The Indian River Lagoon is long and narrow like a river, but it is called a lagoon because it is really a salt-water lake connected to the ocean by inlets. Unlike a river, the lagoon has no particular direction of water flow and the water moves in whatever direction the wind blows. The water in the lagoon comes partly from fresh-water run-off from the surrounding land and partly from ocean water that moves in and out with the tide. Evaporation raises salinity levels by removing fresh water and leaving the salt behind, concentrating it.

   During the dry and windy winter months, the salinity normally starts to rise. As warmer, but still dry spring weather arrives, evaporation increases. Salinity typically peaks in late spring and can reach ocean levels (35ppt, 3.5% salt) or higher and then starts a downward trend as the summer rains begin. A hurricane event dramatically lowers salinity, particularly in the northern lagoon. Salinity in the southern half of the lagoon is typically 2 to 3 % (20 - 30 ppt.) and varies less due to large man made inlets supplying ocean water.

   The sea grasses that grow in the lagoon do best with salinities between 2 to 3 percent (20 - 30 ppt.) Low salinities in the summer stress seagrass at a time when they should be having their best growth. The impact of the freshwater is also compounded by sediment clouding the water making it hard for light to penetrate to reach the sea grass. When seagrass beds decline, it also impacts the fish and shellfish that depend on the seagrass to spawn and hide from predators. Many fish species can tolerate low salinity but often their eggs cannot be fertilized at low levels because as the water becomes fresher, it becomes less dense and the eggs cannot float on the surface to be fertilized. Sustained salinity below 15 ppt will kill clams and other shellfish outright.

   The majority of the lagoon on the maps to the right is dark blue indicating salinities between 2.5 to 3.5 % (25 - 35 ppt.). On the May 2002 map the ongoing drought caused the Mosquito Lagoon and areas marked in white around the inlets to have salinities even higher than the ocean itself.

   In May 2002 the area around Cocoa had a salinity between 2 and 2.5 % (20-25 ppt.) indicated by the slashed line. The Aug 2002 map reflects the drought ending with a rainy summer pouring millions of gallons of fresh water into the lagoon. Evaporation was also limited due to the cloudy skies and high humidity associated with the daily storms. Due to tremendous local rainfall around the Vero Beach area in Indian River County, (marked with a crosshatch on the map) salinites reached lows of less than 1 percent (10 ppt.) This was the lowest salinity we have ever recorded in our 12 years of testing the water. The larger tributaries were impacted by the large amounts of runoff flowing into them. Along with this runoff came large amounts of sediment and other pollutants which add nutrients to the lagoon causing algae blooms and low dissolved oxygen. This May, salinities are lower than last year but still in good ranges. With normal summer rains continuing, we can expect to see the salinities drop as we approach August.