A stable carbon isotope study of dissolved inorganic carbon in hardwater Lake Kinneret (Sea of Galilee)

Nine depth profiles of dissolved inorganic carbon (DIC) and its carbon-13 content [expressed as delta(13)C(DIC)] were measured during 1974/75 in hardwater Lake Kinneret. DIC decreased (and pH increased) in the upper layers of the stratified lake with a concomitant delta(13)C increase. These features were due to enhanced algal photosynthesis, which prevailed during spring and early summer and preferentially utilized dissolved (CO2)-C-12. During the same period, the DIC in the deep waters of the lake gradually increased and delta(13)C gradually decreased, due to in situ decomposition/oxidation of isotopically light organic debris which sank down from the upper layers. In autumn, the deepening of the thermocline added to the epilimnion these richer in DIC but isotopically lighter, hypolimnetic waters. The DIC of the deep layers continued to increase during autumn, but its delta(13)C(DIC) did not, because, in addition to DIC produced by oxidation of organic matter depleted in C-13, dissolution of carbonate particles enriched in C-13 was also taking place. During winter the DIC and delta(13)C(DIC) profiles reflected the mixing of the entire water column. The difference Delta between delta(13)C(DIC) and delta(13)C(plankton) was inversely correlated with the rate of primary productivity (r(2) = 0.85). The fractionation between delta(13)C (CO2)(aq) and delta(13)C(plankton) was correlated to the availability of (CO2)(aq) during most of the yearly cycle, except during the exponential growth period of the algal bloom. During the destratification and the turnover periods, for about 5-6 months, Lake Kinneret emitted CO2 to the atmosphere. The evasion rate of CO2 from the mixed-water column was about 6 x 10(-2) mole CO2 m(-2) day(-1) and may have produced measurable enrichment of delta(13)C(DIC) in the lake water. This situation was reversed during the rest of the year, when enhanced algal productivity prevailed, and the lake absorbed CO2 at a rate of about 0.7 x 10(-2) mole CO2 m(-2) day(-1). This invasion rate of CO2 was much smaller than evasion and did not affect delta(13)C(DIC). On an annual basis, Lake Kinneret is a net source of CO2 to the atmosphere.