Spring Has Sprung—And So Has Runoff

Spring Phosphorus Loading
As winter gives way to spring, melting snow and seasonal rainfall reconnect the surrounding landscape with nearby lakes and streams. During this time, nutrients that have accumulated across the watershed—especially phosphorus—are transported into surface waters through runoff and drainage pathways. This process, known as external loading, is a major driver of nutrient buildup in lakes.
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Sources of phosphorus include fertilizers, soil erosion, stormwater runoff, septic systems, and organic material entering waterways. While spring runoff introduces new phosphorus each year, many lakes also contain significant amounts of “legacy” phosphorus that have built up in sediments over decades.

Internal Phosphorus Loading
Once phosphorus enters a lake, much of it settles to the bottom and becomes part of the sediment through the breakdown of aquatic plants, algae, and other organic matter. Over time, this creates a long-term reservoir of phosphorus within the lakebed.

During the summer months, many lakes form distinct layers, with warmer water at the surface and colder water near the bottom. This layering limits mixing and reduces the amount of oxygen reaching deeper waters. As oxygen levels decline near the lake bottom, conditions can become hypoxic or anoxic.
Under these low-oxygen conditions, phosphorus that was previously bound in the sediments can be released back into the water column. This process, known as internal loading, makes phosphorus available again for algae and cyanobacteria growth.
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As a result, lakes with a history of phosphorus accumulation may continue to experience water quality problems, even if new phosphorus inputs are reduced.

Why This Is a Concern
Excess phosphorus can lead to eutrophication, a process characterized by excessive algae growth and declining water quality. In some cases, these conditions can support harmful algal blooms that produce toxins such as microcystin.
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These blooms can impact recreational use, create unpleasant conditions, and pose risks to people, pets, and wildlife. Additionally, increased algae growth and decomposition can reduce oxygen levels in the water, placing stress on fish and other aquatic organisms and disrupting the overall balance of the ecosystem.
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What You Can Do: Protecting Our Lakes Starts at Home
While spring runoff is a natural process, the amount of phosphorus entering our lakes is heavily influenced by everyday activities across the watershed. Small changes in how we manage our properties can make a meaningful difference in reducing nutrient pollution.

Simple practices—like minimizing fertilizer use, maintaining healthy shoreline vegetation, managing yard waste properly, and ensuring septic systems are functioning correctly—can all help limit the amount of phosphorus that reaches our waterways. Reducing soil erosion and slowing stormwater runoff are also key steps in keeping nutrients on land and out of the lake.

Because everyone in the watershed plays a role, collective action is one of the most effective ways to protect water quality.
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To learn more about practical steps you can take, visit our Best Practices page for guidance, tips, and resources to help protect and improve our lakes.