UC Santa Barbara

Quantifying and determining the oceanographic and climate forcings of variability in phytoplankton community composition (PCC) represents a critical step in understanding and predicting marine ecosystem structure and function. In this dissertation, I analyzed samples and data from an extensive oceanographic time series from the highly productive Santa Barbara Channel, CA (SBC). The goals of this work were to (1) determine the “best-case scenario” for retrieving PCC from remotely sensible hyperspectral bio-optical observations; (2) characterize the dominant oceanographic and climate forcings of PCC variations in the SBC; and (3) integrate HPLC pigment and amplicon sequencing observations of PCC to assess the agreement between methods and the potential to gain novel insights into phytoplankton physiology and ecology and ecosystem function. In Chapter 2, we considered nearly a decade of concurrent HPLC phytoplankton pigment and spectrophotometric phytoplankton absorption coefficient observations. We implemented covariance-based analyses to identify 3-5 phytoplankton pigment communities from the HPLC pigment data, and spectral derivative analysis of phytoplankton absorption to identify phytoplankton absorption features. We then developed a bio-optical modeling approach to retrieve phytoplankton pigment concentrations and PCC indices from phytoplankton absorption derivative spectra with high skill. Notably, we found that absorption features from across the visible spectrum are useful in modeling PCC. In Chapter 3, we applied the bio-optical models from Chapter 2 to the PnB archive of phytoplankton absorption observations to double the length of the PnB HPLC phytoplankton biomarker pigment time series to 20+ years of observations. We then characterized the dominant oceanographic and climate forcings of five phytoplankton groups resolved from HPLC pigment data. Despite the widely documented dominant response of diatoms to seasonal upwelling, our analysis revealed that prymnesiophytes and chlorophytes are the typical “first responders” to seasonal upwelling in the SBC. On multi-decadal time scales, we identified an association between anomalous SBC dinoflagellate blooms and surface ocean advection patterns likely forced by a dominant mode of decadal climate variability, the North Pacific Gyre Oscillation. Finally, Chapter 4 focused on integrating HPLC pigment and amplicon sequencing observations to assess PCC in the coastal ocean. High uncertainty in estimates of biomass contributions of specific phytoplankton groups was often introduced by inter- and intra-group variability in biomarker pigment expression. However, we observed that distinct suites of biomarker pigments covary with unique communities of phytoplankton and other protistan groups. Integrating the two data sets thus provided novel insights into phytoplankton physiology and function, and suggested a path toward monitoring ecosystem structure and function on unprecedented spatiotemporal scales via ocean color remote sensing of phytoplankton biomarker pigment concentrations.