Lake Water Quality Indicators

How Does the VLMP Measure Lake Water Quality?

There are many threats to Maine lakes. Foremost among these threats is the potential for nutrient and biological enrichment of lakes resulting from human activity in lake watersheds. This condition is characterized by declining water clarity (transparency), resulting from an increase in the growth of algae. Excess algae in lake water can cause a disturbance to the normal equilibrium of the aquatic ecosystem. As algae die and decompose, bacteria consume oxygen that is dissolved in the water. Increased algae growth can lead to a decline in oxygen levels over time. Oxygen loss can reduce critical habitat for coldwater fish (trout and salmon), and it can further accelerate the decline of water quality.

The enrichment of lakes with nutrients (especially phosphorus) and algae as a result of human activity is referred to as “cultural eutrophication” (CE). Stormwater runoff from disturbed or developed areas of lake watersheds can carry phosphorus, sediment particles, and other pollutants considerable distances to the lake. Lake watershed boundaries may be situated close to the shoreline, or they may extend for miles away from the lake.

One method of assessing the effects of CE in lakes is to measure the concentration of algae in the water. The Secchi disk is a simple, inexpensive device used to estimate algal concentrations, based on water clarity. Volunteers in the VLMP are provided with a Secchi disk that is attached to a calibrated line, and a viewing scope. They are instructed by VLMP and Maine DEP staff on the method of taking a Secchi disk reading.

All Volunteer monitors are trained to measure Secchi disk transparency. Ideally, readings are taken a minimum of twice each month from May through September or October.

Secchi Disk Transparency
Long Term Data Distribution

Interpreting Secchi Disk Transparency Data

The Secchi disk is a quick, inexpensive and generally reliable procedure for assessing lake water quality. The primary uses of Secchi transparency data are: 1) to characterize or define the existing water quality of a lake, and 2) to track long-term water quality trends.

Secchi disk transparency is an indirect water quality indicator, based on the assumption that water clarity is affected primarily by algal growth. That assumption is reasonable in most cases. However, other factors may also influence transparency, including the amount of sediment that is suspended in the water, and natural water color.

Natural color and suspended sediments vary widely from one lake to another. Color is influenced by the concentration of dissolved organic compounds in the water. These “humic acids” can stain the water in some lakes to the point where light penetration into the water column is substantially attenuated. Shallow lakes may be subject to moderate concentrations of resuspended bottom sediments in the water column, resulting from wind turbulence. Both color and sediment can limit the value of Secchi transparency data as an indicator of algae growth.

All planktonic plants and animals that live in lakes can have an influence on transparency. These organisms undergo seasonal and annual growth cycles, resulting in changes in their overall density and in their location in the water column.

Secchi transparency is often at a low point soon after the ice melts in the spring. That is when the entire lake mixes, or “turns over,” causing nutrients and sediments from the lake bottom to become suspended in the water.

The availability of silica from the bottom sediments stimulates diatoms, a type of algae that typically experiences peak growth in the spring and fall. This can result in a brief period of reduced transparency. As lake water warms and stabilizes during the summer, other types of algae will dominate the water column, depending on water temperature, nutrient levels and other factors.

Many Secchi transparency readings are needed over a period of years in order to detect and track trends in lake water quality with confidence. Because of “natural variation,” several complete seasons of data are typically required to characterize the quality of individual lakes. For this reason, volunteer lake monitors are asked to take a minimum of two Secchi disk readings per month from May through September or October every year.

Dissolved Oxygen and Water Temperature Data

Another indicator of the quality of lakes and ponds is the concentration of oxygen that is dissolved in the water. Oxygen is produced through plant metabolism (photosynthesis), and it is consumed during respiration and decomposition. Dissolved oxygen (DO) levels in lake water are influenced by a number of factors, including water temperature, the concentration of algae and other plants in the water, and the amount of nutrients and organic matter that flow into the water body from the watershed.

An adequate supply of dissolved oxygen in lake water is essential to fish and other aquatic life forms. DO is a sensitive indicator of changes in water quality, and of the ability of a water body to support aquatic life. The gradual loss of DO in the deepest area of a lake may indicate that the ecosystem is out of balance and stressed.

Thermal Stratification
As lake water is warmed in the summer the water in a
deep pond or lake is layered into three levels:
1) warmer (less dense) epilimnion layer at the surface
2) the thin thermocline (transition) layer
3) the cold and deep hypolimnion layer

Biological activity peaks in lakes and ponds during the summer months. It is also at this time that a phenomenon called thermal stratification occurs. The combined influence of the two processes has a pronounced effect on water chemistry, and in particular on dissolved oxygen levels. The isolation of the deep cold water at the bottom of the lake from the surface water during summer stratification prevents the oxygen supply in the deeper water from being replenished. The period of isolation varies from one body of water to another, and depends on the depth of the body, and the influences of weather. Stratification may last from several weeks to a few months.

Some oxygen loss occurs naturally during the summer months as water temperatures rise, because the solubility of oxygen in water is inversely proportional to the water temperature. In other words, cold water is able to contain more oxygen than warm water (all other factors being held equal.) However, as lakes become more biologically productive, and organic matter accumulates in the system, the potential increases for oxygen levels to decline. The loss of oxygen can stress fish and other aquatic biota, and under certain circumstances, it can cause an acceleration in the decline of water quality.

Volunteer monitors are able to measure oxygen concentrations in the water using specially designed chemical kits and a water sampler. This inexpensive method is accurate and reliable, although somewhat time-consuming. The temperature of the water must also be recorded for each oxygen reading. A more costly, but quicker alternative involves the use of a probe that is attached to a cable and meter. The probe simultaneously measures dissolved oxygen and water temperature.

Oxygen concentration and water temperature should be recorded throughout the summer stratification period, from early spring through late summer and early fall, when oxygen levels are often lowest in Maine lakes and ponds. Readings are taken from the water surface to the bottom of the deepest area of the lake, at one-meter intervals (depending on the individual water body.) Dissolved oxygen is measured in milligrams per liter (mg/L) or parts per million (ppm).

Total Phosphorus
Long Term Data Distribution

Total Phosphorus Data

Volunteer monitors can also be trained to measure the concentration of phosphorus in their lakes. The method used by most volunteers involves collecting a sample from a few inches below the water surface at the designated monitoring station. Samples are analyzed for total phosphorus (TP), which includes all of the organic and inorganic forms of the element that may be present in the water in solution or in particulate form.

Phosphorus is the nutrient that most influences the growth of algae in lakes. An increase in the concentration of total phosphorus in the water generally indicates a potential increase in biological productivity of the system. Tracking changes over time in lake phosphorus levels is another way of monitoring water quality. Combined with Secchi transparency readings, TP data can provide additional insight into the complex dynamics of a lake's trophic state and water quality.

Ideally, samples are taken from early summer through the end of the sampling season in September or October. However, because phosphorus sample analysis involves laboratory fees, volunteers may be limited to taking one or two samples during the late summer (about mid-August), when biological activity is at a peak. The VLMP has made arrangements to provide volunteer monitors with special sampling and mailing containers to facilitate the collection of phosphorus data.

As is the case with most indicators of lake water quality, the concentration of phosphorus in lake water varies within individual seasons, and from one year to the next. That is why it is important to collect multiple samples during the monitoring season, when possible. Figure 4 illustrates the variation in total phosphorus samples from a Maine lake within a single season (year) and the annual average concentration for several years. Total phosphorus concentrations are generally reported in parts per billion (ppb).

Chemical Lake Data

Although an increasing number of volunteers are expanding their knowledge and capabilities for monitoring their lakes, the need exists for periodic baseline assessment data for all of the lakes in the program. The VLMP and Maine DEP staff strive to collect additional water quality information for lakes in the program, and other lakes and ponds with special concerns. This is done on a rotating basis, and as resources allow. Baseline sampling of most of the program lakes occurs every three to five years during the late summer. The additional information adds considerable value to data collected by volunteer monitors Data are gathered for the following indicators of lake water quality:

Total Phosphorus Long Term Distribution
Chlorophyll a Long Term Distribution
Color Long Term Distribution
Conductivity Long Term Distribution
pH Long Term Distribution
Total Alkalinity Long Term Distribution
Phytoplankton
Anions and Cations
Zooplankton

Maine Volunteer Lake Monitoring Program

vlmp@mainevlmp.org
24 Maple Hill Road, Auburn, ME 04210
(207)-783-7733
www.MaineVolunteerLakeMonitors.org
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