On using pigment-normalized, light-saturated carbon uptake with satellite-derived pigment for estimating column photosynthesis

[1] Using five Pacific open-sea data sets with five subsets, we linearly correlated observed column photosynthesis (P-t, mg C m(-2) d(-1), from C-14-uptake) with "satellite pigment'' (C-sat, mg m(-3)), simulated from in situ data at the same stations, and modeled P-t by a depth-integrated approach. The correlation (r(2)) of P-t with C-sat and its accuracy improved after combining C-sat with an in situ-based proxy for the light-saturated rate of photosynthesis, P-opt(b) (mg C ( mg chl d)(-1)), for the particular stations. Incorporating station P-opt(b) always led to high r(2) and accuracy of the regression of modeled on observed P-t. However, P-opt(b) cannot yet be determined from space, so that in practice, geographic or seasonal means of P-opt(b) have to be used. For our 10 sets, such means lead to more imprecise (lower r(2) in 9 of 10 cases) and less accurate (for all) results, because P-opt(b) and P-t are often correlated. We expect this error source to be widespread and to afflict all modeling of P-t using a term for light-saturated photosynthesis, but we cannot offer a solution to the quandary. On the basis of combining of data sets, we do not favor the use of a single algorithm employing light-saturated photosynthesis for calculating global P-t by a depth-integrated approach.