Zebra Mussels and Plankton

Introduction Data Collection and Analysis Results The Bottom Line

Results

Preliminary results from those analyses that have been completed to date confirm that zebra mussels have a measurable impact on the plankton community of Lake Champlain.  Chlorophyll a levels declined significantly in all barrels over the course of both 96-hour experiments, most likely the result of a decrease in light intensity (Figure 12); however, statistical analysis (Repeated Measures Analysis of Variance) showed clearly that there was an additional significant effect of treatment in both runs (Run1: F=106.17, p=0.0005; Run 2: F=92.97, p<0.0001). Subsequent Student-Newman-Keuls (SNK) tests showed chlorophyll a levels to be significantly lower in all zebra mussel treatments (zebra mussels alone and zebra mussels + zooplankton) when compared to controls at all times except time 0 (the beginning of the experiment).  In Run 2, chlorophyll concentrations in the zooplankton treatments were generally similar to those in the controls but at 72 and 96 hours, showed slightly lower densities.  Run 2 samples collected at 48 hours were invalid due to a laboratory processing error and were not included in the statistical analysis.

Soluble Reactive Phosphorus (SRP) concentrations changed significantly in all barrels over both runs (Figure 13).  Again using repeated measures ANOVA, we found a statistically significant effect of both treatment and time (using natural log transformed data:  Run 1: F=9.74, p=0.025; Run 2: F=23.77, p<0.001).   In Run 1, SRP levels were higher in the zebra mussel treatments compared to the controls at all dates after the initial sample, but because of the large variability in the data, these differences were only significant at 12 and 24 hours.  In Run 2, the zebra mussel treatments were significantly higher in SRP at all times after 12 hours.  As for chlorophyll, the 72 hour samples from Run 2 were excluded in analysis because of laboratory processing errors.

In Run 2, the SRP concentrations in the zebra mussel treatment were not significantly different from those in the zebra mussel plus zooplankton treatment, nor was the SRP in the zooplankton treatment significantly different from the SRP in the control.  One might have expected that the SRP concentrations in the zebra mussel plus zooplankton treatment would be the highest observed because SRP concentrations were elevated in both the zebra mussel alone and zooplankton alone treatments, but this was not the case.  This conflicting pattern suggests that the effects of zebra mussels and zooplankton are not strictly additive.

Samples were analyzed for microcystin concentration using an Enzyme Linked ImmunoSorbent Assay (ELISA) kit supplied by EnviroLogix.  Concentrations did not change over the experiments and remained at concentrations far below the World Health Organization’s standard for protection of human health (1ppb) and, therefore, microcystin was not considered further in the context of this study.

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