The Varying Radiocarbon Activity of Some Recent Submerged Estonian Plants Grown in the Early 1990s

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Eleven samples of aquatic plants from three Estonian lakes were analyzed for their radiocarbon content in a collaboration between the laboratories in Tallinn and Uppsala. δ13C values for the actual species were compiled to allow normalization of activities measured in Tallinn without δ13C values. The range for well determined species is usually a few per mil and the statistical uncertainty ≥1‰. δ13C values vary considerably for different Potamogéton species and Myriophýllum spp. Lake Äntu Sinijärv and Lake Päidre are hard-water lakes containing 300 and 200 mg HCO3 –/L, respectively. One sample consisted of a carbonate crust deposited on a Ceratophýllum demersum plant in L. Äntu Sinijärv. Its Δ14C value was −147.3 ± 6.7‰ in 1990, whereas the plant had a value of −74.1 ± 8.0‰ (δ13C = −35.0‰). The same species in L. Päidre had a Δ14C value of +8.0 ± 8.8‰ (δ13C = −25.2‰) in 1992. Other species in L. Päidre contained more 14C, from a Δ14C value of about +30‰ to about +155‰, the latter value measured in Tallinn on floating leaves of Nuphar lútea, close to that of the contemporaneous atmospheric CO2. In the third lake, Lake Punso, containing ≤30 mg HCO3 –/L, the stems of Nuphar lútea exhibited in 1990 a memory effect: the activity, Δ14C = 209.6 ± 10.3‰, significantly exceeded that of the contemporaneous atmospheric CO2. However, the floating leaves of the same plant had the Δ14C value 143.1 ± 10.0‰, close to the atmospheric 14C level in 1990. The memory is explained by nutrients stored in the root stock, used when the growth starts.