VARIATIONS IN THE SPECIFIC ABSORPTION-COEFFICIENT FOR NATURAL PHYTOPLANKTON ASSEMBLAGES - IMPACT ON ESTIMATES OF PRIMARY PRODUCTION

Data collected in the estuary and Gulf of St. Lawrence in summer 1989 and 1990 were analyzed to quantify horizontal variations in the spectral values of the specific absorption coefficient of phytoplankton in different natural environments and to examine the consequences of assuming this specific absorption spectrum as constant when estimating primary production. Absorption spectra of total particles were measured with the glass-fiber filter technique, and absorption by phytoplankton was estimated numerically. We analyzed 369 spectra to estimate the actual extent of variations in the specific absorption coefficient by phytoplankton at several spatial scales in the St. Lawrence system. The potential impact of these variations on the estimation of primary production was assessed for various ecosystems with a spectral primary production model. This model takes into account chlorophyll variations with depth together with associated variations in attenuance and variations in partitioning of the various absorbing components in going from eutrophic to oligotrophic waters. Using this model, we found that, at large scale (> 1,000 km), the relative error in estimating primary production can be as high as 24% in oligotrophic waters when a constant specific absorption spectrum is assumed. Field measurements of primary production support results derived from the model.