Simultaneous Measurement of Primary Production by Whole-Lake and Bottle Radiocarbon Additions

Three additions of inorganic14Cwere made during midsummer to the mixed layers of two small lake basins in northwestern Ontario. The incorporation of this14Cinto organic matter was monitored, and the resulting rates of daily primary production measured in the open water were compared with those derived from bottles incubated both in the laboratory and in situ. Sedimentation of particulate14C, regeneration of inorganic14Cfrom organic14Cfixed on previous days, and time scale were examined as potentially important ways in which the whole-lake method differed from bottle incubations. Daily production estimates derived from laboratory incubations and a numerical model agreed well with whole-lake values. In situ bottle measurements that were analyzed in the standard way consistently underestimated whole-lake values, primarily because of the assumption that solar irradiance (I) is linearly related to integral production (P). Investigators without access to a controlled-light incubator could increase the accuracy of their daily production estimates by calculating P vs. I curves from their in situ production data and analyzing them with the numerical model.