Coastal lagoons are estuarine basins where freshwater inflows are trapped behind coastal dune systems, sand spits, or barrier islands which impede exchange with the ocean. They are most frequent in regions where freshwater inflows to the coast are small or seasonal, so that exchange with the ocean may not occur for months or years at a time. Many occupy shallow drowned valleys formed when the sea level was lower during the last ice age and subsequently flooded by postglacial sea level rise. The tidal range is usually small. Accordingly, coastal lagoons are frequently found in warm temperate, dry subtropical, or Mediterranean regions along moderately sheltered coasts. Lagoons are infrequent in wetter temperate and tropical regions where freshwater inflows are sufficient to scour out river mouths and keep them open. Here estuaries are dominated by salt marshes in temperate and mangroves in tropical climes. A particularly good example is the series of coastal habitats on the southern and eastern coastline of Australia which change from open temperate estuaries and salt marshes in the wetter southern regions of Tasmania, through a series of coastal lagoons of varying sizes and ecologies along the south and east coasts, to open subtropical and tropical estuaries, reefs, and mangroves in the warmer and wetter north. A similar, although inverted, sequence can be seen running south along the east coasts of Canada, and the northeastern, central, and southeastern coasts of the USA. The resulting lagoons have varying water residence times, depending on volume, climate, freshwater inflow volumes, and the tidal prism.
Some lagoons are predominantly freshwater or brackish, while others are predominantly marine; so the dominant organisms in coastal lagoons reflect the balance of freshwater and marine influences. All are influenced by the local biogeography. Thus, the dominant species in Northern Hemisphere lagoons are quite different from those in their Southern Hemisphere equivalents. Different coastal regions of the globe differ in their biodiversity; for example, the endemic biodiversity of seagrasses is very high in Australian waters. Nevertheless, two points are worthy of note. First, there is great functional similarity between systems despite differing in the actual species involved. Second, human activity is quickly moving species around the world so that there are large numbers of what might be called 'feral' introduced species in coastal waters close to ports and large cities.
Coastal lagoons are ecologically diverse and provide habitats for many birds, fish, and plants. The interactions between the species in estuaries and coastal lagoons produce valuable ecosystem services. Indeed, the value of ecosystem services calculated for such systems by Costanza et al. was the highest of any ecosystem studied. Lagoons are also esthetically pleasing and desirable places to live, providing harbors, fertile catchments, and ocean access for cities and towns; thus, they have long been the sites of rapid urban and industrial development. Habitat change and other threats to lagoons now compromise these valuable services. All around the world they are threatened by land-use change in their catchments, urbanization, agriculture, fisheries, transport, tourism, climate change, and sea level rise. Coastal waters and lagoons are therefore definitive examples of the problems of multiple use management. Rapid population growth in coastal areas is common in many western countries (particularly the common 'sea-change' phenomenon, in which there is a trend toward rapid population growth along coasts), so the threats and challenges are increasing rapidly. Climate change and sea level rise are also becoming issues to be dealt with. In tropical and subtropical regions there is both evidence of rapid coastal habitat loss and population growth as well as an increased frequency of severe hurricanes. Modified systems impacted by severe hurricanes and tsunamis appear to be more fragile in the face of extreme events and certainly do not degrade gracefully.
Research and the management of coastal systems require a synthesis of social, economic, and ecological disciplines. Around the world there are a number of major research and management programs which aim to apply ecosystem knowledge to the effective management of coastal resources. Current examples include work in Chesapeake Bay and the Comprehensive Everglades Restoration Plan in the USA. In Italy the lagoon of Venice is a classic example. In Australia major programs have been undertaken in coastal embayments and lagoons in Adelaide (Gulf of St. Vincent), Brisbane (Moreton Bay), and Melbourne (Port Phillip Bay). (For details on these programs and useful links, see www.chesapeakebay.net, www.evergladesplan.org, and www.healthywaterways.org.) Land-use change (both urbanization and agriculture) in catchments, together with the use of coastal lagoons for transport and tourism, has led to a combination ofchanges in physical structures (both dredging and construction of seawalls and other barriers), altered hydrology and tidal exchanges, increased nutrient loads, and inputs oftoxicants. The resulting symptoms of environmental degradation include algal blooms (which may be toxic), loss ofbiodiver-sity, and ecological integrity (including the loss of seagrasses and other important functional groups), anoxia in bottom waters, loss of important biogeochemical functions (deni-trification efficiency), and the disturbances caused by introduced, 'feral' species from ships and ballast water.
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