Oxygen dissolves in surface water due to the mixing action of winds and as a by-product of plant photosynthesis. At night plants cannot photosynthesize without sunlight, and instead respire like we do, using up the oxygen dissolved in the water. As a result, the Dissolved Oxygen (DO) is usually lowest in the morning after being consumed all night, with photosynthesis and wind action eventually raising the levels during the course of the day. DO is measured in milligrams of oxygen per liter of water abbreviated as mg/L. A mg/L is one part oxygen to a million parts water. If DO levels drop below 3.9 mg/L some fish and other aquatic organisms can be stressed. If levels drop below 2.0 mg/L many species can die off. Low DO levels are the most common reasons for fishkills.

    This page shows two maps of the dissolved oxygen levels in the lagoon from August, 2002 and then again in January, 2003. Typically oxygen levels in the Lagoon water are higher in the winter and lower in the summer. Runoff from summer rain brings nutrients into the warm lagoon triggering growth of algae. The algae use up oxygen in the water overnight resulting in low oxygen levels in the morning. The August map shows that low oxygen areas are mostly adjacent to major run-off water sources. This includes the Titusville area, Cocoa, the Eau Gallie River, Crane Creek, Turkey Creek, the St. Sebastian River, the Vero Beach drainage canals and Taylor Creek in Ft. Pierce. August had the worst D.O. problems this past year with Xs in the northern Lagoon marking where D.O. levels were low enough to cause fish kills. Summer of 2000 and 2001 showed lower D.O. then this past summer with numerous fish kills resulting. The January D.O. results shown on the other map show no problem areas. Cold winter temperatures reduce algae growth and colder water has a greater capacity to hold D.O.