COASTAL LAGOONS
Michael J. Kennish
Department of Marine and Coastal Sciences,
School of Environmental and Biological Sciences,
Rutgers University, New Brunswick, NJ, USA
Synonyms
Coastal bays; Coastal lakes; Coastal ponds
Definition
Coastal lagoons are shallow brackish or marine water bodies separated from the ocean by a barrier island, spit, reef,
or sand bank (Colombo, 1977; Barnes, 1980; Kjerfve,
1994; Kennish and Paerl, 2010a). Depending on the extent
of the barriers, they may be partially or totally enclosed,
although most are connected at least intermittently to the
open ocean by one or more restricted tidal inlets. Oertel
(2005) called the smaller, totally enclosed systems coastal
lakes or coastal ponds. Those with outlets to the sea are
termed coastal lagoons and coastal bays, depending on
their shapes.
Introduction
Coastal lagoons form on low-lying coasts such as along
the Atlantic and Gulf coasts of the USA, where they are
particularly extensive, covering $2,800 km of shoreline
(Nichols and Boon, 1994). They are much less common
on most other coasts, occupying only $12 % of the coastal
shorelines worldwide. The Antarctic is the only continent
devoid of coastal lagoons, while they are most prominent
along the coasts of Africa (17.9 % of the coastline) and
North America (17.6 %) and less conspicuous along the
coasts of Asia (13.8 %), South America (12.2 %), Australia (11.4 %), and Europe (5.3 %) (Barnes, 1980; Kennish
and Paerl, 2010a).
The size and shape of coastal lagoons vary considerably, although they are usually oriented with their long
axis parallel to the shoreline, as exemplified by the
Barnegat Bay-Little Egg Harbor system in New Jersey
(USA) (Figure 1) (Kennish, 2001). However, some
lagoonal water bodies have a triangular or delta shape with
v-shaped landward margins, as demonstrated by the
Rehoboth Bay and Assawoman Bay in Delaware (USA)
(Oertel, 2005). They range in size from a few square
kilometers up to 10,000 km
2 as in the case of the
expansive Lagoa dos Patos in Brazil (Bird, 1994).
Formation
The genesis of coastal lagoons is closely linked to the formation of coastal barriers which separate flooded basins
landward from the coastal ocean. According to de Beaumont (1845), the barriers form by the upbuilding of bars
and shoals. Gilbert (1885) attributed barrier formation to
the progradation of spits which creates shallow embayments behind them. McGee (1890) advanced an
inundation model of coastal lagoon formation whereby
a rising sea floods lowland areas. Oertel (2005) supported
the models of Gilbert (1885) and McGee (1890) as the two
main modes by which coastal lagoons form.
Physical-chemical characteristics
The basin morphometry and circulation of coastal lagoons
differ considerably from those of larger, river-dominated
estuaries. Coastal lagoons are shallow, generally averaging less than 2–3 m in depth, but depths of up to 30 m have
been recorded in some tidal channels of these systems
(Oertel, 2005; Kennish and Paerl, 2010b). They are generally well mixed by wave and current action. Because
coastal lagoons receive relatively small volumes of freshwater input, tidal exchanges through narrow inlets play
a significant role as a driver of lagoonal circulation. Most
coastal lagoons are microtidal systems.
The physical-chemical processes taking place in coastal
lagoons depend greatly on multiple factors, notably the
size and configuration of the tidal inlets, expanse and
development of bordering watersheds, amount of freshwater input, tidal prism, wind velocity and direction, and
water depth (Alongi, 1998; Kennish and Paerl, 2010a).
As stated by Kennish and Paerl (2010a), “Variations in
precipitation and evaporation, surface runoff, and groundwater seepage, together with fluxes in wind forcing,
account for large differences in advective transport in
lagoonal estuaries. Storm and wind surges, overwash
events, inlet configurations, land reclamation, construction of dams, dikes and artificial bars, as well as channel
dredging events, are important drivers of hydrological
change in these systems.”
Because of the extreme enclosure of most coastal
lagoons by barriers and the limited tidal exchange with
ocean waters, these shallow systems tend to have
protracted water residence times. As a result, coastal
lagoons are susceptible to accumulation of pollutants from
coastal watersheds and airsheds. They are also easily
impacted by overwash events driven by extreme climate
events such as hurricanes that can transport large amounts
of beach and coastal ocean sediments into these backbays.
This was the case in New Jersey when superstorm Sandy
made landfall on October 29, 2012, creating a storm surge
exceeding 4 m in some areas and dumping more than
1.5 million cubic meters of beach sand into Barnegat
Bay-Little Egg Harbor. Similar events have been recorded
for other coastal lagoons impacted by hurricanes and
extratropical storms.
Sediments
Coastal lagoons receive terrigenous sediment from
streams and rivers draining coastal watersheds. These sediments often consist of fine silts and clays, much of which
flocculate and are deposited at the mouth of the influent
systems. Fine-grained sediments also accumulate near
the lagoonal shoreline in proximity to salt marshes which
facilitate deposition of silts and clays. However, in some
140
COASTAL LAGOONS
Michael J. Kennish
Department of Marine and Coastal Sciences,
School of Environmental and Biological Sciences,
Rutgers University, New Brunswick, NJ, USA
Synonyms
Coastal bays; Coastal lakes; Coastal ponds
Definition
Coastal lagoons are shallow brackish or marine water bodies separated from the ocean by a barrier island, spit, reef,
or sand bank (Colombo, 1977; Barnes, 1980; Kjerfve,
1994; Kennish and Paerl, 2010a). Depending on the extent
of the barriers, they may be partially or totally enclosed,
although most are connected at least intermittently to the
open ocean by one or more restricted tidal inlets. Oertel
(2005) called the smaller, totally enclosed systems coastal
lakes or coastal ponds. Those with outlets to the sea are
termed coastal lagoons and coastal bays, depending on
their shapes.
Introduction
Coastal lagoons form on low-lying coasts such as along
the Atlantic and Gulf coasts of the USA, where they are
particularly extensive, covering $2,800 km of shoreline
(Nichols and Boon, 1994). They are much less common
on most other coasts, occupying only $12 % of the coastal
shorelines worldwide. The Antarctic is the only continent
devoid of coastal lagoons, while they are most prominent
along the coasts of Africa (17.9 % of the coastline) and
North America (17.6 %) and less conspicuous along the
coasts of Asia (13.8 %), South America (12.2 %), Australia (11.4 %), and Europe (5.3 %) (Barnes, 1980; Kennish
and Paerl, 2010a).
The size and shape of coastal lagoons vary considerably, although they are usually oriented with their long
axis parallel to the shoreline, as exemplified by the
Barnegat Bay-Little Egg Harbor system in New Jersey
(USA) (Figure 1) (Kennish, 2001). However, some
lagoonal water bodies have a triangular or delta shape with
v-shaped landward margins, as demonstrated by the
Rehoboth Bay and Assawoman Bay in Delaware (USA)
(Oertel, 2005). They range in size from a few square
kilometers up to 10,000 km
2 as in the case of the
expansive Lagoa dos Patos in Brazil (Bird, 1994).
Formation
The genesis of coastal lagoons is closely linked to the formation of coastal barriers which separate flooded basins
landward from the coastal ocean. According to de Beaumont (1845), the barriers form by the upbuilding of bars
and shoals. Gilbert (1885) attributed barrier formation to
the progradation of spits which creates shallow embayments behind them. McGee (1890) advanced an
inundation model of coastal lagoon formation whereby
a rising sea floods lowland areas. Oertel (2005) supported
the models of Gilbert (1885) and McGee (1890) as the two
main modes by which coastal lagoons form.
Physical-chemical characteristics
The basin morphometry and circulation of coastal lagoons
differ considerably from those of larger, river-dominated
estuaries. Coastal lagoons are shallow, generally averaging less than 2–3 m in depth, but depths of up to 30 m have
been recorded in some tidal channels of these systems
(Oertel, 2005; Kennish and Paerl, 2010b). They are generally well mixed by wave and current action. Because
coastal lagoons receive relatively small volumes of freshwater input, tidal exchanges through narrow inlets play
a significant role as a driver of lagoonal circulation. Most
coastal lagoons are microtidal systems.
The physical-chemical processes taking place in coastal
lagoons depend greatly on multiple factors, notably the
size and configuration of the tidal inlets, expanse and
development of bordering watersheds, amount of freshwater input, tidal prism, wind velocity and direction, and
water depth (Alongi, 1998; Kennish and Paerl, 2010a).
As stated by Kennish and Paerl (2010a), “Variations in
precipitation and evaporation, surface runoff, and groundwater seepage, together with fluxes in wind forcing,
account for large differences in advective transport in
lagoonal estuaries. Storm and wind surges, overwash
events, inlet configurations, land reclamation, construction of dams, dikes and artificial bars, as well as channel
dredging events, are important drivers of hydrological
change in these systems.”
Because of the extreme enclosure of most coastal
lagoons by barriers and the limited tidal exchange with
ocean waters, these shallow systems tend to have
protracted water residence times. As a result, coastal
lagoons are susceptible to accumulation of pollutants from
coastal watersheds and airsheds. They are also easily
impacted by overwash events driven by extreme climate
events such as hurricanes that can transport large amounts
of beach and coastal ocean sediments into these backbays.
This was the case in New Jersey when superstorm Sandy
made landfall on October 29, 2012, creating a storm surge
exceeding 4 m in some areas and dumping more than
1.5 million cubic meters of beach sand into Barnegat
Bay-Little Egg Harbor. Similar events have been recorded
for other coastal lagoons impacted by hurricanes and
extratropical storms.
Sediments
Coastal lagoons receive terrigenous sediment from
streams and rivers draining coastal watersheds. These sediments often consist of fine silts and clays, much of which
flocculate and are deposited at the mouth of the influent
systems. Fine-grained sediments also accumulate near
the lagoonal shoreline in proximity to salt marshes which
facilitate deposition of silts and clays. However, in some
140
COASTAL LAGOONS