Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Robert J Orth
Gary A Kendrick
Mark J Brush
Matthew L Kirwan
Sexual reproduction provides submerged aquatic vegetation (SAV) populations unique opportunities for dispersal, genetic mixing, and resilience in the event of catastrophic population declines. Relative to asexual reproduction, sexual reproduction is a risky resource investment and can have a lower probability of success. A wide variety of abiotic and biotic interactions common in both terrestrial and aquatic environments can lead to significant mortality of seeds and seedlings. The goal of this dissertation is to explore the diversity of biological interactions that influence seed and seedling survival in SAV that drive the population dynamics and restoration success of SAV species. A combination of survey and experimental methods were used to test if three biological interactions, disturbance, herbivory, and competition, compromised seedling recruitment for three different SAV species growing in three different coastal environments. Chapter One explored the influence of sediment bioturbators on seedling establishment for the seagrass Posidonia australis in a marine environment. Field surveys demonstrated that dispersed seeds of P. australis overlap with a suite of sediment bioturbators that disturb the sediment in the coastal lagoons of Western Australia. The movement of sediment bioturbators found in these areas (sand dollars, sea stars, and heart urchins) dislodged and moved recently settled P. australis seeds. The overlap in habitat suitability between these animals and P. australis seeds suggests high densities of these animals consistently disturb non-dormant P. australis seeds and may act as a bottleneck to seedling recruitment. Chapter Two explored the role of grazers on seedling recruitment of a freshwater angiosperm. Field surveys recorded high grazing levels of isolated Vallisneria americana propagules in oligohaline areas of the James and Chickahominy Rivers, VA. Camera surveys identified the blue crab, Callinectes sapidus, as a likely herbivore. Subsequent surveys and experiments suggested C. sapidus in this system consumes SAV as part of their diet. The emergence of non-native SAV in the system, but not V. americana, suggests grazing prevents the recruitment of some SAV, but not others. Chapter Three evaluated interactions between adult plants and seedlings and how they impacted seedling establishment of an estuarine seagrass species. Field surveys consistently recorded seedlings establishing among existing Zostera marina shoots in a meso/polyhaline region of Chesapeake Bay, VA. Concurrent experiments indicated that seed supply influenced seedling establishment rates in some areas. Further surveys and experiments showed that negative interactions between seedlings and adult shoots influenced the subsequent survival of these seedlings within existing Z. marina meadows. Results from the research studies in this dissertation, conducted across three diverse coastal habitats, demonstrate that interactions between SAV seeds or seedlings and other biota can be very important in ultimately determining seed or seedling survival. The diverse mechanisms through which biota compromise seedling recruitment and sexual reproduction for SAV observed here suggest there may be many additional, unexplored biological interactions affecting successful sexual recruitment for many SAV species. Because sexual reproduction provides substantial benefits to SAV populations, incorporating risks associated with seedling recruitment into population models and restoration strategies may help better predict SAV population health, resiliency and expansion as well as help optimize SAV restoration efforts.
© The Author
Johnson, Andrew James, "Unexplored Aspects of the Biotic Filter to Seedling Recruitment in Aquatic Environments" (2019). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1563898631.