Shepard, Alicia Kail (2015-08). Responses of Heterotrophic and Autotrophic Pico- and Nano-Plankton to Nutrient Availability and Enrichment across Marine Systems in the Northern Gulf of Mexico. Doctoral Dissertation.
Thesis
It is predicted that ubiquitous marine microbial communities adapt to shifts in their immediate environment that are reflected in changing community abundance, structure, production, interactions and functions. This study describes spatiotemporal microbial dynamics in two unique marine settings with naturally occurring variations in surface water inorganic nutrient concentration, including an estuarine and an open ocean system. Mesocosm experiments were conducted using combinations of inorganic nutrients expected to influence microbial communities in order to support in situ interpretations. A statistical examination of flow cytometric derived microbial groupings, based on physiological rather than taxonomic characteristics revealed important relationships between inorganic nutrients and marine microbial communities. Correlations specifically indicated the importance of temperature, salinity and inorganic nutrients to changes in microbial physiological community structure. Heterotrophic microbes in the Trinity River Basin of Galveston Bay appear to undergo episodic nitrogen limitation that occurs when high temperature stimulates increased cellular metabolic activity and carbon is saturated beyond heterotrophic requirements. A step-wise spatiotemporal co-limitation of autotrophic and heterotrophic fractions of microbial plankton in Galveston Bay exists such that temperature ultimately limits abundance, followed by inorganic phosphorous at a station where nutrient pulses stimulated by freshwater inflows are infrequent. If inorganic phosphorous is available, as occurs in the Trinity River Basin then dissolved inorganic nitrogen becomes the limiting factor. Finally, nutrient limitation processes influence microbial plankton abundance and physiological community structure similarly in the northern Gulf of Mexico, where nutrients are made available by mesoscale circulation and coastal entrainments. Continued exploration into the complex environmental connections to marine microbial ecology is required to better understand and predict microbial impacts on biogeochemical cycles.
