Water Clarity (Secchi Depth)

The Secchi Depths reported from sites volunteers monitored along the entire length of the lagoon were averaged for each month. The results of boat sampling done by MRC staff were included as well. The Secchi Depth is a method for measuring the penetration of light below the surface of the water and thus the limit of visibility of the water. The method uses a Secchi Disk with four alternating black and white painted quadrants (quarters) with a premeasured line attached. The disk is lowered into the water until it can no longer be seen by an observer at the surface or the black can no longer be distinguished from the white areas. This depth is known as the Secchi Depth. Low Secchi Depths usually are caused by large amounts of suspended material or natural or man-made chemicals dyeing the water a dark color. Large concentrations of algae, especially algae blooms caused by excess nutirents will also lower the Secchi Depth. Low Secchi Depths mean that sunlight will not penetrate far below the surface of the water. If light levels become too low, plants which live on the bottom, like sea grasses, may not be able to photosynthesize and will die. This has been a severe problem in many areas of the Indian River Lagoon where decreases in water clarity have caused many sea grass beds to die off. Aquatic plants, such as sea grass, produce oxygen for fish and other aquatic life, as well as provide food, shelter, nurseries and habitat. When seagrasses die off they decompose and the process consumes oxygen increasing the likelihood of fishkills due to low dissolved oxygen in the water. Less sea grass also means less habitat for other organisms such as fish and shellfish. Sea grasses are especially vital for juvenile fish to hide them from predators. Each time the lagoon loses some sea grass cover it loses all the fish and shellfish that would have thrived there. By enlarging the area that drains into the Lagoon through canals and drainage ditches we also increased the amount of sediment transported every time it rains. Development of an area increases the runoff as vegetation is removed for construction and continues to be high if the banks of streams and canals are not allowed to be densely vegetated. Destruction of wetlands and mangroves are other serious factors since they not only stabilize sediments with their root systems they also slow water movement and help drop the sediments from the water. When the flow of runoff is not slowed down it erodes more sediment from the shorelines and helps to suspend bottom sediment as well. Large amounts of suspended material can clog the gills of fish and shellfish and can potentially kill them directly. The suspended matter can contain high levels of heavy metals and other pollutants and can provide a surface on which microorganisms grow.

The width of the Secchi Depth maps are twice the lagoon's true width to show more details of the lagoon. The Comprehensive Plan for the Indian River Lagoon puts a Secchi Depth of 1.5 meters (approximately 4.5 feet) as a goal for the entire Lagoon. Areas of low water clarity (secchi depth < 1 meter) are marked in red. Areas of moderate water clarity (1.0-1.5 meters) are marked in yellow and high water clarity (secchi depth > 1.5 meters) are marked in blue.

One tool in the fight for better water clarity is the Baffle Box. Baffle boxes are concrete structures designed to slow down storm water and cause the coarse sediment to settle out before it reaches the lagoon. Muck dredging projects like one that occured in Crane Creek winter of 97/98 also help increase water clarity by removing the muck sediments which are easily resuspended by boat traffic. What can you do to help? Keep your property well vegetated, do not trample vegetation along shorelines, get involved with our Johnny Mangrove program and help grow and plant mangroves along the shoreline in your area.