The average water clarity has deteriorated in the months of July and August, from 1982 to the present. These are the years of the volunteer monitoring under the Lakes Lay Monitoring Program. Moreover, the trend is in line with the Fish and Game's single monitoring in July 1937 (the Fish and Game's observation was likely taken without a viewscope, so that the lakewater is likely even higher than what is shown in the graph). Crossing the mesotrophic boundary means increasing algae and scum in the lake.
But when we graph the yearly transparency from all the months from which there are observations, there is little or no decline from 1982-2006. What is happening in July and August? Possibly the "July-August effect" is from the summer's greater lake usage, more second home usage in the summer, and more disturbance of streams in the watershed.
Possibly the effect could be from changes in rainfall and runoff patterns, and somehow these changes affect the summer differently from the spring and fall.
Possibly the effect is from temperature. July and August are hotter months than May, June, September, and October, when other observations were made. It seems like a stretch, but possibly the "July-August effect" is a local impact of global warming. Under this hypothesis, with warmer summer lake temperatures there is accelerated growth of algae, and decreasing transparency. True, the lake water temperature might increase in the spring and fall too, so one might think that when these months are included in the graph, we would see an overall decline in transparancy, which are not seeing. But the warming effect is likely nonlinear. Warming the lake water a little in spring and fall might not be enough to make a significant difference in transparancy, but warming the already warm water in July and August may be enougth to make a significant difference in transparancy. This differential difference might be observable in the July-August date alone, but be obscured by the spring, summer, and fall observations.
In any case, we have observations on temperature, rainfall, and population so it is probably possible to distinguish between the hypotheses, or their combination. Learning more about the "July-August effect" about urgency and direction of our efforts. After all, July and August are important months.