The beam attenuation to chlorophyll ratio: an optical index of phytoplankton physiology in the surface ocean?

The particulate beam attenuation coefficient (c(p)) is proportional to the concentration of suspended particles in a size domain overlapping that of the phytoplankton assemblage. c(p) is largely insensitive to changes in intracellular chlorophyll concentration, which varies with growth irradiance (a process termed 'photoacclimation'). Earlier studies have shown that the ratio of c(p):chlorophyll (i.e., c(p)*) exhibits depth-dependent changes that are consistent with photoacclimation. Similar relationships may likewise be expected in the horizontal and temporal dimensions, reflecting changes in mixing depth, incident irradiance, and light attenuation. A link between c(p)* and more robust photoadaptive variables has never been explicitly tested in the field. Here we use five historical field data sets to directly compare spatial and temporal variability in c(p)* with two independent indices of photoacclimation: the light-saturated, chlorophyll-normalized photosynthetic rate, p(opt)(b)-saturation index, E-k. For the variety of oceanographic conditions. and the light considered. a first-order correlation emerged between c(p)* and p(opt)(b) or E-k. These simple empirical results suggest that a relationship exists between a bio-optical variable that can potentially be retrieved remotely (c(p)*) and physiological variables crucial for estimating primary productivity in the sea. (C) 2003 Elsevier Ltd. All rights reserved.