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

The Concept of Limiting Factor for Algae

The concept of a limiting factor is very old, summarized in Leibig’s Law of the Minimum, which used a bucket with staves of different lengths to show that the shortest stave controlled the depth water could attain in the bucket. The theory is simple, perhaps too simple to capture the variation induced by having multiple species present with varying optimal nutrient ratios, light requirements, and maximum growth rates. In reality, multiple factors control the growth of algae, with some species more impacted by one factor than others. There is therefore constant competition going on, with each species increasing in abundance to the extent possible before some species-specific limit is reached based on the relation of the individual species’ needs and available resources.

There are very few generalizations that apply to all algae with regard to limiting factors. Light is very important to photosynthesis, but some algae survive under very low light and some are “facultatively heterotrophic”, consuming organic compounds, bacteria, other algae, and even zooplankton to gain energy rather than depending on photosynthesis. Nitrogen is a key nutrient, but cyanobacteria with specialized cells called heterocytes can convert dissolved nitrogen gas into ammonium. Since our atmosphere is 78% nitrogen, it is virtually impossible to prevent these algae from acquiring some nitrogen. Phosphorus is the closest thing to a sure limiting factor, being relatively rare in the earth’s crust but critical to energy transfer. There is no substitute for phosphorus, and it can be controlled by multiple means in active management practices, so it is the logical target of choice in algae control programs that seek to prevent algae growth to bloom proportions.

Approaching algae management with multiple limiting factors is a good idea. In many cases, best management practices that control phosphorus also limit nitrogen and other nutrients. Increased flushing can reduce detention time such that growth rates are insufficient to generate blooms. Adjusting the fish community to encourage more and larger herbivorous zooplankton can increase algae consumption rates and funnel energy into desirable fish while limiting algae biomass. Yet control over planktonic algae will lead to deeper light penetration, which may provide benthic algae enough light to grow using nutrients available at the sediment-water interface during decomposition or other release mechanisms. Dredging can remove algae resting stages and nutrient reserves, limiting benthic production, but the cost is usually extreme.  

Using the concept of limiting factor(s) for algae is appropriate but more than a little complicated. The situation is far more complex that the simple bucket analogy suggests, and the more we can learn about the lake we want to manage the more successful we are likely to be in achieving our goals.

NECNALMS Leaders Meet

Representatives of all six New England states met in Concord in early December to review programs and get updates on lake issues in each state. The spread of invasive plants and increased frequency of cyanobacteria blooms continue to be the primary biological threats. Retirements and reduced staffing in state agencies represent the primary administrative issue. Funding cutbacks, especially for federal “pass-through” monies, constitute the greatest economic disincentive for lake management. Yet the demand for lake management remains high, and many lake associations and towns have been addressing issues on their own. NECNALMS continues to seek ways to support efforts by New England states to foster effective and sound lake management.

Lake leaders discussed past and future conferences. New Hampshire is due to host NECNALMS in 2018, but with the normal organizers very involved in national efforts by NALMS and changes in policies at the typical conference venues, it is not certain that there will be a 2018 NECNALMS conference. It is possible that NALMS will come to New England in 2019, in which case the NECNALMS leadership will be very involved in planning that conference starting in mid-2018. A decision will be made soon.

The 2017 NALMS symposium was held in early November just outside Denver, CO, and was both well-attended and well-run. The program was diverse and opportunities for interactions were plentiful. The Colorado Lake and Reservoir Management Association was the local host, and did a fine job on the arrangements. NALMS has been experiencing some financial stress relating to federal freezes on programs and funds, but is managing carefully to avoid shortfalls. The 2018 conference will be held in Cincinnati, OH and a decision on the 2019 location should be made this coming spring.

NECNALMS leaders are also very active on the national and international levels through NALMS. Perry Thomas of VT serves as the Region I director, while Amy Smagula of NH is the NALMS Secretary. Jeff Schloss of NH is the conference planner, while Ken Wagner of MA returns to duty at the start of 2018 as Editor-in-Chief of the NALMS peer-reviewed scientific journal, Lake and Reservoir Management.

Forms of Nitrogen

Nitrogen comes in multiple forms but, much like phosphorus, not all are available to all algae and plants. Total nitrogen is akin to total phosphorus, providing a maximum estimate of nitrogen that might be utilized for plant and algae growth, but different species make better use of some forms than others, and the range of nitrogen forms is wider than for phosphorus, so relationships are more complicated.

Nitrogen gas makes up about 78% of the earth’s atmosphere and reaches equilibrium with the aquatic environment, but only some specialized organisms can use nitrogen gas directly. Included are certain cyanobacteria, or blue-green algae, which is why a low ratio of non-gaseous nitrogen to phosphorus favors cyanobacteria; they have access to a nitrogen source that other algae can’t use.

One key set of nitrogen compounds is the sequence from ammonium to nitrite to nitrate, with conversion of ammonium to nitrite and nitrite to nitrate in the presence of oxygen and specific bacteria. The conversion is fairly fast, especially for nitrite, so nitrate should be the most abundant of these three nitrogen forms when oxygen is abundant. When oxygen is used up, as can occur in the deep zone of lakes over the summer during stratification when decomposition uses up oxygen and atmospheric replenishment is minimal, ammonium accumulates. Ammonium and nitrate are differentially preferred by different species of algae and higher plants, and so can affect aquatic biology. Ammonia, which has one less hydrogen molecule than ammonium and is toxic to aquatic animal life, is present as a fraction of ammonium depending on temperature, pH and other water quality factors, upping the stakes for which forms of nitrogen are present at what concentration.

Total Kjeldahl nitrogen (TKN) is determined by a digestion that turns organic nitrogen into ammonium, so this test measures all but nitrate and nitrite nitrogen. Addition of nitrate/nitrite to TKN is functionally equivalent to total nitrogen. The organic fraction (TKN minus ammonium) is not directly available to support plant growth, but decay processes will make some of that organic fraction available over time.

When assessing nitrogen in lakes and tributaries, the minimum testing to gain reasonable understanding of nitrogen influence includes TKN and nitrate/nitrite, although it is useful to have ammonium as well to separate the organic fraction from TKN.


Forms of Phosphorus

Just about everyone working with lakes knows that phosphorus is a key factor in many undesirable features of lakes, most notably algae blooms and. by extension, oxygen and pH fluctuations that impair habitat. But not all phosphorus is created equal. Soluble reactive phosphorus, usually orthophosphate, is the most available form, but is usually only a small fraction of the total phosphorus in any sample. In fact, soluble reactive phosphorus can cycle so fast that its actual measured quantity is not all that important in the interpretation of water quality; low concentrations are normal even in eutrophic lakes.

Total phosphorus is useful as a measure of maximum available phosphorus, but some portion of that total will be refractory, unavailable for uptake by algae. Yet nearly all the empirically determined relationships between phosphorus and other limnological features (e.g., chlorophyll, water clarity) are based on total phosphorus, so measuring total phosphorus is generally an essential part of any lake or tributary monitoring program.

The utility of everything in between soluble reactive and total phosphorus is a matter of some speculation. Empirical work over two decades ago found that total dissolved phosphorus, which is assessed the same way as total phosphorus except that the sample is filtered first, correlates best with algal growth potential. Total dissolved phosphorus is therefore a very useful back-up measurement to go with total phosphorus.

There are other forms of phosphorus that can be measured, and more than one way to measure many of the forms of phosphorus, so there are decisions to be made in any monitoring program that affect results, utility and cost. It is not a simple matter of measuring soluble reactive phosphorus, which is easiest and cheapest to assess. Care should be taken in the choice of phosphorus forms to be measured, the methods for measurement, and the use of resulting data.

Autoanalyzer used for phosphorus measurement

Forming and Running a Lake Association

Creating and managing a lake association is hard work, if you do it right. There are many types of lake organizations, including homeowners associations, watershed alliances, and lake management districts. They differ in the details of formation and their power over members, including taxation, rulemaking, and enforcement. Communication is a central and critical role of each, and can take many forms. Overall, there is a lot to know before starting a lake association and still more to know to run one successfully. NALMS produced a very useful pamphlet on this topic back in the 1980s, which is out of print and out of date, although it still has some great insights and advice. Producing a new, updated version would be another great service for NALMS or NECNALMS to offer. Just sayin’…

Lake Science for Real Estate Agents

If you have a background in lake science and have ever hunted for a lakefront home, you may have been frustrated with real estate agents who don’t know much about the lake itself and use words like pristine for any lake that doesn’t look like a putting green. NALMS developed a program to educate real estate agents about lake features and processes that are important to most lakefront property owners over two decades ago, before the internet was the force it is today. It seems that a rework is in order, one that could link state databases with real estate listings to give searchers useful background information on things like the presence of invasive species, water quality, or management practices that affect lake use, such as drawdown. At the same time, it would be nice to provide basic training for real estate agents so that they would know how to properly describe a lake in terms that a range of buyers could understand. This won’t keep unscrupulous sellers from making inappropriate claims, but it could allow buyer representatives (an increasingly common role for real estate agents) to be better informed and provide what the sellers are really looking for in a lakefront property. Is NALMS listening? Might NECNALMS take this on as a project in New England? Anyone up to the challenge?