The Lagoon Monitor
Dissolved Oxygen- Fish Need to Breath too!
Salinity takes a Nose dive in the St. Lucie and Southern Lagoon
Why does Salinity Matter?
National Estuary Day
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Oxygen dissolves in lagoon 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.
Photosynthesis and wind action eventually raise the levels during the course of the day.
DO is measured in milligrams of oxygen per liter of water (mg/L). This is the same as one part oxygen for one 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 will die.
Low DO levels are the most common reasons for fishkills.
Typically oxygen levels in the Lagoon water are higher in the winter and lower in the summer. Runoff from summer rain washes nutrients from fertilizers and septic tanks 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 maps on this page show data from the Lagoon watch volunteer monitors from June 2003 and the first two weeks of August 2003.
On June 2003, DO was good, over 5mg/L along the lagoon. The area marked X had a DO under 2mg/L and the areas of Cocoa, Eau Gallie, Melbourne Turkey Creek and Taylor Creek had a DO between 3 and 4 mg/L.
In August, more X areas were reported between Titusville and Cocoa and at Taylor Creek. DO was under 4mg/L between Turkey Creek and Sebastian inlet. June 2003 shows the DO conditions before the start of the raining season.
By early August, low DO areas are much enlarged, particularly in the northern sections of the lagoon where circulation is restricted by causeways and has very few inlets to supply well oxygenated ocean water.
In the South where there are large deep inlets, tidal action brings in ocean water and keeps the DO above the danger levels.
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