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

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Occurrence and Duration of Water Quality Conditions Potentially Harmful to Fish

Dissolved Oxygen Concentration, pH, and Temperature

In order to identify periods when conditions of dissolved oxygen concentration, pH, and temperature potentially harmful to fish were present in Upper Klamath Lake and the spatial extent of these conditions, hourly data were collected at each site of conditions when dissolved oxygen concentration was less than 4 mg/L, pH was greater than 9.7, or temperature was greater than 28ºC. These values are based on high stress thresholds for Upper Klamath Lake suckers (Loftus, 2001). The percentage of sites at which a potentially harmful condition was recorded at least once in a day was calculated, providing the “one-reading” statistic. Additionally, the numbers of hourly occurrences of these conditions per day were summed over all of the sites to provide a count of “site-hours” per day of these conditions. The counts per day were divided by the total possible site-hours in the day (24 hours multiplied by the number of sites in Upper Klamath Lake), providing a percentage of site-hours in the day during which potentially harmful conditions were recorded, referred to as the “percent of site-hours” statistic. The “one-reading” statistic provides a measure of the physical presence of that condition in the lake. It also represents the maximum percentage of the lakewide site-hours that would meet the criterion if, at each site where at least one reading met the condition, the condition persisted for an entire 24 hours. The “percent of site-hours” statistic coupled with the “one-reading” statistic, then, provides a measure of the persistence of the condition throughout the day at sites where that condition was recorded. These statistics were graphed as a time series of daily values for dissolved oxygen concentration, pH, and temperature. Graphs of these statistics for dissolved oxygen concentration and pH are shown in figure 24. Only one observation of temperature above 28 ºC was made during the 2005 field season, at the upper sonde at site MDT on August 7 at 7:00 p.m. Because temperature exceeding the criterion was observed only once, a graph of potentially harmful temperature is not shown.

Figure 24 reflects the primary LDOE of 2005, which occurred in late July and early August (fig. 20), and shows that when this event occurred, it was not severe. Most of the 16 Upper Klamath Lake sondes (50–60 percent) recorded a dissolved oxygen concentration of less than 4 mg/L at least once during each day in mid-July through early August, but only 25–35 percent of the site hours recorded these conditions during this period. This pairing of the “one reading” and “site hours” statistics indicates two possible scenarios: either the condition was widespread but marginally persistent throughout the day at all sites recording this condition, or the condition was widespread but persistent at only a few of the sites that logged the condition. Time series of daily median dissolved oxygen percent saturation for each Upper Klamath Lake sonde (fig. 21) indicated that the latter explanation is most likely. (Note that water temperatures were around 21°C throughout Upper Klamath Lake at this time, corresponding to 52 percent dissolved oxygen saturation when dissolved oxygen concentration is 4 mg/L, denoted by the red line on the plots in (figure 21). Sites in and northwest of the trench were the only sites having notably low daily median dissolved oxygen concentration at this time, and only sites in the trench had daily median dissolved oxygen concentration near zero percent saturation. At sites where potentially harmful dissolved oxygen concentrations did not persist throughout the day (sites out of and away from the trench), the previously discussed timing histograms (fig. 23) indicate that when these conditions occurred, they occurred during the early morning hours.

Potentially harmful pH values were measured within a large portion of the lake twice during the 2005 field season, from late June through mid-July and again from mid-August through early September. As with the LDOE previously described, conditions resulting from potentially harmful pH values occurred during these times and may have been persistent throughout the day at few sites when this condition was present. Because of the positive relation between dissolved oxygen concentration and pH, and the decreased dissolved oxygen concentrations at sites in and northwest of the trench, sites with persistently high pH were mostly east and south of the trench.

Ammonia

When high ammonia concentrations occur concurrently with high pH and temperature, a significant fraction of the total concentration is present in the un-ionized ammonia form (fig. 25), which is particularly toxic to aquatic life. The dependence on pH is stronger than the dependence on temperature; at 22ºC and a pH of 9, for example, 31 percent of the ammonia will be in un-ionized form, but at 22ºC and a pH of 9.5, the fraction increases to 59 percent (U.S. Environmental Protection Agency, 1998). Because ammonia concentration tended to peak when the bloom declined (with an associated decrease in pH), the maximum concentrations of un-ionized ammonia, which occurred simultaneously with high pH values, often were mismatched in time from maximum concentrations of total ammonia. Mean lethal un-ionized ammonia concentration to larval and juvenile suckers, as determined by Saiki and others (1999), ranged between 480 and 1,290 µg/L. Concentrations of un-ionized ammonia measured in Upper Klamath Lake did not reach the concentrations determined to be lethal to endangered larval and juvenile suckers. Because ammonia was released into the water column concurrently with AFA bloom decline and decreasing pH, the maximum concentrations of un-ionized ammonia, which occurred simultaneously with high pH values at the peak of the bloom, were offset in time from maximum concentrations of total ammonia.

Although un-ionized ammonia concentrations did not reach lethal values, the combination of low dissolved oxygen and high un-ionized ammonia concentrations was stressful to fish, probably making them more susceptible to disease. Lethal concentrations of un-ionized ammonia might occur for short periods of time or in localized areas more susceptible to extremes in temperature and pH, such as shallow water.

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