Photosynthetic rates in Lake Superior

Summary

Three methods of measuring carbon fixation rates in Lake Superior were employed during a July 26—August 8, 1973 cruise on Lake Superior:

(a)	14C-uptake in clear vs dark bottles
(b)	Δ pH in clear vs dark bottles and
(c)	pH-depth profiles comparing early morning versus late afternoon data, to compute diurnal Δ pH in the upper 50 meters of water. The photosynthetic yields revealed large discrepancies among these three methods. If one assumes that clear vs. dark bottle data represents the average yield in the euphotic zone (36 meters) and that the photoperiod is 14 hours then method (a) yields 14 millimoles/m2/day, (b) yields 111 and (c) yields 200. Because practically all Lake Superior productivity data in the literature are based on 14C data, it seems likely that the estimates of the degree of oligotrophy of Lake Superior have been biased by the low photosynthetic yields obtained by this method.