Date Awarded

2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Kenneth A. Moore

Abstract

The effects of water quality and sediment composition on mid-Atlantic semi-annual and perennial Zostera marina reproductive success, seed-bank viability, and seed germination were elucidated using laboratory and in situ experiments, quantitative field observations, and ecological model simulations. The sediment seed-bank was found to play a large role in the recovery of perennial Z. marina beds in the Chesapeake Bay and in the yearly re-establishment of beds in North Carolina which were determined to have a semi-annual life history. However, the resiliency provided by sediment seed-bank for both semi-annual and perennial Z. marina beds was limited as seeds remained viable for less than one year. When comparing the two life forms, semi-annual Z. marina beds produced a greater proportion of flowering shoots and more seeds than nearby perennial beds. Seed germination was significantly affected by sediment type and burial depth with maximum germination of seeds occurring in sediments containing >3% organic content and buried at depths <3 cm. Model simulations indicate that York River Z. marina beds are currently at their maximum temperature threshold and that projected increases of >1??C in water temperature within the Chesapeake Bay may result in large scale declines. While the sediment seed-bank may provide a mechanism for recovery following one year of increased temperature stress, seed-banks are depleted following large scale germination events and may not provide resiliency to multiple consecutive years of stressful conditions. Further research into the interactive effects of sediment and water column conditions and seed physiology on seed viability are required to gain a more comprehensive understanding of seed-bank dynamics in Z. marina beds. Monitoring of semi-annual and perennial Z. marina beds in North Carolina indicated that multiple life history strategies may be found within one Z. marina bed. Shoots within the semiannual Z. marina bed germinated from seeds, a portion of seedlings flowered during their first year of growth, and all shoots completed their life cycle and died within one year of germination like a semi-annual plant; however, not all shoots flowered and shoots reproduced both sexually and asexually similar to a perennial plant. Since the individual plants found within the semiannual bed did not display the all of the defining characteristics of either perennial or annual Z. marina life histories, this population cannot be completely described by either life history strategy. Research into the development of a semi-annual life history strategy for Z. marina within this site and the possibility of this form occurring at other geographic regions requires additional research. Sexual reproduction is an important component of both semi-annual and perennial Z. marina populations that should be included in ecological studies and models. Although most perennial Z. marina beds rely on asexual reproduction as a primary form of bed maintenance, the ability to reproduce sexually is maintained and, as shown here, may play a large role in the recovery, maintenance, and expansion of these populations. For these reasons and due to the use of seeds in restoration of Z. marina beds within large systems such as the Chesapeake Bay, research into the dynamics of sexual reproduction within existing beds, a better understanding of seed physiology, and additional research into environmental effects (including the sediment) on seed germination and viability are essential.

DOI

https://dx.doi.org/doi:10.25773/v5-1fxp-s868

Rights

© The Author

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