Electronic Theses and Dissertations

Date of Award


Document Type


Degree Name

Ph.D. in Biological Science

First Advisor

Clifford A. Ochs

Second Advisor

James Cizdziel


University of Mississippi

Relational Format



Bacterioplankton are important structural and functional components of river networks yet their biogeographical patterns in these systems are largely unknown. Using Illumina sequencing of the 16S rRNA gene we characterized variation in bacterioplankton community alpha diversity (within-sample richness of operational taxonomic units OTUs) and beta diversity (between-sample differences in composition) (1) along a 1 300-km downstream reach of the Mississippi River (2) over a range of temporal scales in the Lower Mississippi River (LMR) and (3) along cross-sectional transects repeated monthly for six months across the LMR floodplain. Particle-associated assemblages were richer in bacterial OTUs and more productive than free-living assemblages indicating the importance of suspended particles as microhabitats for bacterial diversity and activity. Particle-associated richness steadily increased downstream along the river suggesting greater niche availability and/or colonization of particles with downriver flow. Communities tended to shift abruptly in composition after converging with major tributaries of the network (i.e. the Illinois Missouri and Ohio rivers) however there was an additional but transient shift at Memphis Tennessee. Conversely composition changed more gradually within stretches between these points. In the LMR there was high variation in alpha diversity over daily and weekly timescales. In contrast there was successional change in beta diversity that corresponded to temporal trends in water temperature dissolved nitrogen and chlorophyll a. Particle-associated productivity aligned almost exclusively with temperature. Results indicate that community richness composition and productivity varied at discrete timescales in the LMR in response to different sets of environmental drivers. Across the LMR floodplain variation in communities corresponded to the degree and timing of hydrologic connectivity to the main river channel. During high water floodplain community composition was similar to main channel assemblages. Following hydrologic disconnection floodplain communities were dominated by cyanobacteria and distinct from the river. After prolonged disconnection (4-5 months) there was convergence in composition of floodplain and river communities which were dominated by heterotrophic taxa. Results suggest that succession of floodplain communities was initiated by river inputs at high river stages. In summary bacterioplankton community composition and productivity of the Mississippi River Basin varied over space and time which corresponded strongly to environmental variability.



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