Take Aim – Sediment Phosphorus Testing in September and October
Did you know that your lake’s sediment may be a significant source of phosphorous that is feeding your algae issues? Phosphorus is complex and moves from the sediment into the water column and back into the sediment depending on the time of year. In summer, warm temperatures, low oxygen, and high biological activity release phosphorus from sediments into the water. As fall brings cooler temperatures and lake turnover, oxygen returns to bottom waters, shifting sediments from a phosphorus source to a sink. Organic matter that accumulated over the growing season decomposes, recycling phosphorus back into the sediment. By autumn, much of the system’s phosphorus has settled into the lakebed, where it can be measured.
Sediment Phosphorus Fractionation – What Is It and Why Does It matter?
Nutrient deactivation in lakes is a management practice used to reduce excess phosphorus in the water. Natural products, often aluminum-based, iron-based, or clay-based, are added to the lake and bind with the phosphorus making it unavailable for use by algae. The nutrient deactivation products form a solid compound that settles into the sediment. Once bound, the phosphorus is no longer available to fuel algae growth, which helps improve water clarity and overall lake health.
Knowing how much phosphorus needs to be bound is the first step in the nutrient deactivation process. Simple water quality tests and sediment tests can provide effective estimates for small ponds and lakes. But for larger lakes, where the cost of nutrient deactivation can be high, sediment testing that dials in the types and amounts of phosphorus available, a process called sediment phosphorus fractionization, can be a worthwhile investment to ensure the right amount of nutrient deactivation product is applied. Too little leads to less-than-optimal results and too much product leads to unneeded expense.
Phosphorus Fractions – Identifying the Types of Phosphorus in Your Sediment
Nutrient deactivation in lakes is a management practice used to reduce excess phosphorus in the water. Natural products, often aluminum-based, iron-based, or clay-based, are added to the lake and bind with the phosphorus making it unavailable for use by algae. The nutrient deactivation products form a solid compound that settles into the sediment. Once bound, the phosphorus is no longer available to fuel algae growth, which helps improve water clarity and overall lake health.
Knowing how much phosphorus needs to be bound is the first step in the nutrient deactivation process. Simple water quality tests and sediment tests can provide effective estimates for small ponds and lakes. But for larger lakes, where the cost of nutrient deactivation can be high, sediment testing that dials in the types and amounts of phosphorus available, a process called sediment phosphorus fractionization, can be a worthwhile investment to ensure the right amount of nutrient deactivation product is applied. Too little leads to less-than-optimal results and too much product leads to unneeded expense.
In sediment, phosphorus does not exist in only one form. Some forms are easily released back into the water column, and some forms are tightly locked away and unavailable for use by algae. Scientists use fractionation, a step-by-step analysis, to figure out which kinds of phosphorus are present and in what quantities to determine how likely the phosphorus in your sediment will cause algae problems.
The Main Phosphorus Fractions
The types of sediment phosphorus are categorized by how available the phosphorus is, or can become, for use by algae.
- Loosely bound phosphorus
Readily available phosphorus.- Can easily move back into the water column.
- Readily available phosphorus is the “troublemaker” that feeds algal blooms.
- Redox-sensitive phosphorus
Phosphorus controlled by oxygen levels.- When there is plenty of oxygen in the water, iron in the sediments holds on to phosphorus, keeping it trapped.
- When oxygen runs out (low or no oxygen at the bottom), that bond breaks, and phosphorus is released into the water.
- This is basically the “on-off switch” form of phosphorus that leaks out of sediments when oxygen conditions change.
- Organic phosphorus
Comes from decomposing plant and animal matter in sediments.- This form of phosphorus is not immediately available but can become available slowly as things decompose.
- Metal oxide phosphorus
The form of phosphorus in lake or pond sediments that gets “locked up” by metals like iron, aluminum, or manganese.- These metals form oxides (like rust for iron). The oxides act like sticky glue that grabs onto phosphorus and holds it in the sediment.
- If oxygen is present, the phosphorus stays trapped.
- If oxygen disappears (like at the lake bottom in summer), the “glue” can dissolve, and the phosphorus is “recycled” back into the water.
- Apatite phosphorus
The form of phosphorus in lake or pond sediments that is locked into minerals, mainly with calcium.- It’s part of the crystal structure of a hard mineral called apatite (similar to what makes up bones and teeth).
- Because it’s tightly bound to calcium, it doesn’t dissolve easily.
- This means it’s a very stable, non-recyclable form of phosphorus — once it’s in apatite, it usually stays buried and unavailable to algae.
- So, if metal-oxide phosphorus is “recyclable,” apatite phosphorus is more like permanently stored.
The Big Picture
- Sediment Fractionation tells lake managers how the types and amount of phosphorus present.
- If most of the phosphorus in a lake is readily available, the lake is at high risk for algal blooms.
- If most of the phosphorus is permanently bound to something, it is unavailable for use by algae.
In Short
Sediment phosphorus fractionation is like sorting the mud’s phosphorus into piles of “easy to use”, “sometimes available”, and “locked away”, which helps predict how much nutrient pollution will recycle back into the lake. This data helps lake managers make ultra-specific recommendations on how to deal with the phosphorus to prevent algal blooms from happening in the first place!
September and October are the best times to conduct these studies! Analyzing your sediment now will give you a huge head start on managing your lake effectively next year!
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