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CHAPTER 4

Coastal wetlands: unction and role in reducingimpact o land-based management

G.L. Bruland Natural Resources and Environmental Management Department,University o Hawaii Manoa, USA.

Abstract

Coastal wetlands are among the most productive, valuable, and yet most threat-ened ecosystems in the world. They provide a variety o unctions that reduce theimpact o land-based management on the coastal zone such as slowing the ow o water rom the mountains to the sea, trapping o sediments, and retaining or trans-orming nutrients. Numerous studies have reported that increased soil erosion andnutrient export rom land-based management are threatening estuaries and coastalzones. Coastal wetlands are located at a critical inter ace between the terrestrial andmarine environments and are ideally positioned to reduce impacts rom land-basedsources. There are various types o coastal wetlands including riparian wetlands,tidal reshwater marshes, tidal salt marshes, and mangroves. Some classifcationsystems also consider seagrass beds and coral ree s to be wetlands. Coastal wetlandecosystems vary in their ability to reduce impacts rom land-based management inboth space and time. These wetlands can retain, and trans orm, or sometimes evenact as sources o nutrients and sediments. Some wetland types are more e ectiveat sediment retention and others at nutrient retention. Watershed size, climate, andposition o wetlands in a watershed are other important actors that determine thee ectiveness o coastal wetlands in reducing the e ects o land-based activities.

Wetlands do not appear to be infnite sinks or sediment or nutrients. Once criti-cal sediment and nutrient loading thresholds have been crossed, coastal wetlandsare subject to degradation and even loss. While many coastal nations have devel-oped coastal-zone management policies and legislation, degradation and losses o coastal wetlands continue to occur due to altered hydrology, increased sedimentand nutrient loading, urban development, agriculture, and aquaculture. While wehave made signifcant progress in our ability to restore and create tidal marshes andmangroves, other coastal systems such as seagrass beds and coral ree s appear to


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2 Coastal Watershed Management

be much harder to restore. Thus, it is important that existing natural coastal wet-lands be prioritized or conservation and that best management plans be developedto reduce sediment and nutrient losses rom terrestrial watersheds.

1 Introduction and current status o coastal wetlands

Changes in terrestrial land-use patterns such as agricultural intensifcation andurban expansion have resulted in increased sediment and nutrient loadings thatare transported into coastal areas [14]. For example, the change in nitrogen(N) loading to coastal areas since preindustrial times has increased our old inthe Mississippi River, eight old in rivers o the northeastern United States, andten old in rivers draining to the North Sea [2]. Human activities have increased

sediment loads in rivers by 20% compared to preindustrial times [5]. However,reservoirs and diversions trap approximately 30% o the total sediment load inrivers rom reaching the ocean. While the overall sediment load to the coastal zonehas decreased by 10% [5], there are hotspots in places like the Philippines, Indo-nesia, and Madagascar where sediment loads to coastal zones have dramaticallyincreased in recent years. Other pollutants such as herbicides and heavy metals,that are generated rom land-based activities, have been shown to suppress photo-synthesis in seagrasses and corals and suppress coral ertilization at concentrationso a ew tens o parts per billions [5, 6, 7].

Coastal wetlands provide a critical inter ace between terrestrial and marineenvironments, and their importance to global sediment and nutrient budgets ismuch greater than their proportional sur ace area on earth would suggest [8]. It hasbeen estimated that wetlands provide $4.88 trillion (US) yr 1 in ecosystem ser-vices [9]. These ecosystem services include disturbance regulation, water supply,water quality maintenance, pollination, biological control, ood production, andothers. According to Costanza et al. [9], wetlands are 75% more valuable in termso ecosystem services than lakes and rivers, 15 times more valuable than orests,and 64 times more valuable than grassland or rangelands. Despite the many eco-system services that wetlands provide, they have been subject to conversion toother land uses or millennia and especially during the last 200 years.

Some o the best data on these conversion rates come rom the United States.For example, it has been estimated that the state o Cali ornia has lost over 90% o its original wetland area due to conversion to urban and agricultural land uses [10].Iowa, Illinois, Indiana, and Ohio each have lost greater than 85% o their wetlandsmainly due to conversion to agriculture [10]. In Florida, the Greater Everglades

Ecosystem comprises less than hal its original extent due to drainage and conversionto agricultural and urban land uses [11]. The state o Louisiana is expected to loseanother 181,300 ha o wetlands in next 50 years or an area equal to the size o Washington DC./Baltimore metropolitan region [12]. Wetland losses in Louisianacontinue to occur due to channelization o the Mississippi River, sea-level rise,herbivory rom nutria ( Myocastor coypu ), storm surge rom hurricanes, and impactsdue to oil and gas production. Louisiana accounted or approximately 90% o thecoastal marsh loss in the continental states during 1990s (not including Alaska).


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Coastal Wetlands 3

On a more positive note, wetland loss rates in the United States have decreasedover the last 30 years [13]. In the period rom 1986 to 1997, 98% o wetland lossesin the United States were rom orested and reshwater wetlands, while only 2% o the losses were rom estuarine wetlands [13]. While conversion rates o coastalwetlands have declined in the United States, globally conversion o coastal wet-lands continues and in some places has even accelerated [5].

The coastal land at the continental margin accounts or less than 5% o theEarths land area, yet 17% o the earths human population lives within this zone[14]. Furthermore, approximately 4 billion people live within 60 km o the worldscoastlines [15]. Table 1 lists the share o the total and coastal population that livewithin 50 km o di erent coastal wetland types.

Specifcally, 27% o the earths human population lives within 50 km o an estu-ary (Table 1). Coastal population densities have been estimated to be 100 peopleper square kilometer compared to only 38 people per square kilometer in inlandareas [5]. This not only causes damage to coastal wetlands but also to adjacentseagrass beds and coral ree s (classifed by some as wetlands and others as deep-water habitats; see the next section). Seagrass beds are currently threatened byphysical disturbance, ship activities, dredging, landfll, erosion rom terrestrialsources, growth o aquaculture, and eutrophication [16]. Large-scale declines inseagrass beds have been observed at over 40 locations, 70% o which were due tohuman-induced degradation [16]. Coral ree s are also in serious decline. Thirty percent o all ree s are severely damaged and close to 60% may be lost by 2030; ur-thermore, it has been stated that there are no pristine ree s remaining [17].

The rest o this chapter will include in ormation on wetland classifcation, com-pare and contrast di erent types o coastal wetlands, examine the coverage and

position o wetlands in the watershed, explore the role o coastal wetlands in trap-ping sediment and retaining nutrients, compare soils o natural wetlands to createdand restored wetlands in a case-study example, and identi y uture research needsand directions. Specifcally, the major wetland classifcation systems will be iden-tifed and their classifcation o coastal wetlands will be discussed. This will beollowed by comparing and contrasting the dominant types o coastal and deepwater

Table 1: Share o world and coastal populations living within 50 km o estuaries,mangroves, seagrasses, and coral ree s in 1995 [14].

Types

Human population[millions]

Share o worldpopulation [%]

Share o coastalpopulation [%]

Estuaries 1,599 27 71Mangroves 1,033 18 45Seagrasses 1,146 19 49Coral Ree s 711 12 31Total 5,596

Based on spatially re erenced population data. Due to overlap o some habitat types the fgures do not add up to 100%.


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wetlands including riparian wetlands, tidal reshwater marshes, salt marshes, man-groves, seagrass beds, coral ree s, and kelp orests. The coverage and position o wetlands in the watershed will then be examined in light o reducing impacts romland-based management. Next the methods or assessing sediment retention incoastal wetlands will be discussed ollowed by a summary o studies that investi-gated sediment retention. A similar section on methods or assessing nutrientretention and summary o nutrient retention studies will ollow. This chapter willconclude with a case study that compares soils o natural wetlands to created andrestored wetlands as well as a discussion o uture research needs and directions.

2 Wetland classifcation

Be ore moving into a discussion o the unction and role o wetlands in reducingthe impacts o land-based management, it is important to understand that thereare di erent defnitions o wetlands and that various systems are used or theirclassifcation. Currently, the three most prominent hierarchical wetland classif-cation systems include the U.S. Fish and Wildli e Service (USFWS) Classifca-tion System, the Canadian Classifcation System, and the Ramsar ConventionSystem. The USFWS classifcation system, titled Classifcation o Wetland andDeepwater Habitats o the United States was published in 1979 [18]. This sys-tem includes both wetlands and deepwater habitats. The USFWS System defneswetlands as Lands transitional between terrestrial and aquatic systems wherethe water table is usually at or near the sur ace. This defnition does not includeareas with permanent standing water greater than two m deep. Such areas wouldbe defned as aquatic or marine habitats [18]. The levels o the USFWS Systeminclude Systems , Subsystems , Classes , and Subclasses . Systems are the broad-est level o the classifcation scheme and include Marine, Estuarine, Riverine,Lacustrine, and Palustrine. Coastal wetlands are usually classifed in the Estua-rine System .

The Canadian Wetland Classifcation System defnes a wetland as a land that issaturated with water long enough to promote wetland or aquatic processes as indi-cated by poorly drained soils, hydrophytic vegetation and various kinds o bio-logical activity which are adapted to a wet environment [19]. Classifcation in theCanadian System is hierarchical and has three main levels: Classes , Forms , andTypes [19]. The fve Classes o wetlands in this system include Bogs, Fens,Swamps, Marshes, and Shallow Water Marshes. Coastal wetlands are ound in allo the Classes except or Bog.

The Ramsar Convention defnes wetlands as Areas o marsh, en, peatland, orwater, whether natural or artifcial, permanent or temporary, with water that isstatic, owing, resh, brackish, or salt, including areas o marine water where thedepth at low tide does not exceed six m [20]. This is a more inclusive defnitiono wetlands that incorporates coral ree s and seagrass beds that are not defned aswetlands in the USFWS or Canadian systems. The Ramsar System groups wet-lands into Classes based on their location in the landscape and vegetation [20].It has 32 Classes that are divided into marine/coastal and inland groups.


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Coastal Wetlands 5

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The Hydrogeomorphic (HGM) Classifcation System [21] is unctional classif-cation system that is also worth mentioning. The HGM System is ocused on eval-uating physical, chemical, and biological unctions o wetlands in the feld simply,rapidly, and inexpensively [21]. The HGM system emphasizes two abiotic controlsin maintaining wetland unctions, hydrology and geomorphology [21]. Hydrologycontrols the amount, source, and season o water entering the wetland whereasgeomorphology controls where the water comes rom and whether or not it leaves.The HGM system includes 7 di erent Geomorphic Settings : depressional, riverine,lacustrine ringe, tidal ringe, slope, mineral soil ats, and organic soil ats [21].It recognizes three Water Sources : precipitation, sur ace water, and groundwater aswell as three types o Hydrodynamics : vertical, unidirectional, and bidirectional[21]. Coastal wetlands can have either riverine or tidal ringe geomorphic settings,all three types o water sources, and unidirectional (riverine) or bidirectional (tidalringe) hydrodynamics. Figure 1 illustrates how the our systems mentioned abovewould classi y a common coastal wetland, the tidal salt marsh.

3 Types o coastal wetlands

There are a variety o types o coastal wetlands that occur in di erent landscapepositions, have di erent vegetative communities, and provide di erent unctionsin terms o reducing the e ects o land-based pollution in coastal zones. Spanninga continuum rom salt to resh water, these include mangroves, tidal salt marshes,tidal reshwater marshes, and riparian wetlands (see Fig. 2a). Deepwater habitatsinclude seagrass beds, coral ree s, and kelp orests. These ecosystems are o tennested across the hierarchy o the larger estuarine-coastal system (Fig. 2b).

3.1 Riparian wetlands

Riparian wetlands occur as ecotones or inter aces between aquatic and uplandecosystems, have distinct vegetation and soil characteristics [22, 23], and per ormimportant unctions at the watershed scale [24]. Riparian wetlands are not easilyclassifed or delineated but instead are comprised o mosaics o land orms andcommunities within the larger landscape [23]. Brinson et al. [24] stated that ripar-ian wetlands are characterized by an abundance o water and ertile alluvial soils.They also list three major eatures that separate riparian wetlands rom other typeso wetlands and upland ecosystems that include:

1. Their linear orm as a consequence o their proximity to rivers and streams.2. Energy and material rom the surrounding landscape [upstream watershed]

converge and pass through riparian wetlands in much greater amounts thanthose o any other reshwater wetland systems.

3. Riparian wetlands are unctionally connected to upstream and downstreamwetlands and are laterally connected to upslope [upland] and downslope(aquatic) ecosystems [24].

Thus, riparian wetlands are dynamic, open systems that are subject to large inputso sur ace water, sediments, and nutrients rom orested, agricultural, and urban


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Coastal Wetlands 7

areas upstream in the watershed. These riparian systems have the capacity to retainand trans orm large quantities o these inputs and keep them rom being trans-ported to coastal zones.

In the United States, the most extensive riparian wetland ecosystems are thebottomland hardwood orests o the Southeast. These bottomlands stretch acrossthe Gul and Atlantic coastal plains rom Texas to Maryland and are associatedwith rivers such as the Mississippi, Appalachicola-Chattahoochee, Ogeeche,

(a)

(b)

Figure 2: (a) The estuarine salinity gradient. (b) Hierarchy o the estuarine-coastallandscape. Copywrite (2000) rom Wetlands , 3rd Edition by W.J. Mitschand J.G. Gosselink [27]. Reproduced by permission o John Wiley andSons, Inc.


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Altamaha-Ocmulgee, Pee Dee-Yadkin, Neuse, Tar-Pamlico, and Roanoke. Bottom-land hardwood orests in the southeastern U.S. have been, and continue to be, con-verted to other land uses such as agriculture and urban developments. The NatureConservancy [25] estimated that in 1991 about 2.0 million ha o bottomland hard-wood orested remained in the Mississippi River Alluvial Plain; this area supportedabout 8.5 million ha o bottomland hardwood orest prior to European settlement.The Atlantic coastal plain that stretches rom Florida to Maryland also has exten-sive areas o riparian hardwood orest lining many o the rivers that ow rom thePiedmont to the ocean. Some o these riparian orests remain intact, others havebeen logged, others have regenerated, and many are in the process o being restored[26]. At the global scale there are various other orested wetland systems that areound in oodplains or riparian zones. The Amazon River oodplain in Brazil is anexample o one o these types o systems as is the Zaire Swamps in A rica. Thesesystems per orm similar unctions to the bottomland hardwood systems describedabove but are located in a tropical rather than a temperate setting.

Riparian wetlands located in temperate regions o the Western United States, orin tropical climates such as those in the Hawaiian and other Pacifc Islands, aregenerally quite di erent rom the bottomland hardwood orests described above.These riparian wetlands are located in steep, narrow, and dynamic riparian zones.While oodplain orests o ten have subtle changes in elevation and vegetation, thegradients in steep-sloped riparian wetlands are usually sharp and the visual distinc-tions between community types are clear [27]. These riparian wetlands have alsobeen extensively modifed by human activity [27]. Logging, grazing, conversion toagriculture and urban development have been widespread. These riparian areas areo ten the only at lands available or cultivation and home building. They also tend

to concentrate grazers due to the at terrain and presence o water. Logging alsocontinues to have a major impact on these wetlands. O ten the streams in theseriparian zones are used to move logs rom the orest to estuarine holding pens. Asmany streams were too small to move logs e fciently, they were dammed and theirbanks were cleared to enhance logging and transport o timber [27]. This has leadto extensive erosion rom upland areas and deposition in riparian ecosystems withsubsequent transport o sediment into coastal zones. In some cases, destruction o riparian zones has resulted in oods and burial o natural estuarine habitats undertons o silt and enriched sediment [28]. The value o the ecosystem services pro-vided by riparian wetlands and oodplains has been estimated to be $19,580 ha 1 yr1 [9], which is one o the highest values or any type o ecosystem.

3.2 Tidal reshwater marshes

Tidal reshwater marshes are close enough to the oceans to experience tides, butare above the reach o oceanic saltwater [27]. These coastal wetlands combinemany o the eatures o salt marshes and reshwater marshes. They are similar instructure and unction to salt marshes, with the major di erence being a greaterdiversity in biota due to the reduction in salt stress. Plant diversity is high andmore birds use these marshes than any other marsh type [27]. O ten, the bound-ary between tidal reshwater marshes and salt marshes is di fcult to determine.


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Coastal Wetlands 9

Tidal reshwater marshes have been studied much less than salt marshes or inlandreshwater marshes. Three major types o tidal reshwater marshes have beenrecognized: 1) mature marshes, 2) oating marshes, and 3) new marshes in pro-grading deltas [27].

In a cross-section, elevation in tidal reshwater marshes usually increases slowlyrom the stream edge to the adjacent upland areas. They typically have a slightlyelevated levee along the stream bank where the over owing water deposits mucho its sediment load. The sediments in these marshes are airly organic, especiallyin the oating marshes [27]. The European Union Habitats Directive has declaredthe conservation or coastal reshwater wetlands a priority [29].

3.3 Tidal salt marshes

Tidal salt marshes are ound in coastal areas in the middle and high latitudes.They are common wherever accumulation o sediment is equal to or greater thanthe rate o land subsidence and where there is adequate protection rom destruc-tive waves and storms [27]. From a ar, tidal salt marshes appear to be vast feldso a single species, o ten salt marsh cordgrass ( Spartina alterni ora ). While theirphysiognomy is much simpler than a bottomland hardwood orest, the vegetationo tidal salt marshes does vary across salinity and ooding gradients and provideshabitat or a variety o plants, animals, microbes that are adapted to deal withthe stresses o this environment. Numerous studies have shown salt marshes tobe highly productive and to support the spawning and eeding o various marineorganisms [27]. Thus, salt marshes represent a critical inter ace between terrestrialand marine ecosystems [27].

Chapman [30] divided the worlds salt marshes into the ollowing major geographi-cal groups: artic, northern Europe, Mediterranean, Eastern North America, WesternNorth America, Australasia, eastern Asia, Australia, South America, and the tropics.Although di erent plant associations are dominant in the di erent geographic groups,the ecological structure and unction o salt marshes is similar around the world [27].

Salt marshes are predominantly intertidal and ound in areas that are at leastoccasionally inundated at high tide but not ooded during low tide [27]. The upperand lower boundaries o these marshes are usually set by the tidal range. The lowerboundary is determined by physical stresses such as depth and duration o oodingand the mechanical e ects o waves, sediment availability and erosional orces[30, 31]. The upper boundary was thought to be set by the limit o ooding onextreme tides [32], but more recent research has indicated that the upper boundary

is set by plant competition [31]. Based on elevation and ooding patterns, saltmarshes can be divided into two zones: the high marsh and the low marsh [27].The high marsh is ooded irregularly and can experience at least 10 days o con-tinuous exposure to the atmosphere, while the low marsh is ooded almost dailyand there are never more than 9 continuous days o exposure [27]. Competitivelysuperior plants tend to dominate the high marsh habitats while stress-tolerantplants dominate the low marsh habitats [31].

Another prominent eature o salt marshes is the presence o tidal creeks. Thesecreeks are o ten ound in the low marsh. As the ow in tidal creeks is bidirectional,


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the channels tend to remain stable [27]. Generally, the banks o these tidal creeksare characterized by greater vegetative production than the interior areas o themarsh due to better ushing o salts and toxins rom the tides, more oxygen in thesoils due to higher elevation, and higher nutrient concentrations.

Tidal salt marshes are among the most productive ecosystems in the world,producing up to 80 metric tons ha 1 o plant material (8000 g m 2 yr1) in thesouthern Coastal Plain o North America [27]. They have also been estimated to beone o the most economically valuable ecosystems due to the many services thatthey provide. One study estimated the value o tidal salt marshes and mangrovesto be $9990 ha 1 yr1 [9]. Un ortunately, due to increased construction o dams andreservoirs, sediment delivery to estuarine zones and salt marshes has decreasedconsiderably, and in some cases large areas o salt marsh cannot keep pace withrising sea levels and are experiencing subsidence [5]. In the Southeastern UnitedStates, recent studies have shown that overharvesting o the blue crab ( Callenectessapidus ) has led to explosive increases in the populations o the periwinkle snail( Littoraria irrorata ) [33]. Freed o predatory control, the snails have engaged indestructive grazing o the salt marsh cordgrass and caused large-scale die-o s insalt marshes [33]. Thus, human alterations o salt-marsh trophic dynamics maydecrease the ability o these systems to trap sediments and retain nutrients romland-based sources.

3.4 Mangroves

Mangroves swamps replace salt marshes along coastlines in subtropical and tropi-cal regions [27]. In a ew transitional situations (e.g. Florida) mangroves and saltmarshes coexist. Global mangrove orest cover is estimated to be between 16 and18 million hectares [34, 35]. In the region between 25 o N and 25 o S latitude, man-groves dominate approximately 75% o the worlds coastline [36]. Mangroves aredefned as areas o trees, shrubs, and other plants ound in the intertidal zones andestuarine margins that have adapted to living in saline waters, continually or athigh tides [37]. Mangroves are known or their seemingly impenetrable maze o woody vegetation and their unique adaptations to the double stresses o oodingand salinity [27]. Mangrove swamps provide many critical ecosystem servicessuch as exporting organic matter to adjacent coastal zones, serving as nurseryhabitat or marine organisms, trapping sediments and nutrients, stabilizing shore-lines, bu ering land rom storms, and providing sa e havens or humans in the118 coastal countries in which they occur [27, 34].

The requency and severity o rosts are the main actors that limit the extensiono mangroves beyond tropical and subtropical climates [38]. Mangroves are par-ticularly dominant in the Indo-West Pacifc region where they contain the greatestdiversity o species [27]. Three main types o mangroves have been identifedbased on dominant physical processes and geomorphological characteristics: tide-dominated riverine mangroves, river-dominated riverine ringe mangroves, andinterior basin mangroves [39]. Up to 95% o the detritus generated by the vegeta-tion may be exported rom riverine mangroves to adjacent estuaries and coastalzones, while only 21% o the detritus in basin mangroves may be exported [36].


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Coastal Wetlands 11

Some o the most intact mangrove orests in the world are ound in Malaysiaand Micronesia. Recent studies have shown how important these ecosystems areto the local economies in these countries [39]. The importance o these ecosystemsis related to the goods and services they provide, such as trapping o sediment,processing o nutrients and organic matter, providing ood and habitat or animals,protecting shorelines, and providing plant products such as uelwood, buildingmaterials, woodchips, tannins, honey, and medicinal products [39]. Mangroves arerapidly being converted to other land uses (i.e. urban development, rice felds, oilpalm plantations, and aquaculture sites) in this region at an average rate o 1% peryear [40]. In some countries, greater than 80% o the original mangrove cover hasbeen lost due to de orestation [34]. Currently, the conversion to aquacultureaccounts or 52% o mangroves losses, orest use accounts or 26% o losses, andreshwater diversions account or 11% o losses [35]. The value o ecosystem ser-vices provided by mangroves has been estimated to be $9990 ha 1 yr1 [9].

The Hawaiian Archipelago provides an interesting example o mangrove ecol-ogy and management. Hawaii has no native mangrove species despite having bothsuitable climate and geomorphic settings [41]. However, since their introductionin 1902, mangroves have ourished to such a degree that many people have beenconcerned about their impacts especially their dramatic e ects on native plantcommunity structure [42]. Expensive projects have been undertaken or theirremoval [41] with varying degrees o success.

3.5 Seagrass beds

Seagrass beds are defned as areas with aquatic owering plants that live ullysubmerged in saline waters with a sediment substrate [36]. They occur overso t sediments worldwide, rom the tropics to the boreal margins o everyocean [16]. In higher latitudes, eelgrass ( Zostera spp.) orms dense meadows,while in the tropics, manatee grass ( Thalassia testudinum ) and turtle grass(Syringodium fli orme ) are dominant [5]. Seagrasses are estimated to coverabout 0.10.2% o the global ocean [43]. They are adapted to continuous andcomplete immersion in saline water, and as such, they grow in tidal zonesalthough some can also grow in intertidal waters [16]. Seagrasses provideimportant ecosystem unctions such as biological productivity, habitat or vari-ous marine organisms, trapping sediment, carbon sequestration, and bu eringo wave action [36, 44]. The combined productivity o seagrasses and associ-ated algae make seagrass beds among the most productive ecosystems on earth

[45]. Seagrasses are also ecosystem engineers that provide physical structurethat trans orms eatureless sediment bottoms into diverse and complex habitatsor coastal biota [44].

As with salt marshes, the presence o a ew plants has the e ect o slowingwater movement and permitting the settlement o even more sediment [36]. Sinceseagrasses grow submerged in seawater, they are more likely to be light-limitedthan salt marsh or mangrove vegetation [36]. O ten, the distance to which sea-grasses extend rom the shoreline is related to the slope o the sea oor. Whenadequate sediment is present, seagrasses will extend seaward to the depth at


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Coastal Wetlands 13

Shark Bay and the Gul o Carpenteria in Australia) and removed rom them. Coralree s are particularly abundant where sediment loading and reshwater inputs areminimal [5]. Thus, coral ree s are quite susceptible to changes in terrestrial landuse and losses o riparian wetlands, tidal marshes, mangroves, and seagrass bedsthat can trap sediments rom land-based sources. Major areas o coral ree s occurin the Pacifc and Indian Oceans and the Caribbean Sea. Three main types o coralree s have been defned [36]:

1. ringing ree : a ree ound growing as a ringe attached to a land mass.2. barrier ree : a ree that occurs at some distance out to sea and creates a shallow

lagoon between the ree and the land.3. atoll: an isolated structure surrounded by deep water that tends to orm a ring

o coral with a central lagoon (Fig. 4).

Coral ree s also provide a number o ecosystem unctions such as serving habitator a tremendous diversity o marine species, supporting coastal fsheries, and pro-tecting coastal areas rom storms and marine erosion [36]. Most coral ree s occuralong the coasts o developing countries where the most intensive coastal degrada-tion is occurring [50]. Coral ree s are at risk rom global change processes suchas bleaching and sea temperature increases, as well as various human activitiessuch as coastal development, overfshing, sediment and sewage inputs that lead toeutrophication, dumping o debris and toxic wastes, and oil spills [5]. One studyhas suggested that all current coral ree s will disappear by 2040 due to warmingsea temperatures [17]. Despite the act that they provide habitat or a tremendousdiversity o species, the valuation o ecosystem services provided by coral ree swas estimated to be $6075 ha 1 yr1 [9], which is less than one third o the amounts

estimated or swamps/ oodplains and seagrass/algae beds.

Figure 4: Types o coral ree s. Copywrite (2000) rom Ecology o Coastal Waters by K.H. Mann. Reproduced by permission o John Wiley and Sons, Inc.


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A temperate counterpart to the coral ree is the kelp orest. Kelp orests are temperate marine ecosystems dominated by large brown algae ( Macrocystis spp.).They are characterized by high productivity and diversity [36]. For example, largebrown algae can grow 45 cm d 1, and extend to 60 m in length. Kelp orests are alsoremarkably resilient to disturbances rom wave impacts, storm surges, and otherextreme oceanographic events [51].

4 Wetlands in di erent types o watersheds

Wetlands in temperate versus tropical watersheds can di er in their ability toreduce impacts rom land-based activities (i.e. logging, agriculture, urbanization)on sediment and nutrient loading to the coastal zone. Plant uptake and microbialactivity in temperate and boreal wetlands has been shown to decrease dramati-cally, i not totally, during the cold winter months [52]. In contrast, plant uptakeand microbial immobilization may occur throughout the year in tropical wetlands.However, the intense precipitation that is o ten experienced in tropical watershedshas been shown to be much more erosive and generate greater sediment exportrom terrestrial areas [53]. For example, on the island o Maui (Hawaii), erosiv-ity can vary rom less than 100 to greater than 1800 erosivity units over a span o 20 km [54]. Furthermore, in these small tropical watersheds, the terrestrial andmarine environments are intimately connected. Here, human land-use activities arequickly translated to coastal areas because o high amounts o rain all (sometimesover 5000 mm yr 1) and steep stream gradients [55]. Rain all on the ridgetop canresult in increased sur ace water inputs to the estuary and coral ree s in a matter o

hours. Thus, all lands may be considered coastal in the Pacifc Islands [54]. Thisis quite a contrast to the Mississippi River Watershed that covers about 40% o thecontinental U.S. and or which it would take many days or precipitation in theheadwaters o the upper Midwest to reach the rivermouth at the Gul o Mexico.

Furthermore, many tropical watersheds are located in areas with steep topographyand subject to intense land-use conversion. The combination o erosive rain all,steep topography, and conversion o orests to agriculture and rangeland land haslead to massive increases in sediment transport through tropical watersheds. Thecoastal wetlands in these watersheds may be unable to handle these large uxes o sediments and nutrients, or are themselves subject to conversion to more intensiveland-use such as agriculture or aquaculture.

5 Coverage and position o wetlands in a watershed

In a study in the MidAtlantic United States, Novitski [56] ound that when the percentage o the watershed in lakes and wetlands dropped below 10%, there wererapid increases in ooding. Thus watersheds appear to have critical thresholds o wetland area or ood control and most likely also or sediment and nutrient reten-tion. The position o a wetland in the watershed also in uences its ability to retainsediments and nutrients. Faber et al. [57] delineated three main watershed zones,


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Figure 6: (a) A stream drainage network ollowing the nomenclature o Strahler[174]. (b) Cross sections o riparian wetland oodplains showing howriparian transport and overbank ooding vary with Strahler stream order.(c) Change in length o oodplain a ected by 1 hectare o disturbanceas a unction o oodplain width. Modifed rom [58] with permissiono Wetlands.

(a)

(b)

(c)


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Coastal Wetlands 17

Ultimately, wetlands are needed in both the headwater and the depositional zonesin terms o habitat and biodiversity but also in terms o ood control, water qual-ity, sediment retention, and biogeochemical cycling. However, to meet the goal o improved water quality rom a reduction in nonpoint-source pollution rom land-based sources, restoration o wetlands along lower order streams may be the best

strategy [58].

6 Methods or quanti ying sediment accumulationin coastal wetlands

Short-term (monthly to annual rates) sediment accumulation in wetlands has beenquantifed by a combination o techniques that include horizon markers and sedi-ment traps. Horizon markers have been used to quanti y sediment accumulationin various wetland types [5961]. This method involves laying down marker hori-zons at various points in the wetland. Feldspar, a white material composed o silt- and clay-sized particles [60], is commonly used as a marker horizon, althoughother studies have employed glitter or sand [62]. Sampling consists o taking asmall core o sediments rom the sur ace, down through the marker horizon, andmeasuring the thickness o sediments deposited above these highly visible markerhorizons. A specialized coring device called a cryocorer can be used to collect thecores [63]. The amount o sediment that has been deposited on top o the markercorresponds to the sediment accumulation. Un ortunately, this method only esti-mates sediment depth and not the mass o sediment per unit area.

Mass o sediment per unit area can be measured with sediment traps. Sedimenttraps consist o tiles, petri dishes, or plastic containers that are deployed on the soil

Table 2: Relationships between stream order and other dimensions o stream con-fguration. First our columns are rom Leapold et al. [173] and last twocolumns are rom Brinson [58].

Streamorder

Number

Averagelength[km]

Totallength[km]

Estimatedoodplainwidth [m]

FloodplainSur ace

Area [km 2]

1 1,570,000 1.6 1,526,130 3 75782 350,000 3.7 1,295,245 6 77713 80,000 8.5 682,216 12 81874 18,000 19.3 347,544 24 83415 4200 45.1 189,218 48 9082

6 950 103.0 97,827 96 93917 200 236.5 47,305 192 90828 41 543.8 22,298 384 85629 8 1250.2 10,002 768 7681

10 1 2896.2 2896 1536 4449


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or sediment sur ace [6467]. The material that collects on the trap can be quanti-tatively removed, weighed, and even analyzed or chemical composition.Long-term sedimentation rates in wetlands have been quantifed using radioiso-

topic dating techniques. Both 137Cs and 210Pb profles have been shown to be e ec-tive or these purposes [6870]. This process involves collecting deep soil cores(to bedrock when possible) and sectioning them into fne (i.e. 2 cm) intervals. The137Cs activity in each sample can be measured with a germanium detector. Peak concentrations o 137Cs levels within each core correspond to peak allout levelsrom atmospheric nuclear weapons testing in 1964 [69, 71]. The sediment locatedabove the 137Cs peak is equal to the amount o sediment that has been trapped inthese wetlands since 1964. 210Pb levels can be used to estimate sediment accretionrates over an even longer time intervals, up to 100 years be ore present [7072].Excess 210Pb has been shown to accumulate in depositional environments romboth atmospheric deposition and sedimentation [70, 73]. 210Pb is considered morereliable than 137Cs because it is polyvalent, and thus bound more tightly to mineraland organic soil particles [74].

7 Role o coastal wetlands in trapping sediment

In an analysis o eight temperate watersheds containing wetlands, Phillips [75]ound that less than 65% o the sediment eroded rom upland areas was transportedout o the watersheds. O the sediment that reached streams within the watersheds,2393% was retained by wetlands through which these streams owed [75]. Thus,in watersheds that have su fcient wetland sur ace area and stream lengths associ-ated with wetlands, sediment transport to coastal zones may be low. However,as watersheds develop, construction activities and the increase in the amount o impervious sur ace in the watershed have been shown to cause accelerated siltloading to neighboring estuaries [15, 76]. A number o studies in the tropics havereported that increased soil erosion in terrestrial watersheds is threatening estuar-ies and coastal coral ree s [7781]. The steep topography, intense precipitation,and extensive land-use changes in these watersheds leads to high sediment loadsthat appear to have exceeded the sediment retention capacity o the coastal wet-lands. When this occurs, sediment deposition within estuarine and ree zones cansmother adult corals, kill juvenile corals, and prevent larval recruitment [81].

Mangroves have been shown to initially play a passive role in sediment accumu-lation [27]. Once mangrove vegetation has been established, it then acts to preventerosion and trap sediments. The stems and leaves o mangrove and salt marsh

vegetation slow water velocity and promote sediment deposition, roots and rhi-zomes increase the stability o the sediment, algae help trap fne sediments, oystercolonies modi y the ow o water and sediments, and macroinvertebrates trap sus-pended detritus [27].

A study o a tropical watershed in Palau, Micronesia reported the mangroveringe zone trapped about 44% o the riverine fne sediment ux, which was stillnot enough to prevent degradation o the associated coral ree s [79]. Another studyon the island o Molokai (Hawaii), reported that turbidity was lower on coral


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Coastal Wetlands 19

ree s adjacent to mangroves than on ree s with no adjacent mangroves [82]. Astudy o the Heeia Swamp in Kaneohe Bay, on the island o Oahu (Hawaii),reported that 10 cm o sediment was deposited in 16 months in the mangrove areas[83].

8 Methods or quanti ying nutrient retention andtrans ormation in coastal wetlands

A number o processes must be considered when quanti ying nutrient trans ormationand retention in coastal wetlands. Plant uptake can be determined by harvestingplant tissue throughout the growing season and analyzing plant tissues or N andP concentration [36]. Microbial immobilization is di fcult to quanti y but can

be measured in laboratory dosing studies and with radioisotopic techniques orP [84].One o the dominant N trans ormation processes in coastal wetlands is denitri-

fcation. This process can be measured with in-situ core techniques but there aremany di fculties associated with these methods due to the high background con-centration o nitrogen gas in the atmosphere. The denitrifcation enzyme activity(DEA) [85, 86] is commonly used as an index o denitrifcation potential. TheDEA is use ul or site comparisons because it o ers a method by which the deni-trifcation potential can be compared across di erent soil types [87]. The DEAinvolves amending sieved, feld-moist soils with solutions o glucose and potas-sium nitrate to ensure nonlimiting substrate conditions, and chloramphenicol toinhibit protein synthesis. The resulting slurries are made anaerobic by repeatedushing with N

2gas. The anaerobic slurries are then injected with acetylene to

inhibit N 2 production [85] and then shaken or a specifc time (i.e. 90 min). Atmultiple time intervals (i.e. 30, 60, and 90 min), gas samples are collected romsample jars. Nitrous oxide (N 2O) concentrations o these gas samples can be deter-mined with a gas chromatograph. Nitrous oxide uxes are then calculated as thetime-linear rate o concentration increase in the headspace o the sample jars. TheDEA is then calculated as the short-term rate o N 2O production in the jars and isindicative o the size o the denitri ying enzyme pool present in the soil [85].

Another important nutrient retention unction that occurs in wetlands is P sorp-tion. The P sorption index (PSI) has been widely used to estimate the P retentioncapacity o wetland soils. The index was developed by Bache and Williams [88]and used by Richardson [89] in a seminal study o P sorption across various wet-land types. Numerous studies have established that the PSI: 1) serves as a reliable

gauge o a wetland soils P sorption potential, 2) is less time consuming to mea-sure than multiple-point P sorption isotherms, and 3) acilitates comparison withrelated soil properties [8993]. The PSI can be determined by shaking a sterilizedsoil sample with a known P concentration or 24 h. The di erence in concentrationo inorganic P between the initial and fnal concentration represents the amount o P sorbed. The index is then calculated as X .(log C )1 where X = amount o P sorbed(mg P.100 g soil 1) and C = the fnal inorganic P concentration in solution (mg P .L1).


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Phosphorus ractionation is another common method that is used to quanti yP storage in wetland soils and involves the quantifcation o the distribution o P across various soil pools. These pools are operationally defned, but have gener-ally been equated with bioavailable-P, Ca and Mg bound P, Al and Fe bound P, andresidual pools [9396].

9 Role o coastal wetlands in retaining andtrans orming nutrients

As P is generally considered to be the limiting nutrient in reshwater ecosystems,N is considered to be the limiting nutrient in marine ecosystems, and coastal wet-lands represent transitional zones o both N and P limitation [8, 97], the results

described in this section involve processes dealing with trans ormation and reten-tion o N and P. Retention o N and P in wetlands results rom cumulative uxesinto storage compartments o wetland ecosystems such as microbes, vegetation,plant litter, and soils [98]. Uptake o N and P by emergent wetland plants may behigh during the growing season, but much o this N is released upon senescencein the late all and winter [99, 100]. Rooted wetland plants tend to pre erentiallytake up reduced ammonium nitrogen rather than oxidized nitrate nitrogen [52,101]. In terms o climate, plant uptake is lowest in cold northern-hemisphere zonesand wetlands with stagnant hydrology, whereas plant uptake is greatest in tropi-cal zones and wetlands with active hydrology [52]. Unlike emergent vegetation,trees in orested wetlands provide long-term nutrient storage [99, 102]. Microbialuptake o N and P has also been thought to be a short-term rather than a long-termnutrient sink [84, 89, 98]. A comprehensive study o the e ect o wetlands onwater quality in Minnesota ound that wetlands were more e ective in removingsuspended solids, total phosphorus and ammonia during high- ow periods, butwere more e ective at removing nitrate in low- ow periods [103]. This suggeststhat the ability o wetlands to retain and store nutrients may not only vary on aseasonal scale but also during storm ow compared to the base ow.

Organic matter accumulation and denitrifcation are two o the more dominantand long-term N trans ormation and retention mechanisms. Organic matter accu-mulation involves the storage o N and P in soil organic matter (SOM) pools.These pools are generated rom litter inputs, and o ten escape decomposition dueto the anaerobic soil conditions. Soils with high organic carbon generally also havehigh organic N and P [104]. This processes mentioned above are common to allcoastal wetland types. In addition, di erent coastal wetlands vary in their ability to

retain and trans orm nutrients. These di erences will be explored in the nextsections o the chapter.

9.1 Retention and trans ormation o N and P in riparian wetlands

As a result o their location in the landscape, riparian wetlands interact with bothupstream and upslope sources o nonpoint-source runo and have the ability to


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Coastal Wetlands 21

reduce inputs o N and P to coastal waters [58, 105, 106]. Thus, riparian wetlandsmay be sinks or N and P at the watershed scale and i so, play a central role inmaintaining regional water quality [107]. Retention and trans ormation o N and Pby wetlands involves a combination o biogeochemical processes such as denitri-fcation, P sorption, sedimentation, and organic matter accumulation, plant uptake,microbial immobilization [27, 52, 108].

Riparian wetlands have been shown to support high denitrifcation rates and, incertain cases, to trans orm the majority o nitrate inputs to nitrogen gases [109,110]. Denitrifcation in these riparian wetlands has also shown high spatial vari-ability [111115] due to the presence o patches o organic matter and anaerobicmicrosites in the soil profle [116119]. The term hot spots was coined todescribe these areas o high denitrifcation [116, 120]. Recent studies have shownsignifcantly lower denitrifcation potential in wetlands with nutrient-poor sub-strates (sand, light till) than in wetlands with nutrient-rich substrates (alluvium,dark till) [121], signifcantly lower DEA levels in restored/created wetlands com-pared to natural wetlands [86, 115], and a positive relationship between plant speciesrichness and denitrifcation potential [122].

Soils o riparian wetlands have been shown to have higher P sorption capacitiesthan adjacent uplands or streambanks [90, 107]. Long-term P storage in wetlandsis believed to be controlled by three main processes: (1) deposition o sediment-bound P; (2) sorption o dissolved phosphate; or (3) the storage o organic P bypeat accretion [96, 108]. While signifcant amounts o P can be stored by sedimen-tation [98], these sediments may be resuspended in uture hydrologic events. Con-sequently, sorption and peat accretion are believed to represent the most importantlong-term P-retention pathways [89, 108].

In alkaline wetland soils, P sorption has been shown to be signifcantly corre-lated with calcium and magnesium content, and to a lesser degree with aluminum,iron and SOM content [108]. In acid wetland soils, P sorption has been shown tobe signifcantly correlated with amorphous Al and Fe content [89, 93, 106], and toa lesser degree SOM and particle size [90, 93]. While iron phosphates are solubi-lized when Fe(III) is reduced to Fe(II) under anaerobic conditions, aluminumphosphates are una ected by changes in redox potential [123]. Thus, soluble ironand phosphate may be lost rom wetland soils in reducing conditions, while alumi-num phosphates persist in acid wetland soils [107].

Another layer o complexity is added when we consider that amorphous Al andFe, SOM, and texture exhibit signifcant spatial and temporal variability in riparianwetlands [93, 107, 124, 125]. Beyond a locally random aspect, this spatial vari-

ability may be related to the combined action o physical, chemical, or biologicalprocesses that operate at di erent spatial scales [126]. In natural riparian wetlands,these processes might include overbank ooding, sediment deposition, sur aceruno , erosion, groundwater inputs, fre, tree-throw, root activity, litter produc-tion, and activity o macro and micro soil auna. Each o these processes mayin uence particular locations o the riparian zone with varying degrees o inten-sity. For example, Fig. 7 shows the spatial variability o per cent clay, oxalate


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extractable aluminum, and the PSI o two riparian wetlands in the North CarolinaCoastal Plain [93].

9.2 Retention and trans ormation o N and P in tidal marshes

1.01.82.53.34.04.85.56.37.0

406692118144170196222248

(a) (b)

(c) (d)

(e) (f)

Clay (%)

Site1 Al ox Site2 Al oxAlox

(mg g 1)

Site1 PSI

PSI( X /log C )

Site2 PSI

7.09.011.013.015.017.019.021.023.0

Site2 Clay

0 8 16 24 320

8

16

24

32Site1 Clay

0 8 16 24 320

8

16

24

32

Figure 7: Spatial distribution o % clay (a), oxalate extractable aluminum (Al ox)(c), and the phosphorus sorption index (PSI) (e) at Site 1, and% clay (b),Alox (d), and PSI ( ) at Site 2. Plot size is 32 m by 32 m. Modifed rom

[92].


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Coastal Wetlands 23

Salt marshes are thought to act as N trans ormers, importing dissolved oxidizedinorganic orms o N and exporting dissolved and particulate reduced orms o N [127]. Salt marshes appear to be sinks or total P, but remobilization o phosphate inthe sediments can lead to small net exports o phosphate rom salt marshes [127].

Many studies have documented the act that salt marshes are net exporters o organic material [27]. Salt marshes are generally thought to be N limited [3], but arecent study indicated that while the vegetation in the salt marshes was N limited,the microbial community was P limited [128].

In terms o P retention, a study along an estuarine salinity gradient along theCooper River in South Carolina, demonstrated that there was a trend o decreasingP sorption capacity o intertidal marsh sediments with increasing salinity [129].Specifcally, the reshwater marsh site had the highest P sorption capacity ol-lowed by the two brackish marshes with intermediate P sorption. The salt marshsedimentshad the lowest P sorption capacity. The results were attributed to thedecrease in soil sur ace area across the salinity gradient as well as the changes inmineralogy, ionic strength, and redox chemistry o Al and Fe [129]. Interestingly,under reshwater conditions, Al and Fe hydroxides carry a net positive charge thatacilitates P sorption, whereas under saltwater conditions, Al and Fe hydroxidescarry a net negative charge that inhibits P sorption [130, 131].

9.3 Retention and trans ormation o N and P in mangroves

While the tidally dominated ringe mangroves provide protection o shorelinesrom marine wave action and storms, riverine and basin mangroves are moreinvolved in trapping o sediment and retention o nutrients rom terrestrial sources[39]. Mangroves are considered to be in a steady-state balance between N loss andN fxation, but they are capable o being stimulated to higher levels o produc-tion rom local additions o ertilizers [36]. In addition to retaining sediments,Walsh [83] reported that the high nitrate and phosphate levels in Heeia Streamwere reduced signifcantly in the upper reaches o the swamp, indicating that themangroves may serve as sinks or these nutrients ask well. Mangroves have alsobeen shown to have a high capacity to absorb and adsorb heavy metals and othertoxic substances in e uents [132].

9.4 Retention and trans ormation o N and P in seagrass bedsand coral ree s

While seagrass beds have been shown to trap sediments rom terrestrial sources,their ability to retain and trans orm nutrients is largely unquantifed [133]. Sedi-ments underlying seagrass beds have been shown to have higher carbon contentand lower redox potentials [134] indicating that they may be more e ective at den-itrifcation than adjacent areas without seagrasses or with rocky bottoms. Researchon the deposition o seagrass castings and macroalgae remnants on beaches hasshown these sources to be important or nutrient provisioning to coastal inverte-brates and shorebirds [5]. Likewise, it was ound that over 6 million kilograms dry


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weight o seagrass and algal detritus (20% o the annual production) is depositedeach year on the 9.5 km beach o Mombasa Marine Park in Kenya [134]. Thesestudies provide evidence that seagrasses may act as nutrient sources in coastalzones.

Nutrient cycling in coral ree s is quite complex. Primary producers have beenshown to take up ammonium and nitrate rom the waters surrounding the ree [36].Consumers in the ree environment commonly excrete ammonia [36]. Ree s sup-port a diverse community o bacteria that are involved in processes that produce,trans orm, or consume N such as N fxation, ammonifcation, nitrifcation, denitri-fcation, and processing o organic N compounds [135]. Thus, it is di fcult togeneralize about the role the coral ree s plat in nutrient trans ormation and reten-tion. However, it is clear that when ree s are subject to high sediment and nutrientloads, they do not respond avorably [17]. Thus, there is a great need to e ectivelymanage the agriculture, pasture land, rangeland, and orestland in terrestrial water-sheds, as well as to conserve and restore coastal wetlands that are so important orsediment and nutrient retention at the watershed scale.

10 Case study: comparison o soils rom created,restored and natural wetlands

The United States Clean Water Act (CWA) o 1972 mandates mitigation when-ever natural wetlands are impacted by development. The Army Corps o Engineers(ACoE) has jurisdiction over this process and requires created and restored wet-lands to meet specifc vegetative and hydrologic criteria during a fve-year moni-

toring period to be considered success ul [136]. Vegetative criteria require survivalo a certain per centage o planted species per acre. Hydrologic criteria require thatthe water table be within 30 cm o the soil sur ace or a consecutive period o atleast 12.5% o the growing season. The current process does not require any moni-toring o soil properties or processes [136, 137]. It is interesting that soil has beenomitted rom the mitigation process, as soil plays an integral part in the defnitiono a wetland as stated in the USFWS Wetland Classifcation System [18]. Hydricsoil, along with hydrophytic vegetation, and wetland hydrology are also the threecriteria used to delineate jurisdictional wetlands as defned by the ACoE [138].The lack o consideration o edaphic characteristics in the wetland mitigation pro-cess is a cause or concern or a number o reasons: 1) soil orms the oundation o these developing ecosystems; 2) inadequate soil properties can be detrimental to

vegetative survival and the establishment o wetland hydrology; and 3) soil is themedium or biogeochemical processes that trans orm and retain nutrients [137].Without suitable soil properties, created wetlands (CWs) and restored wetlands(RWs) may never replace the nutrient trans ormation and retention unctions o the natural wetlands (NWs) that were destroyed.

As ew CWs or RWs are assessed beyond what is needed to meet hydrologicand vegetative success criteria [139], the ability o these wetlands to replacenatural wetland unctions is a topic o considerable debate [86, 140142]. It hasbeen stated that the defnitive test o success or CWs and RWs is how closely


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Coastal Wetlands 25

Figure 8: Spatial distribution o soil organic matter (SOM) at Site 1 restored (a)and natural wetland (b), at Site 2 created (c) and natural wetland (d), atSite 3 (e) and natural wetland ( ), and at Site 4 restored (g) and naturalwetland (h) plots. The upper scale applies to Sites 13, while the lowerscale applies to Site 4. Plot size is 32 m by 32 m. Modifed rom [153].

0 8 16 24 320

8

16

24

32

0

4

8

12

16

20

24

28

32 0 8 16 24 320

8

16

24

32

14

2434

44

54

64

74

84

94

SOM (%)

Created/Restored Natural

Site 1

Site 3

Site 4

SOM (%)

Site 2


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they unction like NWs [143]. Un ortunately, only a ew studies have attempted todetermine whether wetland unctions in CWs and NWs are equivalent to those o NWs [86]. The assumption that wetland unction ollows wetland structure, whichunderlies the ACoE monitoring process, is also largely untested [144].

For example, soil properties o CWs and RWs have almost always been shownto di er rom NWs [145]. Created wetlands typically have higher sand and lowerclay content than NWs [141, 146, 147]. This has important implications or wet-land unction, as coarse-textured soils typically have lower water-holding andnutrient-retention capacities than fne-textured soils [148, 149]. Created wetlandsand RWs also usually have lower levels o SOM and higher bulk densities thanNWs [143, 146, 147, 150, 151]. Such soil conditions can lead to low growth andsurvival o planted and colonizing species. Litter layers in CW/RWs are o tenpoorly developed or absent in comparison to that o NWs [86, 144]. As a result o low organic matter and sparse litter, it has been speculated that the microbial com-munities in the soil o CWs are much less viable than those o NWs [152]. Addi-tionally, soil temperatures were reported to be signifcantly higher in CWs than inNWs as a result o a lack o shading by mature trees [141]. Furthermore, microto-pography has been reported to be considerably lower in CW/RWs than in NWs[141, 151].

Bruland and Richardson [153] conducted a study in which they comparedpatterns o spatial variability o soil properties in CWs/RWs and paired NWs inthe North Carolina coastal plain. They ound that the spatial variability o SOMwas lower at some but not all o the CWs/RWs (Fig. 8), and concluded that priorland use and mitigation activities could decrease, increase, or cause no change inthe spatial variability o soil properties in CWs/RWs compared to NWs. In another

study, Bruland et al. [115] ound that the spatial variability o predicted DEAvalues in was much lower in the CWs/RWs than in the NWs or the riverine sitesbut not or the nonriverine sites

11 Future research needs and directions

Existing landscape patterns contain in ormation about the processes that gener-ated these patterns [154]. In various coastal wetlands around the world, the pro-cesses o hydrologic modifcation, sedimentation, and nutrient loading have notonly a ected the structure and unction o these ecosystems but also their spatialextent and distribution [153, 155158]. The use o remote sensing and geographicin ormation systems (GIS) have enhanced scientists capacity to describe patterns

in nature over larger spatial scales and at fner levels o detail than ever be ore[159]. These methods can be used to quanti y anthropogenic impacts to wetlandsat multiple scales, providing valuable in ormation to aid in the design o wetlandand watershed restoration projects. These types o approaches may be especiallyuse ul or the monitoring o seagrasses and coral ree s. Traditional assessment o cover and density o seagrass and coral cover along transects and quadrats gener-ally have an associated error > 30% about the mean [43]. This makes it di fcult todetect reliable changes in seagrass or coral cover. Typically, changes can only be


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Coastal Wetlands 27

detected when they are greater than 5080%. Thus there is a great need to developremote-sensing techniques to more accurately monitor changes in seagrass bedsover large spatial scales [43].

Signifcant strides have been made in coastal-zone management in the last ewdecades and many o the worlds 123 coastal countries now have some orm o coastal management plans and legislation [5]. However, countries with well-developed coastal-zone management plans are still acing loss o coastal wet-lands, overexploitation o coastal resources, user con icts, and indirectdegradation rom activities occurring sometimes hundreds o km rom the coastalzone itsel [5]. Thus, management and policy have not been able to keep pacewith increasing degradation o coastal ecosystems.

One bright spot in this picture is that coastal wetlands such as tidal marshesand mangroves can o ten be restored once they have been degraded. The scienceo salt-marsh and mangrove restoration is relatively advanced compared to res-toration o other ecosystems. Research has shown that the establishment or re-establishment o proper elevation, topography, tidal ushing, and the planting o a ew key species can restore salt marshes and mangroves on relatively shorttime scales [144, 160]. Mangroves appear to be the most amenable to restoration[15] o the coastal wetlands considered in this chapter. Restoration o mangroveshas been accomplish by transplanting vegetative propagules, young trees, ormature trees with considerable success in India and Southeast Asia [161]. Coastalwetland restoration in the United States has been most success ully achieved inestuarine salt-marsh systems [162, 163]. However, studies have shown that,while the vegetative structure o created and restored marshes appears to approx-imate that o natural marshes, the soil properties and macroinverbrate communi-ties in the created and restored marshes may take much longer time scales todevelop [164, 165]. Restoration o seagrass meadows although increasinglycommon, has been raught with di fculties; a worldwide success rate or resto-ration o these systems was estimated to be only 45% [161, 166]. Restoration o coral ree s may be the most di fcult challenge o any ecosystem, and it has onlybeen practiced at small scales and with limited success [133].

Future research, management, restoration, and policy needs to occur at thewatershed or regional scale [167] and involve an interdisciplinary approach toassessing the role and unctions o coastal wetlands in reducing the impacts o land-based management. According to Vivian-Smith [168], wetland restorationshould consider ecological processes and structure at multiple spatial scales (Fig. 9).Hydrologic, vegetative, and edaphic heterogeneity increases the probability that

an optimal habitat will exist at a restoration site or the intended species and betterapproximate the structure and unction o natural wetlands [15, 169]. On a positivenote, there are currently a number o landscape-scale coastal wetland restorationprojects in various phases o planning and implementation that are designed toameliorate the e ects o hydrologic alteration and nutrient loading including theChesapeake Bay [170], Mississippi River basin [171], Florida Everglades [172],and the wetlands o Iraq [157]. However, it has been estimated that a tripling o the


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area o riparian orests and bu er systems would be needed in the Upper MississippiRiver and Ohio River Basins in order to cause signifcant reduction in the N loado the Mississippi River to the Gul o Mexico [171].

Another move in the right direction appears to be the coupling o coastal-zone management with watershed management as has occurred with the EuropeanWater Framework Directive and projects under the LOICZ (Land-Sea Interactionsin the Coastal Zone) initiative [5]. Also, the U.S. Coral Ree s Task Force has

Figure 9: Restoration actions in the coastal wetland landscape ocusing on habitatheterogeneity at di erent spatial scales (microscale, patch, and landscape)that in uence key processes and accelerate ecosystem development.Copyright (2001) rom, Developing a Framework or Restoration, byG. Vivian-Smith [168]. Reproduced by permission o Taylor & Francis,a division o In orma plc.


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Coastal Wetlands 29

established local action strategies to assess land-based threats to coral ree sspecifcally looking at solutions to prevent erosion and nutrient loading to coastalzones rom terrestrial sources. With uture population trends suggesting that theworlds coastal population is expected to approach 6 billion people by 2025 [15],our ability to preserve, protect, manage, and restore watersheds, coastal wetlands,and near-shore habitats is becoming increasingly important.

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