| Summary: | The vertical structure in fluorescence and beam attenuation (at 660 nm) is related to local hydrographic features and the composition of photosynthetic pigments for the western North Atlantic in September. Phytoplankton and covarying material appear to be the major factors affecting the beam attenuation coefficient through changes in species composition and pigment concentration, and through photoadaptation. Detailed pigment analysis combined with measurements of in vivo phytoplankton absorption spectra showed a major regional difference in the specific-absorption spectra of phytoplankton which is directly linked to the structure of the phytoplankton community present in the water column. The results indicate a strong influence of other pigments co-existing with chlorophyll-$\alpha$ in algal cells on the variabilities of the specific-absorption coefficient of phytoplankton. To account for an effect due to pigment composition, absorption spectra of several phytoplankton species were decomposed, after correction for the "particle-size" effect, and the in vivo absorption properties of the major light-harvesting pigments were estimated. A Gaussian shape is suitable, theoretically and empirically, to represent the absorption spectra of individual photosynthetic components. The Gaussian parameters agreed well with the expected pigment compositions of 3 groups of algae, and the peak heights were linearly correlated with the concentrations of the 4 major pigments measured in the samples. The linear relationship did not vary with phytoplankton species. The results give estimates of the in vivo specific-absorption coefficients of photosynthetic pigments which, then, are used to reconstruct the in vivo absorption spectrum of a multi-species samples. Another application of the previous results is to compute photosynthetic-pigment concentrations in seawater from the knowledge of the absorption coefficient of phytoplankton. The contributions due to detrital particles and phytoplankton to total light absorption are retrieved by non-linear regression on the absorption spectra of total particles from various oceanic regions. The model used explains more than 96% of the variance in the observed particle absorption spectra. The resulting absorption spectra of phytoplankton are then decomposed into Gaussian bands, following a similar procedure as the one previously described. Such a decomposition, combined with HPLC data of phytoplankton pigment concentrations, allows the computation of specific-absorption coefficients for chlorophylls-a,-b,-c, and carotenoids. It is shown that these coefficients can be used to reconstruct the absorption spectra of phytoplankton at various locations and depths. Discrepancies that do occur at some stations are explained in terms of particle-size effect. Lastly, these coefficients can also be used to determine the concentrations of phytoplankton pigments in the water, knowing just the absorption spectrum of light by total particulate matter. Thesis (Ph.D.)--Dalhousie University (Canada), 1993.
|
|---|