Date of Award

Spring 5-2-2024

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography

Advisor

Mark Wells

Second Committee Member

Lee Karp-Boss

Third Committee Member

Margaret Estapa

Additional Committee Members

Peter Countway

Charles Trick

Abstract

The concentration of CO2 in the atmosphere, caused by human activities, has led to a rise in global temperature and an increase in surface ocean temperature. Moreover, the amount of CO2 exchanged between the atmosphere and the ocean's surface alters the water's chemistry, decreasing pH. This phenomenon is known as ocean acidification (OA). The growth of marine phytoplankton is affected by environmental conditions, and the changes in ocean temperature and acidity are expected to impact their growth, ultimately affecting primary productivity in the ocean.

In my thesis, I explore how the growth of marine phytoplankton, including coastal diatoms, is affected by multiple stressors, namely increasing temperature and acidity. I conducted laboratory experiments on toxigenic and non-toxigenic bloom-forming diatoms and natural phytoplankton communities in the North Pacific Ocean. Both temperature increase and acidity can directly and indirectly affect the growth of phytoplankton. Warm temperatures over prolonged periods can affect cell metabolism and increase stratification, leading to nutrient limitation. Similarly, decreasing pH can directly affect cell metabolism and alter the chemistry of essential nutrients, including trace nutrient iron (Fe), making it less available.

In Chapter 2, I investigated the impact of prolonged nutrient limitation and temperature on the growth response of two types of diatoms - toxigenic Pseudo-nitzschia spp. and centric Skeletonema costatum. Specifically, I tested the growth response of P. australis and P. pungens under three different growth conditions: nutrient-replete, short-term nutrient limitation (NL), and prolonged NL, followed by nutrient additions to re-stimulate growth. In warmer conditions (20-25°C), the centric diatom S. costatum grew faster than Pseudo-nitzschia spp. under the initial replete and short NL conditions. However, after prolonged nutrient limitation, S. costatum stopped growing, while P. pungens and P. australis proliferated with only a short lag time.

In Chapter 3, I investigated the effect of lower pH on the growth of Fe-replete and Fe-deplete P. multiseries and S. costatum. Decreases in seawater pH due to ocean acidification have been shown to increase the conditional stability constants of some known Fe complexes, decreasing Fe bioavailability to marine phytoplankton. I hypothesized that the reduced iron availability from ocean acidification would minimally impact P. multiseries compared to S. costatum, due to Pseudo-nitzschia's high-affinity iron uptake system. The findings show that the growth of Fe-limited P. multiseries and S. costatum decreased at lower pH, and contrary to our hypothesis, the growth of P. multiseries was more negatively affected than S. costatum.

In collaboration with other laboratories, my field study in Chapter 4 investigated the effect of pH on Fe availability to natural phytoplankton communities across three different ocean zones in the Northeast Pacific Ocean. This research demonstrated that the effect of pH on Fe availability varies by region, likely increasing in the regions dominated by weak ligands (Californian Shelf) and decreasing in the High Nutrient Low Chlorophyll (HNLC) and North Pacific Subtropical Gyre (NPSG) regions where the strong ligands may be present in greater concentrations.

My dissertation research shows that it is crucial to study both the direct and indirect effects of ocean warming and acidification to predict the response of marine phytoplankton under future ocean scenarios. The findings emphasize how the indirect impacts of warming and reduced pH can significantly influence the growth and community composition of marine phytoplankton.

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