Summary and Conclusions


As an extension of the long-term water quality monitoring program on Upper Klamath Lake and to support the U.S. Geological Survey’s effort to determine the causes of juvenile Lost River and shortnose sucker population decline, water samples were collected between 2007 and 2009 and analyzed for intracellular (particulate) and extracellular (dissolved) microcystins and cylindrospermopsins. Measurements using enzyme-linked immunosorbent assays (ELISA) showed that microcystin concentrations at sampled sites were much higher than cylindrospermopsins, which were not present in most samples analyzed. In all years of the study, microcystins occurred primarily in dissolved and large (> 63 µm) particulate forms, rather than in the small (1.5–63 µm) particulate form and at concentrations that may pose a threat to fish and other wildlife. Concentrations of large particulate microcystins expressed volumetrically were highest in 2007, when total nutrient and orthophosphate concentrations were highest, and lowest in 2008. However, when concentrations of large particulate microcystins are expressed gravimetrically, the median concentration was highest in 2009. Samples collected in 2009 contained higher and more variable concentrations of dissolved microcystins. However, most of the 2008 samples contained microcystins primarily in the dissolved fraction although, in 2009, dissolved microcystins comprised less than one-half of the total concentrations measured that year. Between July and September in 2007 and 2009, samples from sites MDT and HDB contained the highest concentrations of large particulate microcystins and exhibited the widest range of values (samples from site HDB were collected only in 2007 and were not analyzed for dissolved microcystins). Between 2008 and 2009, dissolved microcystins were more concentrated in samples from sites MDT, EPT, and MDN (based on median and peak concentrations). Concentrations also were relatively high at MDL, but this site was sampled only in 2008 when concentrations were lower, overall. 


In 2009, the first lakewide Aphanizomenon flos-aquae-dominated bloom, as indicated by chlorophyll a concentrations, occurred while microcystin concentrations generally were lowest. However, after the bloom recovered, microcystin concentrations followed a similar pattern to those of chlorophyll a. Microcystin concentrations (daily median values) also increased with total nitrogen and total phosphorus concentrations, which, in turn, increased rapidly with chlorophyll a during the second bloom of the season. Peak concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) accompanied the minimum in chlorophyll a near the end of July and decreased as total microcystin concentrations began to increase. Ratios of total nitrogen to total phosphorus (TN:TP) decreased sharply, relative to early-season conditions, during the second A. flos-aquae-dominated bloom and as microcystin concentrations increased. Early increases in microcystin concentrations coincided with seasonally high DIN:DIP ratios, but particulate microcystin concentrations continued to rise after a sharp decrease in DIN:DIP occurred, suggesting that intracellular microcystin occurrence was not adversely affected by low DIN:DIP ratios or by the accompanying low ammonia concentrations. 


Changes in median concentrations of total particulate carbon, total particulate nitrogen, and total particulate phosphorus over time followed a pattern similar to that of chlorophyll a and exhibited sharp decreases that corresponded withpeaks in median concentrations of dissolved nutrients. Median total particulate nitrogen to total particulate phosphorus (TPN:TPP) ratios were higher during the first A. flos-aquae bloom than during the second bloom peak in mid-August when total microcystins increased most rapidly.


Understanding the ecological interactions between M. aeruginosa and A. flos-aquae in Upper Klamath Lakeis important for implementing effective lake management, in that activities (including the limitation of phosphorus inputs), which may affect the growth and abundance of A. flos-aquae will likely influence the presence of toxigenic Microcystis aeruginosa. Early in the sample season, nitrogen fixation by the abundant A. flos-aquae population appears to provide new nitrogen to facilitate growth of toxigenic cells (and other non-diazotrophs), which suggests that, because phosphorus plays a major role in regulating the dominant bloom cycle, limited phosphorus availability may be indirectly important for regulating growth of these groups. Later in the season, these species appear to co-exist mutually, rather than compete for resources, and the lower nitrogen to phosphorus ratios (TN:TP and TPN:TPP) measured during this time indicate that, on the cellular level, more nutrients may be allocated to exponential growth rather than to resource-acquisition machinery, which is more heavily supported under competitive equilibrium early in the season when nutrient concentrations are low. 


Results of this study contribute to understanding the spatiotemporal dynamics of microcystin occurrence in Upper Klamath Lake and the possible environmental influences on microcystin concentrations or changes in the biomass of microcystin producers. Analyses of data collected between 2007 and 2009 have led to the formation of testable hypotheses that will become the focus of future monitoring and research. In particular, growth of toxigenic M. aeruginosa and elevated microcystin concentrations appear to be linked with the second A. flos-aquae-dominated bloom in years with a well-defined cycle of growth and decline (two bloom periods typically are observed) as a result of the large increase in dissolved nutrients released from decomposing cells. In addition, although toxigenic M. aeruginosa growth and (or) microcystin production are likely dependent on the release of DIN following the first major A. flos-aquae-dominated bloom decline (A. flos-aquae is diazotrophic and M. aeruginosa is not), time-series and correlation analyses suggest that the occurrence of microcystins in Upper Klamath Lake may be strongly regulated, indirectly, by phosphorus availability, which, in turn, drives the seasonal A. flos-aquae-dominated bloom cycle. Furthermore, ongoing and future collaborative work with fishery biologists, chemists, and specialists in other disciplines is needed to determine if the endangered suckers in Upper Klamath Lake consume toxigenic cyanobacteria through the food chain and if these fish consume microcystins at concentrations necessary to promote significant tissue damage and mortality. Knowledge of the environmental, physiological, and ecological factors that regulate microcystin production in Upper Klamath Lake is critical for effective lake management, minimization of health risks to wildlife, livestock, and humans, and for understanding the risk of toxin exposure to endangered populations of native fish species inhabiting the lake. 


First posted May 30, 2012