Category Archives: In-Lake Management

A Short Primer on Oxygenation for Lake Management

Circulation is one way to oxygenate a lake, but here we consider methods that do not destratify the lake. These are often called hypolimnetic oxygenation, referring to the bottom layer of a stratified lake as the target of the action. In some cases it is important to maintain stratification, either to support a coldwater fishery or to keep separate water layers for some supply function. There are four basic ways to add oxygen to deeper waters without causing mixing and a lot of information to gather and consider when designing an appropriate oxygenation system. 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.

Hypolimnetic oxygenation is a technique for increasing deep water oxygen and managing algae through control of nutrient levels. The central process is the introduction of more oxygen, intended to limit internal recycling of phosphorus, thereby controlling algae. The extra oxygen also reduces the accumulation of other undesirable substances, such as iron, manganese, ammonia and hydrogen sulfide, which makes this process attractive for water supplies. The four general approaches include hypolimnetic aeration chambers, speece cones, diffused oxygen, and sidestream supersaturation.

Hypolimnetic aeration chambers use an air compressor as with whole lake circulation, but in this case the upward plume is contained in a chamber to avoid mixing with the surface waters; water is pulled in, oxygenated by air contact, and returned to near where it came from initially.  Use of air can be effective but is not efficient; transfer is slow and the vertical distance of contact rarely allows more than 30% of the oxygen in air (which is only 21% oxygen to begin with) to be transferred. A lot of water has to be moved through the chamber to reach common oxygen goals, and the cost of power to do that can be substantial.

Speece cones are chambers where deep water is pumped in along with pure oxygen, and the rate of input of each is supposed to balance, such that all the oxygen is dissolved in the water. The oxygenated water is then put back in the deeper part of the lake. Any excess oxygen rises to the surface or is trapped in the chamber; at best this is a waste, and at worst it could cause unintended mixing. Installation of Speece cones in a lake can be challenging, and maintenance can be tedious. This approach can be effective and efficient, but tends to be hard to operate well.

Diffused oxygen is a simple process of releasing small bubbles of pure oxygen into the water column at a deep enough depth to allow complete absorption before the bubbles can rise enough to cause destratification. This generally requires about 20 feet of bubble travel distance, so this approach may not be applicable to lakes with only a thin layer of bottom water. In earlier years, the delivery mechanism had not been perfected, but these days adding diffused oxygen is fairly straightforward and reliable. Liquid oxygen is allowed to convert to gas and moves through delivery lines under the pressure of the gas, resulting in very low power use that offsets the cost of the pure oxygen.

Sidestream supersaturation involves removing water from the targeted deeper water layer, oxygenating it in chambers on land, and putting it back in the deep zone. Speece cones are sometimes used, but rather than placing them in the lake, they are on land and used to supersaturate withdrawn water. As long as the temperature of the withdrawn water has not been increased (which can be a risk when supersaturating), the oxygenated water moves mostly laterally in the target area. This allows thinner layers of bottom water to be oxygenated than would be possible with diffused oxygen. This approach is still being improved, but shows great promise.

Proper application requires the following information:

  • An accurate nutrient budget with a detailed analysis of internal sources of phosphorus
  • The oxygen demand that must be met by the system; calculations and related interpretation for design purposes are best performed by experienced professionals
  • Lake morphometry and stratification data to facilitate choice of system and features for maximum effectiveness
  • Location and details of any land based facilties and the power source

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, pump and/or oxygen storage 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 abundant or an important goal

Hypolimnetic oxygenation is fairly common in drinking water reservoirs but less often found in recreational lakes, as ongoing management and related costs can be substantial. Permits are required, and potential applicants should consult with their local and state environmental agencies.

Four methods of hypolimnetic oxygenation

A Short Primer on Circulation for Lake Management

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

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

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.

What is a Balanced Fish Community?

Fishery management is very challenging. It involves physical, chemical and biological factors, plus the interactions between them. There are multiple life stages for each species, multiple species, and multiple trophic levels. The community is in constant flux, so stability is not a very applicable term. Instead, we tend to think in terms of a balanced fish community, one in which the various components are found in relative proportions that minimize fluctuations in year class strength, overall biomass, and energy flow. There is no one “right” combination, but a balanced fish community will have predator and prey species at a ratio that prevents either from being dominant. A balanced fish community will sometimes have strong temporal variations, such as when young of the year are produced and there are suddenly many small fish present, but predation is expected to foster an annual cycle of increase and decrease that maintains balance.

Range of species and sizes.

Lakes provide a variety of fish habitats and food resources, but not all lakes are created equal and some are better suited for some species than others. A deep lake with a cold bottom layer with adequate oxygen through the summer will support trout much better than a shallow lake with only a warmer upper layer during summer. Species such as pickerel and pike are “sit and wait” predators, doing best in lakes with ample submerged vegetation where they can hide until prey come near, while others such as walleye tend to prowl open water chasing schools of smaller fish. Largemouth and smallmouth bass are intermediate in predator tactics, with slightly different temperature and cover preferences. Prey species have similarly variable preferences, including yellow and white perch, multiple sunfish species, and a suite of minnows. And larger specimens of many prey species become predators of smaller fish. So there are many considerations in managing a fishery, and achieving balance can be elusive, but similarity of the results of successive surveys over 3 to 5 year intervals is a good indication of balance.

Overabundant and stunted sunfish.

In theory, stocking fish can help maintain balance if it compensates for weak year classes of key species, or adds species that occupy habitats and/or consume food resources that are otherwise not fully utilized, but that is not typically how stocking is used in New England states. Rather, gamefish are stocked to support fishing where natural production is inadequate. Prey species are stocked to support gamefish growth, but usually this involves species that overlap with existing species and destabilize the fish community.  Landlocked alewife represent a controversial example; they provide an impressive forage base but out-compete other fish by efficient filtering of all but the smallest zooplankton from the water column. Good fishing is usually the key goal of fish stocking programs, which is not mutually exclusive with community balance, but the two often do not go hand in hand.

Top Down vs. Bottom Up Control of Algae

A lot of research over about two decades led to the conclusion that algal abundance may be strongly influenced by cascading influences from the top of the food web. Certainly low nutrient availability will minimize algae biomass, but just because nutrients are abundant does not always translate into high algae biomass. The reason appears to be that a favorable biological structure can result in rapid consumption of algae as they are produced, preventing the build-up of algal biomass. Having abundant large predatory fish reduces the number of small fish, which in turn reduces predation on zooplankton, allowing greater grazing pressure on algae. Not just any zooplankton will depress algae, however, and the forms that generate the greatest grazing pressure (large bodied herbivores, especially Daphnia) are also the preferred food of many small fish. But a greater biomass of big grazing zooplankton will increase consumption of algae, resulting in the lowest biomass of algae for whatever level of fertility is present.

Small (Chydorus) cladoceran zooplankton

So can the most favorable biological structure possible achieve algae control when nutrients are abundant? The answer appears to be “no”. At phosphorus levels in excess of about 80 ug/L, research in Europe has found that no amount of zooplankton can prevent algae blooms. As blooms are probable at phosphorus levels above about 25 ug/L, there is an intermediate zone where biological structure can make a major difference, but nutrient controls may be necessary to prevent blooms. And since some cyanobacteria grow to large particle sizes at the sediment-water interface before rising in the water column, presenting zooplankton with a difficult challenge for consumption, even low nutrients in the water column and an elevated biomass of large bodied zooplankton may not be able to control cyanobacteria.

Large (Daphnia) cladoceran zooplankton

Creating a favorable biological structure provides benefits and is strongly encouraged. Nutrient control is clearly a critical part of algae management. But there is not much reason for a debate over the relative value of bottom up vs. top down controls; we need every tool we can get to keep our lakes in desirable condition!

Understanding Detention Time and Flushing Rate

The average length of time which water spends in a lake is quite simply the volume of the lake divided by the inflow of water from all sources. The flushing rate is the inverse of detention time, or the amount of time it takes to replace the volume of a lake. This is best understood as an example.

A 100 acre lake with an average depth of 10 feet holds 1000 acre-feet of water, or 43.56 million cubic feet. If surface water inflows average 10 cubic feet per second, that adds 864,000 cubic feet per day. If direct precipitation is 46 inches per year, that adds another 383 acre-feet of water per year, or about 46,000 cubic feet per day. If groundwater in-seepage contributes another 20,000 cubic feet per day, the grand total of water inputs would be 930,000 cubic feet per day, with the surface inflow being dominant in this case (which is not always the case, so adding in direct precipitation and groundwater is important). The volume of 43,560,000 cubic feet divided by an average inflow of 930,000 cubic feet per day yields a detention time of just under 47 days. The flushing rate would be just under 8 times per year.

In the example above, if the inflow was less, the detention time would increase and the flushing rate would decrease. If more water was somehow put in the lake, the detention time would decrease and the flushing rate would increase. One could use sources of outflow to get detention time and flushing rate, with surface outflow, evaporation, and out-seepage of groundwater as the primary outflows (unless there is a major withdrawal of some kind), but the inflow and outflow have to match over the long run for the lake to remain stable, and it is easier to get values for inflow sources than outflows.

A key point is that it is only the volume of the lake and the total inflow or outflow that matter to detention time and flushing rate. One cannot change these values by altering an outlet structure, dredging a channel, or other physical manipulations within the lake. If no more water is added or subtracted from a lake with the same volume, the detention time and flushing rate do not change. Dredging and other lake manipulations may alter the circulation pattern, and if the lake is subdivided into parts, this may affect detention and flushing of one part, but the overall detention time and flushing rate cannot change without a change in lake volume or inflow quantity.

Detention and flushing rate are important to lake function and many management options. A lake with a detention time of less than about two weeks is unlikely to develop algae blooms, as the water does not stay around long enough to let blooms form. Lakes with very long detention times, more than a year, are less subject to watershed influences on a day to day or even season to season basis; there is simply not enough inflow to alter water quality over a short space of time. And most water quality models that allow us to predict changes with specified management actions depend on detention time or flushing rate as an important term in the mathematical model; being off by even a small amount affects calculations and reliability.

Detention time and flushing rate are not constants, however, and vary over time with changing inflows. For lake with long average detention times, this is not a major influence, but for lakes with average detention times of days to a few months, the variation within a year can be meaningful. A lake with an average detention time of a month could experience much lower summer inflows and have the same water present for 3 months, while spring thaw and related snowmelt and rain reduce the detention time to a matter of days in April.

Detention time and flushing rate can vary over space as well. A “dead end” part of a lake may have a much longer detention time and be flushed much less than some area in the main path of inflows, leading to stagnation and possible water quality problems in that dead end cove or segment of the lake. Rerouting water through the dead end may improve circulation and reduced detention time for that area but unless this is new inflow to the system, it will not change the average detention time for the whole lake.

One other related concept is important to understand when managing lakes. If a lake is long and linear, with most inputs at one end and the outlet at the other end, as with certain human-made reservoirs, the movement of water is called “plug-flow” in engineering terms, and is similar to water moving through a garden hose. Each increment of inflow pushes the next increment forward, and while there will be some mixing, water entering the lake at any time will have more or less similar detention time to water entering at any other time, with variation due mainly to changes in inflow rate. An algae bloom that forms in such a lake will be removed after about one flushing, or replacement of the equivalent of about one lake bottom.

But for the more typical lake configuration of a bowl with various input sources and an outlet somewhere along shore, the engineering model is called a “continuously stirred tank reactor”. In this model, each increment of water added is completely mixed with all other water in the lake, and while average detention time is calculable, the actual detention time for any increment of water can be highly variable. Because of the mixing, it takes much longer to get any undesirable water mass flushed out of the system. Just replacing the water volume of the lake one time (one flushing) will not get rid of all of an algae bloom or a spike of phosphorus or an oil spill; it takes 3 to 5 flushings to clear the lake.

Sources of Plankton Blooms

For many years it was generally thought that planktonic algae blooms involved a few resting stages germinating from the bottom or a few cells hanging on from some previous time in the water column encountering adequate nutrients under sufficient light, resulting in enough growth to discolor the water and be called a “bloom” (Note: there is no technical definition of a bloom; it is in the eye of the beholder, although there is general agreement that there should be reduced clarity and increased color involved). This mode of bloom formation is certainly possible and does occur, but for the harmful algal blooms currently getting so much media attention, there are two other modes of bloom formation that seem to be dominant.

One mode is metalimnetic accumulation, which means that algae accumulate in the boundary layer between upper, well-lit, warmer waters (the epilimnion) and lower, darker, colder waters (the hypolimnion). This can only happen where the lake is deep enough to develop a thermal gradient, usually more than 20 feet in New England, but thermal stratification can develop in shallower lakes under the right circumstances. Algae in this zone get enough light from above and enough nutrients from below to grow into a dense layer which may be disrupted by wind or may synchronously rise in response to lower light availability or changes in temperature that signal breakdown of thermal stratification. These blooms are most often evident in late summer or early fall, but can be moved into upper waters by summer storms. Several types of golden algae that produce taste and odor use this mode of growth, and water supplies with intakes in this boundary layer have to deal with them. Several blue-green algae known for toxicity also utilize this mode of growth and are a threat to both human health and lake ecology.

The other mode involves growth at the sediment-water interface, much like algae mats, with an eventual rise in the water column, again like the mats. But we think of these algae as planktonic forms, and never really considered their origin. It appears that these algae, mostly blue-greens and often potentially toxic forms, gain most nutrients from decay or other releases from sediment, accumulate excess amounts in cells, grow to full “maturity” on the sediment, then produce gas pockets in cells and rise over just a few days to form a ready-made bloom at the surface.

These bottom-formed blooms are particularly problematic in lakes known for being “clean” for many decades. The inputs from the watershed over many years were assimilated into the bottom sediments and at some point the balance tipped, such that the accumulated nutrients, especially phosphorus, became available for uptake at the sediment-water interface. It appears that this is related to oxygen loss in the surficial sediment, largely a function of organic build up. The water column may still have low nutrient levels, but the rising blue-greens already have enough extra nutrients to survive for days to a few weeks, and the resulting blooms seems to come out of nowhere to impair lake use and threaten ecological functions. Worse yet, this appears to bring nutrients into surface waters, allowing follow on blooms when the initial algae die off. This mode of bloom formation is both a harbinger of eutrophication and a vector of it.

Planktothix sp., a metalimnetic bloom former

Dolichospermum lemmermannii, a bottom bloom former

Algae Mats

Two groups of algae form almost all the mats: green and blue-green algae. These mats mostly form on the bottom, utilizing nutrients at the sediment-water interface, then move upward as they trap their own photosynthetic gases or accumulate gas released from the sediment under thick tangles of filaments. These mats may continue to grow for a time at or near the surface as a function of stored nutrients from their time on the bottom or from additional nutrients in the water column. Yet ultimately they tend to wind up on the surface, often blown to the edges by wind, in large decaying masses that turn various colors from yellow to blue and may be quite malodorous. In great quantities, they can really detract from the lake experience.

One partial exception includes the “cotton candy” or “cloud” growths of certain filamentous greens, mostly in the Spirogyra group. These algae do get their start at the bottom, but grow upward in a loose, slimy affiliation that looks like a mass of light green cotton candy or a cloud in the water. When you try to grab it, there is not much to grab, but your hand feels slimy. These algae produce a lot of mucilage, hence the slimy feel, and have enough structural strength to expand into these underwater, microscopic, “tinker-toy” conglomerations.

Sometimes a green mat will remain anchored to the sediment while part of it floats upward, creating a pillar in the water. Blue-green mats of Plectonema are brown to black and don’t rise in New England lakes until late summer, if at all, but from about Maryland south they can be a major impediment to lake use from early summer on. In New England, blue-green surface mats are most often chunks of Oscillatoria that break free of the bottom; these are very dark blue to black, often with brown sediment on the underside (still attached from the bottom) and they often have a distinct and unpleasant odor.

Algae mats are a clear indication that nutrients have accumulated in the sediment in water shallow enough for light to penetrate to the bottom. Once formed, algae mats are very hard to kill, as the outer filaments protect the inner filaments to a large degree. Removing the sediment is the most effective approach, but is very expensive and involves an often tedious permitting process. Treating the sediment with a phosphorus inactivator or algaecide before dense mats form is often effective, but results are not permanent.

Green algal mats   

Blue-green algal mats