L
Journal of Experimental Marine Biology and Ecology, 240 (1999) 193–212
Ontogenetic photosynthetic changes, dispersal and survival of
Thalassia testudinum (turtle grass) seedlings in a sub-tropical
lagoon
*
James E. Kaldy , Kenneth H. Dunton
University of Texas, Marine Science Institute, 750 Channelview Dr., Port Aransas, TX 78373, USA Received 30 May 1998; accepted 17 April 1999
Abstract
Northward expansion of Thalassia testudinum (turtle grass) in Laguna Madre is occurring faster than can be explained by rhizome growth. We hypothesized that seedling establishment can account for the measured rates of meadow expansion and that seedling carbohydrate reserves are utilized until the plant is photosynthetically self-sufficient. To address seedling establishment, we estimated seed output, seedling dispersal and survival. Carbon dynamics were calculated from measurements of biomass allocation, non-structural carbohydrate carbon reserves and photo-synthetic parameters in relation to T. testudinum seedling age. Potential seed production calculated
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for 1996 was consistent with field observations and was estimated at 66614 seeds m bare area. Fruits can be positively buoyant for up to 10 days, while seeds were generally buoyant for ,1 day. Water current measurements, made at about the time of seed release, indicate a positive net
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transport of 1.5 km d to the north. Seedling survival in laboratory culture after 6 months was 96% compared to 11% in the field after 1 year. The average root:rhizome1seed:leaf ratio changed from 0:11:1 for a 1 week old plant to 1:3:1 for a 15 month old plant. Seedlings used to determine whole plant photosynthesis ranged in age from about 1 week (0.25 months) to 15
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months. Gross Pmax increased from 80 to 220 mmol O2 gdw sht h , while whole plant
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respiration decreased from 170 to 60mmol O gdw sht2 h . As the photosynthetic parameters changed, the average non-structural carbohydrate carbon (NSCC) reserves of the seeds decreased
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from 24 to 3.0 mg NSCC plant . Subsequent increases in NSCC were the result of rhizome
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development. Daily carbon balance, assessed using Hsat periods of 8–18 h d , predicts that T.
testudinum seedlings become photosynthetically self-sufficient between 2 and 6 months. The
unique characteristics of T. testudinum, including seed buoyancy, high seed production and survival rates, coupled with ontogenetic changes in carbon allocation and production imply that sexual reproduction can be important in the long distance dispersal and colonization for this species. 1999 Elsevier Science B.V. All rights reserved.
*Corresponding author. Present address: Texas A&M University, Department of Oceanography, College Station, TX 77843-3146, USA. Tel.:11-409-845-6939.
E-mail address: [email protected] (J.E. Kaldy)
Keywords: Ontogeny; Seagrass; Seedlings; Dispersal; Survival; Thalassia testudinum
1. Introduction
Clonal plants reproduce by two mechanisms, seed production and dispersal sub-sequent to a flowering event (sexual), and rhizome expansion (clonal growth). Seagrasses are clonal plants that have adapted to the marine environment and complete their entire life-cycle in a saline medium, including flowering, pollen transport and seed germination
˜
(Phillips and Menez, 1988). Although the bulk of seagrass bed expansion probably occurs through clonal growth (Lewis and Phillips, 1980; Phillips et al., 1981; Johnson and Williams, 1982), seeds are important to the maintenance of genetic variation within the population (Laushman, 1993; Alberte et al., 1994; Williams and Orth, 1998) and as agents of long distance dispersal. However, for seeds to contribute to a population they must survive and develop within a dynamic environment. Little is known about the physiological changes associated with seedling development, yet these processes are critical to understanding the factors influencing survival.
Flowering Thalassia testudinum has been documented in Florida (Grey and Moffler, 1978; Lewis and Phillips, 1980; Moffler et al., 1981; Durako and Moffler, 1985a,b, 1987), but is generally considered uncommon in South Texas (McMillan, 1976; Phillips et al., 1981). During 1993, flowering and fruit set was observed in T. testudinum beds in South Texas (Corpus Christi Bay). The seeds were viable and seedlings were maintained in culture for more than 3 years. In a large T. testudinum bed in the Lower Laguna Madre (LLM) near Port Isabel, Texas, flowering was observed during May and June with subsequent seed release during August 1994, 1995 and 1996 (Kaldy, personal observation). In Florida, between 10 and 35% of the T. testudinum shoots flower in any given year (Durako and Moffler, 1985a,b, 1987).
Recent work in LLM found that flowering phenology was similar to Florida populations. During a 2 year study, 13–30% of the shoots flowered and ,10% of the shoots formed fruits. Examination of short shoot flowering scars indicated that 28–40% of shoots flowered at least once during their life-time. Additionally, T. testudinum plants in LLM were shown to allocate 15% of the total above-ground biomass to sexual reproduction and appear to reach reproductive maturity between 1 and 3 years of age (Kaldy, 1997). Propagule production and survival are critical to persistence and expansion of seagrass meadows; however, seed output, dispersal and survival have virtually been ignored in the literature.
Long-term mapping surveys of seagrass in the Lower Laguna Madre indicate that
2
Thalassia testudinum coverage has increased by about 32 km since the opening of the
Gulf Intracoastal Waterway (Quammen and Onuf, 1993; Onuf, 1996a). Enhanced water exchange between the Gulf of Mexico and LLM has reduced hypersalinity, permitting expansion of the competitively dominant seagrass, T. testudinum. Quammen and Onuf (1993) hypothesize that meadow expansion occurs as a result of propagule dispersal and establishment forming small patches with subsequent clonal growth filling in the gaps. This ‘leap-frog’ mechanism may explain linear meadow expansion rates on the order of hundreds of meters per year (Quammen and Onuf, 1993).
Early work on Zostera marina seed dispersal concluded that gas bubbles made seeds buoyant and that water currents could transport seeds more than 200 m (Churchill et al., 1985). Orth et al. (1994) postulated that seed dispersal could occur via rafting of buoyant pre-dehiscent reproductive shoots, but this process was not addressed ex-perimentally. Studies of T. testudinum seed dispersal have not been conducted, even though the seeds and fruits are known to be buoyant (Orpurt and Boral, 1964).
The minimum light requirements of adult seagrass shoots have been assessed for several species, including Zostera marina (Dennison and Alberte, 1986; Dennison, 1987), Halodule wrightii (Dunton, 1994; Kenworthy and Fonseca, 1996; Onuf, 1996b),
Syringodium filiforme (Kenworthy and Fonseca, 1996) and Thalassia testudinum
(Fourqurean and Zieman, 1991; Herzka and Dunton, 1997). In situ photosynthetic work and extensive field monitoring indicate that the minimum light requirements of many seagrasses are .18% surface irradiance (SI) (Dunton, 1994; Dunton and Tomasko, 1994; Kenworthy and Fonseca, 1996; Onuf, 1996b), substantially higher than proposed by Duarte (1991). The high light requirements of seagrasses are related in part to plant architecture; over 80% of the total biomass can be localized in below-ground tissues (Fourqurean and Zieman, 1991; Lee and Dunton, 1996).
Although shoot photosynthesis has been widely studied in seagrasses (Drew, 1978; ´
Libes, 1986; Fourqurean and Zieman, 1991; Perez and Romero, 1992; Dunton and Tomasko, 1994; Kenworthy and Fonseca, 1996; Herzka and Dunton, 1997), there have been no studies of changes in photosynthetic physiology associated with seagrass seedling development. However, several studies have been conducted with mangrove seeds and seedlings. Steinke and Naidoo (1991) found negative net photosynthetic rates in young mangrove seedlings and positive net photosynthesis in older seedlings. Steinke and Charles (1987) measured the depletion of stored carbohydrate reserves during early mangrove seedling development. Chapman (1962a,b) found that environmental con-ditions and wounding could increase mangrove seedling respiration rates and that tissues associated with nutrient transport had high respiration rates. We hypothesize that seagrass seedlings depend primarily on stored carbohydrate reserves, subsidized by autotrophic production until the photosynthetic apparatus of the seedling is capable of supporting the plant’s carbon demands.
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 A&M–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
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.)
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
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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 180mmol 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.
O measurements and each light level was run for 30–45 min. Seedlings were oriented2
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 (Pmax, 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 respiratorysat
quotients of one. Although the Hsatmodel often underestimates production (Herzka and 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 Pmax times the hours of saturating irradiance. Net daily carbon balance was the sum of daily respiration and gross photosynthesis. To examine the impact of Hsat on
21
seedling daily net carbon balance we varied the duration of Hsatbetween 8 and 18 h d . Herzka and Dunton (1998) used Hsat510 as representative of typical summer conditions in Lower Laguna Madre based on continuous measurements of underwater
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light and T. testudinum shoot I values of 300k mmol photons m s . The upper limit of Hsat518 h used in these calculations is unrealistic but is included for heuristic purposes.
2.6. Non-structural carbohydrate analysis
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 24
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
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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
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
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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 Pmax exhibited a significant (P,0.0001) three-fold increase from 78mmol O2
21 21 21 21
gdw sht h at 0.25 months of age to ca. 220mmol O gdw sht2 h at 6 months (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 Pmax values four-fold higher (P,0.05) than plants at 0.25 and 2 months (Table 2). Respiration values exhibited a significant (P50.0041) three-fold decrease from 170mmol O gdw2
21 21 21 21
sht h at 0.25 months to 50mmol O gdw sht2 h at 2 months (Table 2). Pairwise 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
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, n54–6.
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root / rhizome tissues. Average respiration was 25 and 45 mmol O gdw sht2 h for 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 (P50.0025) between ages. Mean alpha values for 2
Table 3
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Comparison of whole Thalassia testudinum plant respiration (mmol O2 gdw sht h ) and respiration 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
Table 4
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Summary of Thalassia testudinum seedling net carbon budget (mmol C gdw sht d ) calculated based on measured photosynthetic and respiration rates for different Hsat periods
Age 8 Hsat 10 Hsat 12 Hsat 15 Hsat 18 Hsat
(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).
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The compensation irradiance (I ) varied between 30 and 70c mmol m s , but was not calculated for seedlings at 1 week since respiration rates were two-fold greater than
22
gross Pmax. The saturation irradiance (I ) increased with plant age from 50k mmol m
21 22 21
s at 0.25 months to 220mmol m s at 15 months (Table 2).
3.5. Seedling carbon budget
Photosynthetic production was calculated from Hsatperiods that ranged between 8 and 18 h per day, while daily respiration was calculated based on 24 h. For all Hsat periods 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 Hsat periods examined.
4. Discussion
4.1. Seed buoyancy and dispersal
Buoyancy is a common mode of propagule dispersal for a wide variety of plants associated with the marine environment. Long-distance dispersal of coconut (Gunn and Dennis, 1976) and mangroves (Clarke, 1993) has been well documented. Some species of seagrass also utilize buoyancy for dispersal. Churchill et al. (1985) found that Zostera seeds could be transport 200 m by attached gas bubbles. Orth et al. (1994) suggested that rafting of pre-dehiscent spathes may function in long-distance dispersal of Z.
Buoyancy and current dispersal are probably important mechanisms to the transport of T.
testudinum propagules.
2
Thalassia cover in LLM has increased by about 32 km to the north during the last 40
years between the initial dredging of the Gulf Intracoastal Waterway and seagrass
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surveys in 1988, equivalent to a 0.6–0.8 km year increase (Onuf, personal communication). Seedling colonization certainly accounts for a fraction of this increase, since seedlings have been identified from established H. wrightii beds and bare areas. Although re-establishment of vegetative fragments has been postulated, recent work concluded that it is very rare (Ewanchuk and Williams, 1996). In LLM, T. testudinum
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rhizome growth is on the order of about 0.6 m year (Kaldy and Dunton, 1999), which cannot account for the measured seagrass bed expansion rates. However, in LLM a seed would only have to be buoyant for 8 h to travel about 0.5 km. These observations and calculations are consistent with the ‘leap-frog hypothesis’ put forward by Quammen and Onuf (1993) to account for rapid T. testudinum expansion in Lower Laguna Madre.
4.2. Potential seed production and survival
To calculate potential seed production in LLM, the system was defined as an 8 by 4
2
km rectangular meadow (32 km ), oriented north–south. Based on aerial photographs, we assumed that 3.7% of the Thalassia meadow was bare substrate, making available
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30.8 km for seed production. Areal seed production (90 seeds m ) was calculated
22
from the average maximum fruit density at the study sites (45 fruits m ) and two seeds
21
fruit (Kaldy, 1997). Since 90% of the seeds lose buoyancy within 24 h, we assumed
21
that the other 10% were lost to the system. Net transport was assumed to be 1.5 km d to the north. As a result of northward transport, the area capable of receiving seagrass
2
propagules was defined as the original 32 km plus an additional 4 by 2 km rectangular
2
area for a total area of 40 km . Seeds were homogeneously distributed at the surface of the water. Furthermore, we assumed that only seeds settling in bare patches would survive and that the percentage of bare area was the same as the original system (3.7%).
9
Total potential T. testudinum seed production in LLM was calculated as 2.49310
2
during 1996. Distributing these seeds homogeneously over the 40 km area yields a
7 22 22
density of 6.23310 seeds km , which is equivalent to 62.3 seeds m . Since there are
2 7
about 1.5 km of bare area, this is equivalent to about 9.34310 seeds in the bare areas.
22
These calculations are consistent with field observations of 66614 seeds m bare area in LLM during September 1996. Applying the 11% survival rate from the survival
22
studies, we predict that about seven seedlings m of bare area will survive to 1 year.
22
However, direct field observations indicate that less than one seedling m of bare area actually survive to 1 year.
area. Additionally, pinfish (Lagodon rhomboides) are known to be herbivorous (Weins-tein et al., 1982) and were observed to pick up and spit out Thalassia seeds, while mullet (Mugill cephalus) were often observed to disturb the sediments in the bare areas around the study sites (Kaldy, personal observation). Ultimately, the establishment of a new genet is the result of a seedling being dispersed to an appropriate habitat, developing in a predictable manner and surviving complex biological interactions. Seagrasses have developed several unique characteristics to facilitate the long-term survival of the species.
4.3. Ontogenetic allocation patterns
Dramatic changes in the morphology and physiology of all organisms occur during the early stages of development. For plants, the most massive and profound changes occur during seedling development. In T. testudinum, the young seedling undergoes rapid development that is reflected in changing plant morphology and physiology. Initial development relies almost exclusively upon stored carbohydrate reserves provided by the ‘maternal’ plant. Physiological independence from reserve materials occurs slowly, but appears to be complete between 2 and 6 months after germination.
To meet the basic and competing requirements of nutrient and light capture, seedlings must allocate their resources to different components. One week after the seeds are released most of their biomass consists of below-ground tissue, with little leaf material present, but by age 15 months seedlings consisted of 3 to 5 short shoots with 10–15 cm of rhizome. The increase in the B /A ratio with increasing plant age, from 9 to 15 months, was the result of substantial rhizome tissue development associated with seedling growth and maturation. The R:R1S:L ratio was dynamic and was driven by changes in all three components of the ratio (Table 1). Seedlings at 15 months exhibited
21
NSCC levels equivalent to 136 mg gdw or about 30% of total biomass, which was similar to the values reported by Lee and Dunton (1996). Young seedlings were morphologically very distinct, while older seedlings (15 months) exhibited biomass allocation patterns and NSCC values characteristic of ‘adult’ T. testudinum plants in LLM.
Total seedling biomass remained nearly constant between 0.25 and 6 months, implying that changes were primarily a re-allocation of existing materials. Total biomass doubled between 6 and 9 months and doubled again between 9 and 15 months (Table 1). These increases in total biomass coincided with high positive Pnet values (Fig. 4), suggesting that seedlings were producing new biomass from photosynthetically fixed carbon (i.e. not carbon re-allocation).
4.4. Photosynthetic parameters
Fig. 4. Net Pmax, plant non-structural carbohydrate carbon (NSCC) reserves and below- to above-ground biomass ratios for Thalassia testudinum seedlings of varying ages. Values are mean6SE, n54.
in the petioles of water-lilly contributes to seedling development (Al-Hamdini and Francko, 1992).
seedling development (Steinke and Charles, 1987). The high respiration rates of young seedlings (Table 2) were the result of rapid developmental changes. In a review article, Yemm (1965) related respiration rates with seed germination and the type of stored reserves for a variety of plant species; seedlings with reserves rich in protein and lipids had high early respiration rates, e.g. about two-fold higher than measured for 1 week old
Thalassia seedlings. Seagrass seeds generally contain large amounts of stored starch,
with limited protein and lipid stores (Kuo et al., 1990, 1991; West et al., 1992). Photosynthetic characters associated with seedlings between 6 and 15 months were comparable to in situ measurements made on ‘adult’ T. testudinum plants in the LLM (Herzka and Dunton, 1997). Gross Pmax rates increased with plant age and were stable after 6 months (Table 2). Alpha, a measure of photosynthetic efficiency, was consistent between age groups, except for plants at 2 months which were lower than all other ages. This may have been related to differences in blade chlorophyll, which was not measured during this study. Although alpha values presented here are high, they were within the range reported by Herzka and Dunton (1997) and Czerny (1994) for laboratory P vs. I
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measurements. The I value for plants at 15 months was 218.6648.6k mmol m s , which is 30% lower than in situ values presented by Herzka and Dunton (1997). Laboratory derived estimates of Ic and Ik were underestimated as a result of the unidirectional light field and the 2plight sensor used in our measurements in contrast to Herzka and Dunton (1997).
4.5. Carbon balance
Daily carbon balance was calculated for a range of Hsat periods (Dennison, 1987) in order to estimate the age seedlings become photosynthetically self-sufficient. Although net photosynthesis on an hourly basis was positive at 2 months (Fig. 3), daily net carbon balance was negative (Table 4). Negative daily carbon balance in young seedlings (0.25 and 2 months) was a direct consequence of the low photosynthetic potential, e.g. low
Pmax and high respiration values. Low rates of photosynthesis during 8–18 h of saturating irradiance could not meet the high respiratory demand throughout the entire day. Carbon balance was calculated using Hsat periods representative of spring and summer conditions when temperatures are normally 258C or greater. Recent work in Texas found that photosynthetic activity in T. testudinum has a temperature optimum near 308C and that photosynthesis is severely depressed at temperatures below 208C (Czerny, 1994).
Daily carbon balance calculations predict that the high respiration and low photo-synthetic rates of young seedlings (,2 months) would lead to negative carbon balance even if plants were exposed to an unrealistic 18 h of saturating irradiance. Preliminary calculations indicate that Thalassia seedlings depend on parentally supplied reserves to meet 50–75% of their daily carbon requirements during at least the first 2 months. Similarly, mangrove seedlings rely heavily on parentally supplied stored reserves, utilizing 70% of NSCC within 14 days of germination (Steinke and Charles, 1987). T.
testudinum seedlings become photosynthetically self-sufficient between 2 and 6 months.
and shoot biomass and density (Kaldy, 1997) indicate that a single shoot requires about 60 days to produce the amount of non-structural carbohydrate carbon in two seeds. These calculations assume that all photosynthesis occurs in the leaves and is allocated to the seeds; however, photosynthetic production from flowers and fruits can account for 50% of the carbon required for seed development (Bazzaz and Carlson, 1979; Bazzaz et al., 1979; Williams et al., 1985). Seeds represent a substantial energy investment by adult plants and seed production may result in reduced shoot growth (Saulnier and Reekie, 1995; Laporte and Delph, 1996). Leaf and rhizome growth rates increased subsequent to fruit dehiscence, indicating an energetic cost associated with reproduction in T. testudinum (Kaldy, 1997).
Several carbon budgets have been developed for seagrasses using laboratory de-termined rates of photosynthesis and respiration (Dennison and Alberte, 1985, 1986; Dennison, 1987; Zimmerman et al., 1991). Generally, estimates of photosynthesis and respiration were based on small pieces of blade material, extrapolated to whole plants. Recently, whole plant respiration has been estimated by measuring the respiration rates for above- and below-ground tissues separately, summing the rates and normalizing to leaf tissue weight (Fourqurean and Zieman, 1991; Herzka and Dunton, 1997). Both of these studies found that shoot respiration rates are substantially higher than other tissues (Fourqurean and Zieman, 1991; Herzka and Dunton, 1997). Data collected on whole and dissected T. testudinum seedlings at 6 and 9 months (Table 3) indicates that the fractionated approach overestimates respiration rates by 25–30%, and therefore is not appropriate for accurate assessment of seagrass carbon balance.
Acknowledgements
This work was completed in partial fulfillment of the Ph.D. requirements from the University of Texas at Austin, Marine Science Department, by J.E. Kaldy. C. Chiscano, A. Czerny, S. Herzka, K. Jackson, K.-S. Lee, Dr. K. Major, H. Miller and C. Weilhoeffer contributed valuable field assistance, lively discussion and comments on previous versions of this manuscript. D. Hockaday kindly provided lab space and logistical support at the University of Texas–Pan American Coastal Studies Lab on South Padre Island. R. Allen from Central Power and Light, Corpus Christi, Texas, provided aerial photographs used to make estimates of bare areas in seagrass beds near the study sites. C. Brown, formerly at the Conrad Blucher Institute for Surveying and Science, Texas A&M University–Corpus Christi supplied the current meter data used in this manuscript. Drs. N. Fowler, E. Ingall, P. Montagna, C. Onuf, J. Raven and D. Tomasko kindly provided comments which improved the manuscript. Two anonymous reviewers provided constructive comments that helped to improve the manuscript. This work was supported by grants to K. Dunton from the Texas Higher Education Coordinating Board Advanced Research Program (grant [003658-419) and the EPA Gulf of Mexico
Program (grant [ MX994713-95-2). Salary support was provided in part by the
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