Tracking phosphorus from farm to lake: a wicked problem

Post by Melissa Motew

When it comes to lake water quality here in the Yahara, former Dane County Executive Kathleen Falk may have said it best when she identified phosphorus as “public enemy number one.”  As many people in the watershed are aware, the primary culprit causing eutrophication in the Yahara lakes and rivers is phosphorus, a nutrient that is necessary for plant growth (and, thus, agriculture), but can be harmful to natural systems when in excess.

This special element draws a lot of attention from researchers, decision-makers, and the media. But identifying the culprit is only the first step in dealing with it. As everyone has learned (or is learning), addressing the issue of excessive phosphorus loading, or the amount of P that enters a waterway, is indeed a wicked problem.

The problems caused by excessive phosphorus are obvious, as they manifest in unpleasant sights and smells, changes to the aquatic ecosystems, and reduced opportunities for people to use the lakes. Now is the perfect time of year to see eutrophic conditions in situ. I biked over the Yahara River in Madison recently and was reminded of the “pea-soup” analogy. Credit: Melissa Motew

The problems caused by excessive phosphorus are obvious, as they manifest in unpleasant sights and smells, changes to the aquatic ecosystems, and reduced opportunities for people to use the lakes. Now is the perfect time of year to see eutrophic conditions in situ. I biked over the Yahara River in Madison recently and was reminded of the “pea-soup” analogy. Photo credit: Melissa Motew

In the Yahara, as in other agricultural watersheds, there are many sources that leach phosphorus into streams, rivers, and lakes, but the primary source is agricultural fertilizer, both the synthetic kind and manure. The abundance of dairy agriculture in the Yahara means that, in addition to all the milk, there is a lot of manure produced every year. Inevitably, some of it ends up in waterways, as runoff carries it off fields.

Understanding and tracking exactly how agricultural phosphorus gets into waterways is a difficult scientific problem. There are biophysical factors to reckon with, including topography, soil type, land features, snowmelt, and precipitation.

There are social factors as well, such as the multitude of land management decisions farmers must make every day. For example, how often, how much, and where to apply fertilizer comes down to farmer choice, and inevitably, this varies across the watershed.

In recent decades there have been many collaborative efforts to curb agricultural runoff and prevent phosphorus from getting into the Yahara’s waterways. Conservation practices, nutrient management plans, manure digesters, and winter manure spreading restrictions are just some of the approaches that have been taken to improve water quality while maintaining the region’s agriculture.

Manure digesters in Dane County. Photo credit: Chloe Wardropper

Manure digesters in Dane County. Photo credit: Chloe Wardropper

We are still waiting to see the downstream effects of those efforts, however, since phosphorus loading into the lakes hasn’t declined yet, nor has lake water quality improved. Why we haven’t seen any improvements to date—despite these significant efforts—is one scientific question the WSC team is currently investigating.

Our team is using computer models to study how phosphorus moves within the watershed, from source to sink—i.e., from fertilizer application on farms to the Yahara lakes. Once our models are fully developed, we’ll be investigating how different factors, such as Best Management Practices (BMPs), increased precipitation, and manure quantity, affect the amount of phosphorus that makes its way into the system.

We also hope to understand the timescales at which changes may occur.  For instance, how long does it take for P loading to Lake Mendota to decline if BMPs are implemented in the northern part of the watershed, an area with a high density of livestock operations and, thus, where much of the P originates—years, decades, centuries, or never? The models will enable us to study such questions for historical time periods as well as future ones, in which conditions and drivers within the watershed are likely to keep changing, along with the health of our rivers and lakes.

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