A Secchi disk reading is a measurement of water clarity. The depth at which the disk disappears is the water clarity (or transparency) depth. The depth of clarity is affected by such factors as: free floating algae, soil particles from erosion, re-suspension of bottom sediments, etc.
What does the Secchi disk measurement tell us?
The Secchi Disk measurement is an indirect measurement of lake fertility and productivity. It measures the depth of light penetration into the lake. Light penetration may be decreased by algae, water color, or particles suspended in the water column. We are most concerned about the abundance of free floating algae which into a lake. Generally, the greater the number of free floating algae, the shallower the Secchi depth reading. The fewer the number of free floating algae, the deeper the Secchi depth reading. Some lakes have a brown color which may be due to indicates the impact of nutrient input humic acid. Other lakes have marl which may cause the water to become "milky".
The purpose of taking Secchi disk measurements is to show changes in water quality from week to week, month to month and season to season. Since each lake is unique, the most valid interpretation of Secchi disk measurements is to identify what is happening to each lake. The seasonal results over a period of years will indicate whether the lake water quality is remaining stable, decreasing or showing improvement. If the readings show a decreasing water quality, the association may want to take a closer look at activities around the lake and within the watershed to determine nutrient input and decide the best possible solutions.
SPRING AND LATE SUMMER PHOSPHORUS
Phosphorus is one of several essential nutrients that algae need to grow and reproduce. For most lakes in Michigan, phosphorus is the limiting factor for algal growth. The total amount of phosphorus in the water is used to predict the level of productivity and eutrophication in a lake. An increase in phosphorus over time is a measure of nutrient enrichment in a lake.
Phosphorus is a naturally occurring element that is found in rocks and soil . Humans use and dispose of phosphorus on a daily basis in common items such as fertilizers, foods, and cleaning agents. Lakes with developed watersheds often receive a portion of this human-generated phosphorus through runoff, septic leachate, and other sources.
Phosphorus is found in lakes in several forms that are in a constant state of flux as environmental conditions change and plants and animals live, die, and decompose in the lake. The various forms of phosphorus are constantly changing and are distributed in different locations of the lake with changing seasons. Because the forms of phosphorus are continuously changing and recycling, it is convenient to measure all of the forms of phosphorus together as total phosphorus.
The late summer phosphorus results, along with chlorophyll and Secchi disk transparency measurements, provide an estimate of the level of productivity, or trophic state, of your lake. These results are used to calculate a set of trophic state indices (i.e., Carlson TSI) for the lake. These indices provide a quantitative means of describing the stage of lake aging, or eutrophication. Using Carlson's TSI, we classify lakes according to their trophic state (i.e oligotrophic, mesotrophic, eutrophic, etc.). Spring overturn, when the lake is generally well mixed from top to bottom, is an opportune time of the year to sample just the surface of the lake to obtain a representative sample for estimating the total amount of phosphorus in the lake. At other times of the year, more extensive water column sampling is needed to determine phosphorus levels in the lake.
Chlorophyll is the green photosynthetic pigment in the cells of plants. The relative amount of algae in a lake can be estimated by measuring the chlorophyll concentration in the water.
The amount of chlorophyll in an algal cell varies among algae species as well as with changing light conditions at different depths within the lake. Changing seasons also create different light conditions which, in turn, affect chlorophyll production . To account for some of this variability, algal chlorophyll is monitored during five mid-month sampling events over the summer season (May through September) using a water column composite sampling technique.
The summer chlorophyll monitoring results, along with total phosphorus, and Secchi disk transparency measurements, provide an estimate of the level of productivity, or trophic state, of your lake. These results are used to calculate a set of trophic state indices (i.e., Carlson TSI) for the lake. These indices provide a quantitative means of describing the state of lake aging, or eutrophication. Using Carlson's TSI, we classify lakes according to their trophic state (i.e., oligotrophic, mesotrophic, eutrophic, etc.)
DISSOLVED OXYGEN AND TEMPERATURE
Dissolved oxygen and temperature are two fundamental measurements of lake productivity. The amount of dissolved oxygen in the water is an important indicator of overall lake health.
Water temperature is a key factor in many important lake processes. For approximately two weeks in the spring and fall, the typical lake in Michigan is entirely mixed from top to bottom, with all the water in the lake at uniform temperature. In the summer most lakes with sufficient depth (greater than 30 feet) are stratified into distinct layers of different temperatures. These layers are referred to as the epilmnion (warm surface waters) and hypolimnion (cold bottom waters) which are seperated by the metalimnion, or thermocline layer, a stratum of rapidly changing temperature. The physical and chemical changes within these layers influence the cycling of nutrients and other elements within the lake.
During the summer, the bottom waters of stratified lakes are susceptible to oxygen depletion. Decomposition of dead plant and animal organic matter in the hypolimnion and sediments deplete the dissolved oxygen acquired during spring mixing and the thermocline acts as a barrier that prevents additional oxygen from the atmosphere and the surface waters from reaching the bottom waters during the summer months. The greater the supply of organic matter from the epilimnion and the smaller the volume of the hypolimniom the more rapid the oxygen depletion in the bottom waters.
The dissolved oxygen and temperature regime of a lake is important to know in order to understand changes resulting from eutrophication and to develop appropriate management plans. A lake’s dissolved oxygen and temperature patterns not only influence the physical and chemical qualities of a lake but the sources and quantities of phosphorus, as well as the types of fish and animal populations.
Temperature and dissolved oxygen are typically measured as surface-to-bottom profiles over the deep basin of the lake. Temperature is usually measured with a thermometer or an electronic meter called a thermistor. Dissolved oxygen is either measured with an electronic meter or by a chemical test. The CLMP uses an electronic meter designed to measure both temperature, with a thermistor, and dissolved oxygen. The meters are calibrated by the volunteer monitors before each sampling event.