The response of photosynthetic absorption coefficients to irradiance in culture and in tidally mixed estuarine waters

The accuracy of models for primary production and light propagation depends on correct assignment of absorption to photosynthetic pigments. The phytoplankton absorption coefficient is comprised of two components: photosynthetic and photoprotective absorption coefficients. A method based on the fluorescent excitation of chlorophyll a is used to quantify the photosynthetic absorption coefficient for phytoplankton grown in culture and sampled from Puget Sound, Washington. The difference spectrum between total phytoplankton and photosynthetic absorption should be equivalent to photoprotective absorption. For cultures, the difference spectra exhibit peaks near 460 and 490 nm and broad-band absorption between 400 and 450 nm. However, for field samples an additional pronounced peak is observed around 440 nm, similar in shape to the chlorophyll a Soret peak. If the 440-nm peak were associated with photosystem I chlorophyll a, the photosynthetic absorption coefficient will be underestimated by ,15% for these samples. Variability in both coefficients is predictable as a function of irradiance. The photosynthetic coefficient varies inversely with growth irradiance, and the photoprotective coefficient varies directly with irradiance. This direct relationship with irradiance accounts for much of the variability in the spectral shape of the total phytoplankton absorption coefficient. The ratio of the photosynthetic absorption coefficient to the total phytoplankton absorption coefficient increases as a function of decreasing irradiance for cultures and for field samples collected from stratified regions of the water column. This ratio is a photoadaptive parameter that can serve to integrate physiological response to irradiance and has the potential to provide estimates of mixed layer dynamics. Modeling light propagation through the water column and determining a photon budget for the euphotic zone requires an estimate of the phytoplankton absorption coefficient. The phytoplankton absorption coefficient often is the most variable optical component of the water column. The composition and quantity of the phytoplankton assemblage affect both the spectrum and the magnitude of the absorption coefficient. Phytoplankton adapt the composition and concentration of photosynthetic and nonphotosynthetic pigments in response to variations in irradiance intensity and spectral composition (Yentsch and Yentsch 1979; SooHoo et al. 1986; Mitchell and Kiefer 1988; Johnsen and Sakshaug 1993). The accuracy of photon budgets and of bio-optical models for ocean productivity is improved by the incorporation of spectral information into the irradiance and absorption coefficient measurements (e.g., Bidigare et al. 1987, 1992; Smith et al. 1989). The accuracy also is improved by 1 Present address: Technology, Planning and Management Corporation, NOAA Coastal Services Center, 2234 South Hobson Road, Charleston, South Carolina 29405-2413.