Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Deborah K. Steinberg


Large gelatinous zooplankton (GZ) blooms of lobate ctenophores, Mnemiopsis leidyi, and scyphomedusae, Chrysaora quinquecirrha , occur throughout Chesapeake Bay and its tributaries. The mechanisms of GZ bloom formation, and the roles GZ blooms play in dissolved organic matter (DOM) and carbon (C) cycling are not fully understood. During 2003--2006, I conducted laboratory experiments and field surveys in the lower York River to determine factors controlling timing and magnitude of GZ blooms, and to evaluate their effects on C cycling. Highest biomass of M. leidyi occurred in early summer (May-June) and in late winter. Peaks in ctenophore biomass in the mesohaline region occurred one-month earlier than in downriver, polyhaline regions, due to higher ctenophore reproduction and larval dispersal upriver. High predation by C. quinquecirrha scyphomedusae on M. leidyi appears to cause the rapid decline in summer ctenophore blooms, and we hypothesize that subsequently medusae become C-limited. High GZ biomass coincides with peaks in microbial biomass, and as DOM is released by zooplankton but consumed by bacteria, these disparate trophic levels may be linked. I measured DOM production by GZ and the response of free-living bacterioplankton to GZ DOM, quantified in terms of bacterial metabolism, and bacteria phylogenetic community composition. Release rate of DOC by both GZ species was high relative to simultaneous release of DON and DOP, and for M. leidyi DOM metabolites were C-rich due to high mucus production in ctenophores. Furthermore, bacterioplankton abundance and production rapidly increased (within 6 hours) in response to uptake of GZ metabolites; however, decreases in bacterial growth efficiencies indicated that increases in bacterial C respiration were greater relative to changes in bacterial biomass. Enumeration of microbial assemblages using the fluorescence in situ hybridization (FISH) technique showed specific bacterial groups, namely gamma-proteobacteria, are responsible for increased metabolism of GZ DOM metabolites. In the context of worldwide increases in GZ, my results have significant implications for C transfer in marine food webs, with the potential for more C to be shunted to the microbial loop away from higher trophic levels.



© The Author