Cycling of calcite in hard water lakes of different trophic states

Based on oxygen (O2), pH, calcium (Ca2+), and carbonate (CO 22 3 ) sediment pore-water concentration profiles, we compare the benthic dissolution of autochthonous calcite in oligotrophic Lake Lucerne and eutrophic Lake Sempach. Despite their difference in trophic state, the two lakes have similar benthic O2 regimes because L. Sempach is artificially oxygenated. This peculiarity enabled the study of the direct effect of trophic state on benthic calcite dissolution. Although areal benthic O2 fluxes did not differ, O2 penetrated deeper into the sediment of the oligotrophic L. Lucerne, where most organic matter (OM, 96%) was degraded aerobically, whereas in L. Sempach, aerobic and anaerobic decomposition contributed about equal amounts to the biogenic carbon dioxide production. Release rates of bicarbonate from L. Sempach sediments exceeded those of L. Lucerne nearly twofold. The HCO 3 : Ca 2+ ratio of benthic fluxes was 3.7 in L. Lucerne compared to 10.3 in L. Sempach, suggesting that in L. Sempach most of the released HCO 3 did not originate from calcite dissolution. Furthermore, benthic calcite dissolution in L. Lucerne exceeded that in L. Sempach by two-fold, despite lower pH values in the pore water of L. Sempach. Differences in benthic microbial decomposition of OM and redissolution of calcite crystals are explained by the longer residence time of OM and calcite in the oxic sediment layer of the oligotrophic lake and the larger weight-specific surface area of its smaller autochthonous calcite crystals. We suggest that trophic state and O2 supply to the sediment are key parameters controlling the cycling of calcite and organic carbon in lakes. The cycling of calcium (Ca2+) and carbonate species (HCO 3 / 22 3 ) is a prominent seasonal process in hard-water lakes. It is driven by biogenic calcite precipitation and its redissolution related to aerobic decomposition of organic matter. Although the processes controlling calcite precipitation are relatively well understood, the factors controlling redissolution of calcite are not. The goal of this study was to help elucidate those factors and thus better understand the cycling of two biogeochemically important elements (calcium [Ca] and carbon [C]) in freshwater systems. Most calcite formed in hard-water lakes precipitates during the early algal bloom in spring and mid-summer, when nutrient-enriched surface waters stimulate intense carbon dioxide (CO2) assimilation. The consequent increase in pH shifts the HCO 3 /CO 22 3 equilibrium, causing calcite supersaturation (Brunskill 1969; Stabel 1986; Gruber et al. 2000). Müller et al. (1998a) reported increasing calcite supersaturation as increasing total phosphorus (P) concentrations in lakes increased, presumably because HPO 22 4 inhibits the formation of crystals from calcite nuclei (Stumm 1992; Dove and Hochella 1993; Hartley et al. 1995). Increasing supersaturation combined with retarded crystallization eventually leads to precipitation around fewer nuclei, producing larger calcite crystals. Therefore, the size of benthic calcite crystals has been calibrated as a proxy indicator of former trophic states in paleolimnology (Lotter et al. 1997, 1998; Teranes et al. 1999). Calcite dissolution depends on the surface area of the crystals and the production of protons due to aerobic decomposition of organic matter, either in the water column or the oxic sediment layer. Contrary to deep oceans, only a small fraction of the particulate organic matter is decomposed during the settling process in shallower lakes. For example, in oxic lakes most diagenetic decomposition of organic matter occurs in the uppermost sediment layer (Jørgensen and Revsbech 1983; Sweerts et al. 1991; Müller et al. 2002; Lorke et al. 2003). Therefore, we inferred that this is also the major site of calcite dissolution, and we focused attention on this narrow zone of intense biogeochemical activity. In this study, we used Ca2+, pH, and CO 22 3 -sensitive liquid membrane electrodes complemented by oxygen (O2) microelectrode measurements to quantify calcite dissolution in undisturbed sediment cores and to document the effect of artificial oxygenation of lakes on the alkalinity of Lakes Baldegg, Sempach, and Hallwil in Switzerland. Based on these data, we conclude that trophic state, lake depth, and O2 supply to the sediment are key parameters defining the burial rate of organic matter and calcite in sediments of hard-water lakes.