Category Archives: In-Lake Management

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

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

Two Main Plant Types

Plant taxonomy aside, there are really two main plant types when we are considering management: annuals and perennials. At this point all those whose eyes glaze over when the Latin genus names start rolling out should be paying attention, and gardeners and home landscapers probably already get it. Much of the decision about what to do rests on whether a plant reappears annually from seeds or other resting stages (propagules) or comes back from root stalks or stems (or never really dies back in winter). Perennial plants are easier to deal with (just like in your garden or lawn); if you kill the plant, it won’t return from its vegetative parts. For annuals, however, killing the plant just buys time; seeds, winter buds, turions, and other structures germinate and form new plants, usually in the following spring. Management strategies therefore have to consider whether the target plants are annuals or perennials, when they produce propagules, and when growth begins.

If one is applying drawdown, it can be effective on perennial plants like the milfoils, but will have little impact on annuals like naiad or most pondweeds. In fact, drawdown often stimulates propagule germination, so a shift from perennials to annuals can be expected over a period of years with continued annual drawdown.

If one is using herbicides, killing the plant before it can drop seeds is essential to eventually gaining control over an annual, but the seed bank may be large enough to allow recovery for years to come. If a contact herbicide is used, the root system of a perennial is usually unharmed and will allow plant regeneration. Systemic herbicides, which move throughout the plant and are intended to kill it all, can provide more lasting results with perennials, but are more expensive than contact herbicides and may be a waste of money for annual plant control.

Benthic barriers kill nearly all plants over which they are placed, but if the barrier is removed, propagules from annual plants are likely to sprout. Hand harvesting practitioners know that it is essential to get the whole plant out, roots included, if a perennial species is to be controlled by that means. Dredging may be the only all-purpose tool for rooted plants, removing the plant and the seed bank, but at a very high cost with a lot of permitting, so we don’t see this approach used much.

Some plants apply a mix of perennial and annual strategies, making them harder to manage. Bigleaf pondweed produces seeds but does not die back completely in most winters and survives many drawdowns and herbicide treatments. This is a native species in New England and is not a major nuisance plant in most cases, but it has created problems for swimming in some lakes. Many perennials do produce seeds, but viability tends to be low; they don’t depend on these for annual persistence, but it does mean that removing a stand of a perennial plant may not be a one-time job.

So be sure to know which plants apply which strategies when considering control; success may depend upon it.

Curlyleaf pondweed, an annual  

Eurasian water milfoil, a perennial


Algaecides are chemicals used to directly kill algae. The Latin root is simple – algae, or microscopic plants, and cide, a killing agent. By far the most common algaecide is copper, in use for over 100 years and very effective against a wide range of algae. There are many formulations, with differences mostly intended to improve effectiveness or duration of activity under various environmental conditions, but the key ingredient remains the copper ion itself. After reactions are complete, the copper remains, and is usually deposited in the sediment. This can’t be good for the lake, but there are few studies that have demonstrated any measurable negative impacts. With repeated treatment, the sediment may be considered hazardous waste if ever dredged, but for the most part the reacted copper appears to be inert. Doses of copper in New England waters rarely exceed 0.2 mg/L as copper, and are often <0.1 mg/L. Larger doses are used in some other parts of the USA, mainly to overcome interference by high suspended or dissolved solids, and these are poor examples to compare with New England applications. Some zooplankton and some trout species may be susceptible to toxic effects at applied doses, but the vast majority of non-target aquatic organisms are not threatened by copper doses used in New England.

A more recent algaecide is peroxide, formed from sodium carbonate peroxyhydrate when added to water. It is an oxidant that impacts cell walls of algae, with groups like cyanobacteria being generally more susceptible than stronger walled forms like some greens and diatoms. It leaves no potentially hazardous residues. The primary drawbacks are that sometimes we want to kill green algae, especially mats of filamentous forms, and peroxide-based algaecides are more expensive than copper alternatives. Still, the generally positive environmental profile of peroxide-based algaecides makes them attractive. Peroxides seem to be more effective than copper on cyanobacteria mats, which are often sources of taste and odor in reservoirs.

There are a few other manufactured algaecides that have specialized applications, but copper and peroxide represent nearly all the market for this type of treatment. It is preferable to limit nutrients to control algae, but this is much easier said than done, and having algaecides as a management option helps make our drinking water safe and our recreational lakes swimmable. Excessive use of algaecides should be avoided, and control of nutrients should be pursued as a long term solution, but algaecide application is a valuable management tool that should not be rejected without careful consideration.

The biggest issue with treatment is the tendency to wait until there is a major accumulation of algae to treat. At that point treatment will lead to a lot of decaying organic matter, release of nutrients, and possibly release of toxins. This latter possibility has led many states to disallow treatment if potentially toxic algae are too abundant. The most effective way to use algaecides is to prevent a bloom, not get rid of one. This means tracking algae on a regular basis, typically weekly, and reacting when problem species start to increase, which is not an easy task.

One other important point about algaecides warrants attention. As noted at the start, these are compounds that directly kill algae. Some regulatory agencies, notably but not exclusively in New York, have defined algaecides as any additive that prevents algae from becoming abundant. Consequently, phosphorus inactivation with aluminum or lanthanum is considered to be algaecide application, and since these are not registered as algaecides with the federal government, treatments using them cannot be permitted. By this line of reasoning, addition of oxygen to the bottom of a lake to keep phosphorus sequestered is also an algaecide application, and addition of water to cause dilution or flushing in a lake would also represent application of an algaecide. This sort of regulatory foolishness hurts sound lake management and highlights why it is institutions that limit success far more than science or economics.

Monitoring Algae

How can one know what algae are in the water? Sometimes the growths are large enough that someone can see enough with the naked eye to make at least a preliminary identification, but for the most part algae identification requires the use of substantial magnification. For many years (literally over 200) we have used lens systems organized into what we call a microscope to magnify algae enough to tell what they are, and there are few substitutes for looking. Even then, considerable training is needed to know what is being observed, leading to bottlenecks in getting algae data to support lake management. Having a trained phycologist (one who studies algae) look at algae under a microscope remains the most preferred option, but some useful substitutes have arisen with digital technology. A digital microscope can magnify an image on a screen for under $1000. That image can be captured as a photo that can be compared with online sources or sent to an expert. The USEPA, Region I, has been pioneering an effort to get lake groups with an interest in harmful algae blooms to use such systems to report algae in their lakes. The accumulation of images can be turned into useful data that allow characterization of bloom frequency, common bloom species, and possibly trends within or among lakes.

The advent of automated systems for detecting and photographing particles makes it possible to process samples quickly, but we are just getting to the point where image libraries can be used by instruments to actually make identifications. The FlowCam, made by Fluid Dynamics in Maine, counts particles and photographs them, allowing the user to catalog and identify them with some expertise. At even a rudimentary level, such systems allow fast assessment of possible threats to lake uses. The Imaging FlowCytobot, developed by Woods Hole MA scientists, has been adapted by PhycoTech of Michigan to actually make identifications, and this technology could meet the need for rapid but taxonomically detailed and accurate data generation. These instruments are expensive, but contracting for identification can be affordable.

While lack of images (visual or digital) limits identification, use of fluorescence systems to determine the amount and types of pigments present can help discern algae blooms. Chlorophyll-a, a photosynthetic pigment common to all algae, can be assessed with fluorescence, which is basically the amount of light at a particular wavelength emitted from a water sample after excitation by light at another wavelength. The amount of chlorophyll-a in different types of algae varies, so this is not a direct measure of biomass, and the quality and amount of natural light and the concentration of non-photosynthetic organic matter in the water can cause variation not related to algal biomass. However, in a general sense, fluorescence can be used to get a rough appraisal of how much algae is in the water. Further, phycobilin sensors that detect pigments specific to cyanobacteria and provide an estimate of how much cyanobacteria are in the sample. There are even more sophisticated systems that excite samples with a range of wavelengths and record fluorescence in a way that allows a relative approximation of multiple algae types in the sample. Calibration and “training” of the instrument for specific waterbodies improves reliability.

Compared to identification and mapping of rooted aquatic plants, algae assessment is more challenging, but effort is needed if one wants to understand all important aspects of a lake.

Invasive Species Primer

So what exactly is an invasive species? There is no textbook answer, or at least not one that everyone agrees on. The most common professional definition is a species not indigenous to the area that does ecological and/or economic damage when it becomes established. It is not merely any species that cause a nuisance, as many native species (such as water lilies or coontail) can do reach nuisance densities and are quite native to New England. It is not a species that invades but maintains a low density or even goes unnoticed, not impairing any use of the lake. And there are such species to be sure. When professionals talk about aquatic invasive species, they are usually referring to plants or animals that arrive at a lake and become a dominant component of the aquatic community, negatively impacting other species or uses of the lake. Several non-indigenous species of Myriophyllum, the watermilfoil genus, qualify, but there are native milfoils as well, some of which are even on various state endangered species lists! The zebra mussel is a great example, not well established in many New England lakes, but causing both economic and ecological harm when it invades. The list of invasive species for each state varies a bit, but there are a few dozen species that just about everyone agrees we would be better off without.

What is it about invasive species that make them objectionable? For the most part, species termed invasive displace other species by some competitive advantage or lack of predators, and become abundant enough to influence lake features that affect lake uses. Dense plant growths can include native species, but among the worst conditions are associated with Eurasian or variable leaf watermifoil, fanwort, and hydrilla, all species that came to New England in the last century and have not been well integrated into aquatic communities. It is possible that at some point balance will be achieved, and some people or even agencies make the argument that we don’t need to act; if we wait them out, the invaders will become part of functioning aquatic systems. This might be true in some cases, but given the track record, it does not seem responsible to wait that long to test the theory.

Invasive animal species, like the zebra mussel or spiny water flea, alter the flow of energy in a lake and affect the aquatic food web. Aggressive snakehead fish similarly impact the food web, but from the top down via predation. Just how much damage is done can vary greatly depending on the condition of the infested lake. Lakes with very healthy native plant communities that cover the bottom in the zone where light is adequate tend to resist colonization by invasive plants, although it is reasonable to expect eventual dominance by the invader. Lakes with no hard substrate or very little calcium in the water column are not likely to support dense populations of zebra mussels, but one can expect that all native freshwater clam shells will be colonized and those species are likely to be eliminated. There can even be some upside, as with clearer water from the filtering effect of zebra mussels, but this tends to favor buoyant cyanobacteria, so ultimately zebra mussels may promote objectionable algae blooms.

Invasive species are analogous to infectious diseases. Not every disease will kill you, but none are considered pleasant or desirable. Living with a disease is highly personal experience, but acting as a vector for that disease is irresponsible. Having an invasive species in a lake and deciding not to act to control it may be a valid position in some circumstances, but the potential impacts on other lakes in the area should be considered in making management decisions. This is a complicated area of judicial, regulatory and scientific interaction, and blanket statements that universally apply are hard to come by.

The cost of impacts vs. the cost of control is also a difficult topic. Actually putting dollar figures on impacts is not always easy, and even accurate estimation of control costs can be challenging. Ideally, eradication is the goal, but that may not be feasible in all cases. There is a whole school of thought on invasion ecology that considers the potential for control along a timeline of species establishment and impact. Often invasive species are not noticed or managed until they have reached the point on the curve where eradication is very expensive. Clearly prevention is the most cost-effective approach to invasive species management, and rapid response is the clear second choice for action, but both of these are given way less attention than they deserve in our monitoring and regulatory systems at the state level. Maintenance and restoration are the more expensive alternatives that apply once an invader has become established, and the cost can indeed by staggering. Millions of dollars are spent annually in New England alone to manage invasive species, rarely with eradication as a result or even a goal.

There is a very real need to enable citizens with an interest in lakes to recognize invasive species and to empower groups to take early action. In Massachusetts, the process and timeline for getting a permit for a rapid response program are the same as for addressing a longstanding infestation or addressing nuisance native species. We have no mandate to control invasive species, but we have laws and regulations that protect many species; if the control of an invasive species conflicts even a little with protection of an endangered species, the chances of getting a permit to control the invader are very slim. A more holistic approach is needed, but until we reach that stage of enlightenment, it is important for lake monitors to recognize invasive species and bring them to the attention of appropriate state agencies.



Ice Out and Its Meaning for Lakes

The annual date of ice out for some lakes is fodder for prognostication and even wagers, but for aquatic plants and animals, that date has deeper ecological significance. Light and temperature are key cues in the aquatic environment, and ice cover keeps lakes cold and dark in late winter. As the air temperature warms, the ice melts, usually leaving open water around the edge and then falling apart over deeper water over a short time period. If that date is earlier, algae and rooted plants can get a head start on spring growth. If that date is later, growth is delayed. Temperature also affects when hibernating aquatic animals, like turtles and frogs, become active. Fish are active even under the ice, as any ice fisherman will tell you, but are more aggressive after ice-out and turn to spawning activities based on temperature cues.

While lakes may not actively manage time, it is a lot like it is for people; if you get up early, you can get a lot more done in a day, and you may not be able to finish your to-do list if you sleep in. As the water warms and light penetrates further without ice, lots of biological processes increase in lakes. Bacteria decompose organic bottom sediments, using oxygen and releasing various substances into the water column. Algae take up nutrients and use sunlight to photosynthesize and make more biomass. Zooplankton eat algae and reproduce more frequently, but small fish also eat zooplankton and limit that trophic level by early summer in most lakes. Fish spawn and make small fish that eat those zooplankton.  In the meantime, rooted plants are growing, either from seeds, various winter buds, or root stocks, anywhere that light penetrates to a hospitable bottom substrate. Benthic invertebrates, often dependent on those plants, grow, reproduce and are eaten by fish or each other. A lake waking up from what seems like a winter sleep is indeed a busy place!

With variation in ice out date from year to year, and weather variation once the ice does go out, the sequence and intensity of cues will vary considerably from year to year, making every year unique to some extent. General patterns of plant growth, algae succession, fish spawning and other biological processes are known, but small changes can make quite a difference. A cold snap or windy period in May can retard stratification or cause a downturn in fish spawning that is not recoverable in that year. A very mild winter like we had going into 2016 can let perennial plants like invasive species of watermilfoil get a very early start (some plants may not even have died back to roots and stems) that outcompetes native species and makes it hard for harvesting programs to keep up. Weather plays a big role, and is influenced by climate change.

Climate change is a popular topic and the subject of spirited debates, but the data clearly show that lakes have been experiencing earlier ice-out dates over the last century (see graph). We seem to be losing a day of ice about every decade, such that based on the period of record going back about 150 years ice-out is now occurring two weeks earlier on average. Just keep in mind that aquatic organisms do not live in the “average”, and lakes have experienced both very late and very early ice out dates in just the last few years.

Ice out dates for various lakes.