119.1616.3 9
4 224.1613.7
261.7617.2 1.6560.07
36.069.7 137.9610.6
15 4
215.3616.5 266.1620.2
1.1460.19 56.069.0
218.6648.6
J .E. Kaldy, K.H. Dunton J. Exp. Mar. Biol. Ecol. 240 1999 193 –212
203
Fig. 3. Net photosynthesis vs. irradiance curves for Thalassia testudinum seedlings of various ages. Measurements of PFD were made using a cosine sensor. All data points are mean6SE, n 5 4–6.
21 21
root rhizome tissues. Average respiration was 25 and 45 mmol O gdw sht h
for
2
rhizome and shoot tissues, respectively. When respiration rates from dissected plants were normalized to shoot biomass, estimates of whole plant respiration were about 30
higher than measurements made on intact plants Table 3. The initial slope of the P vs. I curve
a changed significantly P 5 0.0025 between ages. Mean alpha values for 2
Table 3
21 21
Comparison of whole Thalassia testudinum plant respiration mmol O gdw sht
h and respiration
2
estimated using dissected plants. The same plants were used for both estimates of respiration. Values are mean6SE n, sample size. ND, no data
Age n
Whole plant Sum of dissected
months tissue
6 5
262.9610.9 281.1615.7
9 4
261.7617.2 285.0620.6
15 4
266.1620.2 ND
204 J
.E. Kaldy, K.H. Dunton J. Exp. Mar. Biol. Ecol. 240 1999 193 –212 Table 4
21 21
Summary of Thalassia testudinum seedling net carbon budget mmol C gdw sht d
calculated based on measured photosynthetic and respiration rates for different H
periods
sat
Age 8 H
10 H 12 H
15 H 18 H
sat sat
sat sat
sat
months 0.25
23406.4 23251.2
23096.0 22863.2
22630.9 2
2670.4 2554.2
2438.0 2263.7
289.4 6
273.6 719.4
1165.2 1833.9
2502.6 9
312.0 760.2
1208.4 1880.7
2553.0 15
136.0 566.6
997.2 1643.1
2289.0
month old plants were 2–5-fold lower P , 0.05 than plants at any other age Table 2.
22 21
The compensation irradiance I varied between 30 and 70 mmol m s
, but was not
c
calculated for seedlings at 1 week since respiration rates were two-fold greater than
22
gross P . The saturation irradiance I increased with plant age from 50 mmol m
max k
21 22
21
s at 0.25 months to 220 mmol m
s at 15 months Table 2.
3.5. Seedling carbon budget Photosynthetic production was calculated from H
periods that ranged between 8 and
sat
18 h per day, while daily respiration was calculated based on 24 h. For all H periods
sat
examined, plants older than 6 months had a positive net daily carbon balance, while plants less than 6 months exhibited a negative net daily carbon balance Table 4.
Despite positive net photosynthesis on an hourly basis Fig. 3 the daily net carbon balance for 2 month old plants was negative for all H
periods examined.
sat
4. Discussion
