Lake Sampling Data - North Lake Leelanau

Abstract

Leelanau Conservancy North Lake Leelanau Lake Sampling Program metadata

Reports for "North Lake Leelanau" Size: 4.6 square miles Maximum Depth: 121 feet Average Depth: 43 feet Shoreline: 15 linear miles

Measurements 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 minimum, maximum and average Secchi Depth for all observations between 1990 and 2012. 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 (5.9 meters) occurred in Big Glen Lake. The smallest Secchi Depth occurred Little Glen Lake (8.1 feet or 2.5 meters). This lake also had the highest total phosphorus concentration as shown on the Total Phosphorus Chart. The Secchi Depth of about 7.5 feet (2.3 meters) 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.