Effects of nutrient stoichiometry and environmental conditions on the growth, toxin concentration, and nitrogen uptake of cyanobacteria.

Anthropogenic influences are negatively affecting water bodies by both increasing the concentrations of nutrients as well as contributing to increased temperatures from climate change. Nitrogen (N) and phosphorus (P) are the main nutrient contributors to eutrophication, and as such have become nutrients of high interest with many studies devoted to further understanding their influence. One issue seen with eutrophication is the proliferation of harmful algal blooms (HABs). One of the main phytoplankton groups responsible for HAB events in inland waters are cyanobacteria, of which some members are capable of both N fixation as well as cyanotoxin production. A method to better understand the effects N and P have on N fixation, cyanotoxin production and cyanobacteria prevalence is that of ecological stoichiometry. Ecological stoichiometry utilizes ratios to better understand how elements move through the abiotic and biotic environment, based on organisms' optimum N:P demands. In this dissertation, nutrient stoichiometry is used to determine how the diazotroph Dolichospermum’s cellular N:P changes through time when grown under various N:P levels, as well how N acclimation affects N fixation. We then examine the effects both N:P and salinity have on particulate and dissolved cyanotoxins in a non-diazotroph (Microcystis) and diazotroph (Aphanizomenon). Both genera were grown in N:P 4 and 50 (by mol) media, and when found to reach N limitation in the N:P 4 they were exposed to salinities varying from 0-10 g L.1. Lastly, Lake Waco, Waco, TX USA was sampled spatiotemporally to investigate if phytoplankton community composition and nutrient limitation differed between sites within the lake, as well as how environmental variables influenced different phytoplankton groups. Together, these chapters utilize N:P stoichiometry and other environmental variable to better understand how cyanobacteria respond to environmental changes.