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 small dammed body of water in northern Mexico. The species Cypridopsis vidua was collected every month throughout 1990. Four to ten specimens were measured from each month's collection. Monthly average Mg/Ca ratios range from 0.018 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). The high variability in the data set shows that many measurements of single valves must be averaged to track temperature change, or that many valves must be combined. Valves must also be screened for incomplete calcification as Mg/Ca ratios are significantly higher in poorly calcified valves. If the Mg/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 elemental composition of the valve and that water chemistry is of minor importance. Copyright (C) 2001 Elsevier Science Ltd.