Effect of Phosphorus Amendments on Present Day Plankton Communities in Pelagic Lake Erie

To address questions regarding the potential impact of elevated total phosphorus (TP) inputs (due to relaxed regulations of TP loading), a series of TP enrichment experiments were conducted at pelagic stations in the 3 hydrologically distinct basins of Lake Erie. Results of nutrient assimilation measurements and assays for nutrient bioavailability suggest that the chemical speciation, and not concentration, of nitrogenous compounds may influence phytoplankton community structure; this in turn may lead to the selective proliferation of cyanobacteria in the eastern basin of the lake. Assays with cyanobacterial bioluminescent reporter systems for P and N availability as well as N-tot:P-tot assimilation ratios from on-deck incubation experiments support this work. Considered in the context of a microbial food web relative to a grazing food web, the results imply that alterations in current TP loading controls may lead to alterations in the phytoplankton community structure in the different basins of the Lake Erie system

Investigating the Effects of Organic Ligands on Iron and Copper Availability to Coastal and Oceanic Phytoplankton Using Continuous Cultures

The chemical speciation of Fe and Cu in marine waters impart a strong selective pressure on phytoplankton assemblages by influencing the availability and partitioning of these essential trace nutrients among taxa. Consequently, trace metal speciation establishes interspecific competitive interactions that largely determine the phytoplankton community structure in natural marine waters. The aim of this dissertation was to broaden the understanding of Fe and Cu availability to different phytoplankton by examining the effects of organic complexation on natural community assemblages from coastal and oceanic waters using continuous cultures. Continuous cultures provide an effective platform for investigating subtle responses in natural populations to consistent nutrient flux and speciation, as the community trajectory is amplified by dilution losses of slow-growing and noncompetitive organisms. The successful design and implementation of a novel continuous culture system for shipboard experiments with natural mixed communities is detailed here. Domoic acid (DA), a potent phycotoxin produced by the diatom genus Pseudo-nitzschia, is an Fe and Cu chelator. Dissolved DA (dDA) additions to natural phytoplankton communities from coastal and offshore regions enhanced diatom growth more than Fe or Cu additions alone, and principally selected for Pseudo-nitzschia over all other phytoplankton. These findings are consistent with evidence that Pseudo-nitzschia release dDA to enhance the Cu acquisition needed to initiate an inducible high-affinity Fe uptake mechanism. These experiments provide the first direct evidence that the production of dDA by Pseudo-nitzschia may facilitate a competitive advantage, and explain its success in coastal and offshore assemblages. Three commercially available strong Fe(III)-binding siderophores isolated from terrestrial organisms (desferrioxamine B, ferrichrome and rhodotorulic acid) were used as analogs to marine siderophores to determine Fe availability to different coastal phytoplankton taxa. While all have high conditional constants for Fe(III) in seawater, the complexation characteristics greatly influenced Fe availability, reflected in their effects on chlorophyll biomass, Fe uptake and photophysiology. Although the specific compounds comprising strong Fe and Cu complexing organic ligands in seawater are unknown, these findings strongly suggest their specific chemical nature is likely more important than their metal affinities in determining Fe availability to phytoplankton