print friendly version
screen friendly version
By Scott Williams
VLMP Executive Director
Are you wondering if the lake you monitor was clearer—or less clear—than average last summer? The Maine VLMP and DEP have tracked Secchi transparency (water clarity) data collected by hundreds of volunteer lake monitors over the years in order to be able to answer questions like this. We do so because clear water is at the very top of the list of attributes that the public values for lakes, and as it happens, water clarity is a good indicator of overall water quality for lakes and ponds. Short and long-term changes in the clarity of lake water also provide us with valuable information about the ways in which Maine lakes, both individually and as a whole, respond to influences like the weather. What we learn from the water clarity readings that you take with your Secchi disk also has important implications for the long-term protection of your lake and its watershed.
Thanks to thousands of Secchi disk readings taken by volunteer monitors during the past four decades (3,555 readings alone in 2008!), we have begun to piece together a picture of how lakes and their watersheds are influenced by people and the weather. For starters, we know that the nutrient phosphorus controls the amount of algae that grows in lake water, and the concentration of algae in lakes largely determines how far down into the water we are able to see. We also know that as their watersheds become developed, the amount of phosphorus in lakes increases. Stormwater runoff is the vehicle that carries phosphorus from the watershed to the lake.
You might expect that extended periods of heavy rain would cause lakes to become clearer, because all of that nice, pure rainwater would flush the phosphorus and algae out of the lake. But that basic logic gets complicated when natural forested watersheds become developed with roads, buildings, driveways, lawns, parking lots and more. Only a small percentage of the water in most lakes falls directly into the lake from the sky.
Most of the snowmelt and rain that flows into and through our lakes must first flow through their watersheds.
In a healthy, undisturbed watershed, natural buffer vegetation helps to filter the phosphorus and sediment from runoff. But as a watershed becomes more developed and buffers are removed, more runoff along with more phosphorus and sediment reaches the lake, ultimately causing the water to be less clear. Conversely, and strange as it may seem, less rain often translates to clearer lake water, because without runoff there is less fertilizer (phosphorus) flowing into the water to stimulate the growth of algae.
Now for the disclaimer: This is not a “one size fits all” phenomenon. Not all lakes respond in the same manner to more, or less rain (and snow), because precipitation isn’t the only weather factor that influences lake clarity. To complicate matters further, other factors affect the degree to which individual lakes respond to precipitation, including lake flushing rates, thermal stratification, dissolved oxygen levels, the size of individual lakes and their watersheds, and probably many more.
All of those factors notwithstanding, in recent years we have observed that many, but certainly not all, Maine lakes are generally clearer during years of below average precipitation, and less clear during years when precipitation is above average.
But the relationship between precipitation and lake clarity isn’t always obvious at first glance, as was shown in the data from 2008, when much of Maine experienced some of the heaviest rainfall in years, especially during the middle and late summer. Information obtained from the National Weather Service indicates that for much of the State, cumulative precipitation through the 2008 monitoring season was well above average, with frequent extreme rainstorms documented during the summer. Overall, the Portland area experienced the 3rd wettest year in the 138 years that records have been kept! Certainly, if precipitation is a strong influence on the annual clarity of our lakes, the 2008 data should be noteworthy.
Figure 1 indicates that in 2008, a slightly larger percentage of 418 Maine lakes were clearer than they have been historically. But a nearly equal number were less clear than they have been in the past, and about 13% showed no change. Even though the weather in 2008 was dramatic, the difference between the lakes that were clearer, and those that were not was not particularly dramatic. If anything, we might have expected that there would be substantially more lakes that were less clear, compared to those that were clearer.
The 2008 figures are somewhat of a surprise. In recent years that have been either very wet or very dry, the difference between the percentages of lakes that have been more or less clear has for the most part, been pronounced. One possible reason for the unexpected results in 2008 is that much of the rain during the summer came relatively late in the season (July and August). It is typically the snowmelt and rain that comes early in the open-water season (April and May) that has the greatest influence on the growth of algae in lakes during the summer. Lakes may not have had sufficient time to “process” the phosphorus in the late-summer runoff.
Figure 2 is another form of graphic illustration of the annual water clarity of Maine lakes, compared to their historical averages. The 2008 bar shows the same information as the Figure 1 pie chart. Below this is the bar for 2007, a relatively dry year for Maine, when about 62% of the lakes were clearer than average, and only about 30% were less clear (8% unchanged). In comparing these two years, we see that many lakes that were clearer than average in 2007 became less clear in 2008, so there was actually quite a swing toward reduced lake clarity from one year to the next. In 2006, Maine experienced the wettest month of May on record, and much of the State experienced the wettest months of June and August in many years. The bar for 2006 shows us that a high percentage (54%) of 429 lakes were less clear, compared to only 35% that were more clear, and only 10% were the same as their historical average.
Yet another way of looking at the changes that we see in Maine lakes from year to year is to calculate the annual average Secchi transparency (clarity) for all lakes for which there were data for individual years, and compare this single number from one year to the next. The Secchi disks on the graph in Figure 3 represent the average of the annual averages for all lakes in the dataset for individual years. For example, in 2008, the annual average Secchi clarity for each of the 418 lakes in the set were lumped together and averaged, resulting in an overall lake clarity reading for all of the lakes, which was 5.34 meters.
Looking at the data from this perspective, we see that even though the number of lakes that were more, or less clear than average in 2008 was nearly equal (Figs 1 & 2), Maine lakes as a whole were less clear in 2008 than they were in 2007, a relatively dry summer, when the average transparency for all Maine lakes was 5.7 meters - one of the clearest years for Maine lakes in nearly three decades! But during the wet summer of 2008, the average dropped to 5.3 meters, moving into the range of 5.0-5.5 meters, where most of the annual averages have occurred since the early 1970’s.
Figure 3 also illustrates how some of the clearest years for Maine’s lakes have occurred within a year or two of the least clear years. The most dramatic example of this took place between 1984 and 1985, when the average jumped to from 5.0 to 5.7 meters. What could have caused such a quick and substantial change? If you were fortunate enough to have spent time on a Maine lake in 1985, you might recall that the weather was exceptionally clear, dry, and calm throughout the summer. Very little precipitation occurred in Maine during the lake monitoring period. Without rain there is no runoff, and without runoff, phosphorus from the watershed stays in the watershed and out of the lake, resulting in measurably clearer water, as the graph suggests.
The change from 2006 to 2007 is nearly as dramatic. As we discussed above, May 2006 was the wettest on record for some areas of Maine and much of the summer was very wet throughout the state, resulting in one of the least clear years on record for Maine lakes. But in 2007, a relatively dry year, the average clarity of Maine lakes improved sharply. Please note that it was not until the mid 1970’s that the number of lakes being monitored reached 100 or more, which probably accounts for the wild swings in Secchi transparency during that earlier, brief period.
Many questions remain concerning why some lakes are clearer, and others less clear during dry years, and even whether or not it is the same group of lakes that experience shifts in transparency from year to year. We are working on the answers to these questions. What is certain is that our ability to describe relationships between lakes, the weather, and their environment is the direct result of the efforts of hundreds of dedicated volunteer lake monitors throughout Maine, without whom we would not recognize that these interesting relationships exist.
There’s a simple take-home message about watershed stewardship and lake protection in our observations of the annual volunteer Secchi data:
“Regardless of weather extremes, it is important to use conservation practices to try to prevent stormwater runoff from flowing over the land into streams or lakes. Runoff captured by buffers evaporates, is taken up by plants or filters through the soil recharging the water in your lake without causing excess algal growth and reducing water clarity.”
The data that support this article comes to you courtesy of Maine’s volunteer lake water quality monitors!
Maine Volunteer Lake Monitoring Program
vlmp@mainevlmp.org
24 Maple Hill Road, Auburn, ME 04210
(207) 783-7733
www.MaineVolunteerLakeMonitors.org
copyright 2010 Maine Volunteer Lake Monitoring Program | website comments to: vlmp@mainevlmp.org
