Reproduction of the eelgrass Zostera marina at the species southern distributional limit in the Eastern Atlantic
Abstract
The plant reproductive effort, the seed germination rate and the seedling survival and development of Zostera marina (eelgrass) were assessed in four populations (Fuzeta, Culatra, Barrinha and Armona) at the species’ southern distribution limit in the Eastern Atlantic, the Ria Formosa lagoon. Germinated seeds were individually placed in Petri dishes with natural sandy sediments and kept in a culture chamber at the same temperature and salinity conditions as the natural environment. In addition, seeds from three different depths of Fuzeta population were cultivated in outdoor mesocosms. The populations of Fuzeta and Barrinha showed higher seed production and the seeds produced were heavier than the other populations. The germination of the seeds both in the laboratory and in the outdoor tanks began c. 8–12 weeks after the collection of the flowering shoots at a water temperature of 22 °C. The spontaneous germination in the laboratory (2.4–5.3%) and in the mesocosm experiment (5.6–8.9%) was low and from all the germinated seeds (n = 20) only three reached the seedling stage. The spontaneously germinated seeds from Fuzeta survived for a longer period than those from Barrinha, but only the germinated seeds of Barrinha reached the seedling stage (one-leaf seedling stage). In outdoor tanks, higher seed germination and earlier seedling emergence (2 weeks after seeding) and survival (for 208 days) occurred for the seeds obtained from the shallow meadow. The reproductive effort of Z. marina populations of Ria Formosa showed that flowering shoots and seed traits are site-specific. The low reproductive success indicated by the low germination and seedling survival suggests a bottleneck in the species’ reproductive cycle that may account for the scarce presence of the species in Ria Formosa lagoon. The high water temperatures of Ria Formosa in winter may partly explain this bottleneck. Increased temperatures due to climate change may reduce even further the sexual reproduction of Z. marina in its southern distributional limit in the Eastern Atlantic.
Problem
Seagrasses are marine angiosperms that reproduce both vegetatively through clonal growth and sexually through the production of flowers and seeds (den Hartog 1970; Phillips & Meñez 1988). Successful recruitment from sexual reproduction in seagrasses is extremely low (Hemminga & Duarte 2000). Their reproductive cycle involves several steps from the flowering event to the formation of a new plant, many of them representing bottlenecks (e.g. seed germination, seedling survival) that constrain sexual reproduction success (Hemminga & Duarte 2000; Orth et al. 2006). The processes that influence the establishment of a seedling from a seed are often diverse and complex, and include the site suitability (e.g. nutrients) and safety (e.g. currents protection) among others (Orth et al. 2006). Nevertheless, sexual reproduction is the only way to maintain genetic diversity of seagrass meadows (Ackerman 2006), and may be the only way to recolonise bare sediments or impacted areas (Thayer et al. 1984; Marbà & Walker 1999; Plus et al. 2003; Rasheed 2004; Greve et al. 2005; Lee et al. 2007).
Zostera marina L. (eelgrass) is a seagrass species widely distributed along the coasts of North America, Europe and Japan (den Hartog 1970). The reproductive biology and strategies of the species have been well described, indicating that the reproductive processes are highly variable along its geographic distribution (De Cock 1980; Churchill 1983; McMillan 1983; Phillips et al. 1983; Silberhorn et al. 1983; Hootsmans et al. 1987; van Lent & Verschuure 1994;Meling-López & Ibarra-Obando 1999; Olesen 1999; Morita et al. 2007). There are considerable variations in spatial and temporal patterns of the species’ flowering and seed production, which have been related to latitudinal and local gradients (Ackerman 2006), but also with genotypic variation of the populations (Hemminga & Duarte 2000; Walker et al. 2001). Temperature appears to be critical for all phases of the reproductive event, such as the flower appearance, seed production, seed germination and seedling development, but other environmental factors such as irradiance and nutrients may also play a role in the timing and characteristics of the process (Walker et al. 2001; Ackerman 2006). The water temperature increase due to climate change effects may be affecting the reproductive success of Z. marina (Short & Neckles 1999), in particular the highest summer temperatures at its southern distributional limit in the Eastern Atlantic, the Ria Formosa lagoon (South Portugal). Zostera marina appears to be declining here, where few populations are still thriving. The local populations are characterized by low genetic diversity (Billingham et al. 2003), indicating a small contribution of sexual reproduction in Ria Formosa, where populations are structured as a result of clonal growth and limited seed dispersal (Billingham et al. 2007). Although long seed dispersal distances are possible for this species (up to 150 km; Källström et al. 2008), Billingham et al. (2007) inferred that, in the local populations, most seeds disperse less than 4 m. The understanding of the local reproductive processes from seed formation to seedling development is important to the conservation and recovery of meadows through the use of seeds and as baseline information to monitor future effects of elevated temperatures on Z. marina reproduction at the species’ southern distribution limits. This study aimed to characterise the reproductive potential of plants from four Z. marina populations of Ria Formosa lagoon, through (i) quantification of the plant reproductive effort, (ii) assessment of the spontaneous seed germination rate, and (iii) evaluation of seedling survival and development.
Methods
Study site
This study was conducted in Z. marina populations of Ria Formosa lagoon, Southern Portugal (Fig. 1). Ria Formosa is a mesotidal coastal system covering an area of about 84 km2. The tidal amplitude ranges from 3.50 m on spring tides to 1.30 m on neap tides (Andrade 1990). Water temperature in the lagoon varies between 12 °C in the winter and 27 °C in the summer. Salinity ranges from 35.5 to 36 PSU throughout the year except during heavy rainfalls, when they can be as low as 15 PSU (Falcão 1996). Light availability varies between 10.9 mol·quanta·m−2·day−1 during winter and 52.1 mol·quanta·m−2·day−1 during summer. Zostera marina forms continuous patches that grow in muddy to sandy substrates and extend from the low intertidal to depths of 6 m (Billingham et al. 2003). The populations of Fuzeta, Culatra, Armona and Barrinha (Fig. 1) were assessed for the characterisation of the species reproductive processes.
Map of Ria Formosa lagoon, South Portugal with the location of the studied Zostera marina populations. B: Barrinha, C: Culatra; A: Armona, F: Fuzeta.
Plant reproductive effort
Flowering shoots of Z. marina were harvested in each population during June and July 2007 at the local species’ flowering peak. The flowering shoots were carried to the laboratory and kept in aerated seawater until the seeds matured and fell off naturally. The plant reproductive effort of each population was estimated by the number of spathes per flowering shoot, the number of rhipidia (branches per flowering shoot), the number of spathes per branch, the number of seeds obtained and the seed weight. The wet weight (precision of 0.1 mg) of the seeds was determined individually from a subsample of n = 50, after blotting each seed rapidly on a paper towel. The seeds of each population were stored separately in an aquarium with aerated natural seawater (following Granger et al. 2002) within a culture chamber, with a light intensity of 150 μmol·quanta·m−2·s−1 at field temperature (22 °C) and salinity (35 PSU) conditions, until the germination experiments.
Seed germination and development
During the storage in the culture chamber, Z. marina seeds germinated spontaneously in aerated seawater at ambient conditions. The seeds were monitored every week for the spontaneous rupture of the seed coat and emergence of the cotyledon, i.e. for germination (Hootsmans et al. 1987). Then, the germlings (germinated seeds) or seedlings (when the first chlorophyll-bearing leaf appeared) obtained were individually placed in Petri dishes with natural sandy sediments (0.5% ± 0.1 of organic matter content) and kept in an aerated aquarium with local seawater, within a culture chamber at the same temperature (22 °C), salinity (35 PSU) and light (150 μmol·quanta·m−2·s−1) conditions. The seedlings were monitored twice a week for survival and development. The number of leaves of each seedling was recorded to identify the seedling stage and the time elapsed between stages. Leaf length was measured during the seedling development to estimate the leaf growth, and the number of shoots was recorded to estimate the shoot production.
A germination experiment conducted in outdoor tanks was also done. Zostera marina seeds were obtained at different depths (0.8–1.2, 1.6–1.8 and 2.0–2.4 m) from the population of Fuzeta. Seeds with a rigid seed coat, indicative of seed viability (Orth et al. 1994), were used in the experiment. The seeds were planted in 10-cm diameter plastic pots filled with natural sandy sediments (0.5 ± 0.1% of organic matter content) at 1 cm depth. Ten seeds were planted per pot in a total of nine pots per depth level (90 seeds per depth level). Pots were randomly placed into three replicate tanks (three pots of each depth level per tank). The pots were maintained at 20 cm water depth, with flowing, aerated natural seawater that was independently supplied to each tank, under natural light availability. The pots were monitored every week for seed germination, and for seedling survival and development, as described above. The experiment began in September 2007 and was carried out for 1 year.
Statistical analysis
Prior to statistical analyses, datasets were tested for homogeneity of variances and normality of distributions. Significant differences in reproductive effort datasets (number of spathes per flowering shoot, number of spathes per branch, number of rhipidia and seed weight) were investigated using one-way ANOVA with population as main factor. The seed germination dataset (mesocosmos experiment) was investigated using two-way repeated measures ANOVA with meadow depth and time as main factors. When ANOVA indicated significant differences among meadows and/or time, Tukey’s multiple comparison test was applied to determine which meadow(s) and/or time(s) were significantly different from each other. Significance levels were tested at P < 0.05 (Sokal & Rohlf 1995).
Results
Plant reproductive effort
The reproductive characteristics of the flowering shoots and the weight of seeds of Z. marina populations of Ria Formosa are shown in Table 1. The number of spathes per flowering shoot was significantly higher in the populations of Fuzeta (16.6 spathes·flowering·shoot−1) and Culatra (12.7 spathes·flowering·shoot−1) than in Barrinha (8.0 spathes·flowering·shoot−1) and Armona (7.6 spathes·flowering·shoot−1). The number of spathes per branch and the number of rhipidia were significantly higher in Fuzeta. The total number of seeds obtained was drastically different among populations. A total of 500 and 684 seeds were obtained from Fuzeta and Barrinha populations, respectively, whereas only one and nine seeds were obtained from Culatra and Armona. Hence, the production of seeds per spathe and per flowering shoot was much higher in Fuzeta (0.35 and 6.67 seeds, respectively) and Barrinha (0.41 and 4.30 seeds, respectively) than in Culatra (0.001 and 0.04 seeds, respectively) and Armona (0.01 and 0.13 seeds, respectively) populations. The seeds of Fuzeta (5.2 mg) and Barrinha (6.6 mg) were also heavier than the seeds of Culatra (3.8 mg) and Armona (4.9 mg).
| Fuzeta | Culatra | Barrinha | Armona | P | |
|---|---|---|---|---|---|
| flowering shoots | |||||
| no. spathes per flowering shoot | 16.6 ± 0.6a | 12.7 ± 1.1a | 8.0 ± 0.3b | 7.6 ± 0.5b | <0.001 |
| no. spathes per branch | 4.1 ± 0.1a | 3.3 ± 0.2b | 3.0 ± 0.1b | 2.5 ± 0.1c | <0.001 |
| no. rhipidia | 4.1 ± 0.1a | 3.8 ± 0.2c | 2.8 ± 0.1b | 3.1 ± 0.2bc | <0.001 |
| seeds | |||||
| seed production per spathe | 0.35 | 0.001 | 0.41 | 0.01 | |
| seed production per flowering shoot | 6.67 | 0.04 | 4.30 | 0.13 | |
| seed weight (mg) | 5.2 ± 0.1a | 3.8* | 6.6 ± 0.1b | 4.9 ± 0.5a | <0.001 |
- Different letters denote significant differences among populations. P values are the significance levels of one-way ANOVA.
- *n = 1 (not included in statistical analysis).
The reproductive characteristics of the flowering shoots and the weight of seeds obtained at different depths in the population of Fuzeta are shown in Table 2. The number of spathes per flowering shoot did not vary significantly among depths, although a lower number of spathes per flowering shoot were observed at the medium depth. The flowering shoots of the deeper meadow had a significantly lower number of spathes per branch (3.7 spathes) relative to the medium and shallow meadows (4.4 and 4.2 spathes, respectively), whereas the number of rhipidia was significantly lower in the medium meadow. The total number of seeds obtained varied between 110 (shallow meadow) and 222 (medium meadow). The production of seeds per spathe and per flowering shoot was clearly higher in the medium meadow (0.66 and 13.88 seeds, respectively) than in the deeper (0.28 and 5.79 seeds, respectively) and shallow meadows (0.19 and 3.67 seeds, respectively). The seed weight was not significantly different among the meadows, varying between 5.01 mg in the deeper and 5.62 mg in the medium meadow.
| deeper | medium | shallow | P | |
|---|---|---|---|---|
| flowering shoots | ||||
| no. spathes per flowering shoot | 17.4 ± 1.0 | 14.2 ± 1.0 | 17.1 ± 1.1 | 0.14 |
| no. spathes per branch | 3.7 ± 0.1a | 4.4 ± 0.2b | 4.2 ± 0.1b | 0.005 |
| no. rhipidia | 4.6 ± 0.2a | 3.3 ± 0.2b | 4.0 ± 0.2a | <0.001 |
| seeds | ||||
| seed production per spathe | 0.28 | 0.66 | 0.19 | |
| seed production per flowering shoot | 5.79 | 13.88 | 3.67 | |
| seed weight (mg) | 5.01 ± 0.18 | 5.62 ± 0.23 | 5.10 ± 0.19 | 0.08 |
- Different letters denote significant differences among depths. P values are the significance levels of one-way ANOVA.
Seed germination and development
Spontaneous seed germination in the laboratory was only observed for the populations of Fuzeta and Barrinha (Table 3), the populations where the seeds were heavier (Table 1). Seed germination was higher in Barrinha (5.3%) and seeds were heavier (6.6 mg) than in Fuzeta (2.4% and 5.2 mg, respectively). Germination of the seeds from the Fuzeta population occurred only during August (8–10 weeks after the collection of the flowering shoots), whereas the seeds of Barrinha spontaneously germinated from October to January (from the 12th to the 26th week after the collection of the flowering shoots). The survival of the germlings/seedlings from Fuzeta was much higher (18.0 days) than those from Barrinha (9.8 days). The seedling stage was only reached by the faster developing seeds of Barrinha. The germinated seeds required on average 15 days to reach the seedling stage and were not able to develop further than the 1st leaf-seedling stage. The individual that lived longer did so for a similar period in both populations (35 and 33 days for Fuzeta and Barrinha, respectively).
| Fuzeta | Barrinha | |
|---|---|---|
| germination (%) | 2.4 (n = 12) | 5.3 (n = 36) |
| survival (days) | 18.0 ± 5.6 (range: 6–35, n = 6) | 9.8 ± 1.7 (range: 2–33, n = 22) |
| seedling (days) | – | 15.3 ± 3.3 (range: 12–22, n = 3) |
- The number of seeds were n = 500 for Fuzeta and n = 684 for Barrinha populations.
The germination of Z. marina seeds in outdoor tanks occurred 2 weeks after seeding and 12 weeks after the collection of the flowering shoots, in mid-September, at a water temperature of 22 °C (Fig. 2). Seed germination increased significantly during the first weeks of the experiment. After the 19th week, no further seed germination occurred. The germination rate of seeds from the deeper meadow was lower compared to those from medium and shallow meadows, but no significant differences were found. The cumulative seed germination reached 5.6% in the deeper meadow (18th week of the experiment), 7.8% in the medium meadow (14th week of the experiment) and 8.9% in the shallow meadow (18th week of the experiment). In total, 7.4% of the Z. marina seeds germinated. Seed germination occurred until early January, when the water temperature of the outdoor tanks was around 14 °C and light availability was 14.2 mol·quanta·m−2·day−1.
Cumulative percentage of Zostera marina seed germination from different depth levels of Fuzeta, and the weekly average water temperature of outdoor tanks.
Of all the germinated seeds (n = 20) only three reached the seedling stage (15%). Seedlings germinated between October (seedling #1, shallow meadow) at the 5th week of experiment, and January (seedling #3, deeper meadow) at the 19th week of experiment (Table 4). Longevity of the seedlings varied from 6 to 208 days. Seedling #1 reached the 5-leaf stage in 49 days, seedling #2 reached the 3-leaf stage in 40 days and seedling #3 reached the 1-leaf stage in less than 7 days. The 1- and 2-leaf seedling stages were very short stages (<7 days). The maximum shoot height was 50 mm for seedling #1, 40 mm for seedling #2 and 21 mm for seedling #3. There was a decrease in the leaf number and shoot height when seedlings were degenerating. The total leaf growth of seedling #1 until the 5-leaf stage was 1.50 mm·day−1. This seedling developed two roots of 50 and 240 mm. None of the seedlings produced additional shoots and no rhizome growth occurred.
| seedlings | |||
|---|---|---|---|
| #1 | #2 | #3 | |
| germination | Oct | Dec | Jan |
| longevity (days) | 208 | 84 | 6 |
| max. shoot height (mm) | 50 | 40 | 21 |
| max. leaf stage (no.leaves) | 5 | 3 | 1 |
| time to max. leaf stage (days) | 49 | 40 | <7 |
| plastochrone interval (days) | 26 | 14 | <7 |
| no. roots | 2 | 2 | 0 |
| no. lateral shoots | 0 | 0 | 0 |
Discussion
The reproductive effort of Z. marina populations of Ria Formosa lagoon were site-specific. The populations of Fuzeta and Barrinha showed higher seed production and the seeds produced were also heavier than those from the other populations. Moreover, only the seeds from these two populations spontaneously germinated during storage. The germinated seeds from Fuzeta survived for a longer period than those from Barrinha, but only the germinated seeds of Barrinha reached the seedling stage.
The higher number of spathes per flowering shoot found in Fuzeta and Culatra relative to Barrinha and Armona may be related to the fact that the latter populations are closer to the lagoon inlets and are thus subject to stronger currents and higher sediment dynamics, which may result in the breakage and loss of the spathes. Although the potential for stronger currents may break off the spathes or the reproductive shoots of Z. marina, this has not been studied in the local populations yet. In any case, the number of spathes per flowering shoot of Z. marina populations of Ria Formosa (7.6–16.6 spathes per flowering shoot) was, in general, within the range reported for this species elsewhere (5–18 spathes per flowering shoot; Phillips & Backman 1983; Phillips et al. 1983; Silberhorn et al. 1983; Morita et al. 2007). Because the number of spathes per flowering shoot is inversely related to the water temperature (Silberhorn et al. 1983), a higher number of spathes per flowering shoot was reported for Northern France (20 spathes per flowering shoot; Jacobs & Pierson 1981) and Canada populations (36 spathes per flowering shoot; Keddy 1987).
The medium-depth plants from the Fuzeta meadow produced more and heavier seeds than the plants from other depths, despite the lower number of spathes per flowering shoot and of rhipidia (Table 2). However, these differences were not expressed in the potential success of the seeds, as higher seed germination and earlier seedling emergency (after 2 weeks) and survival (for 208 days) were observed in the shallow meadow. The differences found along the depth gradient suggest the existence of a relationship between the reproductive characteristics of the species and the depth of the meadow. In a study along an intertidal gradient, Harrison (1993) reported earlier and more frequent germination of Z. marina seeds in the low intertidal.
The germination of the seeds both in the laboratory and in the mesocosm experiment began c. 8–12 weeks after the collection of the flowering shoots at a water temperature of 22 °C. Most seeds germinated during autumn, as reported for some Z. marina populations elsewhere (Churchill 1983; Orth & Moore 1983; Phillips & Backman 1983; Moore et al. 1993; Orth et al. 1994, 2003; Plus et al. 2003). However, most studies reported germination during winter and spring (e.g., Harrison 1993; Orth & Moore 1983; Orth et al. 1994, 2003; Phillips & Backman 1983; van Lent & Verschuure 1995; Harwell & Orth 1999; Olesen 1999; Bintz & Nixon 2001; van Katwijk & Wijgergangs 2004; Greve et al. 2005), as seeds tend to remain dormant until temperatures decrease to 15 °C (Moore et al. 1993). Temperature was reported as the main factor determining the timing for germination of Z. marina seeds being the optimal temperature for germination as low as 6–11 °C (Probert & Brenchley 1999) or 10–15 °C (Abe et al. 2008). The high water temperatures of Ria Formosa (15 °C in winter and 21 °C in summer in average) may explain the low germination rates observed in this study compared to those reported elsewhere (Table 5). Increasing temperatures due to climate change may eventually prevent Z. marina from reproducing sexually in its Southern Atlantic distribution limit. The seawater temperature anomalies in Ria Formosa in the last three decades have increased around 1 °C in the autumn/winter period, when the seed germination of the species was observed. In addition to the changes in the patterns of sexual reproduction, the low genetic diversity of Z. marina in Ria Formosa (Billingham et al. 2003) may not provide the critical response for maintaining the species’ ecosystem functioning and for adaptation to temperature change, as resistance to temperature stress is enhanced by genotypic diversity (Ehlers et al. 2008).
| source | location | seed germination | seedling survival |
|---|---|---|---|
| Harrison (1993) | SW Netherlands (FO) | 57% (13%a) | 1% (12 months) |
| Lee et al. (2007) | Jindong Bay, Korea (FO) | 2–6%a | c. 100% (12 months) |
| Plus et al. (2003) | Thau lagoon, France (FO) | 80% (6 months) | |
| van Lent & Verschuure (1994) | SW Netherlands (FO) | 12.5–87.2%a | |
| Churchill (1983) | Long Island, USA (FE) | 76–93% | 0% (12 months) |
| Harwell & Orth (1999) | Chesapeake Bay, USA (FE) | 4.5–14.5% (6 months) | |
| Orth & Moore (1983) | Chesapeake Bay, USA (FE) | <15% | |
| Orth et al. (1994) | Chesapeake Bay, USA (FE) | 3.8–39.8%a | |
| Orth et al. (2003) | Chesapeake Bay, USA (FE) | 1–15%a | |
| Pickerell et al. (2005) | Peconic Estuary, USA (FE) | 6.9% | |
| van Katwijk & Wijgergangs (2004) | Dutch Wadden Sea (FE) | 11–52% | 5–56%* (9 weeks) |
| Bintz & Nixon (2001) | Rhode Island, USA (M) | 74–100% (12 weeks) | |
| Harwell & Orth (1999) | Chesapeake Bay, USA (M) | 50% (6 months) | |
| Moore et al. (1993) | Chesapeake Bay, USA (M) | 80%a | |
| this study | Ria Formosa, Portugal (M) | 5.6–8.9% | 0% (35 weeks) |
- aSeed germination as seedling establishment.
- *Values inferred from data provided by the authors.
The low germination of the seeds held in water (2.4–5.3%) was similar to that reported by Orth & Moore (1983) for Z. marina seeds (0%) under similar conditions for the populations of Chesapeake Bay, which can be considered near the southern distribution limit of the species along the Western Atlantic coast.
The low germination and seedling survival rates (Table 5) suggest a bottleneck in the reproductive life cycle of Z. marina of Ria Formosa, as few seeds established as seedlings and none survived into the adult phase. These reproductive constraints may account for the scarce presence of Z. marina in Ria Formosa, and for the low genetic variability of the species in the lagoon (Billingham et al. 2003). Recruitment from seeds in Ria Formosa seems to be a function of the low seed germination and the low seedling survival. Supporting the low reproductive success observed here is the fact that Z. marina populations of Ria Formosa are genetically structured, mainly due to clonal expansion of individuals, and are characterised by aggregated, not intermingling, clones (Billingham et al. 2007). In addition to the species’ low seed germination and survival, temperature stress, scarce presence and low genetic variability, the short time period between the seed release and the seed germination observed here may also contribute to the low resilience of local populations to cope with the effects of climate change. A longer period between the seed release and germination could allow for larger-scale resilience by potentially enhancing the chance of species adaptation to a wider range of environmental conditions.
The lower weight of Z. marina seeds of Ria Formosa (4.9–6.6 mg) compared with other populations (c. 8 mg; Granger et al. 2002) may also play a role in the lower reproductive success of the species. The weight of the seeds may express their potential to germinate, and of the seedling to develop and survive, assuming that heavier seeds have a larger amount of resources and thus a better chance to succeed (Fenner & Thompson 2004).
The mortality of the seedlings in the mesocosm experiment occurred between January and April, as observed by Churchill (1983), who also could not identify the causes of seedling loss. In the field, seedling growth was reported to be slow during winter but rapidly increasing during spring and summer (Churchill 1983; Orth & Moore 1983; Greve et al. 2005; Lee et al. 2007). Despite the vulnerability and often low survival of Z. marina seedlings, high seedling survival has been reported in disturbed areas (Plus et al. 2003; Greve et al. 2005; Lee et al. 2007; Wisehart et al. 2007). This high seedling survival may be related to the presence of less shading as a result of the loss of adult plants due to disturbance, assuming that seedling establishment is significantly affected by shoot density (Plus et al. 2003) and light availability (Bintz & Nixon 2001). Lower light levels result in morphological changes of the seedlings with serious implications for seedling survival (Bintz & Nixon 2001). Hence, the lower light availability during winter months may have been responsible for diminished seedling fitness and subsequent mortality. Bintz & Nixon (2001) reported that Z. marina seedlings required a minimum of 8 mol·quanta·m−2·day−1 PAR to growth and survive, which was similar to the light available to the seedlings in December (10.9 mol·quanta·m−2·day−1). In addition to light, water temperature may have also accounted for the seedling mortality. Abe et al. (2008) reported that the optimal temperature for Z. marina seedling growth is 20–25 °C, which is higher than the water temperature during the period of seedling mortality (14–19 °C).
To our knowledge there are no studies reporting the development of Z. marina seedlings into adult plants in the laboratory. In mesocosm conditions, van Lent & Verschuure (1995) reported the development of seedlings until the production of flowering shoots, i.e. into the adult phase, for an annual form of Z. marina. In this study, the individual that lived the longest did so for c. 7 months (208 days). Other studies reported the maintenance of Z. marina seedlings in outdoor tanks for 3–6 months (Harwell & Orth 1999; Bintz & Nixon 2001).
In conclusion, the reproductive characteristics of Z. marina populations of Ria Formosa are site-specific. The seeds were able to germinate under natural environmental conditions but very few germinated seeds established as seedlings and none of the seedlings survived into adults. Such low reproductive success limits the recruitment of new meadows and may be related to the local decline of Z. marina populations. The high water temperatures of Ria Formosa may account for the low germination of seeds. Increased temperatures due to climate change may reduce even further the sexual reproduction of Z. marina in its southern distributional limit in the Eastern Atlantic.
Acknowledgements
This study was funded by the EC LIFE-Nature 2006 project ‘Restoration and Management of Biodiversity in the Marine Park Site Arrábida-Espichel (BIOMARES – PTCON0010)’. We are grateful to A. Cunha, O. Diekmann, J. Assis, L. Gonçalves, V. Ferreira, T. Repolho, L. Gonçalves for fieldwork assistance. We thank two anonymous reviewers who greatly improved the manuscript.
