Intra-Meadow Variation in Seagrass Flowering Phenology Across Depths

Variation in the timing of sexual reproduction, as well as differences in the relative investment in asexual vs sexual reproduction, commonly occurs among populations. Less is known about such variation within populations, yet small-scale differences in phenology and reproductive investment have the potential to limit gene flow and consequently the adaptive capacity of populations. We examined within-site variation in sexual reproduction in seagrasses, marine angiosperms that can reproduce asexually via clonal propagation or sexually via flowering, and that form the foundation of critical marine ecosystems globally. Many factors, including light availability, temperature, nutrient availability, and genetic identity, influence the rate and timing of seagrass flower development, yet little is known about how phenology varies across common environmental gradients within individual seagrass meadows. Here, we investigate how the density, morphology, and phenology of eelgrass (Zostera marina) differs across depths within multiple sites in New England. Despite variation in the proportion of reproductive vs vegetative shoots across sites, reproductive investment did not differ across depths within sites. However, a comparison of developmental stages of flowering shoots revealed delays in flower development for deep plants compared with shallow plants for all sites, demonstrating a consistent offset in reproductive phenology across depths. Our results suggest that differences in the timing of flowering may limit gene flow across seagrass meadows, with the potential for repeated genetic divergence in eelgrass populations spanning a depth gradient.