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 km2 as in the case of the expansive Lagoa dos Patos in Brazil (Bird, 1994).

Coastal Lagoons, Figure 1
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Barnegat Bay-Little Egg Harbor, a coastal lagoon located along the central New Jersey coastline (USA). Note the coastal watershed draining to the lagoon and the barrier island system forming the eastern boundary. Figure 1 from US Geological Survey, West Trenton, New Jersey.

Full size image

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 coastal lagoons, the influx of sediments from land sources is minimal, and a significant amount of sediment accumulating in various areas of the lagoonal basin is the result of sediment reworking of the lagoonal floor. This is the case in Barnegat Bay-Little Egg Harbor, New Jersey (Psuty, 2004; Psuty and Silveira, 2009), as well as many other temperate coastal lagoons of North America (Oertel, 2005). Coarser sediments generally are found in proximity to the backbarriers and tidal inlets. These sediments, which are typically better sorted than those near the mainland, primarily derive from marine and backbarrier sources via storm surge and overwash events which build washover fans, and tidal currents through inlets which build ebb-tidal deltas and other sandy deposits in the lagoonal basin.

Biotic production

Coastal lagoons are characterized by high levels of biotic production. This is so because the photic zone extends to the lagoonal floor in most areas, and they usually receive considerable amounts of nutrients from the surrounding watersheds which stimulate primary production. Benthic algal and seagrass production can exceed phytoplankton production in coastal lagoons. In addition, there is strong benthic-pelagic coupling; in coastal lagoons the effects of biogeochemical cycling, bioturbation, and other interactions between the bottom sediments and the overlying water column may be far greater than those in deeper estuaries. Nutrients may be recycled many times before exiting inlets to the coastal ocean due to protracted water residence times which account for high rates of productivity per unit nutrient input (Kennish and Paerl, 2010b).

The range of annual primary production in coastal lagoons is large (∼50–>500 g C m−2 year−1). Based on the classification of Nixon (1995), many coastal lagoons fall within the range of eutrophic conditions (300–500 g C m−2 year−1) or even exhibit hypereutrophic conditions (>500 g C m−2 year−1) (Nixon, 1995). The high primary production in these water bodies, together with the input of organic matter from adjoining wetlands and external systems, supports rich faunal communities, with many species utilizing these environments seasonally. Benthic macrofaunal productivity in coastal lagoons amounts to ∼20–200 g ash-free dry weight m−2 year−1, with zooplankton productivity being as much as 50 % of this amount. Nekton productivity in turn ranges from ∼10 % to 100 % of the zooplankton productivity in these systems (Alvarez-Borrego, 1994). Coastal lagoons also provide ideal nursery and feeding habitats for many marine fauna (Kennish and Paerl, 2010b; Day et al., 2012).

Anthropogenic effects

Coastal lagoons are used for fisheries and aquaculture, energy production, biotechnology, transportation, shipping, and many other human uses (Pauly and Yáñez-Arancibia, 1994; Kennish and Paerl, 2010b). Watersheds surrounding coastal lagoons are often heavily populated and developed because of the great commercial and recreational value of these water bodies, their exceptional ecosystem services, and the access they afford to coastal ocean waters. However, altered land use/land cover of upland areas associated with increasing population growth and development, together with escalating human activities in the coastal lagoons themselves, has impacted their structure and function and compromised their ecological integrity (Kennish and Paerl, 2010b). For example, the removal of natural vegetation, compaction of soils, and construction of impervious surfaces promote nutrient runoff into the lagoons, hastening their nutrient enrichment and eutrophication (Kennish, 1997; Kennish, 2002).

Eutrophication of coastal lagoons and estuaries is on the increase worldwide (Nixon, 1995; Kennish et al., 2008; Kennish, 2009; Kennish and Paerl, 2010a), and it poses the greatest threat to the ecological integrity of these valuable ecosystems (Kennish and de Jonge, 2011). Eutrophication leads to an array of cascading changes in ecosystem structure and function such as decreased dissolved oxygen levels, increased microalgal and macroalgal abundance, occurrence of harmful algal blooms (HABs), loss of seagrass habitat, reduced biodiversity, declining fisheries, imbalanced food webs, altered biogeochemical cycling, and diminished ecosystem services (Nixon, 1995; Kennish, 1997; Kennish et al., 2008; Kennish and Paerl, 2010b).

Because of their extreme enclosure and restricted circulation, coastal lagoons are highly susceptible to accumulation of chemical contaminants such as polycyclic aromatic hydrocarbons, halogenated hydrocarbons, and metals. Bottom sediments serve as a repository and secondary pool of these hazardous substances. Volatile organics and plastics are also a potential threat to organisms inhabiting these environments. Oil spills are particularly detrimental. Pathogens delivered to lagoonal systems in stormwater runoff subsequent to rainfall events frequently compromise their water quality, although such events are usually ephemeral.

The shorelines of many coastal lagoons are altered by housing and bulkhead construction, which interferes with natural processes and directly impacts habitat. The siting of marinas along these shorelines, oil and gasoline leakages from fixed installations, sanitation-tank releases from boats, sewage wastewater discharges, and dredging activities adversely affect lagoonal organisms. Aquaculture operations can markedly degrade water quality in confined areas. In many systems, organic loading contributes to elevated BOD levels and significant oxygen depletion leading to system impairment.

Conclusions

Coastal lagoons are highly productive, enclosed water bodies that are heavily utilized by humans. They are complex physiographic features susceptible to eutrophication and other anthropogenic impacts due to their relatively low freshwater inputs, shallow depths, restricted circulation, poor flushing, limited ocean exchange, and protracted water residence times. As a result, coastal lagoons are beset by similar problems such as depleted dissolved oxygen, habitat loss and alteration, and, in some cases, altered ecosystem structure and function. Indicators of eutrophication are widespread in these shallow water bodies, including elevated chlorophyll a levels, HABs, submerged aquatic vegetation loss, and impacted biotic communities and harvestable fisheries. Progressive eutrophication of coastal lagoons can lead to permanent loss of essential habitat, diminished aquatic life support, and a marked decline in human use. Because of their enclosure, coastal lagoons are also susceptible to chemical contaminant inputs, pathogens, and organic carbon loading. The hardening of lagoonal shorelines, constructing of installations, and dredging of sediments physically alter habitats which also impacts biotic communities and their sustainability.

Cross-references