Unexpected underestimation of primary productivity by ¹⁸O and ¹⁴C methods in a lake: Implications for slow diffusion of isotope tracers in and out of cells

By use of in situ bottle incubations, we determined three variables for the characterization of plankton community primary productivity in Lake Kinneret, Israel: (1) H₂¹⁸O-based primary productivity ( ¹⁸OP), measured by ³⁴O₂ (¹⁸O ¹⁶O) evolution; (2) radiocarbon-uptake-based primary productivity (¹⁴CP); and (3) net O₂ production (NOP), calculated from the rate of O₂:Ar change. Six experiments were conducted in the fall, a period that is characterized by thermocline deepening, erosion of the hypolimnion, and eventual introduction of high concentration of reduced substances into the epilimnion. An additional experiment was conducted in spring. In comparison to net O₂ evolution, the tracer-based methods severely underestimated primary productivity in the fall. This is indicated by unusually high NOP:¹⁸OP and NOP:¹⁴CP ratios (1-2.2 and 2-4, respectively). The latter is considerably different than the typical values of aquatic environments (∼1.4). In contrast to these ratios, the ¹⁸OP:¹⁴CP appears normal (1.6-2.9) and not significantly different from previous results in the lake. Ammonium was the major nitrogen source in the fall experiments, and there is no reason to assume that the cellular composition of phytoplankton was exceptional. It is therefore unlikely that the high NOP:¹⁴CP ratio represents genuine ratios of oxygen and carbon metabolism. We suggest that an explanation accounting for the high values of both NOP:¹⁸O P and NOP:¹⁴CP can be slow diffusion of the tracers in and out of phytoplankton cells.