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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5026

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Dissolved Oxygen Production and Consumption Experiments

Dissolved Oxygen Production and Consumption

The rate of change in dissolved oxygen concentration in light and dark incubation bottles at each level in the water column is shown as time series in figure 16. Rates obtained from the two light bottles were averaged for each rack depth. This was done for dark bottles when there were multiple dark bottles on a rack during an experiment, but only one dark bottle at each depth was used in most of the experiments. Typically, rates from light bottles represented net dissolved oxygen production (positive rates), and rates from dark bottles represented net dissolved oxygen consumption (negative rates). A maximum production rate of 1.47 (mg/L)/ hr of oxygen was measured at site HDB on August 15 and a maximum consumption rate of –0.73 (mg/L)/hr of oxygen was measured at site MDN on August 10 (fig. 16). Because the bottles were filled with unaltered lake water, the experiments measured oxygen production and consumption of the entire planktonic community (phytoplankton, zooplankton, and bacteria). Because AFA typically constitutes more than 90 percent of the phytoplankton by weight during the time of year when these experiments were conducted (Kann, 1997; Perkins and others, 2000), dissolved oxygen production was attributable mostly to photosynthesis by AFA. However, the abundance of oxygen consuming components of the planktonic community relative to AFA is less well known, particularly bacterial biomass. Large quantities of the zooplankter Daphnia plicaria, an amphipod that dominates the zooplankton biomass in Upper Klamath Lake (Kann, 1997), were not observed in bottles during experiments. It is expected that AFA is a major contributor to oxygen consumption observed in the experiments, but without knowledge of the bacterial biomass relative to that of AFA it cannot be conclusively stated that AFA dominates dissolved oxygen consumption processes.

The rate of change of dissolved oxygen concentration in dark bottles sometimes was positive (fig. 16), indicating the unlikely occurrence of oxygen production in absence of light. This problem was largely limited to the end of the field season in September and October. Measured positive changes in dark bottles in the raw data were small (median value of 0.08 mg/L), suggesting that the changes were the result of the accuracy limits of the dissolved oxygen probe, which the manufacturer specifies as ±0.2 mg/L.

Excluding the positive rates of change of dissolved oxygen concentration in the dark bottles, consumption rates measured in dark bottles were comparable to the overnight consumption rates measured in Upper Klamath Lake in 2002 by Lieberman and others (2003). Consumption rates ranged from 0 to –0.73 (mg/L)/ hr of oxygen in this study, whereas consumption rates in the 2002 study ranged from –0.05 to –0.49 (mg/L)/hr of oxygen. 
The larger range of consumption rates determined in this study is attributable to annual variability and the fact that experiments in this study were conducted over a period of 6 months, whereas the 2002 study was conducted during 2 consecutive days in late July and 2 consecutive days in mid-September.

Greater consumption rates typically were measured in dark bottles on the upper incubation rack than in those on the lower incubation rack. Final bottle temperatures tended to be higher in the upper rack bottles than in the lower rack bottles, reflecting the difference in the ambient water temperatures. The higher temperatures could explain the generally higher consumption rates in the bottles on the upper rack.

With the exception of the first experiment at site WMR, upper incubation racks were positioned at 0.5 m below the water surface in the water column during all experiments at all sites (table 2). This placement provided a consistent framework with which to observe patterns in dissolved oxygen production in the upper 0.5 m of the water column. Dissolved oxygen production rates in the upper rack at site MDL were initially high in June, slowed through July, and recovered through August (fig. 16). A similar pattern was observed in the upper rack dissolved oxygen production at site MDN (fig. 16). Experiments were not conducted at sites EPT or MDT during June, but results of experiments done in July and August also show a decrease in upper rack production rates from the mid-July experiment to the late July–early August experiment at both sites, with a subsequent recovery of upper rack dissolved oxygen production rates through August. This pattern of decreasing oxygen production in upper portions of the water column through June and recovery through August is coincident with the patterns in chlorophyll a (fig. 10) and the decrease and recovery of dissolved oxygen concentration in the continuous water quality monitors, as discussed in the section, “Daily Median Water Quality Conditions.” When data from all experiments were combined, upper rack production rates also were positively correlated with the natural logarithm of chlorophyll a concentrations (fig. 17). The fact that no correlation was observed in data from the lower rack likely results from the effect of algal self-shading, as discussed in the section, “Depth of Photic Zone.”

Because these experiments were not done at all sites during all weeks, a lakewide decrease in dissolved oxygen production rates in late July and early August cannot be confirmed, but the similarities of the production rate, chlorophyll a, and time series of dissolved oxygen concentration and percent saturation indicate that reduced AFA dissolved oxygen production contributed to the decreasing dissolved oxygen concentrations observed through July 2005. Because AFA at Agency Lake sites undergoes cycles of large blooms followed by substantial decline similar to the pattern seen in Upper Klamath Lake, decreased oxygen production by AFA also likely contributed to periods of decreasing dissolved oxygen concentrations observed at Agency Lake monitoring sites in July and August.

Depth of Photic Zone

When data from all experiments were combined, an inverse relation was observed between photic zone depth and the natural logarithm of chlorophyll a concentrations (fig. 18). This relation reflects the shading effect that blooms of AFA can have on the water column. During some experiments, rates from light bottles on the lower rack were negative. This happened most notably at site HDB during the August 15 experiment and at site MDL during the July 6 experiment, when the average light bottle change in dissolved oxygen concentration on the lower rack was about –0.21 and –0.27 (mg/L)/hr of oxygen, respectively (fig. 16). Smaller negative rates were measured in light bottles on the lower incubator racks during several dates at sites MDN, EPT, and MDT. Comparison of the depth of the lower rack with the average depth of the photic zone during the experiments at each site (table 2) shows that these instances of net oxygen consumption in light bottles occurred when the bottles were near or below the photic zone. Furthermore, no correlation existed between oxygen production and chlorophyll a concentration in lower incubation racks, whereas a positive correlation was observed in upper incubation racks (fig. 17). These findings indicate that self-shading by AFA during heavy blooms can cause metabolic respiration to exceed photosynthetic production during the daytime in parts of the water column that would otherwise be in the photic zone.

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