Dissolved inorganic carbon (DIC)

Abstract

Various factors control the seasonal succession of the phytoplankton community in Lake Kinneret. One such factor is whether they are CO2 or HCO3-users. In spring, CO2-only users such as the dinoflagellate Peridinium gatunense often predominate and develop a bloom despite of extremely low levels of ambient aqueous CO2 and its rather poor carbon-concentrating mechanism. Based on seasonal dynamics of calcium and carbonate concentrations, mass balance considerations, and the relationships between planktonicδ13C and aqueous CO2 (δ13C), we claim that massive precipitation of calcite which occurs concomitant with the bloom supplies the CO2(aq) needed for the algae to develop and maintain the bloom. Besides riverine inflows that supply most HCO3-, Ca2+, P, N, Se(IV), and perhaps also picoplankton (that seem to serve as nuclei for calcite crystal growth) in the spring, the prevalence of relatively high temperatures and low winds also supports the bloom. The low wind restricts gas exchange and facilitates the gradual accumulated deficiency in CO2(aq) in surface water leading to higher pH and a raise in Ωcalcite to an extent that induces massive calcite precipitation. By the end of this season, the capacity of the lake surface water to precipitate calcite is almost exhausted and bicarbonate users take over. About one third of the autochthonous calcite precipitating from the epilimnion dissolves in the lower water mass (LWM), which is undersaturated with respect to this mineral. However, only 22.5% of the seasonal accumulation of dissolved inorganic carbon (DIC; ~90% HCO3-) in the LWM originates from calcite dissolution while 77.5% originates through direct biodegradation processes under initially oxic and eventually anoxic conditions (mediated mostly by bacterial SO4 reduction) and from the underlying bed sediments through methanogenesis. A potential sink of CO2 and perhaps also of isotopic fractionation in the LWM may be its uptake by anammox organisms during the sub-oxic stage (Chap. 20). The almost steadyδ13C(DIC) of approximately between 8 and 9 %; in the LWM approached in the anoxic stage represents the relative contribution of each of these pools as well as of the primary DIC pool in this layer before stratification commences.