Date of Award

Fall 2008

Project Type


Program or Major

Natural Resources and Environmental Studies

Degree Name

Doctor of Philosophy

First Advisor

Frederick T Short


Light reduction due to anthropogenic impacts is the most widespread cause of worldwide decline of eelgrass, an ecologically important marine angiosperm whose role in supporting overall coastal ecosystem productivity has been widely recognized. Understanding eelgrass plant and meadow responses to light reduction has therefore received significant research interest over the last 30 years, while managers have sought tools to identify critical thresholds for light availability and predict impacts of human-induced disturbances in order to prevent further eelgrass loss. In the present thesis, a review of some of the literature on light reduction and its effects on eelgrass (i) summarized the importance of light as a requirement for eelgrass growth, (ii) highlighted factors that reduce light availability to eelgrass, (iii) summarized eelgrass responses to light reduction to understand its sensitivity to reduced water clarity, and (iv) documented worldwide losses of eelgrass caused by light reduction (Chapter II). An outdoor mesocosm experiment explained eelgrass response to a gradient of light conditions, improving the understanding of the relationship between light availability, growth and survival (Chapter III). The study concluded that at temperatures between 18°C and 23°C, eelgrass plants can thrive at light levels of 58% surface irradiance (SI) and above, and are light-limited at 34% SI and below, but that the minimum light required for long-term eelgrass growth and survival is greater than 11% SI. Finally, a field study at the maximum depth limit of eelgrass colonization was carried out to understand the mechanisms of eelgrass plant adaptation to extreme light reduction (Chapter IV). The results showed that deep edge eelgrass plants were chronically light-limited and that the plants exhibited a greater degree of morphological acclimations to further light reduction in winter than plants at shallower depths. Winter survival of deep edge plants depended largely on wintertime photosynthesis with mobilization of stored reserves playing only a minor role. The thesis concludes with a synthesis (Chapter V) linking all chapters.