PRODUCTIVITY AND LIGHT-DEPENDENT RESPIRATION OF PHYTOPLANKTON, INCLUDING THE DINOFLAGELLATE Karenia brevis: ENVIRONMENTAL EFFECT INTERACTIONS

Abstract

Climate change is altering coastal ecosystems through increasing temperatures, intensifying storm activity, and enhancing terrestrial inputs of nutrients and dissolved organic matter. These environmental changes are expected to influence the physiology and bloom dynamics of harmful algal species such as the dinoflagellate Karenia brevis, which forms nearly annual blooms on the West Florida Shelf (WFS). This dissertation investigated how interacting stressors influence phytoplankton metabolism, with particular emphasis on the primary production and respiration. First, long-term data from 1998 to 2020 were analyzed and the correlations between K. brevis, Peace River discharge, and the ENSO index suggested that freshwater inflow and large-scale climate oscillations play both direct and indirect roles in regulating bloom dynamics on the WFS. To better quantify primary production and respiration, methodological improvements were developed using the H218O technique using Membrane Inlet Mass Spectrometry (MIMS). This approach enables the simultaneous quantification of gross primary production (GPP), net primary production (NPP), and gross respiration (GR; light-dependent respiration) in small-volume incubations, providing a rapid and cost-effective method. Using this approach, opportunistic post-hurricane field experiments and laboratory manipulations examined the combined effects of elevated temperature and humic acid enrichment on the metabolic rates of K. brevis and associated phytoplankton communities. Results showed that warming consistently reduced primary production while enhancing both dark and light-dependent respiration, often shifting the system toward heterotrophic conditions. Complementary laboratory experiments using Pulse-Amplitude Modulated (PAM) fluorometry demonstrated that photosynthetic performance of K. brevis responds to interactions between temperature and nutrient form. While elevated temperatures can affect photosynthetic efficiency, nutrient form and availability can modulate these responses. Together, these findings demonstrate that warming, nutrient inputs, and storm-driven terrestrial runoff interact to regulate K. brevis metabolism and bloom dynamics. Incorporating physiological rates, terrestrial inputs, and climate-scale variability into predictive models will be essential for improving forecasts of harmful algal blooms and ecosystem metabolic balance in a warming coastal ocean.