Date of Award

Spring 5-2021

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Ocean Science and Engineering

Committee Chair

William Monty Graham

Committee Chair School

Ocean Science and Engineering

Committee Member 2

Donald Redalje

Committee Member 2 School

Ocean Science and Engineering

Committee Member 3

Jerry Wiggert

Committee Member 3 School

Ocean Science and Engineering

Committee Member 4

Stephan Howden

Committee Member 4 School

Ocean Science and Engineering

Committee Member 5

Jeffrey Krause


The Mississippi Bight (MSB) is a river-dominated continental margin influenced by multiple large river systems, including the Mississippi River, Alabama and Tombigbee rivers via Mobile Bay, and numerous smaller rivers, creeks, and bayous. This is part of a biologically-rich ecosystem that supports the second largest fishery industry by volume in the United States. Despite our understanding of the linkages between primary production with higher trophic levels, there remains limited studies quantifying these trophic interactions in this system. Microplankton (µm) community dynamics and trophic connectivity between primary producers and heterotrophic protists represent a critical nexus influencing overall biological productivity in this region. These processes were examined using a combination of a novel morphological-based (MBFG) classification system derived from in-flow plankton imaging (i.e. FlowCAM), a suite of in situ biogeochemical and biological measurements, and multivariate statistics to describe patterns of microplankton community composition, biomass, primary productivity, and microzooplankton grazing relative to prevailing physicochemical conditions. Results indicated that the MSB is a highly productive ecosystem, oscillating from a mesotrophic state under reduced river discharge and nutrients to eutrophic conditions during high discharge and elevated nutrient concentrations. Microplankton communities shifted under differing environmental conditions, though diatoms and nanoplankton were predominant throughout the study. Both size fractions (0.6-5 µm and > 5 µm) contributed nearly equivalent proportions to biomass and productivity during low discharge, whereas >5 µm size fraction contributed more biomass and productivity during the spring freshet. Microzooplankton grazing (i.e. ciliates) exerted a significant top-down control on phytoplankton biomass (30-60%) and productivity (> 60%), despite enhanced phytoplankton growth in spring. The effects of environmental variability on biological productivity and ecological resilience in the MSB is best understood within the context of microplankton community dynamics among primary producers and their trophic intermediaries. Results from this research highlight these key ecological relationships that are fundamental to ecosystem function, productivity, and resilience in this system.