Category Archives: In-Lake Management

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.

Source: http://senecacountycce.org/natural-resources/invasive-nuisance-species/invasion-curve

 

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. 

Source: https://www.epa.gov/climate-indicators/climate-change-indicators-lake-ice

 

 

 

 

Zooplankton: The Missing Link

Most people know what algae are, and most have seen both blooms in lakes and pictures taken with a microscope that reveal the fine details of algae cells. Even more people are familiar with fish, even if it is just through the menu at their favorite restaurant. But there are not many fish that eat algae directly; we need an intermediate link to complete the open water food web. That link is the zooplankton, which are small animals, rarely bigger than the head of a pin, that eat mostly algae and are in turn eaten by small fish.

Zooplankton includes small aquatic animals in the water column, mostly crustaceans. The main zooplankters are cladocerans, copepods, and rotifers (see photos), although there are tinier forms (like protozoans) and bigger types (like water mites). They can be filter feeders or selective grazers, picking out what they want from the aquatic soup in which they live. Filter feeders exert the most control over algae, with the filtering capacity proportional to the cube of body length. Consequently, large (>2 mm) bodied cladocerans like Daphnia are preferred for biological control of algae. However, those larger forms also represent the biggest energy “packet” for small fish, and once spring spawning produces the young of the year for each fish species, predation on those larger zooplankton can be intense. Peak zooplankton abundance tends to be in late May or early June, when the quality of algae as food resources is highest, warmer water increases growth rates, and predation by small fish is not yet maximal. This often leads to a clear water phase in even the most nutrient enriched lakes, but it doesn’t last. Types of algae shift toward less edible forms (like many cyanobacteria) and hungry small fish depress the populations of large zooplankton species.


In lakes used by sea-run alewife for spawning, the seasonal pattern tends to be shifted. Spawning alewife don’t eat a lot of zooplankton, but the young of the year live for the summer in those lakes, filtering out zooplankton with their gill rakers and decimating late spring/summer zooplankton. The zooplankton peak in those lakes is usually in winter by evolutionary adjustment. Stocking landlocked alewife in lakes is sometimes encouraged to support trophy gamefish production, but without that long evolutionary adjustment period, this tends to prevent large-bodied, algae-grazing zooplankton from becoming abundant and makes the open water food web much less efficient. It may indeed produce some large gamefish, but at the expense of many other species, possibly the gamefish themselves, as their young need zooplankton for at least a short period in early life.

Common zooplankton of freshwater lakes.

 

Zebra Mussels in Laurel Lake, Massachusetts

Laurel Lake, just off Rt 7 in Lee and Lenox, MA, is the only lake in MA infested with zebra mussels…so far! Zebra mussels got into the lake sometime around 2008 and were discovered in 2009. There has been a lot of discussion over the last decade, but only a small drawdown and boat washing station have been established in response to this problem. Zebra mussel larvae, called veligers, have been able to flow out of the lake and into the Housatonic River each summer, and now at least two reservoirs in Connecticut have become infested. The Laurel Lake Preservation Association (LLPA) has been working with the Towns of Lee and Lenox to fund studies of the lake and possible solutions; a detailed summary of work done from 2010 through 2016 by Water Resource Services Inc. was recently released. Now the LLPA is seeking help at the state and federal level, working to get all agencies with responsibility for or interest in the lake to cooperate on a solution before other lakes in the area become infected. Stay tuned for developments.

Zebra mussels growing on freshwater mussel (Pyganodon).

 

Use of Aluminum to Treat Lakes

Aluminum compounds are coagulants used in water and wastewater treatment to settle solids and pull dissolved solids out of solution. In water treatment these compounds convert impurities into particles that can be settled or filtered. In wastewater treatment aluminum does the same thing, but is also noted for its ability to find phosphorus and lower the fertility of effluents. Over 40 years ago it was hypothesized that aluminum could do in lakes what it did in treatment facilities and might be especially useful for inactivating phosphorus in sediment that was being recycled to create what we now call an “internal load”. While a lot has been learned in the intervening years that makes such treatments more effective, even the earliest treatments provided enough benefits to make continued use worthwhile. Although aluminum treatments will clear the water of most algae, it is not an algaecide, defined simply by the root words as something that kills algae.

Aluminum application to a lake.

While the majority of regulatory agencies in New England appear to understand why and how aluminum is used in lake management, there seems to be some confusion in a few places about aluminum use. The State of New York created a regulatory definition of algaecide that expands coverage to any additive that prevents algae from growing. This would include aluminum, which limits phosphorus availability, and is not on the federal list of registered algaecides, since the EPA does not consider it to be one. Consequently, aluminum treatment cannot be permitted in New York. Of course, by this logic algaecides would also include oxygen if added to keep phosphorus bound to iron and unavailable to algae, and to air used to circulate water, thereby disrupting the growth of many buoyant cyanobacteria. It would also include water used to dilute phosphorus concentrations or even flush a small lake. Arguments about other additives being “natural” simply do not hold water. New England states have generally not bought into this faulty logic, but apparently the Connecticut Department of Health has applied the New York definition of an algaecide in some cases and has not approved aluminum treatments in drinking water supplies, despite approval by CT DEEP for such treatments in recreational lakes.

There is no doubt that control of phosphorus before it enters a lake is preferable where feasible, but there are very real limits on our ability to do that, and where phosphorus has accumulated in a lake, it has to be inactivated to rehabilitate the lake. It is much like fixing a boat that has sprung a leak; the leak needs to be patched, but that won’t get rid of the water that has leaked in. Aluminum treatments offer control of internal loading, and can also be used to treat inflows where watershed management is not yet up to the task or to reduce phosphorus availability in the water column after significant loading events. We have learned over the years how to prevent toxicity, making it a relatively safe technique. Creating regulatory restrictions based on faulty logic or incomplete understanding of the technique hinders lake management in a time when we need every viable technique we can get.

Measuring Oxygen Demand

Knowing how much oxygen is consumed per unit volume of water or area of sediment in a lake is important to understanding lake metabolism and in planning for the provision of adequate oxygen. Systems that mix the lake or add air or oxygen must counteract the oxygen demand to be successful. Measuring demand is tricky, however, since the rate at which oxygen is removed is hard to isolate from factors that put oxygen into the water or allow it to move within water, and consumption of oxygen is not linear over the range from about 2 mg/L down to 0 mg/L.

Laboratory tests can be run in which oxygen loss over a set amount of time is measured in a closed bottle at a standard temperature, and this is useful, but this won’t include sediment oxygen demand, which is often the dominant source. For deep lakes, one can use temperature and dissolved oxygen profiles from a few weeks apart at a time when oxygen is >2 mg/L everywhere to get a reasonable estimate. Oxygen addition from the atmosphere and downward mixing necessitates using values from deeper water, usually below the depth of the thermocline, even if it has not strongly formed yet. Timing is critical. In New England stratification sets up from April into June and is highly weather dependent, but once it sets up, loss of oxygen near the bottom can be rapid and oxygen concentrations can become too low to use in demand calculations. When the oxygen concentration is less than about 2 mg/L, further removal slows down and calculations using low values will underestimate actual demand, something to be avoided when planning oxygen additions.

For proper data, subtraction of later oxygen from earlier oxygen at each depth increment can be summed to yield the mass of oxygen lost over a square meter of lake over time (Table 1). If the temperature has risen between measurements, the water will naturally hold less oxygen, so a correction for temperature-induced oxygen loss must be applied, based on the difference in saturation concentration at the earlier and later temperature measurements. In the example in Table 1, the thermocline forms at about 6 m but was very weak at the time of the April measurements. Yet it was apparent that little oxygen from above was reaching deeper. There was a slight increase in temperature, so the differences first obtained were adjusted down slightly. The difference in total oxygen demand is not large, but can be if the time between measurements is longer.

Table 1. Calculation of Oxygen Demand in a Deep Lake

In shallow lakes it is harder to find a time or place where factors other than oxygen demand are not significantly influential, but we have found one approach that often works.  Photosynthesis ceases overnight in the absence of light, and at least some nights there is calm that limits mixing. Measurements made after dusk and before dawn can be used if oxygen remains >2 mg/L throughout the water column, and the results can be quite striking. Oxygen demand from respiring algae or vascular plants will add to demand from decay, but that is all part of the demand and worth including. The same calculation approach used for the deeper lake is applied to the shallow lake (Table 2), but the whole water column is included.

Table 2. Calculation of Oxygen Demand in a Shallow Lake

For reference, oxygen demand higher than about 0.5 g/m2/d can eventually lead to oxygen depletion in stratified lakes. Values >1.0 g/m2/d will cause depletion in the bottom layer by mid-summer, and values >2.0 g/m2/d will cause oxygen depletion much earlier in summer. Making multiple measurements is recommended to characterize the range of oxygen demand and get a sense for possible error induced by not being able to control all oxygen inputs. With enough measures a pattern usually emerges that allow one to make a fairly accurate estimate of oxygen demand.

Temperature and Oxygen Profiles

If you polled people with a lot of lake assessment experience about what technique provides the most information for the money, chances are that Secchi disk transparency would be the choice, but temperature and oxygen profiles would get at least an honorable mention. The thermal structure of a lake is extremely important to the oxygen regime, which is critical to so many aspects of water quality and lake ecology. Temperature and oxygen profiles collected over the course of the year provide extremely valuable data for understanding your lake.

One needs an instrument that measures temperature and oxygen in water to collect these profiles, and a number of high quality instruments are on the market. YSI and Hach have the biggest market share, but there are others worth considering. Bear in mind that you get what you pay for. Temperature is based on differential movement of metals that respond to temperature change and instruments cannot be calibrated by users; one needs to check against a thermometer or known temperature once in a while, but temperature measures are generally reliable. Oxygen used to be done as a titration in lab or field, then moved to instruments with membranes that measured potential across that boundary, and now are done with luminescence, although all three approaches are still in use. Calibration is essential but not difficult for instruments.

A long enough cable is needed to reach the bottom from the boat or dock, and measurements are typically made every half meter (shallow lakes) to every 3 meters (very deep lakes), with 1 meter intervals most common. If the lake is deep enough to stratify, there will be a transition zone between upper and lower water layers and the thickness of that transition zone may be of interest, so more frequent measures (shorter vertical distance between readings) may be advisable near that boundary, called the metalimnion (with the point of inflection for temperature change called the thermocline). It is also important to get readings near the bottom, or even right in the sediment. Certainly the deepest reading should be no more than a few inches from the bottom, and you may find that oxygen declines sharply as the probe hits the sediment.

Graphing temperature and oxygen profiles provides a useful visual image of what is going on (Figure 1), and the pattern over time can be put on the same graph for comparison (Figure 2). Even shallow lakes may exhibit a decline in temperature, and a difference of only 3 Co can be enough to resist mixing. Even without a discernible change in temperature, there can be oxygen depression or even depletion near the bottom if the oxygen demand is high enough; oxygen doesn’t diffuse all that fast through water, and decay can outpace re-aeration.

Temperature and Oxygen Profile.

 

Oxygen Concentration over Depth and Time at a Deep Station.


So why are these profiles important? Higher temperatures increase metabolism and limit the types of organisms that can be present, perhaps most notably fish; trout do not do well at temperatures much above 21
oC. The temperature structure of the lake tells us about how much mixing is going on over space and time, and in the absence of adequate mixing, oxygen loss can occur. Loss of oxygen affects nearly all biological components of a lake; sensitive species cannot tolerate oxygen much less than 5 mg/L, few aerobic organisms do well at oxygen <2 mg/L, conversion of ammonium to nitrate ceases when oxygen runs out, and release of iron-bound phosphorus can occur at low oxygen concentrations. Much can be predicted about the status of a lake from temperature and oxygen profiles.

Odor in Lakes

When lakes smell bad, we notice. Some basic knowledge of sources of odor may help lake enthusiasts identify odors. Odors can be classified according to some complicated systems, but for our purposes we will stick to some basic descriptions.

Most people recognize “rotten egg” smell, which is hydrogen sulfide (H2S). It is formed when sulfate is metabolized in the absence of oxygen, usually in the bottom of a stratified lake. We sometimes smell H2S around salt marshes, as there are more sulfates in saltwater. In lakes, the presence of H2S means that oxygen has been depleted and the demand has become so great that sulfate (SO4) is being broken down. Other oxidized compounds are usually broken down first, such as nitrates (NO3), so oxygen demand is severe if H2S is being created. Most often the H2S is smelled in bottom water that has been sampled; the H2S tends to stay trapped in the bottom water layer until fall mixing. But with a mixing event, the smell can become detectable.

Certain cyanobacteria (blue-green algae) produce odorous compounds, specifically geosmin and methylisoborneol (MIB), which impart a musty to grassy odor. Often the offending cyanobacteria accumulate as a surface scum, making the odor obvious to those using the lake. If cyanobacteria are abundant enough to produce odor, there is legitimate concern about toxicity, but it is important to know that production of odor and production of toxins are not linked. Still, if enough cyanobacteria are present to create detectable odor, there are enough to create toxins above a safe threshold IF the cyanobacteria are toxin producing forms.

Decaying blue-green scum

Other algae besides cyanobacteria create odor. Actually, any algae will produce some odor if abundant enough, but certain algae can produce specific smells when abundant. Most notable are certain chrysophyta (golden algae), which produce odors such as cucumber, violet, spicy or fishy. No one is likely to confuse these options with a wine tasting event.

Green algae mats

Finally, dead algae tend to give off foul odors usually described as septic or decay. Dying filamentous green algae are particularly malodorous. If mats wash up on shore and start to decay, they are likely to be very noticeable to anyone with a nose.

A good reference to check if you want to know a lot more about odor in water is the American Water Works Association publication M57, called Algae: Source to Treatment.

Climate change impacts quantitatively assessed

Some impacts of climate change have known for years, but others are still surfacing. About a decade ago a representative of the USGS in Maine presented data at the NECNALMS conference for the date that ice cover broke up for multiple lakes in Maine. While there was variability typical of systems influenced by climate change, it was very clear that the date the ice was going out was getting earlier over decades. In about a 60 year period the average ice out date for the 2000s was about 2-3 weeks earlier than it had been in the 1950s. This should have been impressive enough in its own right, but apparently few other than ice fisherman really took notice.

Now we have another interesting measure that might be scarier. Oxygen consumption is an important feature in lakes, causing oxygen to become depleted (called anoxia) in many lakes deep enough to stratify, at which point oxygen can’t rapidly move downward from upper waters and decomposition gradually removes  oxygen from the bottom up to the boundary point, called the thermocline. Fish like trout that need cold water (<21 oC) but high oxygen (>5 mg/L) can get “squeezed”, faced with water too warm above and water with too little oxygen below. There may be no “trout water” during late summer, causing mortality. Further, loss of oxygen in deep water can allow phosphorus bound to iron to be released into the water column where it can support algae blooms, most often cyanobacteria that are favored by this type of release. In shallow water, high oxygen demand is indicative of elevated decomposition, and while complete loss of oxygen in water <15 ft deep is rare, that decomposition releases phosphorus that can fuel algae blooms. So oxygen consumption matters a lot to lake condition.

Data from Long Pond in Brewster and Harwich on Cape Cod, collected by the Natural Resources Department as part of a very useful water quality monitoring program, were plotted in an effort to understand the variation in oxygen consumption observed over time. What was found was that a relatively small difference in temperature, brought on by warmer spring air temperatures, resulted in a major increases in the rate of oxygen consumption, or oxygen demand (see figure). Oxygen demand below about 550 mg/m2/day is considered unlikely to cause severe anoxia, while values higher than 1000 mg/m2/day usually cause most of the bottom water layer to become anoxic in August and values greater than 2000 mg/m2/day will typically cause anoxia in July. Long Pond has an oxygen depletion problem, and it worsens appreciably with increasing temperatures in the deepest water.

Using a statistical technique called regression, the portion of the variability attributable to any tested factor can be evaluated. For Long Pond, and probably many other lakes, change in temperature explains much of the variation in oxygen demand (62% for Long Pond, a high percentage for a single factor). And this doesn’t require a large change either; the range of oxygen demand in Long Pond more than doubles for a temperature increase of only 3 Co (5.4 Fo)! The influence of the current direction of climate change is pushing our lakes toward a higher metabolism, almost like they have a fever, and the implications for all users, human or otherwise, are not good.

Oxygen demand as a function of temperature in Long Pond, MA.