Oral Session 3

Hyperspectral signatures of phytoplankton taxonomy and photo-physiology in a coastal upwelling regime

Coastal upwelling regions contain highly productive surface waters, which exhibit strong variability over small spatial scales. The dynamic nature of these systems requires high-spatial resolution measurements to resolve processes influencing phytoplankton ecology. Previous studies have demonstrated the utility of hyperspectral optical data to characterize phytoplankton taxonomic assemblages in coastal waters, but less work has explored linkages between phytoplankton spectral signatures and their photo-physiology and productivity. During the summer of 2023, we used in situ hyperspectral optical data to investigate changes in the phytoplankton community composition, physiology and productivity across upwelling gradients in the northern California Current System. Whereas phytoplankton pigment measurements indicated that the phytoplankton community was largely dominated by diatoms across our entire study region, measured absorption and attenuation spectra were able to discern subtle changes in the relative contributions of other phytoplankton taxa and particle sizes. We were able to identify two optically-distinct groups that showed differing photo-physiological properties, including photosynthetic efficiency (Fv/Fm) and absorption cross-section, chlorophyll fluorescence to absorption ratio, and maximum photosynthetic rates. The optical grouping explained more of the observed variability in the photo-physiological properties than other oceanographic variables such as sea surface temperature and chlorophyll concentration. Changes in hyperspectral signatures appeared to reflect changes in the age of the upwelling water masses, and a potential signature of iron and silicic acid co-limitation associated with varying shelf width across our study area. Our results demonstrate the utility of high-resolution hyperspectral data to describe phytoplankton taxonomic and photo-physiological variability in highly dynamic upwelling systems.

*Sacchidanandan Pillai, University of British Columbia, Canada, https://orcid.org/0000-0003-4858-9435

Yayla Sezginer, University of British Columbia, Canada

Philippe Tortell, University of British Columbia, Canada

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