THE O-18 O-16 OF DISSOLVED-OXYGEN IN RIVERS AND LAKES IN THE AMAZON BASIN - DETERMINING THE RATIO OF RESPIRATION TO PHOTOSYNTHESIS RATES INFRESH-WATERS/

The concentration and O-18:O-16 ratio of dissolved oxygen were measure d for 23 rivers and lakes of the Amazon Basin during 1988, 1990, and 1 991. With only two exceptions, the rivers and lakes had dissolved oxyg en concentrations that were at 20-90% of atmospheric saturation levels . The delta(18)O of the dissolved oxygen ranged from 15 to 30% (vs. SM OW). The delta(18)O for the lakes were the lowest at 15-23%. delta(18) O < 24.2 parts per thousand (the atmospheric equilibrium value) are th e result of photosynthetic oxygen input. The delta(18)O of the rivers, in contrast, ranged from 24 to 30 parts per thousand. delta(18)O > 24 .2 parts per thousand resulted from respiration. Despite this clear di fference between the delta(18)O for rivers and lakes, these water bodi es had similar levels of oxygen undersaturation. The delta(18)O and di ssolved oxygen concentrations are used to determine the ratio of commu nity respiration (R) to gross photosynthesis (P) rates. R: P varied be tween similar to 1 and 1.5 for lakes and between 1.5 and 4 for rivers. For all rivers and lakes, the measured delta(18)O indicated the prese nce of photosynthetically produced oxygen, with the highest proportion occurring in lakes. The delta(18)O of dissolved oxygen is a unique tr acer of photosynthetic oxygen and provides, through a determination of R : P, a means of quantifying the heterotrophic state of freshwaters.