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
evaporation, which results in fluctuating water temperature and salinity. Lagoons can also be
fragile ecosystems susceptible to pollution effects from municipal, industrial and agricultural
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.
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.