Seagrasses are a group of monocotyledonous submerged plants growing in marine coastal environments (den Hartog 1970; Short et al. 2007). These organisms provide valuable ecological and socio-economic services (Orth et al. 2006), that are imperiled due to the global decline of seagrass populations driven by anthropogenic pressures and climate change (Waycott et al. 2009; Dunic et al. 2021). Restoration of seagrass meadows may compensate for this loss (van Katwijk et al. 2016; Orth et al. 2020). As part of this effort, seed restoration appears as a promising approach due to its less destructive effect on the donor meadows and its potential to generate more genetically diverse restored populations (Sinclair et al. 2021). Harnessing the advantages of seed restoration requires a good understanding of the processes determining seed production, reproductive investment and success. Unfortunately, such knowledge is limited to a few species that are well-studied such as Zostera marina (Reusch 2003), Posidonia oceanica (Jahnke et al. 2015), and Thalassia testudinum (van Tussenbroek et al. 2008; 2016). This knowledge, however, cannot be extrapolated to all seagrass species as their reproductive characteristics are determined by the local environmental (e.g., temperature, light) and ecological (e.g., vertical zonation) conditions. Therefore, it is fundamental to develop population-specific and species-specific assessments of the reproductive phenology and investment (e.g., seed production) of seagrasses when planning restoration efforts.
Seagrass reproductive phenology and investment are highly influenced by temperature and light availability, factors that vary along vertical (von Staats et al. 2021) and latitudinal clines (Ito et al. 2021). In the tropics, these environmental drivers are more stable compared to sub-tropical and temperate zones, which may explain the year-round flowering pattern documented in most tropical seagrasses (Brouns and Heijs 1986; Walker et al. 2001; Rollón et al. 2003). However, even for tropical species, microgeographic environmental gradients can contribute to inter-population differences in reproductive characteristics (Walker et al. 2001; Lee et al. 2007), potentially influencing the capacity of populations to respond to rapidly changing conditions (von Staats et al. 2021). For instance, intra-specific differences in flowering timing and duration have been documented in tropical populations of the seagrass Halophila decipiens in which flowering occurs in April for meadows in Thailand (Lewmanomont et al. 1996), while this process takes place between August and November in South Sulawesi (Verheij and Erftemeijer 1993). Similarly, in the tropical seagrass Halodule wrightii, flowering starts in early spring for populations in the Gulf of Mexico and Caribbean, and in late spring for those in the northern range of the species distribution (McGovern and Blankenhorn 2007). When flowering phenology is consistent across populations, other intra-specific differences may result from microgeographic differences in environmental conditions. In the Indo-Pacific, for example, populations of the seagrass Enhalus acoroides show geographic differences in flowering intensity correlated with spatial changes in light availably influenced by turbidity and water depth (Rollón et al. 2003).
At the population level, changes in reproductive phenology and investment can be related to ecological and temporal trends (e.g., seasonality) that are part of the life histories of these plants (Turschwell et al. 2021). Beyond these natural dynamics, changes in reproductive characteristics can be related to the demographic impacts of rapid and large-scale population declines (Baranano et al. 2022; de los Santos et al. 2019). Such declines typically result in disturbance and the reduction of reproductive success due to pollen limitation (van Tussenbroek et al. 2016). In China, the distribution area and density of E. acoroides have drastically declined over the last decades, and some populations have become fragile due to pollution and physical disturbances (Herbeck et al. 2014; Chen et al. 2015). Furthermore, this dioecious seagrass is a surface-pollinated seagrass species with a lower pollen-ovule ratio of 10:1 (Ackerman 2006). This may result in limited propagule production and plant material to support seagrass habitat recovery. Although E. acoroides has a continuous flowering pattern in other areas of the South Asia region (e.g., Indonesia; Brouns and Heijs 1986), this reproductive process has rarely been noticed in China (Yu et al. 2019). Considering the use of sexual propagules for E. acoroides restoration, we aimed to assess the reproductive phenology of E. acoroides and the effects of fragmentation on the fruit set in Hainan.