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
Limnol. Oceanogr., 52(1), 2007, 329-337 | DOI: 10.4319/lo.2007.52.1.0329
ABSTRACT: 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 (18O16O) 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.