Lake & Watershed Resource Management Associates

P O Box 65; Turner, ME  207-336-2980

LWRMA @megalink.net

 

 

Report on the Health of Great Moose Lake

 

2008

 

Baseline water quality monitoring was conducted for Great Moose Lake on July 10 and August 22, 2008 by LWRMA staff. All parameters and indicators that have been checked during the past several years were measured and sampled, using methods and protocol for lake assessment established by the Maine Department of Environmental Protection. Additional data were gathered throughout the summer monitoring period by certified volunteer lake monitors, John Plummer and Don Childs.

 

Overall, Great Moose Lake experienced a slightly below average year, based on sampling results from the two stations on the lake where monitoring has taken place for the past three decades. This assessment is based on water clarity readings (Secchi transparency), and total phosphorus and chlorophyll-a samples taken at both stations.

 

Lakes and ponds may experience a significant amount of variability from season to season and year to year. In addition to the influences of human activity on lake health (water quality), annual variations in the weather can have a significant effect on indicators of lake water quality. The weather during the 2008 monitoring season was extreme throughout much of the State of Maine. Heavy downpours extended over a period of several days could certainly have been related to the downturn observed in 2008. (see Statewide Perspective on Maine Lakes on page 4)

 

The 2007 water clarity (transparency) average for Great Moose was 4.3 meters at the deep station (01), compared to 4.9 meters in 2007, 4.5 meters in 2006, 4.3 meters in 2005, 5.3 meters in 2004, and the historical average of 4.5 meters for the station.

 

Water clarity at station 02 averaged 4.8 meters, compared to 5. 2 meters in 2007, 5.0 meters in 2006, 4.4 meters in 2005, 5.3 meters in 2004, and the historical average of 4.8 meters at this station.

 

Water clarity (Secchi transparency) is one of three primary indicators of the biological productivity of lake ecosystems, in addition to the nutrient phosphorus (TP) and chlorophyll a (CHL), a pigment that is used to measure the concentration of algae in lake water.

 

The 2008 average total phosphorus (TP) level for station 01 was 13 parts per billion, compared to 9 parts per billion in 2007, 13 ppb in 2006, and the historical average of 9 ppb. The July TP concentration measured 12 ppb, and the August sample measured 13 ppb.

 

The station 02 average was 16 ppb, compared to 10 ppb in 2006 and the historical average of 10 ppb for that area of the lake, based on the July sample concentration of 25 ppb, and 11 ppb in August. The unusually high phosphorus concentration in July cannot readily be explained. It is significantly higher than any previous single sample that has been taken in the lake. Phosphorus samples are easily contaminated with nearly any organic matter that may be in the water, including clusters of algal cells and other material that is extremely fine and difficult to detect visually. It is possible that suspended organic matter in the lake may have been captured in the core sample, causing the elevated reading. The fact that the August sample measured less than one half the concentration of the July sample, and that the chlorophyll-a (algae) sample from July was lower than the July sample is cause to suspect sample contamination.

 

A TP sample taken in August near the bottom sediments at the deepest point in station 01 measured 11 ppb, and a similar sample taken at station 02 in August measured 17 ppb. The sample taken at station 01 was close in concentration to the phosphorus sample taken near the surface. However, the bottom sample taken at station 02 was 6 ppb higher than the surface sample. The higher concentrations of phosphorus near the bottom may have been associated with very low dissolved oxygen levels at the time at both stations. Historical bottom grab samples taken during the late summer have also been relatively high, indicating that low oxygen levels may be playing a role in the release of phosphorus from the bottom sediments. This phenomenon bears watching in the future, because low concentrations of oxygen in the water during the summer, along with indications that phosphorus is being released from the sediments could result in an overall negative impact on water quality under certain circumstances.

 

The 2008 chlorophyll-a average for station 01 was 7.0, compared to 4.0 ppb in 2007, 4.2 ppb in 2006 and the historical average of 4.3 ppb at this station. The station 02 average was 4.5 ppb, compared to 3.5 ppb in 2007, 4.0 ppb in 2006 and the historical average at this station, 4.0 ppb. The substantially higher than average level at station 01 was largely the result of a high value (8.7 ppb) in July. The water was less clear at the deep station on that day, but even so, the CHL level was unusually high, compared to the level at station 02.

 

Dissolved oxygen profiles taken in July and August were similar to those from the past several years. Moderate oxygen depression (less than 5.0 ppm) and some depletion (less than 1-2 ppm) was documented in the deepest several meters of water at station 01 in August, similar to historical conditions documented in this region of the lake.  Severe oxygen depletion was documented at Station 02 in August, with virtually no oxygen found below 7 meters depth. The August, 2008 oxygen profile was similar to ones documented at station 02 for several of the preceding years.

 

Additional water quality indicators monitored in 200 (pH, total alkalinity, water color) were within normal limits for Great Moose Lake, and generally supported the indicators of primary productivity. Water color was more than double the concentration at both stations in August, compared to samples taken one month earlier. Lake color is a function of the concentration of natural humic acids in the water. Humic acids from wetland vegetation and other sources in the watershed may also have been partially responsible for elevating phosphorus levels in the lake in 2008. However, phosphorus concentrations in the lake are very likely primarily related to watershed development.

 

Discrepancies between samples taken at the two stations in 2008, and between the two sampling dates, as well as departures from historical averages for both stations are likely to have been related to the unusual weather during the sampling season. The August 15 visit to the lake followed nearly two weeks of severe rain, which undoubtedly resulted in the flushing of watershed wetlands, as well as moderate runoff from roads, rooftops, lawns and agricultural land. Sediment from soil erosion, phosphorus from sediment particles, fertilizers and human and animal waste may have skewed some samples.

 

 

 

 

 

 

 

 

 

Statewide Perspective on 2008 Lake Water Clarity:

 

To put into perspective the significance of the 2008 water clarity findings, consider that out of 418 Maine lakes that were assessed last year, about 46% were clearer than their historical averages, and about 41 % were less clear than their average. This represents a significant change from 2007, when a much higher percentage of Maine lakes were clearer than they had been historically.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Percentage of Maine Lakes that were clearer, less clear, and the same as their Historical Average in 2008 (Source: Maine Volunteer Lake Monitoring Program Maine Lakes Report

 

It is likely that the reduction in the number of lakes that were clearer than average in 2008 was the result of heavy snow and runoff in the spring and moderate to severe rain throughout much of Maine during the mid to late summer period. Spring runoff from melting snow and rain typically carries a high percentage of the annual phosphorus load to lakes from their watersheds. Information obtained from the National Weather service indicated that Portland, Maine experienced the wettest summer period in 138 years.  

 

Water clarity is one of three primary indicators of the overall biological productivity of lake ecosystems, in addition to the nutrient phosphorus (TP) and chlorophyll a (CHL), a pigment that is used to measure the concentration of algae in lake water. The three indicators, along with dissolved oxygen, are considered to be key measures of the water quality, and overall health of Maine lakes.

 

The chart below shows the extent to which water clarity (Secchi transparency) has varied for Maine lakes over time. The chart shows the average water clarity for all Maine lakes monitored in a given year. Note that this average has, for most years since this information has been tracked, fallen between 5.0-5.5 meters. Variation from one year to the next is influenced by many factors, not the least of which is weather. Maine lakes may be relatively clearer during dry years because stormwater runoff from rainfall carries phosphorus and other pollutants from the watershed to the lake.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Source: Maine Volunteer Lake Monitoring Program 2008 Maine Lakes Report

 

 

The illustration above shows that for the period from 2004-2006, the “average” clarity of Maine lakes dropped substantially. This may have been due to the fact that much of the state experienced above average precipitation during the period. In 2007, Maine lakes as a whole were significantly clearer, most probably due to reduced precipitation during the winter, spring and early summer months, when a high percentage of watershed phosphorus loading typically occurs for lakes. But in 2008, along with a lower percentage of lakes being as clear as they were in 2007, the overall water clarity for Maine lakes dropped to 5.35 meters, as the graph above illustrates.

 

The graph shows that a number of similar changes have occurred historically. Some of the “clearest” years have been those during which drought has recently occurred, such as 1985 and 2002 and 2003, which followed the severe statewide drought of 2001.

 

Each lake and pond responds in a unique way to the influences of weather, changes in land use in the watershed, and other forces upon the ecosystem. That is because of the wide range of physical, chemical and biological characteristics of each lake basin and its watershed. Most lakes and ponds experience moderate levels of natural annual variability.

 

Water clarity (Secchi transparency) is one of four primary indicators of the biological productivity of lake ecosystems, in addition to the nutrient phosphorus (TP), chlorophyll a (CHL), a plant pigment used to measure of the concentration of algae in lake water, and the concentration of dissolved oxygen in deep areas of the lake during the summer months.

 

 

Prepared by Scott Williams, Aquatic Biologist