The development of an algae bloom depends upon local conditions and site-specific characteristics. But they generally occur where there are high levels of nutrients, principally phosphorus, together with warm, sunny and calm conditions.
The main source of nutrients in Lake George is sediment, which has accumulated behind the dams since they were constructed over 100 years ago. This sediment resulted from poor upland soil conservation practices that were generally followed prior to 1960.
What kinds of options are there for removing the algae? Is it harmful?
Typically, the first steps taken target the control of the external sources of phosphorus and can include: encouraging the use of phosphorus free fertilizers; improving agricultural practices, reducing urban run-off; and restoring vegetation buffers around waterways.
Lakes are very slow to recover after excessive phosphorus inputs have been eliminated. Furthermore, it’s extremely difficult to achieve recovery of lake conditions without additional in-lake management. This is due to the fact that lake sediments become phosphorus rich and can deliver excessive amounts of phosphorus to the overlying water. When dissolved oxygen levels decrease in the bottom waters of the lake (anaerobic conditions), large amounts of phosphorus trapped in the bottom sediments are released into the overlying water. This process is often called internal nutrient loading or recycling.
Alum is used primarily to control this internal recycling of phosphorus from the sediments of the lake bottom that result in algae. On contact with water, alum forms a fluffy aluminum hydroxide precipitate called floc. Aluminum hydroxide (the principle ingredient in common antacids such as Maalox) binds with phosphorus to form an aluminum phosphate compound. This compound is insoluble in water under most conditions so the phosphorus in it can no longer be used as food by algae organisms. As the floc slowly settles, some phosphorus is removed from the water. The floc also tends to collect suspended particles in the water and carry them down to the bottom, leaving the lake noticeably clearer. On the bottom of the lake the floc forms a layer that acts as a phosphorus barrier by combining with phosphorus as it is released from the sediments.
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