Eelgrass ecosystems have a wide variety of ecological functions in which living tissues and detritus may be a food source for many marine animals. In this study, we conducted a laboratory simulating experiment to understand the trophic relationship between the eelgrass Zostera marina L and the sea cucumber Apostichopus japonicus. A mixture of decaying eelgrass debris and seafloor surface muddy sediments was used as food to feed A. japonicus, and then specific growth rates (SGR) and fecal production rates (FPR) were measured. According to the proportion of eelgrass debris, we designed five treatment diets, i.e., ES0, ES10, ES20, ES40, and ES100, with eelgrass debris accounting for 0%, 10%, 20%, 40%, and 100% in dry weight, respectively. Results showed that diet composition had a great influence on the growth of A. japonicus. Sea cucumbers could use decaying eelgrass debris as their food source; and when the organic content of a mixture of eelgrass debris and sediment was 19.6% (ES40), a relatively high SGR (1.54%·d−1) and FPR (1.31 g·ind.−1 d−1) of A. japonicus were obtained. It is suggested that eelgrass beds can not only provide habitat for the sea cucumber A. japonicus but can also provide an indirect food source for the deposit feeder. This means that the restoration and reconstruction of eelgrass beds, especially in coastal waters of China, would be a potential and effective measure for sea-cucumber fisheries, in respect to both resource restoration and aquaculture of this valuable species.

Eelgrass beds in coastal waters of China have declined substantially over the past 30 years. In this study, a simple new transplanting technique was developed for eelgrass (Zostera marina L.) restoration. To assist in anchoring single shoots, several rhizomes of rooted shoots were bound to a small elongate stone (50–150 g) with biodegradable thread (cotton or hemp), and then the bound packet was buried at an angle in the sediments at a depth of 2–4 cm. This stone anchoring method was used to transplant eelgrass in early November 2009 and late May 2010 in Huiquan Bay, Qingdao. The method led to high success. Three month survivorship of the transplanted shoots at the two transplant sites was >95%. From April 20 to November 19, 2012, the following characteristics of the 2009 and 2010 transplanted eelgrass beds were monitored: morphological changes, shoot density, shoot height, leaf biomass, and sediment particle size. Results showed that the sexual reproduction period of the planted eelgrass was from April to August, and vegetative reproduction reached its peak in autumn. Maximum shoot height and biomass were observed in June and July. After becoming established, the transplanted eelgrass beds were statistically equal to natural eelgrass beds nearby in terms of shoot height, biomass, and seasonal variations. This indicates that the transplant technique is effective for eelgrass restoration in coastal waters.

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.