Master of Science (M.Sc.)
Virginia Institute of Marine Science
Sexual reproduction and the production of seeds are important for the resilience of all angiosperm species. For clonal species, such as the seagrass Zostera marina, resource allocation is complicated because these species reproduce both asexually and sexually, and the factors contributing to allocation to these two processes remains unknown. The goal of this study was, therefore, to investigate the importance of critical light, nutrient, and rhizome resources on Z.
marina sexual reproduction and flowering intensity. To evaluate the importance of sediment nutrients on Z. marina flowering intensity two distinct field manipulative experiments and one field survey were initiated: 1. Sediments within established Z. marina were fertilized during two periods of active growth (spring and fall) at two contrasting (estuarine and coastal lagoon) locations and the subsequent development of flowering was recorded, 2. Rhizome segments were transplanted to adjacent unvegetated environments to investigate the effects of sediment nutrients on individual shoots, 3. Surveys within and between three locations with apparent differences in sediment structure were designed to investigate the relationship between sediment conditions and flowering intensity. Rhizome segments were also transplanted to adjacent unvegetated environments to test the effect of shoot availability (with each segment having one, two, or three shoots) on sexual reproduction. To evaluate the importance of light and rhizome resources to flowering, two experiments and one field survey were also initiated: 1.In situ light availability was reduced using neutral density shading to test if light availability affected flowering intensity, 2. A field survey using fixed piers as shade structures was also used to determine if long-term shading influenced Z. marina flowering, 3. Cutting of rhizome connections of in situ plants was used to test if acute stress to below ground tissue prior to the development of flowering shoots would influence investment in sexual reproduction.
The addition of supplemental nutrients to the sediment during the fall growth period increased the number of spathes per flowering shoot the following spring relative to control plots at both estuarine and coastal lagoon locations. Similarly, field surveys across three locations demonstrated a direct relationship between ammonium availability and the percentage of flowering shoots. Although short term in situ shading did not significantly affect flowering intensity, measurements of flowering intensity around piers revealed lower percentages of flowering shoots directly beneath piers than areas one or three meters perpendicular to the pier. Eleven percent of transplants also produced more flowering shoots than the initial shoots planted indicating the development of flowering shoots can occur on shoots less than three months old.
Combined, these results indicate resource availability can influence sexual reproduction. Increasing belowground plant and nutrient resources increased investment in both vegetative and sexual reproduction, whereas, long term reductions in light resources were found to only decrease sexual reproduction. This suggests that the duration and magnitude of resource availability are both important in determining the allocation of resources towards sexual reproduction, and through this investment the degree of resiliency in seagrass populations in an increasingly stressed coastal environment.
© The Author
Johnson, Andrew J., "The Influence of Environmental Factors and Resource Availability on Zostera marina Flowering Intensity" (2015). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539617953.