All posts by NE

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!

Evaluating Risk Associated with Cyanobacteria

Cyanobacteria have taken center stage in many newscasts in the last couple of years, and have gotten more attention from the USEPA and state environmental agencies in the last 3 years or so than in the two decades before that. Cyanobacteria are not new; they are among the oldest organisms on the face of the earth, and are adapted to life under a wide range of conditions. But they do best in warm, freshwater with elevated phosphorus levels and a low ratio of nitrogen to phosphorus. Urbanization and agricultural use of land tends to create conditions that favor cyanobacteria, and the warmer temperatures related to climate change are fostering increases in blooms. All of this has been well documented in scientific literature with minimal bias; all political wrangling aside, we understand why cyanobacteria are becoming a more common problem and recognize the risks associated with these organisms.

But how is that risk assessed? New England states have a range of systems in place, but most fall short of addressing the issue to the extent necessary to allow appropriate response. The World Health Organization came up with thresholds of 20,000 and 100,000 cells/mL for low, moderate, and high risk, but cell counts are approximate and do not directly translate into toxin concentrations, which is where the risk really lies. Most states adopted a compromise concentration of 70,000 cells/mL, but it is certainly not true that 69,999 cells/mL is acceptable while 70,001 cells/mL represents extreme hazard. Thresholds create hard boundaries along a gradient with a lot of important variation. Some states now encourage toxin testing, but the ability to economically and rapidly test for the range of known toxins is very limited. Consequently, lakes get posted with warnings to avoid contact when cyanobacteria levels are elevated without actual confirmation of a hazard. Warnings do not legally prevent lake use, but rational people usually take those warnings seriously.

So what is the actual risk? This is a tough question. There is no doubt that some cyanobacteria produce toxins, including liver, nerve and skin toxins, but many species do not produce toxins. We know which species potentially produce toxins, and have identified the genes that allow toxin production, but because a species can produce toxins does not mean that it will. Many very dense blooms contain no toxins. Major die off of a bloom of toxin-producing cyanobacteria can result in elevated toxins in the water without a high cell concentration, although rarely for more than a few days. Risk is a matter of toxicity and exposure, and we don’t know enough about either to make definitive statements in many cases. Posting a lake for elevated cyanobacteria can help control exposure, but failure to follow up on toxicity leaves open questions and impairs our future ability to predict risk. We still have a lot of work to do to characterize cyanobacterial risk.

Managing in a Regulatory Climate

Many years ago when someone or a group felt that something about a lake was not right, they organized and changed it. Lake level was lowered, sediment was scraped, herbicides were applied, shorelines were bulkheaded, and so on. Many of these actions were appropriate, but many were not and most have impacts to other aspects of lake ecology that were not being considered or mitigated. In some cases the results were disastrous, not curing the targeted problem and making things worse in other ways. The dawn of ecological thinking and the incorporation of such thought into government resulted in many environmental laws, starting in the 1960s and peaking in the 1970s, with follow up ever since. Many environmental agencies were created, some completely new (the federal EPA) and others altered to redirect their mission (the federal USACE). State agencies underwent similar formation or transformation. The basic premise was that we needed to regulate how we treated the environment to avoid catastrophes while “fixing” perceived problems.

It is good that we have such agencies and the laws and regulations that guide them, but like most institutions over time, the mission is sometimes perverted or focus is lost. Incremental adjustments to regulations or changes in definitions create unforeseen consequences for legitimate activities. People use the system as a weapon instead of a tool. Agendas get created, processes become more complicated, and it becomes harder to manage. Lake management is suffering from just such a progression.

The fundamental problem with permitting systems is that they are managed by regulatory agencies with little or no obligation to solve a problem. Permits are not applied for because it is a fun thing to do; we request permits because we want to solve a problem. It is rare to have a regulatory staffer say “Oh, I see your problem. And I understand why you want to do what you have laid out in this application. There are a few issues we have to address, but let me see if I can find a way to make this work…” Instead, the regulator views his/her job as preventing environmental damage, not solving an existing environmental problem. If, after a frustrating process of submission, hearings, delays, and revisions,  you give up trying to solve the problem, that is just as good as the agency issuing a denial. The permit system has been upheld. The potential risks from the proposed action have been averted. It doesn’t matter to the regulator that the problem has not been solved.

If someone is advancing a clearly detrimental request, say asking to fill a cove of a public lake full of endangered species to build a supermarket accessible by boat, all to make a few bucks, it is pretty easy to rule that out. But most requests have merit and raise management issues that need attention. What happens when the request is to control an invasive species known to reduce biodiversity, one that will impact some of the endangered species in that same lake, but the options for control may do some temporary harm to the lake? This is a balancing act, but one in which the actual problem may be given very limited consideration. After all, no state in New England (or anywhere for that matter) has a law saying that if an invasive species is found it must be addressed, but most states do have a law that says you can’t harm a protected species or damage wetlands, of which lakes are usually considered to be a category.

Any action that reduces the invasive species may be rejected under current regulatory systems if it is perceived as being harmful to some interest of an applicable law, such as taking individuals of an endangered species, and the burden of proof is on the applicant. The regulatory agency is under no obligation to properly justify its assumptions at that stage (you would have to go to court for that) and can require all sorts of studies of the applicant before rendering a final decision, which may still be unfavorable. And if the applicant gives up and the invasive species eliminates some endangered species in the lake? Apparently that is acceptable, as opposed to losing some of the endangered population in the process of eliminating and invasive species and saving the endangered species in the long run!

The problem is twofold: 1) Lack of big picture thinking by agency staffers, partly attributable to having limited background in aquatic ecology and lake management, and 2) Vesting authority in a regulatory process without any obligation to solve the problems that result in requests for permits. Both of these are solvable, but appear to require political intervention; scientific logic and economic sense are just not getting the job done these days.

Zebra mussels killed this mollusk

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

NALMS 2017 International Symposium Registration Open

The 2017 NALMS conference will be held in Westminster, CO, just outside Denver. This annual gathering showcases the best in lake management through presentations, exhibits and networking. This is a gathering of researchers, consultants, governmental representatives and lake enthusiasts. If you work on lakes, you really can’t afford to miss it. But if you just love your lake and want to know more about how to keep it in its best condition, this is a golden opportunity. The best experts in lake management are readily accessible and enjoy interacting at this gathering. It is always a great event, starting with workshops and progressing through 3 days of interactive sessions. This year it all starts on Monday, November 6th and ends on Thursday, November 9th. Check out details at the NALMS website at

Your Lake and You Publication Available

For many years NALMS has had this newspaper-style publication available, one that has a lot of basic info on lakes and can be customized for any particular lake, association, or state. Your Lake and You has recently been updated and is provided as an electronic file that may be very useful to your lake group. As explained on the NALMS website, the 2017 online edition of the Your Lake & You! booklet is an updated version of the 8-page newspaper and helps explain to homeowners the steps they can take to protect the lakes they live on and love. This wonderful resource is loaded with basic lake information, strategies for taking better care of lakes, and descriptions of resource publications. Find it online at

Lakes Appreciation Month Wraps Up

In case you missed it, July was officially “Appreciate Your Lake” month. For members of NALMS, July represents a time to reflect on how we value lakes, usually with a focus on individual favorites. A number of people were interviewed on National Public Radio’s program Here and Now, segments of which can be found online hereOf course, you can appreciate your lake any time, and some of us like our lakes even better at other times of year besides summer. But summer is the key season to most, and July marks the annual kick off of the Secchi Dip In, an annual collaborative data collection effort that has resulted in a magnificent data base that allows us to know what the distribution of water clarity is for all regions of the USA. If you measured Secchi transparency in July 2017, please submit your data to this valuable data base. Check out the NALMS website for more on Lakes Appreciation Month and what you can do for your lake!


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

Spring Fish Kills

A lot of fish agency staff would like to have that proverbial nickel for every phone call they get about dead fish in the spring. Consultants get this too, and one can pretty much count on a couple of calls a week about dead fish in late May and June, but sometimes right after ice out. There are lots of things that can kill fish, including disease, angling mortality, toxic substances, low oxygen, and high temperatures, but the vast majority of fish-kills boil down to two main influences: low oxygen under the ice and spawning stress in late spring.

First, consider what defines a fish-kill. Two dead fish washing up on shore doesn’t really qualify; most agencies apply some number like 50 to a species or 100 to multiple species. It would be very wasteful to investigate every dead fish that shows up. If 50 fish (or whatever reasonable threshold is applied) of a species can be found at once, that is considered to represent some kind of event. Likewise, if 100 fish of multiple species are found in a short time span, that suggests something more than typical die off. So please don’t get on the phone to your fish agency or consultant when 5 dead sunnies float to shore. By all means, take a lap around the lake and see if this is a widespread phenomenon, in which case a call may be in order, but don’t panic over a few dead fish.

So if we do find enough dead fish to raise an alarm, what might this mean? The first conclusion that many jump to is that there is something undesirable in the water and fish are dying from it. Well, that may be true, but even if so, that doesn’t mean people will die from it if they go in the water, but that is a leap that many people make. If dead fish are plentiful after the ice goes out, it probably means that oxygen got too low and less healthy fish died. This could be one species that lives in the affected area (catfish on the bottom come to mind), or it could be multiple species that coincide in an isolated area (a shallow bay where thick ice met the bottom off shore and prevented them from leaving). These winter fish-kills may not be detected until the ice goes out, but the fish did not likely die after ice-out.

The other, and far more common cause of spring fish-kills is spawning mortality. Many species, most notably perch and sunfish, spawn in shallow areas in the spring. Perch tend to be earlier spawners, and the water is colder and better oxygenated, so they are less likely than sunfish to experience stress while spawning. Sunfish, on the other hand, start spawning in May in much of New England and may continue to do so well into summer. They make nests in shallow water subject to fluctuations in oxygen and temperature within and among days. They don’t eat but they do defend the nests against other fish. The energy balance just doesn’t work out in some cases, and some fish die. If we have extremely variable weather, the stress is often greater.

If a June fish-kill is one species, all of similar size, most commonly 5-6 inch sunfish, you can pretty much count on it being spawning mortality. If the kill involves multiple species or a wide range of sizes, then there might be something worth reporting. It should be noted that the timing of fish-kills in late spring often coincides with a period of herbicide application, leading to another leap to a conclusion that is rarely justified. Over 20 years of fish-kill investigation in Massachusetts, covering hundreds of investigated events, only a handful were linked to herbicides and these were almost always a function of low oxygen created by dying vegetation; toxicity to fish from properly applied herbicides is very, very rare.

Spring fish-kill