Forcing water in a lake to circulate can oxygenate deeper waters and can also exert some direct control over algae growth, but there are risks and limits. The Practical Guide to Lake Management in Massachusetts, available online from the DCR within the Mass.gov website, has a useful review of this technique, and a 2015 publication available from the Water Research Foundation has considerable additional detail.
Whole lake circulation is a technique for management of algae that tends to affect nutrient levels. The central process is the introduction of more oxygen, intended to limit internal recycling of phosphorus, thereby controlling algae. Other important processes may apply as well, however. Circulation strategies increase turbulence and minimize stratification. Whole lake artificial circulation is also referred to as destratification or whole lake aeration. Thermal stratification and features of lake morphometry such as coves create stagnant zones that may be subject to loss of oxygen, accumulation of sediment, or algal blooms. Artificial circulation minimizes stagnation and can eliminate thermal stratification or prevent its formation. Movement of air or water is normally used to create the desired circulation pattern, and this has been accomplished with surface aerators, bottom diffusers, and water pumps. Algae may simply be mixed more evenly in the available volume of water in many cases, but turbulence, changing light regime and altered water chemistry can cause shifts in algal types and reduce biomass.
The use of air as the mixing force also provides some oxygenation of the water, but the efficiency and magnitude of this transfer are generally low. In some instances, wind or solar driven pumps have been used to move water. For air mixed systems, the general rule is that an air flow rate of 1.3 cubic feet per minute per acre of lake (9.2 m3/min/km2) will be needed to maintain a mixed system. However, there are many factors that could require different site specific air flow rates, and undersizing of systems is the greatest contributor to failure for this technique. The objective is to move at least 20% of the target water volume per day, and in cases where oxygen demand is very high, it may be necessary to move all the water every day.
Algal blooms are sometimes controlled by destratification through one or more of the following processes:
- Introduction of dissolved oxygen to the lake bottom may inhibit phosphorus release from sediments, curtailing this internal nutrient source.
- In light-limited algal communities, mixing to the lake’s bottom will increase the time a cell spends in darkness, leading to reduced photosynthesis and productivity.
- Rapid circulation and contact of water with the atmosphere, as well as the introduction of carbon dioxide-rich bottom water during the initial period of mixing, can increase the carbon dioxide content of water and lower pH, leading to a shift from blue-green algae to less noxious green algae.
- Turbulence can neutralize the advantageous buoyancy mechanisms of blue-green algae and cause a shift in algal composition to less objectionable forms such as diatoms.
- When zooplankton that consume algae are mixed throughout the water column, they are less vulnerable to visually feeding fish. If more zooplankters survive, their consumption of algal cells may also increase.
The main drawbacks relate to the difficulty of maintaining mixed conditions in a lake. It is very hard to mix a lake from top to bottom, and mixing near the bottom may entrain sediment and increase turbidity by resuspended particles. And it gets harder to mix a lake during prolonged hot, dry periods as the lake heats up. The energy necessary to overcome a thermal gradient increases as the water warms; the energy required to mix layers with temperatures of 20 an 25 C is much higher than for layers with temperatures of 10 and 15 C, even though both have 5 C differences.
Further, not all of any lake is deep enough to impose a light limitation, and mixing may actually make nutrients more available to algae. This situation can be greatly aggravated if the mixing system is not run continuously or is undersized, as nutrients may build up in deep water then be moved upward by the mixing system. Providing enough power to mix the entire target area and distributing that power so that it is effective are critical aspects of any circulation system.
Proper application requires the following information:
- An accurate nutrient budget with a detailed analysis of internal P sources
- Data related to each of the five possible control mechanisms (oxygenation/P inactivation, light limitation, pH/carbon source adjustment, buoyancy disruption, and enhanced grazing) should be analyzed and evaluated in terms of potential algal control. Specifically:
- Is there anaerobic release of phosphorus that can be mitigated by oxygenation of deep waters?
- Is the mixing zone deep enough to promote light limitation of algae?
- Is there a large amount of carbon dioxide in the bottom waters that could be mixed to the surface to favor the growth of non-blue-green algae?
- Is mixing predicted to counteract the buoyancy advantage of blue-greens over other algae?
- Will a dark, oxygenated refuge be created for zooplankton?
- Reliable estimate of the oxygen demand that must be met by the system
- Reliable estimate of the amount of air necessary to mix/destratify the lake
- Lake morphometry data that facilitates choice of aerator type and placement of aerators for maximum effectiveness
- Location and details of compressor and power source
- Monitoring to track oxygen and nutrient levels after implementation
- Monitoring to track water clarity and algal types and quantity
Factors favoring the use of this technique include:
- A substantial portion of the P load is associated with anoxic sediment sources within the lake
- Studies have demonstrated the impact of internal loading on the lake
- External P load has been controlled to the maximum practical extent or is documented to be small; historic loading may have been much greater than current loading
- Hypolimnetic or sediment oxygen demand is high (>500 mg/m2/day)
- In addition to phosphorus management, control of other reduced compounds such as hydrogen sulfide, ammonia, manganese and iron, is desired
- Adequate phosphorus inactivators are present for reaction upon addition of oxygen
- Shoreline space for a compressor or pump is available where access is sufficient, power is available, and noise impacts will be small
- The lake is bowl shaped, or at least not highly irregular in bathymetry (few separate basins and isolated coves)
- Long-term application of the technique is accepted
- Coldwater fishery habitat is limited or not a concern
There are many possible circulation systems available. Air-driven systems are most common, and deliver compressed air to diffusers placed appropriately to maintain mixed conditions. Pumps that move water up or down are also popular. Pumps that move the water upward run the risk of worsening surface water quality if undersized. Pumping oxygenated water with algae from the surface to greater depth is theoretically more sound.
Permits are required, and potential applicants should consult with their local and state environmental agencies.
Whole lake circulation with diffused air