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During incubation of seawater in bottles, the decrease in dissolved oxygen is often nonlinear over time scales frequently used to measure respiration. Numbers of bacteria always increase, and rates of assimilation of dissolved leucine often increase exponentially. This suggests that sample handling disrupts the previously existing food web, leading to shifts of trophic state and unbalanced growth. Potential errors in measuring respiratory rate can be minimized by documenting these variables. Automated precision Winkler titration (1, 11, 13, 25) is the most widely used method for measuring changes in oxygen concentrations in seawater. We have undertaken an analysis of the precision Winkler method to examine the assumed linearity of respiratory rate during the usual 24-h dark incubations and to ascertain how best to measure respiratory rates in the future. Measurements of respiratory rates in the upper mixed layer of the open ocean are in the range of 0.01 to 1.0 ,uM 02 h-1 (24). In order to measure rates in this range

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