Resolving phytoplankton photoprotective : photosynthetic carotenoid ratios on fine scales using in situ spectral absorption measurements

Abstract

Temporal changes in phytoplankton pigments and spectral absorption were evaluated during June 1998 in East Sound, Orcas Island, Washington. High‐resolution vertical profiles of in situ spectral absorption were obtained with a Wet Labs ac‐9 (nine‐wavelength absorption and beam attenuation meter), and pigment concentrations were determined for discrete water samples using high‐performance liquid chromatography (HPLC). Fucoxanthin was the most abundant carotenoid, indicating the dominance of diatoms. We computed a slope index to evaluate changes in shapes of the in situ particulate absorption coefficient (ap) spectra, ap slope = (ap488 − ap532)/(ap676(488 − 532 nm)). A clear linear relationship was seen between ratios of photoprotective : photosynthetic carotenoids (PPC : PSC) and these ap slopes. While pigment package effects may alter the absorption spectra, in our data set we still found a significant relationship between pigment ratios and in situ ap slopes. Retrieval of this relationship was facilitated by the low and relatively constant detrital absorption coefficient (ad) values in our study area. Similar relationships were found between PPC:PSC ratios and the estimated phytoplankton absorption coefficient (aph) spectra. High PPC:PSC ratios and steeper ap slopes were associated with high‐light levels. Our results suggest that in situ absorption measurements can be used to estimate PPC:PSC ratios in areas where the ad contribution is low or can be estimated. These variations in pigment ratios and spectral absorption reflect photoacclimation responses and/or changes in phytoplankton species composition and suggest in situ absorption measurements may be used to estimate pigmentation changes over fine temporal and spatial scales.