Spatial and temporal variation of bacterial community structure and ecosystem properties in two temperate estuaries

Bacterial processes are essential to the functioning of ecosystems. Little is known, however, about the ways that bacterial communities are structured in space and time or how variation in structure influences the kinds of processes or process rates occurring in the environment at the scale of ecosystems. Knowledge about these relationships is essential to understanding the fundamental properties of natural systems and predicting system behavior. The goals of this dissertation were to determine if the spatial and temporal distribution of free-living estuarine bacterial communities were correlated with spatial and temporal distributions of environmental conditions. And to determine if the observed patterns and correlations were alike for spatial gradients or time in two estuarine types (a classic coastal plain estuary and a lagoon). The estuarine systems studied were geographically close to one another so that climate and land use were alike. One system was on the Atlantic coast-side and the other on the Chesapeake Bay-side of Virginia's Eastern Shore.; Reconnaissance sampling based on the community-level physiological profile method served as the framework for developing a sampling scheme that would capture major spatial and temporal variability within these systems. Cross hybridization of whole-community bacterial DNA extracts was used to assess community compositional similarity and relative diversity in the systems spatially, along the estuarine gradient from freshwater to seawater, and temporally, at mid-estuary stations. Spatial variation of community structure was primarily correlated with the freshwater to seawater gradient. Temporal variation was predominantly seasonal over a two-year period in the mid-estuary. Discriminant analysis and Mantel correlations were used to link the communities' genetic similarity to environmental conditions. Both analyses showed a positive relationship between composition and environmental conditions. The observed spatial and temporal patterns of community composition were correlated with spatial distributions of salinity and phosphate, and temporal distributions of water temperature and nitrate, respectively. This work is significant because there were common (spatial and temporal), repeatable (temporal) community patterns, suggesting that community assembly in these systems is not a stochastic process at the scales of these observations.