Aquatic metabolism and ecosystem health assessment using dissolved O2 stable isotope diel curves

Dissolved O-2 concentration and delta O-18-O-2 diel curves can be combined to assess aquatic photosynthesis, respiration, and metabolic balance, and to disentangle some of the confounding factors associated with interpretation of traditional O-2 concentration curves. A dynamic model is used to illustrate how six key environmental and biological parameters interact to affect diel O-2 saturation and delta O-18-O-2 curves, thereby providing a fundamental framework for the use of delta O-18-O-2 in ecosystem productivity studies. delta O-18-O-2 provides information unavailable from concentration alone because delta O-18-O-2 and saturation curves are not symmetrical and can be used to constrain gas exchange and isotopic fractionation by eliminating many common assumptions. Changes in key parameters affect diel O-2 saturation and delta O-18-O-2 curves as follows: (1) an increase in primary production and respiration rates increases the diel range of O-2 saturation and delta O-18-O-2 and decreases the mean delta O-18-O-2 value; (2) a decrease in the primary production to respiration ratio (P: R) decreases the level of O-2 saturation and increases the delta O-18-O-2 values; (3) an increase in the gas exchange rate decreases the diel range of O-2 saturation and delta O-18-O-2 values and moves the mean O-2 saturation and delta O-18-O-2 values toward atmospheric equilibrium; (4) a decrease in strength of the respiratory isotopic fractionation (alpha(R) closer to 1) has no effect on O-2 saturation and decreases the delta O-18-O-2 values; (5) an increase in the delta O-18 of water has no effect on O-2 saturation and increases the minimum (daytime) delta O-18-O-2 value; and (6) an increase in temperature reduces O2 solubility and thus increases the diel range of O-2 saturation and delta O-18-O-2 values. Understanding the interplay between these key parameters makes it easier to decipher the controls on O-2 and delta O-18-O-2, compare aquatic ecosystems, and make quantitative estimates of ecosystem metabolism. The photosynthesis to respiration to gas exchange ratio (P: R: G) is better than the P: R ratio at describing and assessing the vulnerability of aquatic ecosystems under various environmental stressors by providing better constrained estimates of ecosystem metabolism and gas exchange.