Lake Sampling Program
The lake program consists of laboratory measurements that are designed to determine the concentration of algal nutrients (Total Phosphorus and Nitrate) and the magnitude of the algal community (Chlorophyll). In addition several measurements are made on site to further characterize the overall water quality of the lakes. These measurements include: Secchi Depth and vertical profiles of Conductivity, Dissolved Oxygen, Oxidation-Reduction Potential, pH, and Temperature. Note that the data summaries shown here are average values and may include samples taken from various lake depths. Furthermore, the number of samples may be different for various years and lakes. Thus, the plots and graphs provide only a general indication of the ranking of the water quality of lakes and streams in the area and are not intended to be a rigorous statistical analysis.
Secchi Depth Chart
Water transparency is measured with a black and white disk called a Secchi disk. The Secchi disk is lowered into the water until it cannot be seen. The depth at which it is no longer visible is called the Secchi depth. The water transparency, or Secchi depth, is dependent on the density of suspended materials in the water. The chart below shows the average Secchi Depth for all observations between 1990 and 2007. The plot provides a general indication of the ranking of Secchi Depth in area lakes. Observe that the largest Secchi Depth of just over 19 feet occurred inBigGlenLake. The smallest Secchi Depth occurred Little Glen Lake. This lake also had the highest total phosphorus concentration as shown on the Total Phosphorus Chart. The Secchi Depth of about 7.5 feet means that a six foot person standing in Little Glen Lake up to their nose in water will just be able to see their toes. Finally despite the significant variation among the various lakes, the Secchi Depths are high compared to many other lakes in the State and are currently indicative of high water quality conditions. Continued vigilance and careful and thoughtful planning are essential to maintaining these desired conditions.
Trophic State Index Chart
The Trophic State Index (TSI) is estimated by using water quality measurements and a mathematical formula (Carlson, 1977). TSI numbers are calculated using three different lake measurements: water transparency, and phosphorus levels, and chlorophyll-a.
Chlorophyll-a is the green pigment that algae use to capture the energy of the sun. It is used as an indication of how much algae are in a lake.
All lakes fall into one of three trophic states: eutrophic, mesotrophic, or oligotrophic. Eutrophic lakes have a TSI value greater than 55 and are considered highly fertile, or productive. They often have an abundance of algae and high phosphorus levels. This high algal growth decreases the transparency of the water and gives the water a greenish or brown color. Mesotrophic lakes have a TSI value from 40 to 55. Due to lower nutrient availability in mesotrophic lakes, they are less productive. Being less productive, there is less algal growth and clearer water. Oligotrophic lakes have a TSI value of less than 40. These are the least productive of the lakes and have the clearest water.
All lakes in the Conservancy monitoring program are oligotrophic. The least productive are North Lake Leelanau andBigGlenLake. LittleGlenLakeis the most productive as a result of its shallow depth.CedarLakeis also showing sign of eutrophication likely due to high residential development in the watershed.
Carlson, Robert E. 1977. A trophic state index for lakes. Limnology and Oceanography. 22: 361- 368.
Secchi, North Lake Leelanau, 2002-2007
The Leelanau Conservancy database has the capability to output data in both graphical and tabular formats. The graphical displays can be either bar graph data summaries or time plots. This figure shows a typical time-series plot where the measurement of Secchi depth for North Lake Leelanau is displayed over the period from 2002 to 2007. This plot shows the seasonal variation as well as the long-term trend of Secchi depth. Note that the lowest Secchi depth occurs in mid-summer while higher values are measured in the winter and spring and that the lowest Secchi depth of about 9 or 10 feet has remained relatively constant over the measurement period. Other time plots for various lakes, parameters, time-periods, and depths can be generated using the full capabilities of the Microsoft Access database.