The Indian River Lagoon

The IRL’s seagrass beds, mangroves, oyster reefs, salt marshes, tidal flats, scrubland, beaches and dunes nurture more than 3,500 species of plants, animals and other organisms. This rich biodiversity is largely due to the lagoon’s unique geographic location, at the transition between cool, temperate and warm, subtropical climate zones.

Designated as an “estuary of national significance” by the U.S. Environmental Protection Agency, the IRL also provides enormous human benefits, supporting thousands of jobs and generating $7.6 billion annually to Florida’s economy.

What Is a Lagoon?

Lagoons are shallow coastal bodies of water, separated from the ocean by a series of barrier islands which lie parallel to the shoreline.

Inlets, either natural or man-made, cut through the barrier islands, and permit tidal currents to transport water into and out of the lagoons. Because lagoons tend to be shallow, water temperature and salinity can fluctuate drastically due to precipitation and evaporation.

Lagoons are classified into three main types:

• Leaky lagoons have wide tidal channels, fast currents and unimpaired exchange of water with the ocean.
• Choked lagoons occur along high-energy coastlines, and have one or more long, narrow channels which restrict water exchange with the ocean. Water circulation within this type of lagoon is dominated by wind patterns.
• Restricted lagoons have multiple channels, well-defined exchange with the ocean, and tend to show a net-seaward transport of water. Wind patterns in restricted lagoons can also cause surface currents to develop, thus helping to transport large volumes of water downwind.

The Indian River Lagoon is a restricted-type lagoon.

Dynamics of the IRL

Florida’s Indian River Lagoon (IRL) system actually consists of three lagoons: the Mosquito Lagoon, which originates in Volusia County; the Banana River, in Brevard County; and the Indian River Lagoon, which spans nearly the entire coastal extent of Brevard, Indian River, St. Lucie and Martin counties.

Other properties of lagoons may change depending on its size and physical characteristics. The Indian River Lagoon, for example, is significantly longer than it is wide.

The Role of Wind

Lagoons like the IRL tend to be well-mixed because they are heavily influenced by wind patterns.

Winds enhance vertical mixing in the water column, and also influence surface currents that ensure lateral mixing of estuarine water. This results in virtually no change in observed salinity from the surface to the bottom. In contrast, salinity in well-mixed estuaries decreases horizontally with distance from the ocean.

Winds also influence the direction of the lagoon’s water flow. Currents can switch from north-flowing to south-flowing, or be completely stagnant, depending on the prevailing winds.

Inlets and Tides

The southern portion of the IRL exchanges water with the ocean through three jettied, human-made inlets, all of which differ in size. These and other features divide the IRL into three sub-basins: the southern sub-basin, between St. Lucie Inlet and Fort Pierce Inlet; the central sub-basin, between Fort Pierce Inlet and Sebastian Inlet; and the northern sub-basin, the remaining area of the lagoon north of Sebastian Inlet.

Each of the sub-basins in the IRL experiences somewhat different tidal amplitudes, current speeds, and tidal excursion (the horizontal transport distance associated with either ebb or flood tide). Tidal amplitude, current speed and tidal excursion are all lowest in the northern sub-basin, and increase to the south.

One important exception to this pattern occurs around inlets. In these areas, current speeds during maximum ebb or flood tides can exceed 1 meter per second due to the constricting effects of narrow inlet channels.

However, tidal transport decreases as distance from the inlet increases. Wind takes over as the primary water transport process in the interior of the Lagoon.