196 J
.E. Kaldy, K.H. Dunton J. Exp. Mar. Biol. Ecol. 240 1999 193 –212
study, and areal seed production estimates Kaldy and Dunton, 1999 were used to calculate potential seed production and survival in Lower Laguna Madre, Texas. The
second objective was to examine ontogenetic changes in the photosynthetic physiology and resource allocation patterns of T
. testudinum seedlings. The age seedlings become physiologically independent from parentally supplied stored carbon reserves was
determined by developing a daily carbon balance based on photosynthetic measure- ments. We also compared two methods of estimating whole plant respiration rates and
examined partitioning of biomass and non-structural carbohydrate reserves.
2. Materials and methods
2.1. Current measurements Water current data were obtained from the Conrad Blucher Institute for Surveying and
Science Texas AM–Corpus Christi, which maintained instrumentation on a platform about 2 km south-east of the study site. The platform, designated Station 2, was located
within the T . testudinum meadow Fig. 1. Water current was measured at mid-depth 75
cm with an acoustic doppler velocimeter ADV, Sontek Inc., San Diego, CA, USA current meter. The ADV measures North–South, East–West and vertical current
components u, v and w at a frequency of 8 Hz. Current measurements were obtained for 9 min during every hour and integrated to estimate daily net displacement during
August and September 1996.
2.2. Propagule buoyancy During August 1995 and 1996 the duration of fruit buoyancy was assessed using field
collected fruits and laboratory culture methods. Fruits were collected either floating in the wrack line 1995 or harvested directly from the plants 1996. Fruits collected from
the wrack were assumed to have detached from the shoot on the day of collection. Collections were made at both the shallow site and Station 2 Fig. 1 in the Lower
Laguna Madre, Texas 288089N and 978129W. Fruits were transported to the lab and placed in buckets supplied with running seawater. The number of non-dehiscent floating
fruits was counted and recorded at least once daily. When fruits dehisced, the seeds and pericarp were removed from the culture. Twelve fruits were cultured during 1995 and 16
fruits were cultured during 1996. The duration of seed buoyancy was assessed using the seeds collected from these experiments. Buoyant seeds were placed in a bucket supplied
with running seawater, the number of floating seeds was counted and recorded at least once daily. During 1995 buoyancy was assessed for 15 seeds, while over 50 seeds were
monitored during 1996. Potential propagule dispersal was calculated as the product of buoyancy duration and daily net current displacement.
2.3. Seedling survival Seedling survival was monitored in a naturally formed bare area within a monotypic
J .E. Kaldy, K.H. Dunton J. Exp. Mar. Biol. Ecol. 240 1999 193 –212
197
Fig. 1. Site map of Lower Laguna Madre, Texas, showing the position of the shallow site and Station 2. The shaded portion represents the approximate delineation of Thalassia testudinum. Adapted from Brown and
Kraus, 1997.
T . testudinum bed in the Lower Laguna Madre near Port Isabel, Texas. Large patches of
bare sand bottom are a common feature of seagrass beds in LLM and appear to be the result of smothering by macroalgal mats Kaldy, personal observation. This site was
located about 0.5 km west of a shallow site see Kaldy and Dunton, 1999 on the eastern side of the Gulf Intracoastal Waterway GIWW at 1.3 m depth Fig. 1.
198 J
.E. Kaldy, K.H. Dunton J. Exp. Mar. Biol. Ecol. 240 1999 193 –212
Long-term seedling survival in the field was assessed using seedlings from the 1995 year class. During September 1995 numbered plastic surveyors flags were placed next to
35 naturally settled individual seedlings in a bare area. Survival was assessed several times as water visibility permitted during the 1 year period from September 1995 to
September 1996. Seedling disappearance was attributed to mortality. In terrestrial plants, seedling establishment within a grass stand is rare Harper and White, 1974; Harper,
1977; therefore, seedling mortality in the bare area is not applicable to the entire seagrass bed.
Seedling survival estimates were normalized to the percentage of bare area 3.7 occurring within the T
. testudinum meadow. Bare area occurrence was estimated from aerial photographs taken near the study site during 1991. These were the only available
data; therefore, we assumed that the amount of bare area within the meadow was consistent between years. During September 1996 the density of naturally settled
seedlings in the bare area was estimated from counts in 10 replicate quadrats 35335 cm.
During 1995, seedling survival was also assessed using laboratory culture methods. Briefly, 25 seedlings were transplanted into each of five 8 l culture vessels containing
22 21
3–4 cm of clean beach sand. Seedlings were exposed to about 14 mol photons m d
from fluorescent lamps. Frequent water changes were made and each culture was aerated to prevent stagnation. A flow through system was not used since it promoted extensive
growth of algal epiphytes. Seed survival was assessed monthly by counting the number of live individuals in each tank from August 1995 to January 1996.
2.4. Photosynthesis vs. irradiance curves During August 1994, fruits and seeds of Thalassia testudinum were collected near the
Gulf Intracoastal Waterway in Lower Laguna Madre Fig. 1. Fruits were allowed to dehisce in aquaria supplied with running seawater, while seeds were placed directly into
laboratory culture. The culture vessel consisted of an 8 l glass aquarium containing 100–150 seeds and 3–4 cm of clean beach sand. Illumination of about 100 mmol
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photons m s
was supplied by fluorescent lamps. Frequent water changes were made to maintain salinity and aeration was used to prevent stagnation. Seeds obtained during
August 1995 were cultured under similar conditions, except that seedlings received
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about 180 mmol photons m s
. All photosynthetic data were from plants in the 1994 year class YC, except for the 15 month plants which were from the 1995 YC.
Photosynthesis vs. irradiance P vs. I curves were generated using the Rank Brothers polarographic oxygen electrode system and the Endeco YSI pulsed dissolved oxygen
electrode system Dunton, 1994; Herzka and Dunton, 1997. Oxygen evolution from young plants, 0.25 and 2 months of age, was measured using the Rank brothers system;
between successive light levels plants were allowed to acclimate for 6 min to achieve constant oxygen evolution rates Dunton and Tomasko, 1994. Older plants were
examined using the Endeco system because plant size dictated the use of larger chambers. Plants at 6 and 9 months were incubated in 250 ml chambers and at 15
months in 1 l chambers. There was no discrete acclimation period between light levels for plants examined with the Endeco system, since 2–5 min elapsed between successive
J .E. Kaldy, K.H. Dunton J. Exp. Mar. Biol. Ecol. 240 1999 193 –212
199
O measurements and each light level was run for 30–45 min. Seedlings were oriented
2
perpendicular to the horizontal light field, which was measured with a cosine 2p sensor attached to a LI-1000 datalogger LI-COR, Lincoln, NE, USA. Chambers were
equipped with a stirring mechanism to prevent stratification. In all cases, whole plants root, seed, rhizome and shoot material were incubated.
Seawater passed through a 15 mm filter was used as the medium for all P vs. I incubations. Photosynthesis and respiration measurements were corrected for bacterial
and phytoplankton production. All experiments were conducted at 258C and 26–28‰ salinity. Seedlings were acclimated to incubation conditions for at least 24 h prior to
running the P vs. I curves. Dark respiration rates were always measured first and illumination proceeded from the lowest to highest light level. Oxygen evolution
consumption rates were plotted for each photon flux density PFD from 0 to about 1000
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mmol photons m s
and normalized to dry weight of shoot material. The Smith– Talling function was fitted to the data and the photosynthetic parameters P
,
max
respiration, a, I and I were calculated following Henley 1993.
k c
The affect of dissecting whole plants into above- and below-ground fractions on estimates of whole plant respiration was examined by measuring respiration rates on
fractionated plants subsequent to determination of P vs. I parameters. Plants at 6, 9 and 15 months were dissected into above- and below-ground fractions and transferred to
incubation chambers with fresh media. Above-ground biomass was defined as all live leaf biomass including the sheath to the point of intersection with the vertical rhizome.
Measured respiration rates for above- and below-ground fractions were then summed and normalized to the dry weight of the associated shoot material. Dissected plant
material was prepared approximately 2 h prior to the dark incubations to minimize the effects of wounding.
2.5. Seedling carbon budget Daily net carbon balance was estimated for seedlings using the saturating irradiance
H model following Dennison 1987 and assuming photosynthetic and respiratory
sat
quotients of one. Although the H model often underestimates production Herzka and
sat
Dunton, 1998 it provides realistic first-order estimates. Daily respiration was calculated as hourly dark respiration times 24 h, while daily gross photosynthesis was calculated as
gross P times the hours of saturating irradiance. Net daily carbon balance was the
max
sum of daily respiration and gross photosynthesis. To examine the impact of H on
sat 21
seedling daily net carbon balance we varied the duration of H between 8 and 18 h d
.
sat
Herzka and Dunton 1998 used H 5 10 as representative of typical summer
sat
conditions in Lower Laguna Madre based on continuous measurements of underwater
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light and T . testudinum shoot I values of 300 mmol photons m
s . The upper limit
k
of H 5 18 h used in these calculations is unrealistic but is included for heuristic
sat
purposes. 2.6. Non-structural carbohydrate analysis
The non-structural carbohydrate carbon NSCC content of the seedlings used in the P
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.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
