Rivers and lakes that connect with the ocean and contain a mixture of salt and freshwater are called estuaries. The Indian River Lagoon is an estuary that is long and narrow like a river but it is really a salt water lake connected to the ocean. Unlike a river, the lagoon has no particular direction of water flow and moves in whatever direction the wind blows. Near inlets the lagoon flow is influenced by ocean tides which push heavy salt water into the lagoon while less dense freshwater remains near the surface of the lagoon. Salinities range from completely fresh in some of the rivers flowing into the lagoon to up to 35 parts of salt per thousand parts water (35ppt.) near the ocean inlets (35ppt = 3.5% salt). The concentration of salt in the lagoon is critical for the plant and animal species that live in it. Wetlands that historically did not flow to the lagoon were drained by canals that now bring more then twice the amount of freshwater the lagoon historically received (see article on back cover). The dredging of the Sebastian, Ft Pierce and St Lucie Inlets increased salt water flow into the lagoon near those inlets. Overall man’s effect has been to increase salinity near the inlets and decrease them at a distance from the inlets.

On the right are maps showing the average salinity levels in the lagoon for August 2001 and January 2002. On these maps, low salinity (<15 ppt or 1.5% salt) is indicated in white. Portions of most of the fresh water tributaries to the lagoon would naturally be expected to have salinities in this range. However salinity less than 15 ppt in the lagoon itself can kill shellfish like clams and oysters. Salinities in the range of 15-20 ppt are marked on the maps with a diagonal hatch. When these areas occur in the main body of the lagoon they are considered "stressed". Important fish species like the spotted sea trout can tolerate salinities below 15 ppt but require a salinity above 20 ppt to reproduce successfully. Moderate salinities (20-25 ppt) are indicated in light blue. Most of the seagrasses native to the lagoon fare much better in salinities in a range of 20-30 ppt. High salinities in the range of 25-35 ppt are shown in dark blue. Salinity in the ocean is very close to 35 ppt all year round.

In August 2001, the average salinity in the central lagoon (South Brevard, North Indian River County) showed stressed levels of salinity with small areas near the shore showing salinities too low to sustain shellfish. This pattern has become typical during rainy periods. When rainfall lands on wetlands or vegetated areas, it is filtered into the ground to become valuable ground water. When rainfall lands on our growing urban areas the rain cannot penetrate streets, driveways, parking lots and roofs and instead runs across the surface, picking up pollutants and flowing into ditches and then into the lagoon.

By January 2002, except for a small area near Melbourne Beach, the salinity in this section of the Lagoon had recovered to levels of salinity above 20 ppt. (See Map opposite page.) This is primarily due to a relatively quiet fall hurricane season not adding freshwater and the typical low levels of rainfall we experience in the winter. As spring and summer rains start we can expect freshwater runoff from urban areas to once again lower the salinities in this area. North of Titusville, the lack of nearby urban areas, large canals or inlets results in more gradual seasonal changes in salinity. In this more pristine portion of the lagoon, most freshwater arrives as direct rainfall into the lagoon and not as runoff carrying fertilizers and other pollutants. The salinity is created not by inlets bringing in ocean water, the salinity in this northern area is caused by evaporation concentrating the salts already present in the lagoon water. To the south, the Ft. Pierce and St Lucie Inlets brought enough salt water from the oceans to maintain high salinities throughout the year, though freshwater runoff still brought its share of pollutants. The main channel of the St. Lucie River, (which is also an estuary) displayed a similar trend to the central lagoon, with low salinities in August in response to freshwater runoff, and higher salinities in January in response to a typically dry winter.