Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Richard L. Wetzel

Committee Member

Robert J. Orth


Dissolved nutrient concentrations and invertebrate grazing activity regulate epiphytic biomass. Because epiphyton may limit light and carbon at leaf surfaces and the consequent productivity of submerged macrophytes, factors which influence epiphytic biomass may indirectly affect macrophyte abundance. I measured the simultaneous effects of water column nutrients (ambient or 3x ambient concentrations of nitrogen and phosphorus) and grazing (presence or absence of epifaunal community) on epiphyton and macrophytes seasonally in eelgrass (Zostera marina L.) microcosms on lower Chesapeake Bay. Grazing was more important than nutrients in controlling accrual of total epiphytic biomass, although effects on epiphytic components varied; numbers of diatoms responded to grazing, whereas numbers of cyanobacteria responded to nutrients. Numbers of heterotrophic microflagellates mimicked those of bacteria. The indirect effects of nutrients and grazing on macrophytes depended upon the relative magnitude of each factor and the physiological demands of the macrophyte. Under low grazer densities of early summer, macrophyte production (g m&\sp{lcub}-2{rcub}& d&\sp{lcub}-1{rcub}&) was reduced with grazer removal and nutrient enrichment independently. In contrast, under high densities of late summer, production was reduced by enrichment with grazers absent only. There were no macrophyte responses to treatment during the spring and fall, regardless of differences in epiphytic biomass; this may have been related to comparatively low light requirements of eelgrass at low temperatures. I used a simulation model to extrapolate microcosm results to predictions for community persistence. The model included ranges of environmental variables specific to lower Chesapeake Bay, where declines in eelgrass abundance in recent decades were correlated with nutrient enrichment, reduced grazer populations, and increased turbidity. Simulations indicated that neither nutrient enrichment nor loss of grazers alone would limit eelgrass survival, but together would cause community instability. Simulations indicated further that with grazers present, nutrient enrichment with a slight decrease in submarine irradiance would cause macrophyte loss. Measured rates of epiphytic accrual on artificial substrata in situ suggested that with grazers present, light reduction actually reduced the absolute rates of biomass accumulation despite nutrient enrichment. Predictions for macrophyte community stability must thus consider the relative effects of both direct (acting on macrophytes) and indirect (acting via epiphyton) environmental controls.



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