Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


The relative contributions to organic matter production and the interactions between submerged vascular plants and their associated microalgae assemblages were investigated in seagrass communities characteristic of the lower Chesapeake Bay. The studies were conducted in three parts; the first compared production and respiration of the major autotrophic components in adjacent seagrass communities dominated by Zostera marina and Ruppia maritima, respectively. Annual production for the two communities differed; in the Z. marina area microalgal (i.e. phytoplankton and benthic microalgae) production dominated during the summer months, whereas in the R. maritima area, the macrophyte-epiphyte complex dominated throughout the growing season. Both areas exhibited high annual gross production rates (1580 gC m('-2) in the Z. marina area and 1000 gC m('-2) in the R. maritima area) of which the microalgae accounted for 45% and 36% in the two communities respectively. The ratio of net production to dark respiration (P/R) exceeded 1.0 for each of the components, suggesting export and/or burial of carbon from the system. The second series of studies investigated specific interactions between Z. marina and its epiphytic microalgae. Two sites were examined, where previous observations had been made of differing epiphytic colonization patterns. The two seagrass ecosystems differed markedly in epiphytic abundance, community structure, and productivity and respiration of the epiphytic complex. Based on gross morphological characteristics of the seagrass host, differences in nutrient conditions could exist at the two sites, where the hypothetically enriched site coincided with a flourishing epiphytic community. Effects of nutrient enrichment and light reduction on epiphytic growth were examined directly in the third phase of this study using controlled microcosm experiments. Both nutrient enrichment and light reduction led to enhanced epiphytic productivity and biomass, as well as increased light attenuation associated with epiphytic growth. Direct reduction in ambient light also stimulated epiphytic production relative to that of the seagrass host. Reduced abundance of plant leaves in the nutrient enriched systems perhaps indicated some signs of stress to Z. marina. This study suggests that nutrient enrichment and light reduction in the water column could increase epiphytic growth and production, possibly at the expense of the macrophyte.



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