Green Lake is located in Fayetteville, New York (~43° N). It is a meromictic, alkaline lake that is actively producing carbonate as large bioherms and calcite ledges along the lakeshore. In addition, carbonate sheaths are forming around the roots and stems of the vegetation (Carex sp., Agrostis sp., Eleocharis sp.) that live on the lake periphery. Five sites around the perimeter of Green Lake were sampled for water and carbonate chemistry. Temperature and pH (7.4) were measured at each site and neither varied around the lake during the sampling visit. Measured d ¹⁸O values of the lake water were -8.55 to -8.6 ‰ VSMOW. Previous studies record a range in DIC d ¹³C values of the lake water from –10‰ during the winter to -6.5‰ during the summer (Thompson et al., 1997). Over 15 carbonate reedcasts were collected and analyzed for carbon and oxygen isotopes. In these reedcasts, d ¹³C values record a large variation, ranging from –4.6 to -1.0‰ VPDB. However, most of the measured values were between -4.5 and -3.5‰. These values indicate precipitation in equilibrium with waters having DIC d ¹³C values between -6.6 and -5.5‰. In three reedcasts, much higher d ¹³C values were measured indicating that they formed in waters with higher DIC values. We suggest that the range in carbon values in the reedcasts is due to seasonal productivity when organisms pull out light carbon leaving the water enriched in ¹³C. In contrast, d ¹⁸O values were less variable, ranging from –10.3 to –8.8‰. This range of values is consistent with calcification at temperatures between 18 and 25° C. Most d ¹⁸O values, however, suggest formation in waters near 22-23° C, implying calcification during the warm months. Thus, the monotonic increase in d ¹³C implies that the calcification of these reedcasts occurred over one year, whereas the d ¹⁸O values limit precipitation to the summer. Our preliminary data indicate that the stable isotopic signature recorded in carbonate reedcasts is in equilibrium with lake waters.