Eleven (2002) to 14 (2003 and 2004) continuous water-quality monitors that measured pH, dissolved oxygen, temperature, and specific conductance, were placed in Upper Klamath Lake to support a telemetry tracking study of endangered adult shortnose and Lost River suckers. Samples for the analysis of chlorophyll a and nutrients were collected at a subset of the water-quality monitor sites in each year. The seasonal pattern in the occurrence of supersaturated dissolved oxygen concentrations and high pH associated with photosynthetic activity, as well as the undersaturated dissolved oxygen concentrations associated with oxygen demand through respiration and decay in excess of photosynthetic production, were well described by the dynamics of the massive blooms of Aphanizomenon flos aquae (AFA) that occur each year.
Data from the continuous monitors provided a means to quantify the occurrence, duration, and spatial extent of water-quality conditions potentially harmful to fish (dissolved- oxygen concentration less than 4 milligrams per liter, pH greater than 9.7, and temperature greater than 28 degrees Celsius) in the northern part of the lake, where the preferred adult sucker habitat is found. There were few observations of temperature greater than 28 degrees Celsius, suggesting that temperature is not a significant source of chronic stress to fish, although its role in the spread of disease is harder to define. Observations of pH greater than 9.7 were common during times when the AFA bloom was growing rapidly, so pH may be a source of chronic stress to fish. Dissolved oxygen concentrations less than 4 milligrams per liter were common in all 3 years at the deeper sites, in the lower part of the water column and for short periods during the day. Less common were instances of widespread low dissolved oxygen, throughout the water column and persisting through the entire day, but this was the character of a severe low dissolved oxygen event (LDOE) that culminated in the start of a fish die-off in 2003.
Documented evidence indicates that LDOEs played a role in three fish die-offs in the mid-1990s as well. In the historical context of 15 years of climate and water-quality data, 3 out of 4 of the recent fish die-off years, 1996, 1997, and 2003, were characterized by low winds and high temperatures in July or August coincident with the start of the die-off. High temperatures accelerate the oxygen demanding processes that lead to a LDOE. The role of low winds remains inconclusive, but it could include the development of stratification in the water column and/or the alteration of the wind-driven circulation pattern.
At a site centrally located in the study area, die-off years could be successfully identified in the historical data by screening for water characterized by exceptionally low chlorophyll a concentration, exceptionally low dissolved oxygen concentration throughout the water column (not just near the bottom), and exceptionally high ammonia concentration and water temperature, just prior to or coincident with the start of a fish die-off. These conditions indicate that a severe decline in the AFA bloom and conversion of most of the organic matter into inorganic form had taken place.