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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 barrier islands and permit tidal currents to transport water into and out of the lagoons. Because lagoons are characteristically shallow, they are strongly influenced by precipitation and evaporation, which results in fluctuating water temperature and salinity. Lagoons can also be fragile ecosystems susceptible to pollution effects from municipal, industrial and agricultural runoff. Lagoons are classified into 3 main types: leaky lagoons, choked lagoons, and restricted lagoons. 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. 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 (Kjerfve 1986). The Indian River Lagoon is a restricted type lagoon (Smith 1987, 1990).

A. Circulation patterns in a well mixed estuary. Redrawn from Brown et al. 1989.

Many restricted 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 insure lateral mixing of estuarine water (Figure A). This results in a vertical profile of the water column where virtually no change in salinity is observed from the surface to the bottom. In contrast, salinity in well mixed estuaries does decrease horizontally with distance from the ocean (Figure B).

B. Vertical profile of salinity in a well mixed estuary. Redrawn from Brown et al. 1989.

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 southern portion of the lagoon exchanges water with the ocean through 3 jettied inlets (Sebastian, Fort Pierce, St. Lucie), all of which differ in size. These and other features have led researchers to delimit 3 sub-basins in the IRL (Figure C). The southern sub-basin is defined as the area between St. Lucie Inlet and Fort Pierce Inlet; the central sub-basin lies between Fort Pierce Inlet and Sebastian Inlet; and the northern sub-basin is the area north of Sebastian Inlet.

C. Inlets in the southern portion of the IRL delimit 3 sub-basins.

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). Data from studies by Pitts (1989), Smith (1990) and Liu (1992) have shown that 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 m/sec-1 due to the constricting effects of narrow inlet channels.

It is important to note that although tidal forcing is important near inlets mouths, volume transport in the interior of the Indian River Lagoon is principally a wind driven process (Pitts 1989 Smith 1990 Liu 1992). The shallow nature of coastal lagoons, combined with the constricting effects of inlets aid in decreasing tidal transport as distance from the inlet increases. Thus, wind becomes the primary transport process in the interior of the Indian River Lagoon.