A Short Primer on Dilution and Flushing for Lake Management

Adding low nutrient water and keeping water moving through a lake can help prevent algae blooms, but there are caveats 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. Lake waters that have low concentrations of an essential nutrient are unlikely to exhibit algal blooms. While it is preferable to reduce nutrient loads to the lake, it is possible to lower (dilute) the concentration of nutrients within the lake by adding sufficient quantities of nutrient-poor water from some additional source.  High amounts of additional water, whether low in nutrients or not, can also be used to flush algae out of smaller, linear impoundments faster than they can reproduce.  

When water low in phosphorus is added to the inflow, the actual phosphorus load will increase, but the mean phosphorus concentration should decrease. Dilution or flushing washes out algal cells, but since the reproductive rate for algae is high (blooms form within days to a few weeks), only extremely high flushing rates will be effective without a significant dilution effect. A flushing rate of 10 to 15% of the lake volume per day is appropriate to minimize algal biomass build-up. That means that the entire volume of the lake has to be exchanged in about a week, which will be a tall order in anything but a smaller pond with a substantial source of additional water.

Outlet structures and downstream channels must be capable of handling the added discharge for this approach to be feasible.  Qualitative downstream impacts must also be considered.  Water used for dilution or flushing should be carefully monitored prior to use in the lake.  Application of this technique is most often limited by the lack of an adequate supply of water.

The classic example of successful flushing is Moses Lake in Washington, which is not a small pond, but had a lot of river water available to flush it. Gene Welch and other studied this project over many years and concluded that flushing worked well for both water quality management and biological improvements. Examples in New England are much harder to come by. The lake in Look Park in Northampton MA has been successfully flushed by diverting a nearby river through it when necessary, but that is the only example that comes to mind.

Dilution is even harder to implement, as it requires very clean water. With phosphates often used to meet the anti-corrosion rule under the Safe Drinking Water Act, use of a public water supply may not result in lower phosphorus concentrations. The MWRA has refilled at least one emergency supply reservoir after winter drawdown with water that might otherwise have gone to consumers, and it has improved the phosphorus concentration. Few such cases are known, however. And even where clean dilution water is available, this may not prevent cyanobacteria from growing at the sediment-water interface using nutrients available there, then rising in the water column to form a bloom when available phosphorus near the surface is minimal.

If dilution or flushing seems like a viable alternative for a lake, the following information is needed to evaluate potential success:

  • Accurate hydrologic and nutrient budgets to allow evaluation of potential benefits
  • Assessment of probable in-lake effects and an evaluation of downstream impacts
  • Reliability of source water
  • Routing information for new water source
  • Monitoring program to track changes in detention time, nutrient levels and water clarity

Factors favoring the use of this technique include:

  • Actual reduction in nutrient inputs from identifiable sources is not practical, either for technical or jurisdictional reasons
  • Water level fluctuation will not differ greatly from pre-treatment conditions
  • Adequate water of a suitable quality is available for dilution or flushing
  • Downstream problems with water quantity or quality will not be caused

Permits for such use of water are typically required, and potential applicants should consult with their local and state environmental agencies.

Warning: Public water may contain high P!

Moses Lake in Washington, successful flushing.

The Value of Natural Shoreline

Beyond the buffer zone lies the actual lake shore, which can vary tremendously among lakes without any human intervention. Yet with “management” of the shoreline by people, we get an extremely wide range of conditions and features. Dense vegetation, lawns, natural rocks, riprap, or any combination thereof is possible. And the adjacent shallow water area is just as important, with conditions ranging from a jumble of rocks and woody debris to open sand under natural conditions and often a sterile open water habitat when people decide to “clean up” the nearshore zone.  

Findings from work in Vermont more locally and the National Lakes Assessment more nationally have documented how important the condition of the shoreline and nearshore zone is to overall lake function and species diversity. More fish, more birds, more reptiles and more amphibians can all be expected when the shoreline is in a natural condition, documented in multiple papers in Lake and Reservoir Management, including a 3-paper series by Kaufman et al. in 2014, based on data generated in the National Lake Assessment.

A useful environmental guideline is that if we don’t clearly understand all the linkages in a habitat, leave it as natural as possible for best results. The reasoning behind this is that the condition of the habitat is a complex function of many factors and has evolved into what it is over time as a function of those factors, and disturbing those conditions is not likely to have a desirable result unless we have a clear understanding of all the factors and how they interrelate. Much as with buffer zones, however, there is a human tendency to want to organize the shoreline and nearshore zone, to give it order and aesthetic appeal based on some innate sense of what it should look like. Beauty may be in the eye of the beholder, but functionality can be objectively assessed and rarely corresponds to what most people think is pretty.

New Hampshire and Vermont have shoreland protection laws that encourage natural shorelines, but the protection of the nearshore zone seems less clear (or enforceable). The Wetlands Protection Act in Massachusetts (and similar statutes in most New England states) provides some protection of both shorelines and the nearshore zone, although that protection is subject to interpretation by locally empowered commissions. There is generally solid recognition of the value of lake edges in New England, but actual management of this zone is not as well developed as we might like.

Pretty but not very functional as habitat

Much better habitat, not the best access

Buffer Zones for Lake Protection

Buffer zones are intended to provide a vegetative filter for runoff approaching a waterway, acting to trap particulates and absorb flows to minimize the entry of contaminants into streams and lakes. Buffer strips provide additional habitat benefits, and may be the best way to prevent problems by acting on a watershed-wide basis. The desirability is undeniable, but effectiveness is a complex function of buffer zone width, slope, soils and vegetative features. Engineered buffer zones can be very effective, but may not be overly natural or attractive. Natural buffers have been found to require substantial width (>100 feet, with removal still increasing at widths >500 feet), and few landowners are willing to concede enough land to buffers to maximize effectiveness. Any buffer is a good thing, but if buffers are to be a major factor in water quality management, they need to be wide enough and well-structured to hold and process runoff.

Perhaps the biggest impediment to use of buffers is a societal tendency to favor open lawn areas and avoid high and dense vegetation. Sociologists have suggested that this is a primal response related to personal safety, as though tigers might be hiding in the high grass by the lake! Landscape architects have suggested that it is an aesthetic issue, with disorganized, natural growths suggesting sloppiness and lack of care by the owner. Robert Kirschner of the Chicago Botanical Garden gave a great talk called “What will the neighbors think?” a few years ago at the NECNALMS conference, in which he described the steps lakefront (or streamfront) property owners could take to make effective buffer strips look organized and acceptable within the neighborhood. In Maine, the LakeSmart program recognizes shorefront owners who maintain landscapes that minimize runoff impacts on lakes. We need both a cultural shift in what is “acceptable” and application of clever guidance that facilitates effective buffers without losing aesthetics.

A landscaping compromise to add buffer functionality without sacrificing aesthetics.

Alewife in Lakes

This is a hot button topic in several New England states, eliciting citations of contradictory research and a lot of emotion, depending on one’s priorities for lake management. The fundamental issue is that alewife (usually Alosa pseudoharengus, but other related herring species are sometimes involved) feed by straining water through gill rakers for plankton, much like baleen whales filter out krill and larger plankton in the ocean, leaving very low biomasses of zooplankton for other fish to eat. Where adult alewife enter lakes from the ocean  as anadromous fish, spawn, and leave, the juveniles remain during summer and decimate the zooplankton, but those lakes tend to evolve ecologically, with zooplankton reaching winter maxima and managing to support the food web. But where a landlocked alewife population is established, zooplankton density may be depressed all year long, severely limiting food for small fish that feed visually on large zooplankton. Alewife may provide a valuable food base for some gamefish, but do so at the expense of the rest of the fish community.

Restoring a historic alewife run is therefore a very different policy decision than stocking alewife into a lake where they will be landlocked. For fishing enthusiasts, the potential for trophy gamefish is attractive when a landlocked alewife population is established. But for those interested in clear water, the alewife will promote the greatest amount of algae possible for whatever level of fertility is present. In a lake with enough phosphorus to support algae blooms, the presence of alewife all year is almost a guarantee of blooms, whereas a different fish assemblage might minimize bloom potential by favoring zooplankton control of algae.

If stocking of alewife was accompanied by a nutrient control program to limit the potential for algae blooms, it might be more palatable, but fishery agencies that are the normal advocates for such stocking have never been in the business of overall lake management, and are not known to be advocates for nutrient control. This lack of a more holistic approach to lake management has led some to refer to fishery agencies as government of the fish, by the fish, and for the fish. At the same time, those focused on water clarity have been called Secchi disk worshipers and such a narrow focus is also not holistic. Those promoting anadromous alewife runs are on pretty sound ground, but the debate goes well inland from coastal areas.

Alewife, Alosa pseudoharengus