Plants that live under the water, such as seagrasses, produce oxygen for fish and other aquatic life, as well as provide food, shelter, nurseries and habitat. Like any plant, seagrasses need sunlight to produce energy in a process called photosynthesis. When the lagoon becomes murky, light is not able to penetrate the water very far and if the seagrasses are unable to photosynthesize for long enough they die. Each time the lagoon loses some seagrass it loses all the fish and shellfish that would thrive there. The maps on the right show the results of water clarity testing done by the Lagoon Watch volunteers in August and on the last two Lagoon Days. On Lagoon Day about 100 monitors sample the Lagoon at the same time providing a unique snapshot of water quality.

Secchi depth is a method for measuring the penetration of light below the surface of the water. The method uses a Secchi disk with a checkerboard pattern of four alternating black and white painted quarters with a pre-measured line attached. The disk is lowered into the water until it can no longer be seen from the surface or the black can be no longer distinguished from white areas. This depth is known as the Secchi depth. Low Secchi depths usually are caused by large amounts of suspended materials or natural man-made chemicals dyeing the water a dark color. The higher the secchi depth the deeper the seagrass can survive. The goal for water clarity in the Lagoon is one and a half meters (almost five feet).

On April 20, 2002, the Spring Lagoon Day, water clarity overall was very good, over 1 meter in most areas. Only in Melbourne near the Eau Gallie River was it less than that. Many areas, showed results over 1.25 meters (marked in dark blue), pretty good for the Lagoon overall. The data for August of this year shows much lower secchi depths. Summer rains drained sediment and fertilizers into the lagoon clouding the waters and triggering algae growth that keeps light from penetrating to seagrasses. In the north from Melbourne all the way to Titusville, average Secchi readings were less that 1 meter. The same was true for southern Brevard, Vero Beach and the southern Lagoon to St. Lucie Inlet. The Fall Lagoon Day event on November 16th showed that the Lagoon water clarity improved as cooler weather and reduced rainfall reduced the impact of runoff on the Lagoon’s systems.