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Protection vs. Restoration

There has been a lot of discussion at the annual NALMS symposium over the last 5 years about the value of watershed management. Much of this discussion was a reaction to federal and state directives to focus on watershed management to solve lake problems, as we know from many years of experience that some problems cannot be solved with watershed management (e.g., internal phosphorus loading, excessive plant density) and watershed management has proven very difficult over large areas under multiple jurisdictions. In articles in LakeLine and presentations at NALMS meetings a few people have made statements that have been interpreted as opposing watershed management, but these statement are being misinterpreted in the context of protection vs. restoration.

First of all, there is very little lake restoration going on. We often rehabilitate lakes, altering them to meet use goals, but that is not the same thing as restoration. For lakes created by erecting a dam, true restoration would mean removing the dam and eliminating the lake; this is not usually what lake managers have in mind when they use the word “restoration”. But whether we call it restoration or rehabilitation, reducing nutrient levels, algae blooms and nuisance vascular plant growths is rarely achieved by watershed management once the lake gets to the point of supporting such growths. Once in a while we find the “smoking gun” and can implement a focused watershed management plan, but usually it is a slow, incremental process that very rarely moves the lake adequately in the right direction. Consequently, in-lake efforts are often needed to meet use goals, including phosphorus inactivation, dredging, herbicides, harvesting and other commonly applied methods.

Protection, however, is another matter. If a lake is in a desirable state, it is wrong to assume it will always be that way. Watershed influences can be gradual or catastrophic, but their presence is undeniable. If the watershed is large enough (>10:1 ratio with lake area is a commonly cited threshold), inputs over many years will eventually change the character of the lake, and water quality issues become more probable when the watershed is >50 times the area of the lake, even with no human activities in the lake. However, human activities greatly accelerate loading of sediment, nutrients and a variety of other contaminants. Thresholds as low as 6% development in the watershed have been cited as resulting in measurable changes in water quality, and at development levels on the order of 25-30% it is rare not to see deterioration of water quality. Watershed management is therefore essential to protecting lakes, but the potential to adequately protect the lake declines as the percentage of development or agriculture increases.

Developed vs. Forested Land.

Watershed management is a logical component of any lake management plan. It is wrong to hold an invasive plant control project hostage until the applicant produces a watershed management plan, but it is entirely reasonable to expect holistic lake management programs to incorporate watershed management. If protection of desirable features is the goal, watershed management is a must. If rehabilitation of degraded conditions is needed, watershed management is not likely to be the whole answer. Keep protection and restoration elements of any plan separate when discussing lake management to avoid controversy over the role of watershed management.

Agriculture by a Lake.

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.

 

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 NEC-NALMS Blog

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 LakesInfo@nec-nalms.org.