Bridge: So looking at the total amount of phosphorus in a lake doesn’t tell you much?

Bratton: They used to do these forecasts based upon total phosphorus. Now, if you listen closely, researchers talk about “bioavailable phosphorus.” But in terms of total acres of land farmed today versus decades ago, it’s almost the same. Total phosphorus applied is either the same or less in most cases. Crop yields have gone up, not because farmers are applying more fertilizer, but because of better engineered strains of corn or soybeans.

Bridge: Does this type of nuance — they idea that total phosphorus loads only tell you part of the story — muddle the political debate about how to fight algal blooms, particularly when the agriculture industry argues against more regulations?

Bratton: There’s a temptation of some in the agriculture sector to quantify their actions without translating them into impact. They’ll say, “We’ve changed our tillage practices and there’s less erosion. We’ve prevented so many pounds of sediment from entering the lake.” All of that is true. But to get the full picture, we’ve also got to acknowledge this has sent more dissolved phosphorus in the lake. At the end of the day, if there’s more algal blooms, the problem’s not solved.

Bridge: Why is more dissolved phosphorus entering the lake?

Bratton: There are three primary contributors: One is more spring runoff, total volume of flow into the lake has increased over the past decade. And less tilling of farmlands yields more dissolved phosphorus in two ways: If you apply fertilizer but don’t till it in, rains more easily wash it off. And if you don’t till the soil on a regular basis, you form macropores, little pipes down into the soil.

Bridge: This doesn’t mean farmers should start tilling again, right? But what other actions could they take?

Bratton: You really have to look at the whole system in an integrated way. If we’re not going to till as aggressively, you can’t just sprinkle fertilizer on the surface. We still need to find a way to incorporate it without tilling. Different types of tools will actually drill the phosphorus in (without creating pores that allow phosphorus to easily wash into the water.)

Bridge: What other factors encourage blooms?

Bratton: Not every part of the Great Lakes has blooms. Usually you have a high-yield ag area that’s heavily ditched, drained, tiled and used to be a swamp. And you couldn’t farm a swamp, so people who settled in these areas had to drain it. The Maumee Basin (Ohio’s watershed that feeds Lake Erie), the Saginaw Basin (around Lake Huron along Michigan’s thumb), parts of the Fox River Basin (around Green Bay, Wisc.) were extensively drained and trenched, dropped the water table by two, three, four feet or more. That made them farmable, but the downside is that all the nutrients trapped in those systems — whatever was lost from the fields — short-circuited into the lakes. And the waters are generally shallow, warm, sluggish and have longer retention time.