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.E. Kaldy, K.H. Dunton J. Exp. Mar. Biol. Ecol. 240 1999 193 –212
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.
Fruits of T . testudinum are globose 1–3 cm diameter greenish in color, have a
distinct ‘beak’ at the top and are buoyant Orpurt and Boral, 1964. The green color of the flowers and fruits implies that these organs are photosynthetically active and provide
carbon to developing seeds Bazzaz and Carlson, 1979; Bazzaz et al., 1979; Williams et al., 1985. When the fruits are ripe, they dehisce, releasing 1–3 seeds, which have
already germinated, are metabolically active and have formed rudimentary leaves Orpurt and Boral, 1964. In Lower Laguna Madre, fruit seed release seems to be
synchronized such that the majority of propagules are disseminated within a window of a few weeks Kaldy, 1997.
J .E. Kaldy, K.H. Dunton J. Exp. Mar. Biol. Ecol. 240 1999 193 –212
195
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.
The ecological importance of Thalassia testudinum, ‘synchronous’ seed release, immediate metabolic activity i.e., no dormancy, ease of collection and culture created a
model system to study seedling development. Our first objective was to estimate seed output, seedling dispersal and survival. Seedling buoyancy and water current data were
used to make preliminary estimates of seedling dispersal. Seedling survival, from this
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
