Runoff from tilled fields can carry chemical fertilizers, pesticides, and soil to nearby trout streams via overland flow and groundwater. As large a problem is nitrogen fertilizer leaching downward past plant roots. Nitrogen, in the form of nitrates, moves through the soil, and ends up in groundwater or surface water bodies. This can occur when fertilizer is applied to fields, but not entirely taken up from plants. Instead, it is washed away by rain or irrigation, percolating through the soil to groundwater or nearby streams. Practices that retain or slow runoff and increase infiltration, reduce fertilizer and pesticide use, and reduce erosion are beneficial to maintaining the health of our trout streams.

Both manure management and continuous living crops can play important roles in reducing nitrates and improving overall nutrient management in agriculture.

Research tells us, however, that practices focused on keeping plants in the ground- whether through cover crops, buffer strips, or restoring and retaining wetlands-are most effective at reducing nitrogen loads that wash or leach into our streams.

The risk of water pollution is especially high in the “karst” geology which makes southeast Minnesota’s Driftless area such a phenomenal trout mecca. This karst region has porous bedrock close to the surface which creates the large, cold springs which in turn produce the amazing spring creeks concentrated here. Unfortunately, the porous bedrock also allows contaminants on the surface to easily reach this groundwater and our beloved trout streams.

The Minnesota Pollution Control Agency’s monitoring of surface waters shows elevated nitrate levels, particularly in Minnesota’s southeastern corner. Very high nitrate levels in groundwater and trout streams harm both humans and aquatic ecosystems.

• Nitrates reduce the size and potentially survival of trout populations.
• Nitrates act as nutrient enrichment which can lead to algal blooms, oxygen depletion, and a general deterioration of trout habitat.
• Increased nitrates in streams can decrease suitability of habitat, especially for spawning and nursery areas, and stress adult fish which can make them more susceptible to disease and reduce their reproductive success.
• Increased nitrates have been shown to decrease the number of aquatic insects available as a trout food.

What’s next?

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Read more about Fishkills, Nitrates, Neonics, and Groundwater.

Resources

Figure 1:Nitrogen Load Reduction, Data from the INRS (IDALS, IDNR, ISU CALS). Images courtesy of Iowa Soybean Association.