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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.


Paper on aluminum treatments on Cape Cod approved for publication

A peer-reviewed article covering a dozen treatments of Cape Cod, MA lakes for control of internal phosphorus loading has been approved for publication in Lake and Reservoir Management. The paper is likely to appear in the June issue, but will be available online before then. Entitled “Aluminum Treatments to Control Internal Phosphorus Loading in Lakes on Cape Cod, Massachusetts”, it is authored by Kenneth J. Wagner, Dominic Meringolo, David F. Mitchell, Elizabeth Moran and Spence Smith and details projects from 1995 through 2016 where lakes and the underlying sediment were treated with aluminum sulfate and sodium aluminate to bind up surficial phosphorus bound to iron. That iron-bound phosphorus can be released when oxygen levels are low, as is usually the case in stratified lakes in New England. This internal source of phosphorus is often enough to support algae blooms, and comes with a low ratio of nitrogen to phosphorus, which tends to favor cyanobacteria. Consequently, inactivation of iron-bound phosphorus has been found to eliminate cyanobacterial blooms and increase water clarity in kettlehole lakes on Cape Cod.

Kettlehole lakes have limited surface inflow and depend on precipitation and groundwater as water sources. While the watershed is still important to lake condition over many years, it is that long-term accumulation of available phosphorus in the bottom sediments that drives conditions within any year. The review of treatments over the past two decades suggests some variability in results, but a positive impact on water quality overall for years after treatment. Two lakes have now been treated twice, one after about a decade and the other after 20 years. Surface phosphorus concentrations were reduced by 61% on average, while bottom phosphorus decreased by 84%. Chlorophyll-a, a pigment indicative of algae abundance, declined on average by 81%. These decreases translated into an increase in water clarity of 136% (more than a doubling from an average of 1.9 to 4.4 m) and a decrease in oxygen demand in deep water of 61%. Greater water clarity and more oxygen in deeper water were good for fish and other aquatic organisms as well as benefiting people with regard to water supply and recreation.

Aluminum application in process at Cliff Pond, Brewster, MA

2016 Drought Highlights: Role of watersheds in lake condition

Looking down view tube at Secchi disk

Precipitation and flows were well below normal in spring and summer of 2016. For example, at Morses Pond in Wellesley, MA, there was no winter snow pack to speak of and precipitation in May through August was about half of the average for the previous decade (7.2 vs 14.2 inches). The situation was similar all over New England, and while evaporation exceeded precipitation during summer and caused low water levels, the reduction of nutrient inputs also resulted in high (sometimes record) water clarity. Blooms of algae were less common in lakes that are tightly linked to watershed inputs on a seasonal basis, which includes most impoundments on streams and river and other lakes with watersheds more than about 20 times the area of the lake. Phosphorus and chlorophyll-a concentrations were lower than average for a respective 72 and 80% of lakes surveyed by LEA in Maine, leading to Secchi transparency values higher than average for 72% of surveyed lakes. Unless internal recycling is the dominant source of phosphorus to a lake, reduced precipitation translates into less runoff, lower nutrient inputs, and higher water clarity.

The importance of a watershed to lake condition is clearly demonstrated, but that importance is mediated through two key processes: weather pattern and land management. In 2016 the weather did a lot of what we strive to do with land management, minimizing the transport of nutrients and other contaminants to lakes. We can’t control the weather, and having less water entering our lakes has its downsides (e.g., lower water levels, more impact from rooted plants), but the importance of watershed management to minimize nutrient inputs when the weather is not cooperating is underscored. If we can’t put a dome over our watershed and only open the roof when we want the water, we have to manage the watershed to limit inputs to the lake.

But what is the potential for watershed management to provide the benefits observed in 2016 as a result of low precipitation? The better than average conditions were associated with precipitation about 50% below normal. Nutrient loading is not necessarily proportional to water inputs, and we would expect disproportionately more loading with larger storms, but it seems reasonable to assume that we would need at least a 50% reduction of loading through watershed management to reap the same benefits provided by the 2016 weather pattern. Based on years of evaluation by the USEPA, phosphorus removal by best management practices rarely averages more than 50%, although well designed infiltration facilities can achieve 90% reductions. However, not all watershed soils are suitable for infiltration systems, so what all this means functionally is that we will be hard pressed to provide the level of watershed management necessary to maintain the conditions we observed in 2016.

We can view 2016 as setting the bar for potential lake condition with regard to nutrients, algae and water clarity. Low precipitation limited inputs, and while there were some negative effects of having less water, the water quality was about as high as could be expected in New England lakes. If your lake was not appreciably better than in other recent years, internal loading sources were most likely dominant or there is another source (e.g., direct discharge or extensive storm water piping that limits load reduction on the way to the lake) that requires attention. Yet for those lakes that did exhibit better than average conditions in 2016, maintaining those conditions by watershed management will require superior effort, as the practical limits to best management practices will necessitate application all over the watershed to achieve the level of loading reduction experienced in 2016.


The New England Chapter of the North American Lake Management Society is alive and well, but it has been hard to put out a newsletter on a consistent basis. At our recent leadership meeting, it was felt that newsletters, while interesting, are often dated before they arrive and there are so many competing newsletters that they don’t get the attention they may warrant. Blogs, on the other hand, offer immediate information in categories that can be archived and searched, and are online resources easily accessed by most people today. NECNALMS is therefore going to use the blog format to keep in touch with lake professionals and interested citizens beginning in 2017.
The primary categories into which anticipated posts fall include educational opportunities, watershed management, in-lake management, state news and alerts, legislation and policies, and funding. Others may be employed as the need arises. Anyone can submit something for posting, but be advised that this is not a completely open forum. Quality control and peer review will be employed as necessary to verify the accuracy of statements and avoid undue alarm, controversy and incivility. That does not mean we do not plan to tackle difficult issues or avoid contentious debate, but it does mean that we discourage posting opinions masquerading as documented reality and encourage civil interchanges of reasoned discourse. Alternative interpretations are welcome; “alternative facts” are not. Potential postings should be submitted to