New Insights into Different Reproductive Effort and Sexual Recruitment Contribution between Two Geographic Zostera marina L. Populations in Temperate China

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.

Keywords: Zostera marina L.; clonal growth; flowering shoot; population recruitment; seed bank; seedling; sexual reproduction; temperature regime.

Figure 1

Zostera marina study sites of Swan Lake (SLL) and Huiquan Bay (HQB) as well as location of Changshan Island (Dalian). Red triangular symbols represent the locations of seagrass beds. CS (Center site) and PS (Patch site) represent the center of continuous distributed area and patch area of eelgrass in SLL, respectively.

Figure 2

Water temperatures of Swan Lake (SLL) and Huiquan Bay (HQB). Red line represents the temperature of HQB; black line represents water temperature of SLL. Values are means ± sd.

Figure 3

Light intensity of the canopy of Zostera marina in Swan Lake (SLL) during the study period.

Figure 4

Sediment grain sizes at Swan Lake (SLL) and Huiquan Bay (HQB).

Figure 5

Temporal changes in the vegetative shoot density (A), shoot height (B), and biomass (C) of Zostera marina in Swan Lake. Values are means ± sd.

Figure 6

Temporal changes in the reproductive shoot density (A) and height (B) of Zostera marina at the two study sites in Swan Lake. Values are means ± sd.

Figure 7

Temporal changes in the seed bank densities of Zostera marina at the two study sites in Swan Lake. Values are means ± sd. Different capital or small letters denote significant differences (p < 0.05).

Figure 8

Densities of seedlings, seedling shoots and overwintering shoots, and the number of shoots per seedling of Zostera marina at the patch site (A) and the center site (B) in Swan Lake in 2015. Values are means ± sd.

Figure 9

The seedling heights of Zostera marina in Swan Lake (A) and Huiquan Bay (B). Values are means ± sd.

Figure 10

Comparison of the germination percent (GP) and germination value (GV(D)) of Zostera marina seeds from different sources, Swan Lake (SLL) and Huiquan Bay (HQB), subjected to two temperatures (10 and 15°C). (A,B) GP at 10 and 15°C, respectively; (C,D) GP and GV(D) after 4 weeks, respectively. Values are means ± sd. Different lowercase letters (a,b) in (C) and (D) denote significant differences (p < 0.05).

Figure 11

The reproduction and recruitment cycle of Zostera marina in Swan Lake (A) and Huiquan Bay (B), China. The colored lines and rectangles represent the accidental and the main timing of different stages within the reproduction and recruitment cycle, respectively; O-Clonal growth and S-Clonal growth represent clonal growth of overwintering shoots and seedlings, respectively; W-shoots, S-shoots, T-shoots, and F-shoots refer to overwintering shoots and shoots from their clonal growth, seedling shoots, total shoots and flowering shoots, respectively. Values (means) are based on the investigation in 2015 and represent the peaks of each stage. Four processes are involved in this cycle: (A) i, seed germination from mid-March, seedling establishment beginning in April and completed at mid-June and seedling S-Clonal growth beginning in May; ii, W-shoot clonal growth beginning in mid-April; iii, rapid clonal growth of mixed S- and W-shoots beginning in June, flowering proceeding from May and seed dispersal mainly occurring in mid-July, and at the same time seeds are being dispersed into the sediment (seed bank); iv, shoots that did not flower decline with time and some of them overwinter, entering the next cycle. (B) i, seed germination from October, seedling establishment beginning from mid-October and finishing in mid-December, S-Clonal growth beginning in April; ii, W-shoots begin clonal growth at the same time; iii, rapid clonal growth of mixed S- and W-shoots beginning in June, flowering proceeding from mid-April and seed dispersal mainly occurring in mid-July, and at the same time seeds are being dispersed into the sediment (seed bank); iv, shoots that did not flower decline with time and some of them overwinter, entering the next cycle.