June precipitation appears to be positively correlated to algae blooms in Lake Erie. Looking at monthly precipitation data readily-available from Weather Underground and comparing it to the Western Lake Erie Algae Bloom Severity Index, I was surprised by the strength of the correlation (0.6, 0.85 with an “outlier” removed). June precipitation was the only month to correlate to the Severity Index in my data set and I was surprised that March, April and May precipitation did not.

The Western Lake Erie Algae Bloom Severity index 2002-20015 is a model developed by a research consortium including NOAA and several other groups from academia, government and the private sector.

Background
Harmful algae blooms (HABs) are a serious and familiar water quality issue facing Lake Erie and something I’ve been interested in since the early 1970s when eutrophication received a fair amount of attention in the media, especially pertaining to pollution of Lake Erie.

Around 1971, after some heated debate involving scientists, the soap industry, politicians, and government agencies, states and municipalities rolled out a patchwork work of phosphate detergent restrictions and discharge goals. A good summary of the early work linking eutrophication to detergent phosphates and the governmental processes that dealt with the controversial issue is available in this document published by the University of Colorado in 1994.

Following the reduction of phosphates in municipal wastewater, the severity of HABs diminished in the 1980s according to data collected by the National Center for Water Quality Research at Heidelburg University. However, in the past 15 or 20 years, the algae blooms have returned in a big way. The Severity Index, represented by this bar graph covers years 2002-20015.

Approach
This was a quick and easy investigation. Based on background reading, course work and listening to limnologists over the years, “summertime phosphorus” seems mainly responsible for algae blooms. If runoff provides the plant-available phosphorus to water bodies that “bloom” in summer, then the seasonal timing of the runoff should be important. To get a reasonable representation of monthly precipitation in the Western Lake Erie watershed, I picked six weather recording stations from Weather Underground’s historic data set. The approximate locations of the weather stations are shown on the map. All of the stations, listed below, are at airports near rivers that drain to Western Lake Erie.

City (Airport Weather Station) – Watershed

Adrian, MI (KADG) – River Raisin

Ann Arbor, MI (KARB) – Huron River

Bowling Green, OH (KTDZ) – Portage River

Defiance, OH (KDFI) – Maumee River

Fort Wayne, IN (KFWA) – Maumee River

Lima, OH (KAOH) – Ottawa & Auglaize Rivers

Next, I calculated the average June precipitation (2002-2015) from the six weather stations and compared those numbers, using correlation coefficients and linear regression, to the corresponding yearly Severity Index values from the bar graph.

The first graph shows the June precipitation plotted against the annual Severity Indices. All but one of the points line up pretty well along the trend line. the “outlier” represents 2011 when the Severity Index was high despite low precipitation in June. More about the “outlier” in a minute. The correlation coefficient is calculated for all years 2002-2015.

The second graph with the 2011 data point excluded shows a neater arrangement along the trend line and the correlation coefficient jumps to 0.85.

What about the 2011 “outlier?” It turns out that the weather stations received heavy rainfall over a period of 3 days during the last week of May. In other words, almost in June.

What are we to make of this?

1. The data suggest that heavy late spring and early summer rains (late May and June) are associated with high rates of runoff and phosphorus loading leading to HABs, but heavy runoff at other times of the year is less significant.
2. The ability to predict the severity of HABs based on precipitation during late May and June potentially gives water treatment authorities several weeks to prepare for algae-related problems later in the summer.
3. By late May and June, soil temperatures have warmed considerably. which promotes microbial activity in the soil. It is also a time when farmers are actively working or finishing work in their fields: disking, planting, fertilizing, applying herbicides. Do these factors affect the amount and plant-availability of phosphorus running off in June? Probably.
4. Are there more management practices farmers can use, which they are not already using, to mitigate the water quality impacts of heavy June runoff?

Aside from taking some vulnerable land out of production, I’m not sure there is.

Finally, based on the linear equations shown on the graphs above, I expect the Severity Index for 2016 to be somewhere around 4 or 5. The official Index from the NOAA consortium is due out later this fall.