Abstract
Seagrasses are important components of global coastal ecosystems, and the eelgrass Zostera marina L. is widely distributed along the Atlantic and Pacific coasts in the temperate northern hemisphere, but limited datum related to the contribution of sexual reproduction to population recruitment have been reported. This study aimed to understand eelgrass sexual reproduction and population recruitment in Swan Lake (SLL), and Huiquan Bay (HQB) was included for comparison. Random sampling, permanent quadrats or cores and laboratory seed germination-based experimental methods were employed. The flowering, seed production, seed banks, seed germination, seedling survival, and seedling growth of eelgrass were investigated from July 2014 to December 2015 to evaluate the contribution of sexual reproduction to population recruitment. Results indicated a dominant role of asexual reproduction in HQB, while sexual reproduction played a relatively important role in SLL. The highest flowering shoot density in SLL was 517.27 ± 504.29 shoots m−2 (June) and represented 53.34% of the total shoots at the center site. The potential seed output per reproductive shoot and per unit area in SLL were 103.67 ± 37.95 seeds shoot−1 and 53,623.66 ± 19,628.11 seeds m−2, respectively. The maximum seed bank density in SLL was 552.21 ± 204.94 seeds m−2 (October). Seed germination mainly occurred from the middle of March to the end of May, and the highest seedling density was 296.88 ± 274.27 seedlings m−2 in April. The recruitment from seedlings accounted for 41.36% of the Z. marina population recruitment at the center site, while the sexual recruitment contribution at the patch site (50.52%) was greater than that at the center site. Seeds in SLL were acclimated to spring germination, while in HQB, they were acclimated to autumn germination (early October–late November). Seed bank density in HQB was very low, with a value of 254.35 ± 613.34 seeds m−2 (early October). However, seeds in HQB were significantly larger and heavier than those in SLL (size: P = 0.004; weight: P < 0.001). The recruitment from seedlings accounted for as low as 2.53% of the Z. marina population recruitment in HQB. Our laboratory seed germination experiment, which was conducted in autumn, showed that the seed germination percent in HQB was significantly greater than in SLL at optimal germination temperatures (10 and 15°C; P < 0.001). A laboratory seed germination test at suitable temperature may be a potential novel approach to identify the ecological differences among different geographic populations. It is suggested that the Z. marina population recruitment may have different strategies and adapt to specific local conditions, such as in SLL and HQB, and the temperature regime may control morphological and phonological variations.
Highlights
Seagrasses, submerged marine angiosperms, are widely distributed along the tropical and temperate coastlines of the world
Asexual reproduction is the main contributor to the population recruitment of seagrass meadows (Duarte and Sandjensen, 1990; Procaccini and Mazzella, 1998; Hemminga and Duarte, 2000; Rasheed, 2004), while successful population recruitment from sexual reproduction, which is constrained by bottlenecks along the reproductive cycle of seagrasses is extremely low (Hemminga and Duarte, 2000; Cabaco and Santos, 2009)
At Huiquan Bay (HQB), in the last servey on 6 June 2015 when seedling could be distinguished from adult shoots through rhizomes and/or original seed coats, densities of seedling shoots were 12.67 ± 7.02 seedling shoots m−2 (Table 4), and densities of adult shoots were 487.50 ± 88.39 shoots m−2
Summary
Seagrasses, submerged marine angiosperms, are widely distributed along the tropical and temperate coastlines of the world. Sexual reproduction is the only way to maintain the population’s genetic diversity (Ackerman, 2006; Reynolds et al, 2012, 2013), which improves the ability of seagrasses to withstand adverse environments and recover from disturbance more rapidly (Coyer et al, 2004; Hughes and Stachowicz, 2004, 2009; Ehlers et al, 2008; Cabaco and Santos, 2012) It is important for colonizing new habitats and in recolonizing areas of large-scale decline or complete destruction by dispersing seeds from parental meadows (Howe and Smallwood, 1982; Thayer et al, 1984; Marba and Walker, 1999; Harwell and Orth, 2002; Plus et al, 2003; Rasheed, 2004; Greve et al, 2005; Lee et al, 2007; Kallstrom et al, 2008; Jarvis and Moore, 2010; Kendrick et al, 2012). These results indicate the increasing importance of seeds in seagrass conservation and restoration and have led to recent studies on sexual recruitment (Orth et al, 2000, 2012; Jarvis and Moore, 2010; Tanner and Parham, 2010; Furman et al, 2015; Balestri et al, 2017)
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