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Reproduction and recruitment of the seagrass Halophila stipulacea
Aquatic Botany 85 (2006) 345–349 www.elsevier.com/locate/aquabot
Short communication
Reproduction and recruitment of the seagrass Halophila stipulacea Torleif Malm * Department of Biology and Environmental Science, Kalmar University S-391 82 Kalmar, Sweden Received 26 November 2005; received in revised form 22 May 2006; accepted 31 May 2006
Abstract The small seagrass species, Halophila stipulacea is abundant in the subtidal zone of the Bay of Eilat, Red Sea, southern Israel. Early life history characteristics of this species were investigated in summer 2002 by means of field surveys and outdoor experiments. Monospecific stands were found at depths of between 2 and 20 m. Reproduction began in late May and ripe pericarps were found for 1 month starting from the beginning of August. The ratios of female versus male plants were 0.9 at depths of between 2.5 and 10 m and 0.5 at depths of between 12.5 and 15 m. The proportion of reproductive branches was significantly larger in the shallow (2–5 m) than in the deep (7–15 m) populations, i.e., 20 11% versus 6 10%, respectively. Ripe seeds were predominantly produced at depths of between 2 and 5 m. Experimental studies demonstrated that full sunlight completely inhibited seedling growth at a depth of 30 cm; no macroscopic seedlings could be observed after 40-day exposure to full sunlight. If exposed to 90% photosynthetic active radiation (PAR) but protected from ultraviolet radiation (UVR), the number of macroscopic seedlings increased to 7.4 2.3% of the planted seeds. If protected from both UVR and 80% of the PAR, the number of macroscopic seedlings increased to 22.5 4.0% of the planted seeds. UVR exclusion and 80% PAR reduction also significantly increased the rhizome growth rates of seedlings in the first month after germination (0.14 0.04 mm day1) compared with only UVR exclusion (0.04 0.02 mm day1). The absence of H. stipulacea from the uppermost part of the subtidal zone (depths of 0–2 m) may be due to light inhibition of germling growth and uprooting by occasional storms. # 2006 Elsevier B.V. All rights reserved. Keywords: UVR; PAR; Stress; Disturbance; Seedling and germination
1. Introduction The quantification of sexual reproduction, shoot population dynamics and plant growth is essential if one is to forecast the development of seagrass populations (Hemminga and Duarte, 2000). The importance of sexual recruitment is generally assumed to be low in seagrass populations (Rasheed, 1999). However, small seagrass species such as Halophila spp. are usually more sexually fecund than larger species are (Kenworthy, 2000). The species in the genus Halophila are classic r-strategy species: they are sexually fecund, rapidly growing, good colonisers that can exploit abundant resources, but are weak competitors (Williams, 1990) with a low tolerance of disturbance (Preen et al., 1995; Duarte et al., 1997) due to their size and shallow root system. Populations of small seagrass species may also be more stressed by various environmental factors, such as low light (Huong et al., 2003), drought (Tanaka and Nakaoka, 2004) and high sulphur
* Tel.: +46 70 398 93 667; fax: +46 156 221 87. E-mail address: [email protected]. 0304-3770/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2006.05.008
concentrations (Morris and Virnstein, 2004) than larger species are. High light levels are thought to be an important seagrass stressor at lower latitudes, as shallow tropical and subtropical ecosystems are subjected to high levels of sunlight, including both ultraviolet radiation (UVR, 280–400 nm) and photosynthetic radiation (PAR, 400–780 nm) (Bjo¨rk and Beer, 1999). Variations in cloudiness and aerosols greatly affect the level of UVR and PAR that reach the sea surface in these regions (McKenzie et al., 2003). The depth of light penetration differs considerably between marine areas, depending on the concentrations of absorbing and scattering substances that reduce the transparency of the water (Ha¨der et al., 2003). The level of UVB (280–320 nm) at a depth of 5 m was as high as 12% of the surface value in very clear mid-ocean water but only 0.4% at the same depth in the turbid waters of an inshore reef (Dunne and Brown, 1996). Photoinhibition or photodestruction in the intertidal and upper subtidal zones in tropical areas has been demonstrated for several species and has also been cited as an explanation of differences in the vertical distribution of seagrass species (Durako et al., 2003). Significant photoinhibitory responses for
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both fluorescence and oxygen evolution were found in Halophila ovalis blades after less than 1 h of exposure to 500 mmol m2 s1 of sunlight, while 1 h of exposure to 1000 mmol m2 s1 led to photo damage (Ralph and Burchett, 1995). Apart from direct effects on plant physiology, increased UVR in shallow water may also harm the diazotrophic bacteria on the seagrass blades (Aas et al., 1996; Pereg-Gerk et al., 2002), which in turn may cause a nitrogen shortage for the plants. The effects of high radiation levels on the germination and seedling growth of tropical seagrass species has to the best of my knowledge not been considered in literature. The dioecious seagrass Halophila stipulacea (Forsk.) Ascherson forms dense populations on shallow soft bottoms in the Bay of Eilat, southern Israel. The species has a wide spatial range in the Red Sea. In the late 1970s, the species was the most abundant seagrass in the northern part of the Red Sea, including in the Bay of Eilat (Lipkin, 1979). Its vertical distribution extended from the lower part of the lower intertidal zone to depths of more than 30 m, with the highest biomasses occurring at depths of 3–10 m. It has since been noted that stands of H. stipulacea undergo periodic denudations and the species has almost completely disappeared from the intertidal zone to a depth of 4–5 m along the coasts of the northern part of the Bay of Eilat (own observations, D. Zakai, personal communications). In 2002, a series of studies and experiments were performed in the Bay of Eilat to investigate how the depth distribution of the seagrass and variation in UV and PAR exposure influence the early life cycle of H. stipulacea. The zero hypothesis of the study was that sex quota, degree of sexual reproduction and seedling growth rate are independent of light conditions and stand depth. 2. Materials and methods 2.1. Research area The study was carried out in the northern part of the Bay of Eilat, Red Sea (298 320 4800 N; 348 570 3300 E), from May to August 2002. Surface water temperatures in the bay range between 25 and 27 8C in August and between 21 and 22 8C in February– March (Gertman and Brenner, 2004). The salinity of the surface water is steady at 41 PSU (Gertman and Brenner, 2004) and there are no permanent watercourse discharges into the Bay (Le Houerou, 2003). The northern shore is urbanised, with some 150,000 inhabitants living in Eilat and Aqaba. The natural, undisturbed sediment in this area is fine alluvial sand, but partly due to fish farming, these sediments have been enriched by organic material (Angel et al., 1992). There are also signs of eutrophication in the water, with increased phytoplankton and filamentous macroalgal growth (Lazar and Erez, 2004). 2.2. Methods The field study was carried out using SCUBA diving on May 25, June 25, August 8 and August 25, 2002. The branch, i.e., a physiologically independent unit, approximately 20 plasto-
chrons long and sometimes with shorter lateral branches of 1–5 plastochrons, was used as the statistical unit for the investigation. On each diving occasion, 100 H. stipulacea branches were collected at each of six different depths, i.e., 2.5, 5, 7.5, 10, 12.5 and 15 m. At each depth, the branches were collected randomly in a 100 m2 area. The material was transported under moist and cool conditions to the laboratory, where every branch was checked for male and female flowers and pericarps. Pericarps that fell easily off the mother plants were put in seawater and if they burst and released seeds within 12 h they were considered as ripe. The time needed for each pericarp to burst was measured. The number of seeds from each of 10 pericarps from 2.5 and 5 m depth, respectively, were counted and the size of 10 seeds from each pericarp was estimated using microscopy. Underwater PAR irradiation was measured at noon on clear days in mid May and August and on September 13, 2002, using the external light sensor of a DIVING-PAM underwater fluorometer (Walz, Effeltrich, Germany). In August 2002, an experiment to examine the growth rate of seedlings under different light conditions was carried out using a water table standing on the beach near the shore. Natural sediment was collected from a depth of 5 m and poured into 0.4 m 0.4 m basins that were 0.4 m deep. The basins were filled two thirds deep with sediment and submerged in the water table (0.5 m3) in flowing seawater (2 L min1) one week before the start of the experiment. The water temperature in the experimental basins reached 32 8C at noon and fell to 26–27 8C at night. The daytime temperature was approximately 5 8C higher than that of the surface seawater 100 m from shore, but there was no difference in temperature between the 22 different treatments. Thirty seeds from each of 10 seed capsules were sown in rows 1 cm apart in each basin. The seeds were buried 0.5 cm deep in the sediment. Light filtration was obtained using polycarbonate plastic shields. For the treatments, the basins were covered with plastic shields filtering either 100% UVR and 10% PAR, or 100% UVR and 80% PAR or were left uncovered and fully exposed to sunlight (the light-filtering capacities of the plastic shields are as specified by the producer, I-CA Enterprises). Each light treatment was replicated three times. The lateral extension of each seedling was measured as the growth rate. The experiment continued for 40 days. To investigate whether H. stipulacea seeds were still viable after prolonged exposure to full sunlight, the seeds exposed to full sunlight in the first experiment were covered with polycarbonate shields filtering 100% UVR and 80% PAR for an additional 40 days. 2.3. Statistical analysis Frequency data on number of fertile branches, flowers, pericarps and male and female branches were analysed with StatisticaTM 99 using the chi-square test. The experimental data regarding seedling growth rate and the field data regarding the number of seeds per pericarp and seed size were analysed with StatisticaTM 99 using one-way analysis of variance (ANOVA). The homogeneity of variance was tested using Levin’s test and
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log transformation of the data was done when necessary. Data given in percentages were arcsine square root transformed to approximately normal distribution before analysis. Throughout the text, standard deviation is used as the measurement of variance. 3. Results The highest PAR level at the surface was reached in August 2002 and similar levels were reached in May and September (Fig. 1). About half the surface PAR was filtered out at a depth of 2.5 m; 11% of surface PAR (the theoretical light compensation limit) was measured at a depth of 7 m in May, 12 m in August and 5 m in September. A continuous monospecific H. stipulacea meadow was found along a 500-m-long stretch of the northern shore of the Bay of Eilat. No other seagrass species were found in the area. The meadow extended vertically from a depth of 2–20 m, but was fragmented at both extremes of its depth range, with continuous stands occurring from depths of 5.5–10 m. For 2 days in March 2002, a thunderstorm with south-westerly winds reaching 15 m s1 struck the northern part of the Bay of Eilat. The impact on the seagrass meadows was obvious to depths as great as 5 m. Recolonisation started in September and by November 2002 the coverage between 2 and 5 m depth was approximately the same as before the storm. The first flowers were observed at the end of May and the last seed capsules were seen at the end of August. The frequency of flowers was significantly (x2 = 86.9, d.f. = 10, p < 0.01) lower in the deep part of the meadow (depths of 7.5–15.0 m) than in the shallow part (2.5–5.0 m) (Fig. 2a). The sex ratio was biased, with a significantly higher proportion of female than male branches 10 (x2 = 143.70, d.f. = 10, p < 0.01) (Fig. 2a) with one exception at a depth of 12.5 m in June 2002. The production of flowers was concentrated at the 3–4 nodes closest to the rhizome apex. The first unripe pericarps were observed at end of June and ripe seed capsules were common in August (Fig. 2b). A few ripe seed capsules were observed at a depth of 7.5 m, but successful reproduction was concentrated at depths of 2.5–5.0 m (Fig. 2b).
Fig. 2. (a) Proportion of fertile branches of Halophila stipulacea along a depth gradient off the northern shore of the Bay of Eilat, May–August 2002 (black: males, white: females). (b) Timing of flower and seed production of Halophila stipulacea in a depth gradient off the northern shore of the Bay of Eilat, May– August 2002.
Ripe seed capsules became detached from the mother plant and floated on the surface for 184 27 min before they turned inside out and dispersed their seeds, which did not float. Seed production per pericarp did not differ significantly between depths of 2.5 m (39.1 4.7 no.) and 5.0 m (40.5 5.2 no.), nor did seed size differ significantly between depths of 2.5 m (1.2 0.4 mm) and 5.0 m (1.1 0.3 mm). The number of mature pericarps found at depths greater than 5 m were too few to allow statistical comparison. In the experiment, no macroscopic seedlings appeared in the basins exposed to full sunlight. The number of seedlings emerging was significantly higher in the basins exposed to 0% UVR and 20% PAR (22.6 1.1%) than in those exposed to 0% UVR and 90% PAR (7.4 0.4%) (ANOVA, F 1.5 = 505, p < 0.001). The rhizome growth rate of seedlings exposed to 0% UVR and 20% PAR was significantly higher (0.14 0.03 mm day1) than that of seedlings exposed to 0% UVR and 90% PAR (0.04 0.02 mm day1) (ANOVA, F 1.5 = 29.8, p = 0.005). No seedlings appeared from the basins exposed first to full sunlight for 40 days and then exposed to 0% UVR and 20% PAR for additional 40 days. 4. Discussion
Fig. 1. Solar irradiation measured at noon, off the northern shore of the Bay of Eilat at different depths on three occasions, May–September 2002.
The fragmented growth pattern of shallow H. stipulacea populations in the Bay of Eilat is likely partly due to uprooting
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by infrequent but regular storms or gales originating in east Africa. This assumption is in line with the results of Preen et al. (1995), who identify uprooting by storms as an important factor limiting the vertical distribution of seagrass populations. Recolonisation along the northern shore of the Bay of Eilat probably happened mostly by means of seeds, since most recruits were small, resembled the seedlings obtained in the experiment and first appeared 1 month after the end of the reproductive season. However, no recolonisation was observed in the very shallow part of the area, in the 0–1 m depth interval. The experimental results of this study suggest that limited seed germination or reduced germling survival, both due to high solar irradiation, may be important factors restricting H. stipulacea from the intertidal and the shallowest subtidal zones. The daytime temperature in the experimental setup exceeded the surface temperature in the sea by approximately 5 8C, which may have made the germlings in the experiment more sensitive to UVB light than the plants in the investigated field area were. However, 32 8C is not an extreme water temperature for the area, being found in protected bays further south along Sinai Peninsula where H. stipulacea was formerly reported to grow all the way up to the lower intertidal zone (Lipkin, 1979). The fragmented growth pattern found in the deep meadows is likely the result of low light limitations. The light compensation level for the growth of most seagrass species is reached at 11% of the surface PAR (Duarte, 1991). This irradiation level was measured at a depth of approximately 12 m in August along the northern shore of the Bay of Eilat, that is, at approximately the same depth at which the meadows were observed to become fragmented. Many clonal angiosperms react with increased sexual reproduction to both frequent (Xie et al., 2001) and occasional (Romme et al., 1997) disturbances; this also seems to be true of certain species of seagrass (Gallegos et al., 1992). The observed hydrodynamic disturbance of the shallow part of the H. stipulacea meadow may explain the pronounced differences in sexual reproduction between the branches growing in shallow and in deep water found in this study. A strong female-biased sex ratio in flowering shoots, similar to the overall sex ratio observed in this study, has earlier been reported in other dioecious seagrass species (Waycott et al., 1996). Williams (1995) speculates that while males are scarce, pollen is not, because of a high pollen-to-ovule ratio. Thus, the occurrence of fertilised ovules is not limited by pollen availability and there is no need to produce equal number of male and female flowers for optimal fertilisation. Other researchers assume that seagrass species are pollen limited, especially in deeper meadows where low water turbulence limits the distribution of pollen (Verduin et al., 2002). This assumption is in line with our observations along the northern shore of the Bay of Eilat, where female flowers found growing deeper than approximately 5 m very infrequently developed ripe seeds. Several rapid examinations in the area, carried out from August to November 2002, failed to find any seedlings growing deeper than 5.5 m. Apart from the low fertilisation rate in the deeper meadows, this could be because the seeds were carried
away from the area due to the long dispersal time before the pericarps burst. Lacap et al. (2002) estimated the average dispersal distance of floating fruits of the species Enhalus acoroides and Thalassia hemprichii to be 41 and 23 km, respectively, with floating times similar to that of H. stipulacea found in this investigation. In conclusion, light may be an important factor influencing the upper limit and vertical distribution of H. stipulacea in the Bay of Eilat. However, according to measurements of global irradiation made at Sede Boqer, 200 km north of Eilat (Faiman et al., 2004) and satellite measurements of the stratospheric ozone layer above Jerusalem, 350 km north of Eilat (Total Ozone Mapping Spectrometer, NASA, USA), neither UVR nor PAR has increased over the past 15 to 25 years. The turbidity of the water in the area has not changed either over the past 15 years (IUI database). If H. stipulacea has become more sensitive to UVR and/or PAR, local factors such as pollution and altered ecological interactions are more likely explanations; these hypotheses still have to be confirmed or rejected by additional investigations. Acknowledgements I would like to express many thanks to Professor Sven Beer at the University of Tel Aviv for his support and hospitality during my post-doctoral year in Israel. I also wish to thank the staff and students of the InterUniversity Institute in Eilat for their kind help with the diving and equipment. A special acknowledgement is extended to Ms. Tom Elrom for her patient help during the long and laborious SCUBA-diving excursions. Professor Lena Kautsky gave important feedback on the manuscript. This work was funded by the Swedish Academy of Science and the Department of Plant Sciences, Tel Aviv University. References Aas, P., Lyons, M.M., Pledger, R., Mitchell, D.L., Jeffrey, H.W., 1996. Inhibition of bacterial activities by solar radiation in nearshore waters and the Bay of Mexico. Aquat. Microb. Ecol. 11, 229–238. Angel, D., Kros, P., Zuber, D., Mozes, N., Neori, A., 1992. The turnover of organic matter in hypertrophic sediments below a floating fish farm in the oligotrophic Bay of Eilat (Aqaba). Isr. J. Aquacult. 44, 143–144. Bjo¨rk, M., Beer, S., 1999. Effects of desiccation, re-hydration and high-light stress on the rate of photosynthesis electron transport in intertidal seagrasses. Aquat. Bot. 65, 1–4. Duarte, C.M., 1991. Seagrass depth limits. Aquat. Bot. 40, 363–377. Duarte, C.M., Terrados, J., Agawin, N.S.R., Fortes, M.D., Bach, S., Kenworthy, W.J., 1997. Response of a mixed Philippine seagrass meadow to experimental burial. Mar. Ecol. Prog. Ser. 147 (9), 285–294. Dunne, R.P., Brown, B.E., 1996. Penetration of solar UVB radiation in shallow tropical waters and its potential biological effects on coral reefs; results from the central Indian Ocean and Andaman Sea. Mar. Ecol. Prog. Ser. 144, 109–118. Durako, M.J., Kunzelman, J.I., Kenworthy, W.J., Hammerstrom, K.K., 2003. Depth-related variability in the photobiology of two populations of Halophila johnsonii and Halophila decipiens. Mar. Biol. 142 (6), 1219–1228. Faiman, D., Feuermann, D., Ibbetson, P., Medwed, B., Zemel, A., Ianetz, A., Liubansky, V., Setter, I., Suraqui, S., 2004. The Negev Radiation Survey. J. Sol. Energy Eng. 126, 906–914.
T. Malm / Aquatic Botany 85 (2006) 345–349 Gallegos, M.E., Merino, M., Marba, N., Duarte, C.M., 1992. Flowering of Thalassia testudinum Banks ex Koenig in the Mexican Caribbean: agedependence and interannual variability. Aquat. Bot. 43, 249–255. Gertman, I., Brenner, S., 2004. Analysis of water temperature variability in the Bay of Eilat, IET Project No. 12. InterUniversity Institute, Eilat, Israel, 13 pp. Ha¨der, D.-P., Kumar, H.D., Smith, R.C., Worrest, R.C., 2003. Aquatic ecosystems: effects of solar ultraviolet radiation and interactions with other climatic change factors. Photochem. Photobiol. Sci. 2, 39–50. Hemminga, M.A., Duarte, C.M., 2000. Seagrass Ecology. Cambridge University Press, Cambridge, UK, 310 pp. Huong, T.T.L., Vermaat, J.E., Terrados, J., Tien, N.V., Duarte, C.M., Borum, J., Tri, N.H., 2003. Seasonality and depth zonation of intertidal Halophila ovalis and Zostera japonica in Ha Long Bay (northern Vietnam). Aquat. Bot. 75 (2), 47–157. Kenworthy, W.J., 2000. The role of sexual reproduction in maintaining populations of Halophila decipiens: implications for the biodiversity and conservation of tropical seagrass ecosystems. Pac. Conserv. Biol. 5, 251–259. Lacap, C.D.A., Vermaat, J.E., Rollon, R.N., Nacorda, H.M., 2002. Propagule dispersal of the SE Asian seagrasses Enhalus acoroides and Thalassia hemprichii. Mar. Ecol. Prog. Ser. 235, 75–80. Lazar, B., Erez, J., 2004. Nutrient measurements at Stations A1, B1. IET Project No. 9. InterUniversity Institute, Eilat, Israel, 4 pp. Le Houerou, H.N., 2003. Bioclimatology and phytogeography of the Red Sea and Aden Bay basins: a monograph (with a particular reference to the highland evergreen sclerophylls and lowland halophytes). Arid Land Res. Manage. 17, 177–256. Lipkin, Y., 1979. Quantitative aspects of seagrass communities, particularly of those dominated by Halophila stipulacea, in Sinai (Northern Red Sea). Aquat. Bot. 7, 119–128. McKenzie, L.R., Bjo¨rn, L.-O., Bais, A., Ilyasd, M., 2003. Changes in biologically active ultraviolet radiation reaching the Earth’s surface. Photochem. Photobiol. Sci. 2, 5–15.
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Morris, L.J., Virnstein, R.W., 2004. The demise and recovery of seagrass in the Northern Indian River Lagoon, Florida. Estuaries 27 (6), 915–922. Pereg-Gerk, L., Sar, N., Lipkin, Y., 2002. In situ nitrogen fixation associated with seagrasses in the Bay of Eilat (Red Sea). Aquat. Ecol. 36, 387–394. Preen, A.R., Long, W.J.L., Coles, R.G., 1995. Flood and cyclone related loss, and partial recovery, of more than 1000 km2 of seagrass in Hervey Bay, Queensland, Australia. Aquat. Bot. 52, 3–17. Ralph, P.J., Burchett, M.D., 1995. Photosynthetic responses of the seagrass Halophila ovalis (R. Br.) Hook. f. to high irradiance stress, using chlorophyll a fluorescence. Aquat. Bot. 51, 55–66. Rasheed, A.M., 1999. Recovery of experimentally created gaps within a tropical Zostera capricorni (Aschers.) seagrass meadow, Queensland Australia. J. Exp. Mar. Biol. Ecol. 235, 183–200. Romme, W.H., Turner, M.G., Gardner, R.H., Hargrove, W.W., Tuskan, G.A., Despain, D.G., Renkin, R.A., 1997. A rare episode of sexual reproduction in Aspen (Populus tremuloides Michx) following the 1988 Yellowstone fires. Nat. Areas J. 17, 17–25. Tanaka, Y., Nakaoka, M., 2004. Emergence stress and morphological constraints affect the species distribution and growth of subtropical intertidal seagrasses. Mar. Ecol. Prog. Ser. 284, 117–131. Verduin, J.J., Backhaus, J.O., Walker, D.I., 2002. Estimates of pollen dispersal and capture within Amphibolis antarctica (Labill.) Sonder and Aschers. ex Aschers. meadows. Bull. Mar. Sci. 71, 1269–1277. Waycott, M., Walker, D.I., James, S.H., 1996. Genetic uniformity in Amphibolis antarctica, a dioecious seagrass. Heredity 76, 578–585. Williams, S.L., 1990. Experimental studies of Caribbean seagrass bed development. Ecol. Monogr. 60, 449–469. Williams, S.L., 1995. Surfgrass (Phyllospadix torreyi) reproduction: reproductive phenology, resource allocation, and male rarity. Ecology 76, 1953– 1970. Xie, Z.W., Lu, Y.Q., Ge, S., Hong, D.Y., Li, F.Z., 2001. Clonality in wild rice (Oryza rufipogon Poaceae) and its implications for conservation management. Am. J. Bot. 88, 1058–1064.
Short communication
Reproduction and recruitment of the seagrass Halophila stipulacea Torleif Malm * Department of Biology and Environmental Science, Kalmar University S-391 82 Kalmar, Sweden Received 26 November 2005; received in revised form 22 May 2006; accepted 31 May 2006
Abstract The small seagrass species, Halophila stipulacea is abundant in the subtidal zone of the Bay of Eilat, Red Sea, southern Israel. Early life history characteristics of this species were investigated in summer 2002 by means of field surveys and outdoor experiments. Monospecific stands were found at depths of between 2 and 20 m. Reproduction began in late May and ripe pericarps were found for 1 month starting from the beginning of August. The ratios of female versus male plants were 0.9 at depths of between 2.5 and 10 m and 0.5 at depths of between 12.5 and 15 m. The proportion of reproductive branches was significantly larger in the shallow (2–5 m) than in the deep (7–15 m) populations, i.e., 20 11% versus 6 10%, respectively. Ripe seeds were predominantly produced at depths of between 2 and 5 m. Experimental studies demonstrated that full sunlight completely inhibited seedling growth at a depth of 30 cm; no macroscopic seedlings could be observed after 40-day exposure to full sunlight. If exposed to 90% photosynthetic active radiation (PAR) but protected from ultraviolet radiation (UVR), the number of macroscopic seedlings increased to 7.4 2.3% of the planted seeds. If protected from both UVR and 80% of the PAR, the number of macroscopic seedlings increased to 22.5 4.0% of the planted seeds. UVR exclusion and 80% PAR reduction also significantly increased the rhizome growth rates of seedlings in the first month after germination (0.14 0.04 mm day1) compared with only UVR exclusion (0.04 0.02 mm day1). The absence of H. stipulacea from the uppermost part of the subtidal zone (depths of 0–2 m) may be due to light inhibition of germling growth and uprooting by occasional storms. # 2006 Elsevier B.V. All rights reserved. Keywords: UVR; PAR; Stress; Disturbance; Seedling and germination
1. Introduction The quantification of sexual reproduction, shoot population dynamics and plant growth is essential if one is to forecast the development of seagrass populations (Hemminga and Duarte, 2000). The importance of sexual recruitment is generally assumed to be low in seagrass populations (Rasheed, 1999). However, small seagrass species such as Halophila spp. are usually more sexually fecund than larger species are (Kenworthy, 2000). The species in the genus Halophila are classic r-strategy species: they are sexually fecund, rapidly growing, good colonisers that can exploit abundant resources, but are weak competitors (Williams, 1990) with a low tolerance of disturbance (Preen et al., 1995; Duarte et al., 1997) due to their size and shallow root system. Populations of small seagrass species may also be more stressed by various environmental factors, such as low light (Huong et al., 2003), drought (Tanaka and Nakaoka, 2004) and high sulphur
* Tel.: +46 70 398 93 667; fax: +46 156 221 87. E-mail address: [email protected]. 0304-3770/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2006.05.008
concentrations (Morris and Virnstein, 2004) than larger species are. High light levels are thought to be an important seagrass stressor at lower latitudes, as shallow tropical and subtropical ecosystems are subjected to high levels of sunlight, including both ultraviolet radiation (UVR, 280–400 nm) and photosynthetic radiation (PAR, 400–780 nm) (Bjo¨rk and Beer, 1999). Variations in cloudiness and aerosols greatly affect the level of UVR and PAR that reach the sea surface in these regions (McKenzie et al., 2003). The depth of light penetration differs considerably between marine areas, depending on the concentrations of absorbing and scattering substances that reduce the transparency of the water (Ha¨der et al., 2003). The level of UVB (280–320 nm) at a depth of 5 m was as high as 12% of the surface value in very clear mid-ocean water but only 0.4% at the same depth in the turbid waters of an inshore reef (Dunne and Brown, 1996). Photoinhibition or photodestruction in the intertidal and upper subtidal zones in tropical areas has been demonstrated for several species and has also been cited as an explanation of differences in the vertical distribution of seagrass species (Durako et al., 2003). Significant photoinhibitory responses for
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both fluorescence and oxygen evolution were found in Halophila ovalis blades after less than 1 h of exposure to 500 mmol m2 s1 of sunlight, while 1 h of exposure to 1000 mmol m2 s1 led to photo damage (Ralph and Burchett, 1995). Apart from direct effects on plant physiology, increased UVR in shallow water may also harm the diazotrophic bacteria on the seagrass blades (Aas et al., 1996; Pereg-Gerk et al., 2002), which in turn may cause a nitrogen shortage for the plants. The effects of high radiation levels on the germination and seedling growth of tropical seagrass species has to the best of my knowledge not been considered in literature. The dioecious seagrass Halophila stipulacea (Forsk.) Ascherson forms dense populations on shallow soft bottoms in the Bay of Eilat, southern Israel. The species has a wide spatial range in the Red Sea. In the late 1970s, the species was the most abundant seagrass in the northern part of the Red Sea, including in the Bay of Eilat (Lipkin, 1979). Its vertical distribution extended from the lower part of the lower intertidal zone to depths of more than 30 m, with the highest biomasses occurring at depths of 3–10 m. It has since been noted that stands of H. stipulacea undergo periodic denudations and the species has almost completely disappeared from the intertidal zone to a depth of 4–5 m along the coasts of the northern part of the Bay of Eilat (own observations, D. Zakai, personal communications). In 2002, a series of studies and experiments were performed in the Bay of Eilat to investigate how the depth distribution of the seagrass and variation in UV and PAR exposure influence the early life cycle of H. stipulacea. The zero hypothesis of the study was that sex quota, degree of sexual reproduction and seedling growth rate are independent of light conditions and stand depth. 2. Materials and methods 2.1. Research area The study was carried out in the northern part of the Bay of Eilat, Red Sea (298 320 4800 N; 348 570 3300 E), from May to August 2002. Surface water temperatures in the bay range between 25 and 27 8C in August and between 21 and 22 8C in February– March (Gertman and Brenner, 2004). The salinity of the surface water is steady at 41 PSU (Gertman and Brenner, 2004) and there are no permanent watercourse discharges into the Bay (Le Houerou, 2003). The northern shore is urbanised, with some 150,000 inhabitants living in Eilat and Aqaba. The natural, undisturbed sediment in this area is fine alluvial sand, but partly due to fish farming, these sediments have been enriched by organic material (Angel et al., 1992). There are also signs of eutrophication in the water, with increased phytoplankton and filamentous macroalgal growth (Lazar and Erez, 2004). 2.2. Methods The field study was carried out using SCUBA diving on May 25, June 25, August 8 and August 25, 2002. The branch, i.e., a physiologically independent unit, approximately 20 plasto-
chrons long and sometimes with shorter lateral branches of 1–5 plastochrons, was used as the statistical unit for the investigation. On each diving occasion, 100 H. stipulacea branches were collected at each of six different depths, i.e., 2.5, 5, 7.5, 10, 12.5 and 15 m. At each depth, the branches were collected randomly in a 100 m2 area. The material was transported under moist and cool conditions to the laboratory, where every branch was checked for male and female flowers and pericarps. Pericarps that fell easily off the mother plants were put in seawater and if they burst and released seeds within 12 h they were considered as ripe. The time needed for each pericarp to burst was measured. The number of seeds from each of 10 pericarps from 2.5 and 5 m depth, respectively, were counted and the size of 10 seeds from each pericarp was estimated using microscopy. Underwater PAR irradiation was measured at noon on clear days in mid May and August and on September 13, 2002, using the external light sensor of a DIVING-PAM underwater fluorometer (Walz, Effeltrich, Germany). In August 2002, an experiment to examine the growth rate of seedlings under different light conditions was carried out using a water table standing on the beach near the shore. Natural sediment was collected from a depth of 5 m and poured into 0.4 m 0.4 m basins that were 0.4 m deep. The basins were filled two thirds deep with sediment and submerged in the water table (0.5 m3) in flowing seawater (2 L min1) one week before the start of the experiment. The water temperature in the experimental basins reached 32 8C at noon and fell to 26–27 8C at night. The daytime temperature was approximately 5 8C higher than that of the surface seawater 100 m from shore, but there was no difference in temperature between the 22 different treatments. Thirty seeds from each of 10 seed capsules were sown in rows 1 cm apart in each basin. The seeds were buried 0.5 cm deep in the sediment. Light filtration was obtained using polycarbonate plastic shields. For the treatments, the basins were covered with plastic shields filtering either 100% UVR and 10% PAR, or 100% UVR and 80% PAR or were left uncovered and fully exposed to sunlight (the light-filtering capacities of the plastic shields are as specified by the producer, I-CA Enterprises). Each light treatment was replicated three times. The lateral extension of each seedling was measured as the growth rate. The experiment continued for 40 days. To investigate whether H. stipulacea seeds were still viable after prolonged exposure to full sunlight, the seeds exposed to full sunlight in the first experiment were covered with polycarbonate shields filtering 100% UVR and 80% PAR for an additional 40 days. 2.3. Statistical analysis Frequency data on number of fertile branches, flowers, pericarps and male and female branches were analysed with StatisticaTM 99 using the chi-square test. The experimental data regarding seedling growth rate and the field data regarding the number of seeds per pericarp and seed size were analysed with StatisticaTM 99 using one-way analysis of variance (ANOVA). The homogeneity of variance was tested using Levin’s test and
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log transformation of the data was done when necessary. Data given in percentages were arcsine square root transformed to approximately normal distribution before analysis. Throughout the text, standard deviation is used as the measurement of variance. 3. Results The highest PAR level at the surface was reached in August 2002 and similar levels were reached in May and September (Fig. 1). About half the surface PAR was filtered out at a depth of 2.5 m; 11% of surface PAR (the theoretical light compensation limit) was measured at a depth of 7 m in May, 12 m in August and 5 m in September. A continuous monospecific H. stipulacea meadow was found along a 500-m-long stretch of the northern shore of the Bay of Eilat. No other seagrass species were found in the area. The meadow extended vertically from a depth of 2–20 m, but was fragmented at both extremes of its depth range, with continuous stands occurring from depths of 5.5–10 m. For 2 days in March 2002, a thunderstorm with south-westerly winds reaching 15 m s1 struck the northern part of the Bay of Eilat. The impact on the seagrass meadows was obvious to depths as great as 5 m. Recolonisation started in September and by November 2002 the coverage between 2 and 5 m depth was approximately the same as before the storm. The first flowers were observed at the end of May and the last seed capsules were seen at the end of August. The frequency of flowers was significantly (x2 = 86.9, d.f. = 10, p < 0.01) lower in the deep part of the meadow (depths of 7.5–15.0 m) than in the shallow part (2.5–5.0 m) (Fig. 2a). The sex ratio was biased, with a significantly higher proportion of female than male branches 10 (x2 = 143.70, d.f. = 10, p < 0.01) (Fig. 2a) with one exception at a depth of 12.5 m in June 2002. The production of flowers was concentrated at the 3–4 nodes closest to the rhizome apex. The first unripe pericarps were observed at end of June and ripe seed capsules were common in August (Fig. 2b). A few ripe seed capsules were observed at a depth of 7.5 m, but successful reproduction was concentrated at depths of 2.5–5.0 m (Fig. 2b).
Fig. 2. (a) Proportion of fertile branches of Halophila stipulacea along a depth gradient off the northern shore of the Bay of Eilat, May–August 2002 (black: males, white: females). (b) Timing of flower and seed production of Halophila stipulacea in a depth gradient off the northern shore of the Bay of Eilat, May– August 2002.
Ripe seed capsules became detached from the mother plant and floated on the surface for 184 27 min before they turned inside out and dispersed their seeds, which did not float. Seed production per pericarp did not differ significantly between depths of 2.5 m (39.1 4.7 no.) and 5.0 m (40.5 5.2 no.), nor did seed size differ significantly between depths of 2.5 m (1.2 0.4 mm) and 5.0 m (1.1 0.3 mm). The number of mature pericarps found at depths greater than 5 m were too few to allow statistical comparison. In the experiment, no macroscopic seedlings appeared in the basins exposed to full sunlight. The number of seedlings emerging was significantly higher in the basins exposed to 0% UVR and 20% PAR (22.6 1.1%) than in those exposed to 0% UVR and 90% PAR (7.4 0.4%) (ANOVA, F 1.5 = 505, p < 0.001). The rhizome growth rate of seedlings exposed to 0% UVR and 20% PAR was significantly higher (0.14 0.03 mm day1) than that of seedlings exposed to 0% UVR and 90% PAR (0.04 0.02 mm day1) (ANOVA, F 1.5 = 29.8, p = 0.005). No seedlings appeared from the basins exposed first to full sunlight for 40 days and then exposed to 0% UVR and 20% PAR for additional 40 days. 4. Discussion
Fig. 1. Solar irradiation measured at noon, off the northern shore of the Bay of Eilat at different depths on three occasions, May–September 2002.
The fragmented growth pattern of shallow H. stipulacea populations in the Bay of Eilat is likely partly due to uprooting
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by infrequent but regular storms or gales originating in east Africa. This assumption is in line with the results of Preen et al. (1995), who identify uprooting by storms as an important factor limiting the vertical distribution of seagrass populations. Recolonisation along the northern shore of the Bay of Eilat probably happened mostly by means of seeds, since most recruits were small, resembled the seedlings obtained in the experiment and first appeared 1 month after the end of the reproductive season. However, no recolonisation was observed in the very shallow part of the area, in the 0–1 m depth interval. The experimental results of this study suggest that limited seed germination or reduced germling survival, both due to high solar irradiation, may be important factors restricting H. stipulacea from the intertidal and the shallowest subtidal zones. The daytime temperature in the experimental setup exceeded the surface temperature in the sea by approximately 5 8C, which may have made the germlings in the experiment more sensitive to UVB light than the plants in the investigated field area were. However, 32 8C is not an extreme water temperature for the area, being found in protected bays further south along Sinai Peninsula where H. stipulacea was formerly reported to grow all the way up to the lower intertidal zone (Lipkin, 1979). The fragmented growth pattern found in the deep meadows is likely the result of low light limitations. The light compensation level for the growth of most seagrass species is reached at 11% of the surface PAR (Duarte, 1991). This irradiation level was measured at a depth of approximately 12 m in August along the northern shore of the Bay of Eilat, that is, at approximately the same depth at which the meadows were observed to become fragmented. Many clonal angiosperms react with increased sexual reproduction to both frequent (Xie et al., 2001) and occasional (Romme et al., 1997) disturbances; this also seems to be true of certain species of seagrass (Gallegos et al., 1992). The observed hydrodynamic disturbance of the shallow part of the H. stipulacea meadow may explain the pronounced differences in sexual reproduction between the branches growing in shallow and in deep water found in this study. A strong female-biased sex ratio in flowering shoots, similar to the overall sex ratio observed in this study, has earlier been reported in other dioecious seagrass species (Waycott et al., 1996). Williams (1995) speculates that while males are scarce, pollen is not, because of a high pollen-to-ovule ratio. Thus, the occurrence of fertilised ovules is not limited by pollen availability and there is no need to produce equal number of male and female flowers for optimal fertilisation. Other researchers assume that seagrass species are pollen limited, especially in deeper meadows where low water turbulence limits the distribution of pollen (Verduin et al., 2002). This assumption is in line with our observations along the northern shore of the Bay of Eilat, where female flowers found growing deeper than approximately 5 m very infrequently developed ripe seeds. Several rapid examinations in the area, carried out from August to November 2002, failed to find any seedlings growing deeper than 5.5 m. Apart from the low fertilisation rate in the deeper meadows, this could be because the seeds were carried
away from the area due to the long dispersal time before the pericarps burst. Lacap et al. (2002) estimated the average dispersal distance of floating fruits of the species Enhalus acoroides and Thalassia hemprichii to be 41 and 23 km, respectively, with floating times similar to that of H. stipulacea found in this investigation. In conclusion, light may be an important factor influencing the upper limit and vertical distribution of H. stipulacea in the Bay of Eilat. However, according to measurements of global irradiation made at Sede Boqer, 200 km north of Eilat (Faiman et al., 2004) and satellite measurements of the stratospheric ozone layer above Jerusalem, 350 km north of Eilat (Total Ozone Mapping Spectrometer, NASA, USA), neither UVR nor PAR has increased over the past 15 to 25 years. The turbidity of the water in the area has not changed either over the past 15 years (IUI database). If H. stipulacea has become more sensitive to UVR and/or PAR, local factors such as pollution and altered ecological interactions are more likely explanations; these hypotheses still have to be confirmed or rejected by additional investigations. Acknowledgements I would like to express many thanks to Professor Sven Beer at the University of Tel Aviv for his support and hospitality during my post-doctoral year in Israel. I also wish to thank the staff and students of the InterUniversity Institute in Eilat for their kind help with the diving and equipment. A special acknowledgement is extended to Ms. Tom Elrom for her patient help during the long and laborious SCUBA-diving excursions. Professor Lena Kautsky gave important feedback on the manuscript. This work was funded by the Swedish Academy of Science and the Department of Plant Sciences, Tel Aviv University. References Aas, P., Lyons, M.M., Pledger, R., Mitchell, D.L., Jeffrey, H.W., 1996. Inhibition of bacterial activities by solar radiation in nearshore waters and the Bay of Mexico. Aquat. Microb. Ecol. 11, 229–238. Angel, D., Kros, P., Zuber, D., Mozes, N., Neori, A., 1992. The turnover of organic matter in hypertrophic sediments below a floating fish farm in the oligotrophic Bay of Eilat (Aqaba). Isr. J. Aquacult. 44, 143–144. Bjo¨rk, M., Beer, S., 1999. Effects of desiccation, re-hydration and high-light stress on the rate of photosynthesis electron transport in intertidal seagrasses. Aquat. Bot. 65, 1–4. Duarte, C.M., 1991. Seagrass depth limits. Aquat. Bot. 40, 363–377. Duarte, C.M., Terrados, J., Agawin, N.S.R., Fortes, M.D., Bach, S., Kenworthy, W.J., 1997. Response of a mixed Philippine seagrass meadow to experimental burial. Mar. Ecol. Prog. Ser. 147 (9), 285–294. Dunne, R.P., Brown, B.E., 1996. Penetration of solar UVB radiation in shallow tropical waters and its potential biological effects on coral reefs; results from the central Indian Ocean and Andaman Sea. Mar. Ecol. Prog. Ser. 144, 109–118. Durako, M.J., Kunzelman, J.I., Kenworthy, W.J., Hammerstrom, K.K., 2003. Depth-related variability in the photobiology of two populations of Halophila johnsonii and Halophila decipiens. Mar. Biol. 142 (6), 1219–1228. Faiman, D., Feuermann, D., Ibbetson, P., Medwed, B., Zemel, A., Ianetz, A., Liubansky, V., Setter, I., Suraqui, S., 2004. The Negev Radiation Survey. J. Sol. Energy Eng. 126, 906–914.
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