Phosphorus From Within
The Saga of Internal Phosphorus Loading
In an ideal world, a lake with frequent algae blooms could be “fixed” by reducing the load of nutrients to it from the landscape. It seems simple. If too much phosphorus enters the lake, the algae grows to nuisance levels. To reduce the algae, you reduce the nutrient inputs, right? That’s the first step, but it may not be the last.
The problem is excess phosphorus that has probably been flowing into the lake for years. In most cases, more phosphorus enters a lake than leaves it, and the excess is deposited on the lake bottom. Eventually, some of that excess phosphorus will return to the water column to fuel algae growth.
Phosphorus entering a lake from the watershed is called external loading. By contrast, phosphorus originate from within the lake is called internal loading. Internal loading occurs as phosphorus moves from the bottom sediments in either particulate or dissolved form.
Internal loading of particulate phosphorus can result from mixing, typically by wind or boat traffic. As the water is churned, phosphorus-rich sediments are agitated and introduced to the water above, making it turbid and muddy looking. Certain fish and invertebrates that live or feed in the sediments can agitate the bottom as well. Carp are a common culprit. As they go about their lives the only way they know how, they stir up the lake’s sediments. This process is called bioturbation, simply meaning that the turbidity is biological in origin.
Another particulate form of phosphorus is algae itself. Some species of algae actually have a resting stage in which they hang out at the bottom of the lake. During that time, they may pull phosphorus out of the sediments and incorporate it into their cells. When environmental conditions are just right, they float to the surface, carrying all of that phosphorus with them.
Examples of particulate phosphorus loading
Dissolved P Loading
Phosphorus bonds readily to other molecules. That’s how it becomes attached to soil particles and subsequently enters our lakes. The soil particles are heavy and sink to the bottom of the lake. When oxygen is present at the lake bottom, the bonds between phosphorus and whatever insoluble particle it’s attached to are quite strong, and phosphorus tends to stay on the bottom. However, the bonds can be broken when bottom water becomes anoxic (without oxygen). In this situation, phosphorus can enter the water column above. Additionally, bacterial processes create enzymes that can break the bonds holding phosphorus in the sediments, and warm weather can speed these processes by increasing the metabolism of the bacteria.
“Although you might reduce the amount of phosphorus entering a lake there may not be an immediate reduction in the amount of algae, thanks to internal loading.”
Dissolved phosphorus loading
This all means that although you might reduce the amount of phosphorus entering a lake there may not be an immediate reduction in the amount of algae, thanks to internal loading. Eventually equilibrium will be reached with no net release of phosphorus into the water from the sediments. For this to happen, the phosphorus from the top layer of sediments will have to slowly leach into the water and be removed by flushing the lake out with “cleaner” water. This process may take decades, and waiting that long can be a real disappointment to those who have worked so hard to clean up the lake.
An alternative to waiting for the equilibrium to occur naturally is to stop or slow the release of phosphorus from the sediments. Such an action should only be taken after the external load has been reduced. The most drastic option is to dredge the lake bottom. This both increases the lake’s volume and removes the top layer of sediments, which are likely the most nutrient-rich. The chemical ‘alum’ (aluminum sulfate) can also be added, which not only pulls phosphorus out of suspension, but forms a barrier that serves to “lock” phosphorus on the bottom. Aeration is an option that helps keep oxygen near the bottom, thus preventing the phosphorus from switching to a soluble form. Fish removal and plant management are other options for reducing the impact of internal loading. All of these management options come at a cost, though. Dredging has the greatest up-front cost by far, but the other options have recurring maintenance costs that must be considered.
Regardless of the method used to keep internal loading in check, nutrient management starts first and foremost with reducing the external load. Until you can stem the flow of nutrients into a lake, you won’t be happy with the results of any mangagement. And even then, thanks to internal loading, it could take years before any measurable reduction is seen in the amount of algae. This is an important point to consider for folks involved with lake clean-up. Once the external loading is dealt with, it may only be patience or more money that will finish the job.