Spatial variations in phytoplankton pigment ratios, optical properties, and environmental gradients in Oregon coast surface waters

In situ optics and hydrographic measurements, along with discrete samples for calibration, were used to assess spatial variations in phytoplankton characteristics and taxonomic composition (pigment ratios, relative particle size distribution, and chlorophyll a (chl a ) concentration) in Oregon coast surface waters during August 2001. The relationships between environmental parameters (nutrients, light, temperature), photoprotective:photosynthetic carotenoid (PPC:PSC) ratios, and in situ spectral absorption and beam attenuation from ac‐9 measurements were also evaluated. We computed a “slope” index to evaluate changes in shapes of the phytoplankton absorption ( a ph ) spectra: a ph slope = ( a ph 488 − a ph 532)/[ a ph 676 · (488–532 nm)]. Significant linear relationships were seen between PPC:PSC ratios and the a ph slope parameter for surface samples grouped by location, date, and water mass characteristics. We observed high spatial variability in the phytoplankton photophysiological and taxonomic indicators derived from our optical measurements. Lower PPC:PSC ratios and larger particle size distributions were seen near the coast in colder upwelled water compared to locations farther offshore with warmer, more nutrient deplete water. Temperature and photosynthetic available radiation explained 42% of the variability in PPC:PSC ratios for all surface samples and 61% for surface samples with <2 μM dissolved inorganic nitrogen. Temperature alone explained 87% of the variability in PPC:PSC ratios (estimated from ac‐9 absorption data) and 72% of the variability in relative particle size distribution (estimated from ac‐9 beam attenuation data) for Heceta Bank samples collected over 4 days, inshore and offshore of an upwelling front.