Western Lake Erie clean up: Phosphorus control or zebra mussel effect? An example of the value of long-term data

The arrival of the zebra mussel (Dreissena polymorpha) in the late 1980's increased the potential "improvement" of water quality owing to its filter feeding habit. Several investigators have reported dramatically improved water clarity and significant reductions in Lake Erie phytoplankton after the arrival of zebra mussels. Two interventions, one planned (phosphorus loading control) and the other inadvertent (zebra mussel invasion) have had a combined impact on Lake Erie. When only relatively recent data (5-10 years) arc analyzed, there is a tendency to associate the present improved state of water quality of western Lake Erie (in regards to clarity, "greenness", level of algal interference with cater treatment, recreational use, etc.) solely with the arrival of zebra mussels. A much longer term data set is needed to place the water quality effects of the zebra mussel invasion in its proper perspective relative to the effects of phosphorus loading control.The purpose of this "bulletin" is to illustrate the differences in interpretation of change that result when evaluations of a short-term (5-10 years) data record are contrasted with a longer-term data set. The data record consists of nearly 30 years of planktonic Chlorophyceae (green algae) density data (and two dominant chlorophyte genera, Pediastrum and Scenedesmus) measured in samples collected weekly from the Union Water System intake (north shore, western Lake Eric. at Kingsville, Ontario).