AQUATIC METABOLISM AND ECOSYSTEM HEALTH ASSESSMENT USING DISSOLVED O2 STABLE ISOTOPE DIEL CURVES

E-ISSN： 1939-5582|18|4|965-982

ISSN： 1051-0761

Source： Ecological Applications, Vol.18, Iss.4, 2008-06, pp. : 965-982

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Abstract

Dissolved O2 concentration and δ18O‐O2 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 O2 concentration curves. A dynamic model is used to illustrate how six key environmental and biological parameters interact to affect diel O2 saturation and δ18O‐O2 curves, thereby providing a fundamental framework for the use of δ18O‐O2 in ecosystem productivity studies. δ18O‐O2 provides information unavailable from concentration alone because δ18O‐O2 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 O2 saturation and δ18O‐O2 curves as follows: (1) an increase in primary production and respiration rates increases the diel range of O2 saturation and δ18O‐O2 and decreases the mean δ18O‐O2 value; (2) a decrease in the primary production to respiration ratio (P:R) decreases the level of O2 saturation and increases the δ18O‐O2 values; (3) an increase in the gas exchange rate decreases the diel range of O2 saturation and δ18O‐O2 values and moves the mean O2 saturation and δ18O‐O2 values toward atmospheric equilibrium; (4) a decrease in strength of the respiratory isotopic fractionation (αR closer to 1) has no effect on O2 saturation and decreases the δ18O‐O2 values; (5) an increase in the δ18O of water has no effect on O2 saturation and increases the minimum (daytime) δ18O‐O2 value; and (6) an increase in temperature reduces O2 solubility and thus increases the diel range of O2 saturation and δ18O‐O2 values. Understanding the interplay between these key parameters makes it easier to decipher the controls on O2 and δ18O‐O2, 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.