Quantifying Variation in Zostera marina Seed Size and Composition at the Species’ Southern Limit in the Western Atlantic: Implications for Eelgrass Population Resilience

At the southern limit of Zostera marina in the Western Atlantic, sexual reproduction has provided a mechanism of recovery following thermal stress–related interannual losses. To understand the natural variability in seed quality in eelgrass meadows, flowering shoots were collected weekly to bi-weekly during the 2018 flowering season (March–June) from three sites in Topsail Sound, North Carolina (NC). Seeds were also collected during the period of maximum flowering (May) and examined for changes in seed composition (carbohydrates and proteins) and viability across seed sizes and between sites. The results of the present study show that while eelgrass phenology is following recognized temperature patterns, flowers matured 2–4 weeks earlier in the year than was previously documented. NC eelgrass populations also produced smaller seeds at greater densities than more northern perennial North American populations. In addition, viability did not differ across seed sizes suggesting that seed weight at the time of seed release may not be a reliable measure of seed quality. However, absolute carbohydrate and protein content did increase with seed size. More energy resources in larger compared with smaller seeds may result in better seed germination success. There was also site-based variation in seed length, width, area, and carbohydrate content despite similar temperature environments across the sites, indicating that local-scale factors may affect Z. marina seed size and composition. Information on variations in phenology, seed production, seed size, and seed composition across the species’ geographic range is necessary to better predict eelgrass response to biophysical disturbances and climate change.