Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Natural ecosystems are strongly affected by changes in resource supply (bottom-up forces) and by changes in upper trophic levels (top-down forces). The extent to which these processes impact a system depends largely on the responses of organisms at middle trophic levels. In seagrass beds, a group of mid-level consumers known as mesograzers form a critical link in the chain of impact, connecting seagrass and epiphytic algae with predatory fishes and crustaceans. I observed dramatic seasonal and interannual changes in mesograzer abundance and species composition in eelgrass (Zostera marina) beds of lower Chesapeake Bay, Virginia, and endeavored to explain the top-down and bottom-up causes and consequences of those changes with field studies and controlled experiments. A field cage experiment showed that grazing, predation and nutrient enrichment all had strong effects on the eelgrass community, but that the effects of each factor varied for different community components (Chapter 1). A second experiment delved deeper into the predation dynamic by manipulating the diversity of both predators and mesograzers in macroalgal mesocosms. Increasing predator diversity increased the strength of predation, but increasing mesograzer diversity conferred resistance to some types of predation (Chapter 2). to assess the influence of top-down and bottom-up forces in a more natural context, I analyzed the long-term changes in biotic and abiotic components of an eelgrass bed at the Goodwin Islands National Estuarine Research Reserve. I found that abiotic processes had strong effects on both consumer and resource abundance, and could therefore initiate either top-down or bottom-up control of eelgrass community structure (Chapter 3). to examine this top-down and bottom-up control in more detail I explicitly compared the ecological relationships seen in the field to those observed in mesocosm experiments. Mesocosm experiments tended to find a greater influence of top-down effects and a lesser influence of bottom-up effects, relative to field observations (Chapter 4). Finally, I took a snapshot of the eelgrass food web itself by examining the gut contents and stable carbon and nitrogen isotopic ratios of predators, mesograzers, and plants. I found that direct grazing on eelgrass does occur, but that microalgae and detritus provide the main trophic support for the epifaunal community (Chapter 5). Overall, my results suggest that both top-down and bottom-up forces control eelgrass community structure via mesograzers, but that top-down control in the field is more subtle and more intimately tied with bottom-up control than has been indicated by some manipulative experiments.



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