Surface pigments, algal biomass profiles, and potential production of the euphotic layer: Relationships reinvestigated in view of remote‐sensing applications

Abstract

Maps of surface chlorophyllous pigment (Chl a + Pheo a) are currently produced from ocean color sensors. Transforming such maps into maps of primary production can be reliably done only by using light‐production models in conjunction with additional information about the column‐integrated pigment content and its vertical distribution. As a preliminary effort in this direction, ∼4,000 vertical profiles of pigment (Chl a + Pheo a) determined only in oceanic Case 1 waters have been statistically analyzed. They were scaled according to dimensionless depths (actual depth divided by the depth of the euphotic layer, Z e) and expressed as dimensionless concentrations (actual concentration divided by the mean concentration within the euphotic layer). The depth Z e, generally unknown, was computed with a previously developed bio‐optical model. Highly significant relationships were found allowing 〈C〉tot the pigment content of the euphotic layer, to be inferred from the surface concentration, C̄pd, observed within the layer of one penetration depth. According to their C̄d values (ranging from 0.01 to >10 mg m−3), we categorized the profiles into seven trophic situations and computed a mean vertical profile for each. Between a quasi‐uniform profile in eutrophic waters and a profile with a strong deep maximum in oligotrophic waters, the shape evolves rather regularly. The well‐mixed cold waters, essentially in the Antarctic zone, have been separately examined. On average, their profiles are featureless, without deep maxima, whatever their trophic state. Averaged values of ρ, the ratio of Chl a to (Chl a + Pheo a), have also been obtained for each trophic category. The energy stored by photosynthesizing algae, once normalized with respect to the integrated chlorophyll biomass 〈C〉tot is proportional to the available photosynthetic energy at the surface via a parameter ψ*, which is the cross‐section for photosynthesis per unit of areal chlorophyll. By taking advantage of the relative stability of ψ*, we can compute primary production from ocean color data acquired from space. For such a computation, inputs are the irradiance field at the ocean surface, the “surface” pigment from which 〈C〉tot can be derived, the mean ρ value pertinent to the trophic situation as depicted by the C̄pd or 〈C〉tot values, and the cross‐section ψ*. Instead of a constant ψ* value, the mean profiles can be used; they allow the climatological field of the ψ* parameter to be adjusted through the parallel use of a spectral light‐production model.