THE RELATION BETWEEN DISSOLVED OXYGEN AND OTHER PHYSICOCHEMICAL PROPERTIES IN BARTON SPRINGS, CENTRAL TEXAS

Dissolved oxygen (DO) is a critical component of karst water quality and is necessary for the health and reproduction of aquatic biota at karst springs. To better understand the relation between DO and other physicochemical properties, 6 years of daily DO data collected during January 2004–April 2010 from Barton Springs in Austin, Tex., were analyzed. Barton Springs is the only known habitat of the endangered Barton Springs salamander, Eurycea sosorum, whose probability of survival decreases by 50% at a DO concentration of less than 3.4 mg/L. For the 6 years examined, DO ranged from 4.0 to 8.5 mg/L. A two-segment multiple linear regression model was developed using DO, spring discharge (Q), and temperature (T), with a breakpoint of Q=70 ft³/s separating the segments of the model. Q explained most of the variability in DO when Q was less than 70 ft³/s (positive correlation with Q) (r²=0.88), and T explained most of the variability in DO when Q was greater than 70 ft³/s (inverse correlation) (r²=0.67). When only the data collected after a change to an optical DO sensor (November 2006) was considered, an improved relation between DO and a combination of Q and T was obtained when Q was more than 70 ft³/s (r²=0.86). The relations between DO, Q, and T did not hold, however, in the hours to days following storm recharge. For example, DO was observed to increase by as much as 55% in 18 hours following one storm, and was closely correlated with turbidity and specific conductance, indicating that simple mixing with recharging surface water likely controls the DO concentration. These results indicate that species that depend on sufficient amounts of DO in Barton Springs for survival, such as the Barton Springs salamander, might be under increased stress if discharge decreases, temperature increases, or other processes that reduce DO concentrations intensify.