The ecology and morphology of eelgrass (Zostera marina L.) and its associated macrophyte community in relation to environmental variables

Shallow coastal lagoons are subject to anthropogenic nutrient enrichment and disturbances that result in loss of seagrass ecosystems. These losses have generated a desire by coastal managers for tools to monitor existing seagrass meadows, and to restore those that have been lost. The following Chapters in this dissertation address aspects of these issues. Chapter 1 documents a nearly 16 month study of Zostera marina seasonal and year-to-year leaf and rhizome morphology and growth in Ninigret Pond, R.I. Individual shoots produced approximately 0.11 leaves per day from March to December. Shoot surface area and the length of the longest leaf on a shoot were significantly different between summers. The estimates of the plastochrone interval obtained using rhizome and leaf tagging were closely correlated making rhizome tagging an option for assessing shoot dynamics. Chapter 2 is a collation of data from two mesocosm experiments in which the concentration of nutrients in plant tissue are correlated to N and P loading rates. The ranges in tissue nutrient concentration are then used to construct a preliminary first order index of nutrient loading rates. This endeavor was moderately successful for N but not for P. Chapter 3 is an investigation, using mesocosms, of the single and interactive effects of nutrient loading and summer water temperature on phytoplankton, drift macroalgae, and eelgrass. Phytoplankton and macroalgae responded to nutrient enrichment in early and late summer, respectively. Significant declines in eelgrass can be expected under conditions of nutrient enrichment coupled with higher summer water temperatures. Chapter 4 is a study of the amount of light required for Z. marina seedlings that have the potential to be used as a restoration tool. Seedlings were subjected to three light levels over their first summer. Decreasing the light available to eelgrass seedlings decreased lateral shoot formation and biomass and increased the length and width of leaves. Reducing light to 10% of incident compromised survival, shoot growth, size and biomass.