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Scientific Investigations Report 2008–5201

Scientific Investigations Report 2008–5201

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Summary and Conclusions

Data from multiparameter continuous water quality monitors, physical water samples, and meteorological stations were collected from Upper Klamath and Agency Lakes, Oregon, in 2006 to assess water quality conditions and processes. The data show that the factors controlling water quality processes included Aphanizomenon flos-aquae (AFA) dynamics, temperature, bathymetry, and circulation patterns. These factors induced extremes in pH and dissolved oxygen concentrations that were potentially harmful to fish. Water quality processes that were potentially harmful to fish were temporally coincident but spatially different, thereby reducing refuge from potentially harmful waters.

Diurnal photosynthesis–respiration processes and the seasonal pattern of changes in AFA biomass lead to variation in dissolved oxygen, orthophosphate, and ammonia concentrations, pH, and specific conductance values. In previous years, AFA biomass, measured as chlorophyll a concentrations, increased markedly, inducing increased pH, then decreased, resulting in low dissolved oxygen concentrations. In 2006, chlorophyll a concentrations increased in early July and then decreased in late July–early August before recovering in mid-August. Daily lakewide medians for Upper Klamath Lake showed a seasonal pattern of dissolved oxygen and pH that followed patterns in AFA bloom, decline, and recovery, similar to previous years. Dissolved oxygen concentrations and pH increased in early July, followed by a decrease in dissolved oxygen and pH values in late July when the AFA bloom declined. Additionally, orthophosphate and ammonia concentrations increased during the bloom decline in late July–early August. The bloom recovery period in mid-August resulted in an increase in dissolved oxygen concentrations and pH, and a decrease in ammonia and, to a lesser extent, orthophosphate concentrations. Dissolved oxygen and pH generally were high before and after the bloom decline in Upper Klamath Lake, whereas these conditions occurred only before the bloom decline in Agency Lake. As in 2005, differences in water quality dynamics between the two lakes were attributable to differences in bloom dynamics.

In Upper Klamath Lake, the seasonal maximum in lakewide daily median temperature coincided with the seasonal minimum in lakewide daily median dissolved oxygen. A strong inverse relation between temperature and dissolved oxygen percent saturation was observed in the 2006 data during the time of bloom decline and recovery, although it is unclear whether temperature itself was the cause of the decline. In both 2005 and 2006, the seasonal minimum dissolved oxygen coincided with the season maximum temperature, but seasonal maximum temperatures were higher and seasonal minimum dissolved oxygen was lower in 2006 than in 2005. In Agency Lake, more variability was noted in the relation between dissolved oxygen and temperature, but the seasonal maximum in lakewide daily median temperature coincided with some of the lowest dissolved oxygen concentration values of the 2006 season. Dissolved oxygen concentrations recovered rapidly from seasonal lows in both lakes concurrently with a decrease in temperatures from the seasonal maximum in late July 2006.

Bathymetry influenced overall patterns of water quality conditions as well as the spatial distribution and severity of water quality conditions potentially harmful to fish. Results of dissolved oxygen production and consumption experiments showed that depth of the photic zone was inversely proportional to chlorophyll a concentrations. In the trench, much of the water column lies below the photic zone, where oxygen-consuming processes dominate. As a result of algal decay during bloom decline, dissolved nutrient concentrations such as ammonia were greatest at the trench sites. Calculations of 24-hour change in dissolved oxygen indicate that the deep trench areas were characterized by oxygen consumption and shallow areas of the lake were characterized by oxygen production. Waters in the trench area also were more likely to have potentially harmful low dissolved oxygen concentrations. These conditions continued to occur in trench areas well after dissolved oxygen concentrations had recovered in most of Upper Klamath Lake, lasting sometimes for multiple consecutive days.

In contrast, potentially harmful pH conditions occurred for much of July, August, and September 2006 in the shallow waters outside of the trench in Upper Klamath Lake. These conditions also were prevalent in Agency Lake, but only during June and mid-July. Calculations indicated that gas bubble formation, which may lead to gas bubble disease in fish, may occur in the shallow waters of Upper Klamath and Agency lakes. Because dissolved oxygen production and pH are directly correlated, bubble formation and potentially harmful pH conditions may occur simultaneously over much of both lakes. Potentially harmful low dissolved oxygen and high pH occurred during the same day in Upper Klamath and Agency Lakes, but these same-day occurrences were not frequent or widespread. Water temperature potentially harmful to fish (28°C) was exceeded at the three shallowest nearshore sites.

Circulation patterns were an additional influence on water quality and conditions potentially harmful to fish. Occurrences of potentially harmful pH and possible gas bubble formation were particularly common and long lasting at site SSR. Prevailing northerly winds concentrated AFA along the southern shore of the lake, increasing the potential for photosynthetic oxygen production and its associated increase in pH. The most instances of low dissolved oxygen concentrations potentially harmful to fish were deep in the water column at sites in the trench area on the western shoreline of the lake. These conditions typically were measured at near-bottom sites at the northern end of the trench. Under prevailing wind and current patterns, water flows from south to north through the trench, so water at the northern end of the trench has spent the most time in the areas of the lake where oxygen-consuming processes dominate photosynthetic production. Seasonal patterns in meteorological conditions were similar to those observed in 2005, suggesting similar circulation patterns between these years.

The network of continuous water quality monitoring stations, physical water samples, and meteorological stations provides information critical to understanding lakewide water quality dynamics in Upper Klamath and Agency Lakes. The network provides water quality data at a high temporal resolution that can be related to bloom conditions, weather, bathymetry, and currents. This network is central to future water quality modeling efforts. Continued operation of the monitoring stations to collect a long-term data set also will enable the identification of water quality status and trends in Upper Klamath Lake. Future monitoring would be enhanced by the addition of sites to better quantify conditions that describe the dynamic between Howard Bay and the trench. Other monitoring efforts could include analysis of total dissolved gases and relations between temperature and algal health.

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