<p>Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental and climatic changes. The carbon isotopic composition (<em>δ</em><sup>13</sup>C) of such carbonates depends on the <em>δ</em><sup>13</sup>C of dissolved inorganic carbon (DIC). Their oxygen isotopic composition (<em>δ</em><sup>18</sup>O) is controlled by the <em>δ</em><sup>18</sup>O of the lake water and on water temperature during carbonate precipitation. Lake water <em>δ</em><sup>18</sup>O, in turn, reflects the <em>δ</em><sup>18</sup>O of precipitation in the catchment, water residence time and mixing, and evaporation. A paleoclimate interpretation of carbonate isotope records requires a site-specific calibration based on an understanding of these local conditions.</p> <p>For this study, samples of different carbonate components and water were collected in the littoral zone of Lake Locknesjön, central Sweden (62.99° N, 14.85° E, 328 m a.s.l.) along a water depth gradient from 1 to 8 m. Samples from living organisms and sub-recent samples in surface sediments were taken from the calcifying alga <em>Chara hispida</em>, mollusks from the genus <em>Pisidium</em>, and adult and juvenile instars of two ostracod species, <em>Candona candida</em> and <em>Candona neglecta</em>.</p> <p>Neither the isotopic composition of carbonates nor the <em>δ</em><sup>18</sup>O of water vary significantly with water depth, indicating a well-mixed epilimnion. The mean <em>δ</em><sup>13</sup>C of <em>Chara hispida</em> encrustations is 4 ‰ higher than the other carbonates. This is due to fractionation related to photosynthesis, which preferentially incorporates 12<sup>C</sup> in the organic matter and increases the <em>δ</em><sup>13</sup>C of the encrustations. A small effect of photosynthetic <sup>13</sup>C enrichment in DIC is seen in contemporaneously formed valves of juvenile ostracods. The largest differences in the mean carbonate <em>δ</em><sup>18</sup>O between species are caused by vital offsets, i.e. the species-specific deviations from the <em>δ</em><sup>18</sup>O of inorganic carbonate which would have been precipitated in isotopic equilibrium with the water. After subtraction of these offsets, the remaining differences in the mean carbonate <em>δ</em><sup>18</sup>O between species can mainly be attributed to seasonal water temperature changes. The lowest <em>δ</em><sup>18</sup>O values are observed in <em>Chara hispida</em> encrustations, which form during the summer months when photosynthesis is most intense. Adult ostracods, which calcify their valves during the cold season, display the highest <em>δ</em><sup>18</sup>O values. This is because an increase in water temperature leads to a decrease in fractionation between carbonate and water, and therefore to a decrease in carbonate <em>δ</em><sup>18</sup>O. At the same time, an increase in air temperature leads to an increase in the <em>δ</em><sup>18</sup>O of lake water through its effect on precipitation <em>δ</em><sup>18</sup>O and on evaporation from the lake, and consequently to an increase in carbonate <em>δ</em><sup>18</sup>O, opposite to the effect of increasing water temperature on oxygen-isotope fractionation. However, the seasonal and inter-annual variability in lake water <em>δ</em><sup>18</sup>O is small (~0.5 ‰) due to the long water residence time of the lake. Seasonal changes in the temperature-dependent fractionation are therefore the dominant cause of carbonate <em>δ</em><sup>18</sup>O differences between species when vital offsets are corrected.</p> <p>Temperature reconstructions based on paleotemperature equations for equilibrium carbonate precipitation using the mean <em>δ</em><sup>18</sup>O of each species and the mean <em>δ</em><sup>18</sup>O of lake water are well in agreement with the observed seasonal water temperature range. The high carbonate <em>δ</em><sup>18</sup>O variability of samples within a species, on the other hand, leads to a large scatter in the reconstructed temperatures based on individual samples. This implies that care must be taken to obtain a representative sample size for paleotemperature reconstructions.</p>