| dc.description | The effects of different communities of phytoplankton on optical properties of surface waters in the coastal ocean were examined. Data from large data sets were used to develop semi-analytical expressions describing quantitatively how idealized phytoplankton assemblages could affect apparent optical properties (AOPs), such as diffuse attenuation and surface reflectance. Spectral absorption by idealized phytoplankton assemblages was simulated by varying the parameters governing pigment packaging and accessory pigments as a function of trophic status represented by the concentration of chlorophyll plus pheopigments ( C mg m--3). Backscattering and absorption by other optically active components were also parameterized as functions of C. The model reproduced central trends in apparent optical properties with C and agreed to within 20% of empirical models that relate diffuse attenuation to radiance ratios within a range of C between 0.5 and 30 mg m--3. Sensitivity analyses and independent data suggested that changes in phytoplankton community structure account in part for observed deviations from the central trends. Variability in the spectral shape of the absorption coefficient in distinct communities of phytoplankton in surface waters was related to two major ecological factors: cell size and taxonomic composition. It was found that when the cell size range of the dominant organism is known, approximately 80% of the variability in spectral shape of the phytoplankton absorption could be explained. More than 95% of the variability in all observed spectra was accounted for when phytoplankton absorption was parameterized using two constant spectral shapes and a size parameter. This new parameterization was integrated in the former model, so that AOPs could be estimated for specifying cell sizes of phytoplankton dominating the community. Forward modeling was used to select relationships between diffuse attenuation and ratios of surface reflectance with minimum influence of backscattering and CDOM plus detrital absorption. This approach provided a tool to observe and monitor changes in the nature of a phytoplankton community (i.e., dominant cell size) using optical measurements only. Although refinement in the model is still required, the results suggested that mechanistic approaches to interpreting bio-optical data are feasible and can provide simple parameters relating optical signatures to ecological features of phytoplankton. | en_US |
