The toxic mixotrophic dinoflagellate Karenia spp. forms blooms almost annually in the Gulf of Mexico, especially on the West Florida Shelf (WFS). Blooms typically initiate in early fall but can persist from months to years. Daily, Karenia vertically migrates to the surface water during the day, possibly experiencing changes in temperature, light, nitrogen (N), and prey type and availability. Therefore, this dissertation aimed to examine the interplay between Karenia’s photo-autotrophic and phago-mixotrophic metabolism and the short-term fluctuations in environmental conditions to understand how these factors may relate to the conditions under which Karenia spp. are found in the WFS.Title of Dissertation: INTERACTIONS BETWEEN NITROGEN AND TEMPERATURE ON THE METABOLISM OF THE RED-TIDE MIXOTROPHIC DINOFLAGELLATE KARENIA SPP. IN SUPPORT OF PREDICTIVE MODELS: IMPLICATIONS FOR BLOOM DYNAMICS ON THE WEST FLORIDA SHELF So Hyun (Sophia) Ahn, Doctor of Philosophy, 2023 Dissertation directed by: Professor Patricia M. Glibert, Marine Estuarine Environment Sciences A culture of K. mikimotoi balanced photon flux pressure (light availability) with consumption in overall metabolism when pulsed with 15N-NO3-, 15N-NH4+, or 15N-urea over the range of 15-25°C as shown by photosynthetic fluorescence. However, when shifted to 30°C, cells were significantly stressed, but urea-enriched cells showed a smaller decline in fluorescence, implying that urea might induce a photoprotective mechanism by increasing metabolic “pull.” Studies conducted with natural K. brevis winter and summer populations during 2021 showed that thermal history played a critical role. Unusually, summer blooms had higher biomass but were stressed photosynthetically and nutritionally. However, 15N-urea enriched summer cells had higher uptake rates as well as carbon (C) and N cell-1, especially in warmer waters, showing differential thermal responses based on N forms. Mixotrophy grazing measurements showed that K. brevis grazed both the picoplankter Synechococcus as well as the cryptophyte Rhodomonas. Grazing did not selectively target specific qualities of Synechococcus (based on differing N and P of the prey growth media), but ingestion rates were a function of prey-to-grazer ratios (R2=0.76) as well as prey amounts (R2=0.71). NanoSIMS confirmed 15N incorporation from Synechococcus in K. brevis. In natural communities of K. brevis, ingestion rates were also significantly related to prey-to-grazer ratios (p < 0.01) and by temperatures (p < 0.05) to a lesser degree (R2= 0.75) when incubated at ambient (24°C) and ambient temperature ± 5°C (19, 29°C). The grazer effects on the photosynthetic performance of grazer and prey were also examined. Grazing on Synechococcus indirectly reduce the photosynthetic performance of prey, especially at warmer temperatures but had little or no effect on the photosynthesis of K. brevis itself.