ESTIMATING PRIMARY PRODUCTION AT DEPTH FROM REMOTE-SENSING

By use of a common primary-production model and identical photosynthet ic parameters, four different methods were used to calculate quanta (Q ) and primary production (P) at depth for a study of high-latitude Nor th Atlantic waters. The differences among the four methods relate to t he use of pigment information in the upper water column. Methods 1 and 2 use pigment biomass (B) as an input and a subtropical, empirical re lation between K-d (diffuse attenuation coefficient) and B to estimate Q at depth. Method 1 uses measured B, but Method 2 uses B derived fro m the Coastal Zone Color Scanner (subtropical algorithm) as inputs. Me thods 3 and 4 use the phytoplankton absorption coefficient (alpha(pb)) instead of B as input, and Method 3 uses empirically derived alpha(ph )(440) and K-d values, and Method 4 uses analytically derived alpha(ph )(440) and a (total absorption coefficient) values based on the same r emote measurements as Method 2. When the calculated and the measured v alues of Q(z) and P(z) were compared, Method 4 provided the closest re sults [for P(z), r(2) = 0.95 (n = 24), and for Q(z), r(2) = 0.92 (n = 11)]. Method 1 yielded the worst results [for P(z), r(2) = 0.56 and fo r Q(z), r(2) = 0.81]. These results indicate that one of the greatest uncertainties in the remote estimation of P can come from a potential mismatch of the pigment-specific absorption coefficient (alpha(ph)), which is needed implicitly in current models or algorithms based on B. We point out that this potential mismatch can be avoided if we arrang e the models or algorithms so that they are based on the pigment absor ption coefficient (alpha(ph)). Thus, except for the accuracy of the ph otosynthetic parameters and the above-surface light intensity, the acc uracy of the remote estimation of P depends on how accurately alpha(ph ) can be estimated, but not how accurately B can be estimated. Also, m ethods to derive alpha(ph) empirically and analytically from remotely sensed data are introduced. Curiously, combined application of subtrop ical algorithms for both B and K-d to subarctic waters apparently comp ensates to some extent for effects that are due to their similar and i mplicit pigment-specific absorption coefficients for the calculation o f Q(z).