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Unexpected underestimation of primary productivity by 18 O and 14 C methods in a lake: Implications for slow diffusion of isotope tracers in and out of cells
Author(s) -
Yacobi Yosef Z.,
Perel Naomi,
Barkan Eugeni,
Luz Boaz
Publication year - 2007
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2007.52.1.0329
Subject(s) - isotope , radiochemistry , primary productivity , diffusion , stable isotope ratio , productivity , primary (astronomy) , oxygen 18 , chemistry , environmental science , nuclear physics , physics , astrophysics , thermodynamics , macroeconomics , organic chemistry , nutrient , economics
By use of in situ bottle incubations, we determined three variables for the characterization of plankton community primary productivity in Lake Kinneret, Israel: (1) H 2 18 O‐based primary productivity ( 18 OP), measured by 34 O 2 ( 18 O 16 O) evolution; (2) radiocarbon‐uptake‐based primary productivity ( 14 CP); and (3) net O 2 production (NOP), calculated from the rate of O 2 : 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 O 2 evolution, the tracer‐based methods severely underestimated primary productivity in the fall. This is indicated by unusually high NOP: 18 OP and NOP: 14 CP 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 18 OP: 14 CP 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: 14 CP ratio represents genuine ratios of oxygen and carbon metabolism. We suggest that an explanation accounting for the high values of both NOP: 18 O P and NOP: 14 CP can be slow diffusion of the tracers in and out of phytoplankton cells.