Temperature controls monthly variation in Ostracode valve Mg/Ca: Cypridopsis vidua from a small lake in Sonora, Mexico

The molar Mg/Ca ratio was measured in live-collected ostracodes from a smal l dammed body of water in northern Mexico. The species Cypridopsis vidua wa s collected every month throughout 1990. Four to ten specimens were measure d from each month's collection. Monthly average Mg/Ca ratios range from 0.0 18 to 0.032 and are correlated with water temperature. An anomalously cool July is reflected in lower Mg/Ca ratios in that month's collection. The Mg/ Ca ratio responds to temperature in this way: T-o = (Mg/Ca -0.0103)/0.00066 . The rapid valve chemistry response to temperature change in the lake can occur because the life span of C. vidua is very short (similar to 30 d). Th e high variability in the data set shows that many measurements of single v alves must be averaged to track temperature change, or that many valves mus t be combined. Valves must also be screened for incomplete calcification as Mg/Ca ratios are significantly higher in poorly calcified valves. If the M g/Ca of valve is modeled as a temperature-dependant partition coefficient, the residuals on a linear regression are significantly larger. This implies that ostracode biology and temperature play a significant role in the elem ental composition of the valve and that water chemistry is of minor importa nce. Copyright (C) 2001 Elsevier Science Ltd.