solution

The 1971 edition of Dr. Seuss’s The Lorax included lines describing the dire fate of the “humming fish” after their pond was polluted:

“They’ll walk on their fins and get woefully weary in search of some water that isn’t so smeary. I hear things are just as bad up in Lake Erie.”

The last line was removed in the 1985 edition after Dr. Seuss realized that Lake Erie is “the happy home of smiling fish” again after the input of a key culprit of lake eutrophication, phosphorus, was successfully reduced, particularly in the Maumee River basin. The input of phosphorus from the Maumee River has since stablized. However, since the late 1990s, harmful algal blooms have returned to western Lake Erie. Some suggested that the widespread use of an inorganic form of phosphorus fertilizer is to blame. Because much of the phosphorus in western Lake Erie are from Maumee River, we can use the long-term monitoring data from Heidelberg University to evaluate whether this hypothesis is supported by data.

• Plot daily SRP (soluble reactive phosphorus) concentration against time. Can you see an increasing trend over time?

• Input of nutrient to a lake is better measured by the mass loading rate (the product of flow and concentration). Plot the daily SRP loading rate against time. Is there a temporal trend in loading rate?

• If a temporal trend is not obvious, it is often because of the large daily fluctuations. Calculate the annual total of SRP loading rates and plot them against the respective years. Is there a trend?

• If we repeat the above steps for TP, we will see that the annual TP loadings are more or less the same in the last 20 years. Could the return of harmful algal blooms in the last 20 years be caused by increased proportion of SRP in the total phosphorus? Plot the ratio of SRP over TP over time (both at the daily and annual scales).