Beds of seagrass and other submersed angiosperms have been shown to reduce water velocities and water-column dissolved nutrient and seston concentrations. In eutrophic waters, these effects could reduce algal biomass, enhancing light availability to the surface of the leaves and therefore, increasing seagrass growth. Small seagrass beds (1-10m diameter) should have little influence on water flow and water quality, but there is little research on the effect of bed size on these factors. To investigate the effects of seagrass bed size on these interactions, I developed a numerical ecosystem simulation model and used a spatial simulation model. I also conducted mesocosm and field measurements to determine if the expected relationships were evident in reality. I measured water quality, sediment characteristics, epiphyton mass, and hydrodynamic characteristics across beds of the seagrass Ruppia maritima L. in mesohaline Chesapeake Bay. I also measured net community nutrient uptake in mesocosms. Field measurements of water transport and nutrients were used to calibrate a spatial model of water and nutrient flow through Ruppia beds. This model was used to determine the potential effects of water flow velocity and bed size on nutrient gradients. An ecosystem simulation model was constructed and used to investigate the effects of nutrient supply rates and grazer densities on epiphytic algae and macrophyte growth. Simulation model results showed the controlling effect of nutrient loading rate on epiphytic algal and Potamogeton perfoliatus L. biomass. Potamogeton growth rate was highest at low nutrient loading rates, which allowed the angiosperms to reduce nutrients to levels that reduced algal growth. Grazer effects were greatest at intermediate loading rates. Spatial modeling simulations showed the potential influence of bed size and current velocity on water quality changes in shallow water. In the field, ammonium and dissolved inorganic carbon (DIC) decreased, and dissolved oxygen increased with distance into large (> 300 m diameter) beds of one meter tall, moderate density Ruppia. Water quality was little changed in beds smaller than 100 m wide. Epiphyton mass was generally variable, but decreased with distance into beds under low dissolved nutrient conditions in the fall of 2001. Epiphyton dry weight was related to total suspended solids. Large, dense, seagrass beds in shallow water, may have a gradient of trophic conditions from outside to inside, while the surrounding water should dictate conditions in small beds.