Relations Between Microcystin Concentrations and Other Water Quality Variables


Correlation analysis was performed to identify potentially important associations between microcystin concentrations measured from 2007 to 2009 and environmental factors that have been previously associated with microcystin occurrences in other systems and with the growth and decline of A. flos-aquae (these factors may similarly influence growth and decline of toxigenic M. aeruginosa) in Upper Klamath Lake. Laboratory studies of batch culture incubations, for instance, show that high concentrations of nitrogen and phosphorus in freshwaters may favor the growth of toxic Microcystis strains over nontoxic ones (Vézie and others, 2002). In addition, many experimental studies have shown that intracellular microcystin content or M. aeruginosa growth is favored by elevated concentrations of phosphorus accompanied by low nitrogen to phosphorus ratios (Schindler, 1977; Kotak and others, 2000; Xie and others, 2003; Rantala and others, 2006; Chen and others, 2009); increases in total or dissolved inorganic nitrogen (Te and Gin, 2011); increases in nitrogen and phosphorus (Vézie and others, 2002; Wilhelm and others, 2011); higher water temperature and pH (Dokulil and Teubner, 2000; Jacoby and others, 2000; Te and Gin, 2011); or dissolved oxygen concentrations (Te and Gin, 2011). In the current study, correlation analysis was performed with total microcystin concentrations and with the dissolved and large (> 63 µm) particulate fractions separately to determine if changes in tested parameters correlated with microcystins during the different stages of a potential toxigenic M. aeruginosa bloom cycle; the presence of microcystins in the particulate fraction was used to represent a M. aeruginosa bloom period, and higher dissolved microcystin concentrations were assumed to occur as the bloom declined (analogous to using chlorophyll a as a biomass indicator for A. flos-aquae in this system). Linear correlations of high significance are extremely difficult to achieve when the important connections may be more tied to patterns of succession or sequences of events. Therefore, it should be emphasized that correlation analysis was used in this study to identify potential environmental influences on microcystin occurrence (based on results of previous studies) and to support data trends used to form hypotheses concerning the sequence of events required to promote growth of toxigenic cells and the associated increase in microcystin concentrations in Upper Klamath Lake. Among the variables tested, total microcystins (data collected in 2008 and 2009) correlated significantly (p < 0.05) with DIN:DIP ratio; p-values in correlations between water temperature and water column stability (RTRM) measured at site MDN also were near significance at p < 0.1 (table 4). Dissolved microcystins measured in 2008 and 2009 correlated significantly with only water column stability at site MDN, and more variables, TN:TP ratio, DIP, DIN:DIP ratio, pH, and water column stability at site MDN significantly correlated with large particulate (cell associated) microcystin concentrations measured in all years. The negative correlation between large particulate microcystins and DIN:DIP ratio was the most statistically significant relation found in the analysis, and p-values less than 0.1 also were found between microcystins in the large particulate fraction and total phosphorus and dissolved oxygen concentrations. The low p-values obtained from the positive correlations of microcystins in the large particulate fraction with total phosphorus and DIP, along with the significant negative correlations with TN:TP and DIN:DIP ratios (table 4) and increasing concentrations of total phosphorus and DIP over each sample season (fig. 2), suggest a possible association between phosphorus availability and the presence of cell associated microcystins (in the large particulate fraction) at sites sampled in all years (MDN and WMR). Water column stability measured at site MDN also correlated significantly or nearly significantly with all forms of microcystins measured, illustrating the tendency for toxigenic cells to produce intracellular microcystins under low mixing (stable water column) conditions. 


First posted May 30, 2012