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"Fr Secchi was an astro- physicist and scientific advisor to the Pope. " _-., :w HISTORY For all its simplicity, the Secchi Disk is the most economical, quickest, easiest and wide- ly used tool to measure water quality. It may even be the oldest monitoring tool still being used. It dates back to 1865 when Fr. Pietro Angelo Secchi was asked by Commander Cialdi, head of the Papal Navy, to measure transparency in the Mediterranean Sea. Fr. Secchi was an astroph}'sicist and scientific advisor to the Pope. Since 1865 various sizes and colors have been used. The two most commonly used variations are an all-white metal disk and a disk painted in black and white quad- rants. The Minnesota Pollution Control Agency (MPCA) uses an all-white, 8-inch diameter notched metal disk. The notched sides are for rope storage when the disk is not in use. The attached waterproof rope is marked in meters or feet for easier reading. WI IAT DOES A SECCHI DISK MEASURE? Simply put, a Secchi disk measures how far down into the water a person can see, or how much light passes through the water. This is called water transparency It provides an indirect measure of the suspended particles in the water. In many cases this is an indication of the amount of algae in the water.. The three environmental factors causing decreasing Secchi disk measurements are quantity and type of algae, quantity anal type of other suspended materials such as soil particles and color of the water. Algae abun- dance is related to the amount of nutrients present, especially phosphorus and nitrogen. Nutrients may reach the lake from sources such as sewage treatment plants, septic tanks, and lawn and agricultural fertilizer. Suspended particles may come from sources such as resuspension from the lake bottom, construction sites, agricultural fields, and urban storm runoff. Transparency serves as an early warning that activities on the land are affecting a lake. GETTING THE BEST SECCHI DISK MEASUREMENTS Despite its long use, there is no universally accepted protocol for taking measurements. In some cases Secchi depth is where the disk is just visible, while in others it is the depth where it disappears. The most commonly recommended Secchi reading is the average of the depth at which the disk disappears and then reappears. Since measurements are only as good as the equipment, protocols and observ- er, it is important to establish guidelines for each. One very important item is the estab- lishment of the sampling sites. The main site is usually in the deepest part of the lake, as determined by a bathymetric map or depth gauge. It should be located away from the shoreline, weed beds, underwater bars, points, islands, or stream inlets as they can all influ- ence water transparency. In lakes with several bays secondary sites can be established. The following are some additional guidelines to maintain the reliability of your results. Equipment guidelines: o Keep the disk clean, protected from. scratches and direct sunlight o Use a disk with a matte finish • Use non-stretching rope • Check rope markings each }'ear as the rope may shrink and/or the marks fade • Re-mark equipment as necessary Protocol guidelines: o Establish written protocols so all samplers are doing the same thing • Use established sampling sites • Take readings from June-September • Take readings at least once per month- weekly is even better • Take readings as close to noon as possible (lO:OO A.M.-3:00 eM.) • Take readings on the shady side of the boat • Avoid readings when the lake is choppy or rough Page 3 Sandra Holm, CITIZEN MONITORING PROGRAM DIRECTOR, MINNESOTA LAKES ASSOCIATION Photo Courtesy of Sandy Holm