Coastal Wetland Ecology and Challenges for Environmental Management

Abstract

• Coastal wetlands are plant communities at the land-sea interface. Two common types of coastal wetlands are salt marshes and mangrove swamps. Marshes are dominated by nonwoody grasses and shrubs; mangrove swamps are dominated by trees.

• The global distribution of salt marshes and mangroves is governed by temperature: most mangrove species cannot tolerate freezing temperatures, so they grow in warmer tropical and subtropical latitudes. Marshes are more common in cooler temperate latitudes.

• Salt marshes and mangroves overlap in some subtropical regions; these areas may experience shifts in species composition in response to climate change. The dynamics and ecological consequences of these shifts are important topics for future research.

• Plants in coastal wetlands are adapted for abiotic stressors including prolonged inundation, which causes soil anoxia, and high salinity.

• Salt marshes exhibit predictable zonation patterns, where the distribution of species within a site varies with small changes in elevation. These zonation patterns are driven by species-specific adaptations to abiotic stressors and by interspecific competition. Zonation patterns in mangrove swamps are more variable.

• Coastal wetlands provide a variety of ecosystem services to human communities: wetlands can improve water quality, store nutrients, and buffer against erosion and storm surge and provide nursery habitat for commercially and recreationally important fishery species.

• Current management issues in coastal wetlands include encroaching suburban and agricultural development, sea level rise, nutrient enrichment and eutrophication from agricultural runoff and treated sewage discharge, and freshwater diversion.

• The policies regulating development on coastal wetlands are complex and dynamic. Restoration is the most common approach to mitigate for anthropogenic impacts. An understanding of wetland ecology is crucial to making wise decisions concerning the nature and direction of restoration projects.

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Introduction

Coastal plant communities are broadly defined as those habitats shaped by terrestrial and marine influences. Many, though not all, coastal habitats can be defined as wetlands; the ecology and management of those habitats are covered in this chapter. Wetlands are defined by the United States Army Corps of Engineers by the presence of three features: (1) wetland hydrology, inundation or saturation for at least part of the growing season; (2) hydric soils, soils that are anoxic (containing little or no oxygen for at least part of the growing season; this condition usually develops when soils are inundated with water); and (3) hydrophytic vegetation, vegetation adapted to wet conditions.

The coastal wetlands covered in this chapter are often located within estuaries . An estuary is a semi-enclosed body of water where freshwater from rivers or streams mixes with oceanic waters, creating brackish (slightly salty) conditions. Tidal movement and riverine freshwater input are variable, causing spatial and temporal variations in salinity (Fig. 1). Freshwater input supplies estuaries with sediment, organic matter, and critical nutrients such as nitrogen, phosphorus, and iron. Tidal marine input brings in animal larvae and other essential nutrients such as sulfate and bicarbonate. The combination of these freshwater and marine inputs makes estuaries highly productive habitats.

Fig. 1

Left: typical salinity gradient in the Galveston Bay estuary (Texas, USA), depicting lower salinity (light blue shades) near the riverine inputs and higher salinity (dark blue shades) near the marine input. Right: salinity gradient in the bay during an exceptional drought in 2011 (Data provided by the Galveston Bay Estuary Program)

History

Many early human cultures lived in harmony with wetlands, using these productive habitats to obtain food, fuel, and shelter. However, beginning in the 1700s, and perhaps even earlier, many agriculture-based cultures viewed wetlands as fallow areas with no cultivation value and as breeding grounds for disease-carrying insects. For decades, wetlands were drained for agriculture or cleared and filled for development. By the mid-twentieth century, the resultant wetland losses totaled more than 50 % worldwide; up to 80 % of that loss may be attributable to agricultural expansion (Dahl 1990).

By the 1970s, however, the links between wetland habitats and vital coastal ecosystem services – fishery support, erosion control, water quality improvement – had become better understood. The rate of development slowed, impacts became better managed, and restoration began in earnest. Now, the need to protect and manage these habitats has emerged as a top priority in coastal management. These ecosystem services and management and restoration challenges will be discussed in more detail later in the chapter.

Stressors

Freshwater and marine inputs can augment estuarine productivity, but those inputs also create abiotically stressful conditions. Plant communities are particularly strongly influenced by salinity and flooding, which is usually accompanied by anoxia (no oxygen) or hypoxia (low oxygen) in the soil.

Most plants in coastal wetlands are halophytes – tolerant of high salt levels. Halophytes can withstand some amount of salt in their tissues, but even the most halophytic species must be able to avoid excessive salt accumulation. High concentrations of salt ions can have many negative impacts on plants: salt ions can be toxic, create an osmotic imbalance that prevents uptake of water even when inundated, and repel and prevent uptake of positively charged nutrients like NH₄ ⁺ (ammonium). At the ecosystem level, saline coastal wetlands often have lower plant biomass but faster rates of decomposition, which in turn yields slower rates of nitrogen accumulation relative to freshwater and brackish wetlands (Craft 2007; Craft et al. 2009). Furthermore, potential denitrification, or the conversion of nitrate (NO₃ ⁻) to biologically inert nitrogen gas (N₂), is generally lower at higher salinities (Fig. 2). This is a critical step in the removal of nitrogen from wastewater in treatment wetlands (this topic will be discussed in more detail later in the chapter).

Fig. 2

Simplified conceptual model depicting the relationships between plant productivity, salinity, oxygen availability, and nitrogen and sulfur cycling in wetland sediments. The gray cloud represents the relative size of the oxygenated rhizosphere. Solid arrows represent active processes; dashed arrows represent inhibited or reduced processes

Most halophytes have some mechanism, such as storage in vacuoles or high concentrations of glycolipids and sterols in cell membranes, to help halophytes exclude salt from metabolically active parts of cells. Other common salt avoidance mechanisms include secretion, where salt is excreted from the plant through specialized glands, usually on the leaves; storage, where plants concentrate salt in the bark or older leaves that are then sloughed off or dropped; succulence, where plants store water to dilute internal salts; and external exclusion, where plants produce waxy substances such as suberin to block salt uptake through the root epidermis.

Coastal wetlands can be inundated by tides for extended periods of time, and plant and animal respiration quickly use up the biologically available oxygen in tidal flood waters. This causes hypoxic or anoxic conditions in wetland soils; these conditions may be temporary or can persist for weeks or longer. Low oxygen conditions facilitate the decomposition process, where bacteria reduce sulfate (SO₄ ²⁻) to hydrogen sulfide (H₂S). The production of sulfides generates a “rotten egg smell” in many wetlands. This is a natural process, but sulfides can be toxic at high concentrations or inhibit nutrient uptake by vascular plants (Fig. 2). To reduce sulfide production and oxygenate wetland plant roots, a common adaptation in wetland plants is aerenchyma tissue. Aerenchyma refers to internal spaces that extend from the leaves to the roots, providing a low-resistance internal pathway for the transport of oxygen from the leaves above the water to the submerged tissue (Fig. 3). Aerenchyma forms from the collapse of cortex cells in “programmed cell death” (apoptosis). Through aerenchyma, oxygen is transported to the roots to be used for metabolic processes. The subsequent oxygenation of the rhizosphere (zone surrounding the roots of plants) can lower sulfide production and reduce sulfide toxicity. If, however, sulfide production is extremely high, aerenchyma can become occluded by callus tissue (cells that grow over wounds), leading to plant dieback events.

Fig. 3

Rhizome cross sections of two wetland plants, showing the hollow spaces forming the aerenchyma tissue. (a) Spartina alterniflora, a low-elevation grass species with extensive aerenchyma. (b) Spartina patens, a mid- to high-elevation grass species with less aerenchyma tissue (Photo credit A.R. Armitage)

Both salinity and low soil oxygen levels can potentially impact nitrogen cycling in coastal wetlands, largely because some steps in the nitrogen cycle are oxygen dependent, and others require anoxic conditions. The simplified conceptual diagram in Fig. 2 illustrates some of the key interactions among salinity, oxygen levels, and the nitrogen cycle. High salinity is linked to lower primary productivity, thus lowering oxygen production and transport to the rhizosphere. Lower oxygen levels in the rhizosphere facilitate the anaerobic reduction of sulfate to hydrogen sulfide. Hydrogen sulfide (H₂S) is toxic at high concentrations, which further reduces productivity and creates a feedback that maintains anoxic conditions. Nitrogen fixation, the conversion of atmospheric nitrogen (N₂) to ammonium (NH₄ ⁺), is an anaerobic process that occurs at a relatively rapid rate in most anoxic wetland soils. However, H₂S blocks ammonium uptake, further reducing productivity and contributing to the feedback loop that maintains anoxic soil conditions. Denitrification is also lower at high salinity, in part due to the salt-mediated inhibition of nitrification, an aerobic (oxygen dependent) process that converts ammonium into nitrites and then nitrates (Fig. 2).

Different types of wetlands are typically defined by the character of their plant communities. Swamps are wetlands dominated by trees or shrubs; marshes are primarily composed of herbaceous, nonwoody vegetation such as grasses, rushes, sedges, and forbs. Both swamps and marshes can occur in marine and freshwater habitats; this chapter will focus on two common types of coastal marine wetland communities: salt marshes and mangrove swamps.

Salt Marshes

Salt marshes are defined as those marshes subjected to regular tidal flooding by salt water. Salt marshes occur in estuaries and along marine coastlines, primarily in temperate latitudes. In tropical regions, the short-stature grasses and forbs in salt marshes are generally outcompeted by the taller vegetation in mangrove forests, which will be addressed later in this chapter. A typical salt marsh can be subdivided into several zones based on elevation relative to sea level (Fig. 4). Each of these zones varies in salt and flooding stress; the plants in each of these zones are adapted to those conditions.

Fig. 4

Zonation patterns in a salt marsh. In this picture, the marsh border zone is dominated by the marsh elder, Iva frutescens. The high marsh zone is comprised of grasses such as Spartina patens (marsh hay; lighter green) and the rush Juncus roemerianus (black rush; darker green). The low marsh zone is dominated by the grass Spartina alterniflora (smooth cordgrass) (Photo credit A.R. Armitage)

Zonation

The border zone between salt marshes and nontidal upland habitat is characterized by plants that can grow in moderately saline soils but are intolerant of flooding. Plants in this “marsh border” zone along the high tide line often lack aerenchyma tissue, making them sensitive to flooding and associated soil anoxia. For example, the marsh elder, Iva frutescens, a typical marsh border plant in the Gulf of Mexico, experiences reduced growth and higher mortality if the roots are inundated for as little as 8 % of the growing season (Fig. 5; Thursby and Abdelrhman 2004).

Fig. 5

Excerpt from Fig. 7 in Thursby and Abdelrhman (2004). Relationship between mean stem diameter for older stems of Iva frutescens and the duration of flooding (as percent of growing season) at the root zone (10 cm below soil surface). Percent flooding values are based on elevation measurements made near the same location that the stem samples were taken. Vertical bars are ±2 SE. The means are of 10 stems except for Fox Hill Cove (FOX) and Jenny Creek (JEN) (n = 30) and Mary Donavon Marsh-1 (DON1) (n = 20); (p < 0.01) (Reprinted with permission from Springer-Verlag)

Below the marsh border zone is a large zone broadly often referred to as high marsh. This zone covers a relatively wide elevation range that encompasses a variety of flooding regimes. In this zone, salts tend to accumulate in the soils due to regular but brief tidal flooding followed by evaporation, especially in the more seaward region of the zone. Soil salinities can be more than double that of ambient floodwater. Despite this stressor, plant diversity tends to be high relative to lower elevations (Fig. 6), in part because there are many different adaptations to salt stress. Few plants in this zone are tolerant of prolonged flooding – many have reduced or absent aerenchyma (Fig. 3b).

Fig. 6

Simplified conceptual model depicting the relative importance of abiotic stressors and biotic interactions at different elevations within salt marshes. The predominant factor in each elevation zone is highlighted in the boxes at the top of the graph

The lowest vegetated elevation zone in a salt marsh is the low marsh. Soil salinity is close to that of ambient floodwater. Plant species in this zone must be able to produce extensive aerenchyma in order to withstand prolonged flooding (Fig. 3a). Few plant species can survive the anoxic conditions associated with extensive flooding, so the low marsh zone has relatively low plant diversity. On the east and Gulf coasts of the United States, the low marsh zone is dominated by Spartina alterniflora (Fig. 4). This grass species occurs in all tidally flooded zones of salt marshes, but it grows taller at lower elevations than at higher elevations (Fig. 7). The mechanisms driving this morphological variation are complex; genetic differences and environmental influences both contribute to tall- and short-form morphology.

Fig. 7

Tall and short forms of Spartina alterniflora (Photo credit A.R. Armitage)

Within marsh zones, a microhabitat called a salt pan can form. Salt pans are unvegetated or sparsely vegetated patches, usually in the high marsh, that are characterized by very saline soil. There are several mechanisms for the formation of salt pans (Boston 1983). For example, wrack (floating organic debris) deposition can cover underlying vegetation (Fig. 8a). When it is eventually washed out following a high spring tide (during full or new moon phases), the ground underneath will be devoid of vegetation. Alternative mechanisms of salt pan formation include ice scouring, which can remove large clumps of marsh vegetation in the winter, or waterlogging in small topographic depressions, which can cause mortality of established plants. In all cases, after initial formation of the bare patch, evaporation will rapidly raise soil salinity, often to more than twice as high as ambient seawater. High salinity will depress seed germination and inhibit plant invasion, preventing recolonization and maintaining the salt pan microhabitat for long periods of time. Vegetation in salt pans is typically restricted to a few individuals of extremely salt-tolerant species (e.g., Sarcocornia spp.) and blue-green algae (cyanobacteria) (Fig. 8b). Although these microhabitats have little vegetation, they provide important roosting habitat for many coastal bird species (Fig. 8c).

Fig. 8

(a) Wrack deposition in the high marsh zone of a salt marsh. Wrack has accumulated between stands of Borrichia frutescens (sea oxeye daisy, with yellow flowers) and short-form Spartina alterniflora. Previously covered patches that have turned into salt pans are visible in the background. (b) Fully formed salt pan with sparse succulent vegetation and cyanobacterial mats (visible as blackened patches on the soil). (c) Black skimmers (Rynchops niger) roosting in a salt pan (Photo credit A.R. Armitage)

Case Study: Plant-Animal Facilitation in a New England Salt Marsh

Salt marshes in New England are dominated by smooth cordgrass, Spartina alterniflora . This species is particularly well adapted to frequently flooded low elevations, where it co-occurs with several marsh fauna species. The marsh grasses and fauna have a close facultative mutualistic relationship, where each benefits from the other, though they do not completely rely on each other for survival. One common faunal group in salt marshes is comprised of fiddler crabs (Uca spp.), which excavate extensive burrows. In a set of experiments, Bertness (1985) removed crabs from high-density, low-elevation zones and added crabs to low-density, high-elevation zones. These experiments revealed several mutualistic interactions between crabs and smooth cordgrass. Crab burrowing activity oxygenates the sediment, augments drainage, and increases the decomposition of organic matter, all of which increase smooth cordgrass above- and belowground productivity. Crabs benefit from this association as well – smooth cordgrass roots substantially increase the integrity of crab burrows. This positive feedback between smooth cordgrass and fiddler crabs is strongest within the low marsh elevation, just above the marsh vegetation-water interface. Burrows excavated at the marsh edge, in softer, wetter sediment with few roots, will rapidly collapse. At high marsh elevations, denser root mats interfere with the ability of fiddler crabs to excavate burrows. Therefore, the strength of the fiddler crab-smooth cordgrass facilitation is greatest at the upper edge of the low marsh, where there is a maximized mutual benefit for plants (anoxia stress is alleviated) and crabs (burrow integrity is increased).

Another common animal in New England salt marshes is the ribbed mussel (Geukensia demissa ). These bivalves require a surface for attaching anchoring filaments, and smooth cordgrass stems and roots provide a suitable substrate (Bertness 1992). Mussels can be particularly dense along the seaward edge of the tall smooth cordgrass zone. The anchoring filaments bind smooth cordgrass stems together, which in turn increases sediment stabilization and decreases erosion. Mussels deposit waste products that provide nutrients for plant growth (Jordan and Valiela 1982), resulting in increased aboveground and belowground productivity (Fig. 9; Bertness 1984). Mussels also benefit from this association – mussel growth and survivorship is higher for mussels in smooth cordgrass beds (Stiven and Kuenzler 1979). Smooth cordgrass benefit mussels by providing an attachment substrate and may also supply organic matter as an indirect food source (Bertness 1984).

Fig. 9

Excerpt from Fig. 3 in Bertness (1984). Summary of aboveground Spartina alterniflora parameters in mussel manipulation experiments done on the marsh edge during the 1981 and 1982 growing seasons. Control quadrats; mussel removal quadrats (±SE) (All data are for 0.25-m² quadrats). *P < .05, ANOVA in comparison to control within years. **P < .01, ANOVA in comparison to control within years (Reprinted with permission from the Ecological Society of America)

Summary: Salt Marshes

In summary, zonation in salt marshes is driven by abiotic stressors, interspecific competition, and facultative mutualistic plant-animal interactions. The variation in the relative importance of these factors across salt marsh elevation zones is summarized in the conceptual model in Fig. 6. In the low-elevation zone, prolonged inundation and associated soil anoxia limit plant assemblages to a few species, though facilitative plant-animal interactions somewhat ameliorate this stress. Salinity stress is the primary abiotic stressor at higher elevations. Many of low-elevation plant species can survive at higher, less stressful elevations, but are competitively excluded from those less stressful habitats. This pattern was succinctly described by ecologist Mark Bertness (1991): “Zonation patterns are maintained by competitive dominants restricting the distribution of competitive subordinates to physically stressful habitats.”

Mangroves

Mangrove swamps are dominated by halophytic (salt tolerant) trees that live at the land-sea interface. In an example of convergent evolution, mangrove species evolved from non-mangrove plant lineages independently many different times. In fact, mangroves occur in over 30 families of dicots (class Magnoliopsida). Therefore, trees that are called “mangroves” are not necessarily closely related in an evolutionary sense. Mangrove species differ in their stress adaptations and in their degree of stress tolerance. However, most mangroves are intolerant of freezing temperatures, which limits their distribution to tropical and subtropical latitudes (Fig. 10; Giri et al. 2011). There are over 65 species of mangroves worldwide, with the highest diversity in the Indo-Pacific and Indian Oceans; about four species occur in North America and the Caribbean.

Fig. 10

Excerpt from Fig. 1 in Giri et al. (2011). Mangrove forest distributions of the world – 2000 (Reprinted with permission from Blackwell Publishing Ltd)

Mangrove Stress Adaptations

Mangroves and salt marshes experience similar abiotic stressors, particularly high salinity and prolonged flooding. In addition to the general adaptations described earlier, many mangroves have specialized structural modifications that facilitate survival in these harsh tidal coastal environments.

To adapt to saline waters, the roots of many mangroves are suberized. Suberin is an extracellular glycerolipid polymer found in the cell walls of many plant species. In plant roots, suberin is found at the hypodermis, where it blocks apoplastic (extracellular) transport into the root, and at the endodermis, where it limits transport into the stele. In mangroves, it is particularly concentrated in the epidermis and hypodermis of roots (Fig. 11; Pi et al. 2009). It forms a thick, waxy layer that effectively blocks apoplastic salt uptake by the plant; in some species, over 90 % of the salt in seawater can be excluded by this substance. The suberin layer also reduces radial oxygen loss from the roots (Pi et al. 2009) and may lower transpiration rates and increase water-use efficiency (Baxter et al. 2009).

Fig. 11

Excerpt from Fig. 4 in Pi et al. (2009). Cross sections of root tip, basal zone (4 cm from the root tip), and mature zone (8 cm from the root tip) of Excoecaria agallocha, Lumnitzera racemosa, and Bruguiera gymnorrhiza (cross sections with thickness of 10 μm were made and photographed, scale bars equal to 200 μm; E+H epidermis and hypodermis, Ar aerenchyma air spaces, Ct cortex, SW suberized walls) (Reprinted with permission from Elsevier BV)

Another adaptation to salinity found in about a third of all mangrove species is vivipary, which is a reproductive strategy where there is substantial development of the zygote while still attached to the parent tree. In some mangrove species, the seed embryo will penetrate through the fruit pericarp and grow to a considerable size before dispersal, producing characteristic propagules with elongated hypocotyls (embryonic trunks) (Fig. 12a). In other species, the zygote does not penetrate the pericarp (fruit wall) before dispersal, but the hypocotyls will emerge shortly after release from the parent (Fig. 12b).Among dicots, true vivipary – sexual development on the parent tree –is relatively rare and occurs mostly in mangroves. About 30 of the 33 plant species known to exhibit true vivipary are mangroves (Elmqvist and Cox 1996). Pseudovivipary – asexual development on the parent tree – occurs in several other groups of plants in extreme climates with high abiotic stress levels, such as deserts or alpine environments (Elmqvist and Cox 1996). In all cases, this jump start on seedling development helps protect young plants from the abiotic stressors in the environment by facilitating rapid establishment soon after dispersal. In mangroves, vivipary protects new, vulnerable seeds from salt water stress, allows nutrient uptake from the parent plant under low salt stress, and reduces chloride inhibition of germination. Propagules can float after being released from the parent tree, facilitating long-distance dispersal. Rooting is initiated when favorable habitat is encountered.

Fig. 12

(a) Propagules of the red mangrove, Rhizophora mangle, still attached to the parent tree. (b) Rooted propagule of the black mangrove, Avicennia germinans (Photo credit A.R. Armitage)

A striking morphological characteristic of many mangroves is their complex aerial root structures, which primarily function as adaptations to flooded conditions. Aerial roots that extend from the mangrove trunk are termed prop roots, and those that protrude upward from lateral belowground roots are called pneumatophores (Fig. 13). The aerial portions of these “roots” are covered with large pores called lenticels. Air is taken up through the lenticels and transported through the aerenchyma tissue to the belowground root system (Fig. 11), thus delivering the oxygen necessary for root cellular metabolism in otherwise hypoxic or anoxic soils.

Fig. 13

Mangrove aerial root structures. (a) Prop roots on a juvenile red mangrove, Rhizophora mangle. (b) Pneumatophores extending upward from lateral roots of a juvenile black mangrove, Avicennia germinans (Photo credit A.R. Armitage)

Zonation

In concept, intertidal zonation patterns are dictated by physiological responses of each species to abiotic stressors that vary along tidal gradients. Mangroves are somewhat plastic in their internal and external morphology, so some species can occur at a range of elevations, and zonation patterns are variable within and among geographic regions of the world. A wide variety of factors, including shoreline topography, tidal and freshwater influence, salinity, and sediment characteristics, influence mangrove distribution along elevation gradients. Thom (1984) identified no fewer than eight distinct geomorphic and biological settings that have unique mangrove zonation patterns. This section will focus on some of the most common types of mangrove tidal “zones,” with specific emphasis on the species common to Caribbean mangrove swamps.

The land-sea interface, often referred to as fringe mangrove habitat, is characterized by permanently flooded soils, giving the plants constant exposure to salt water. The soils generally have low oxygen content, though they are not necessarily anoxic (McKee 1993). Oxygenic phototrophs such as diatoms and other eukaryotic algae inhibit nitrogen fixation, thereby maintaining low soil nitrogen content in fringe mangrove soils (Fig. 14; Lee and Joye 2006). This aerobic activity also facilitates sulfide oxidation, reducing the buildup of toxic sulfides (Fig. 14; Sherman et al. 1998). In the Caribbean, the red mangrove (Rhizophora mangle) dominates this fringe habitat. With its characteristic, prominent prop roots (Fig. 13a), red mangroves form an iconic image of the Caribbean coastline. Prop roots are covered with lenticels and contain aerenchyma tissue, enabling red mangroves to survive in permanently flooded soils. Red mangroves also have heavily suberized roots that can block up to 99 % of salt uptake from the flooding seawater. The long, thin propagules characteristic of red mangroves (Fig. 12a) are an additional adaptation to the salt water environment.

Fig. 14

Excerpt from Figs. 1, 3, and 5 in Sherman et al. (1998). Changes in mangrove and soil characteristics with increasing distance from the shoreline (Reprinted with permission from Springer-Verlag)

The zone above the fringe habitat is difficult to succinctly characterize. In some areas, this zone is called a transition habitat that contains a mix of species. In other areas, this drier habitat is called a basin habitat and is dominated by just one or two species. In general, the flooding duration in mid-elevation habitats is relatively short, facilitating the diffusion of oxygen from the atmosphere into the soils. As in the fringe habitat, nitrogen and sulfide accumulation rates are relatively low (Fig. 14). The shorter flood periods allow mangroves in this zone to have somewhat reduced aerial root structures. In the Caribbean, black mangrove (Avicennia germinans) is characteristic of this zone. The pneumatophores of this species can extend upward out of the ground for several meters away from the primary tree trunk (Fig. 13b). Like prop roots, pneumatophores have aerenchyma and lenticels to facilitate gas exchange and root aeration. Black mangroves roots are suberized, but not as heavily as red mangrove roots. Black mangroves manage excess salt uptake by secreting salt through numerous small salt excretion glands scattered across leaf surfaces. The production of small but numerous propagules (Fig. 12b) facilitates seedling survival in saline soils.

The highest elevations in mangrove swamps sometimes transition to terrestrial or freshwater habitat, but in other cases, they are characterized as dwarf mangrove habitat. Dwarf habitat is essentially basin habitat that is so infrequently flooded or is otherwise abiotically stressful that the trees are stunted in height. In these habitats, soils are generally anoxic, facilitating sulfate reduction and the accumulation of sulfides (Fig. 14). Nitrogen fixation also occurs in the anoxic soil, increasing total nitrogen concentration in the soil. If this habitat is occasionally tidally influenced, then the soils will be saline. In general, the duration of flooding is relatively short, so mangroves at this elevation have more adaptations for managing salt than for flooding. In the Caribbean, white mangroves (Laguncularia racemosa) are characteristic of this zone, though they can occur at lower elevations as well. White mangroves can develop small pneumatophores or reduced prop roots if prolonged flooding occurs, but are frequently found at higher elevations and without aerial roots. White mangroves usually occur in saline soils, so they have moderately suberized roots and large salt excretion glands on the leaves. Like many other mangrove species, white mangroves produce propagules to reduce salt stress on seedlings.

Case Study: Plant-Animal Interactions on Mangrove Islands in Florida

Small islands with dense stands of red mangroves are common along Caribbean and Florida coastlines. Some of these islands are used as rookeries by nesting birds (e.g., herons, egrets, pelicans, cormorants). During the nesting season, copious amounts of guano (bird feces) are deposited on the rookery islands, and mangroves take up some of the excess nutrients. Trees on the enriched rookery islands produce more branches and flowers than trees on non-rookery islands (Fig. 15; Onuf et al. 1977). This example illustrates how important nonconsumptive relationships can be in structuring plant communities. In this case, mangroves provide birds with nesting habitat, and the birds benefit the plants by supplying nutrients for growth. The indirect interaction between birds and plants demonstrates that bottom-up forces, in this case resource supply, can influence both plant and bird fitness and productivity (Fig. 16).

Fig. 15

Excerpt from Fig. 4 in Onuf et al. (1977). Mean numbers (± SE) of leaves, branches, and flowers added per 1-cm diam. main stem in high- (solid line) and low- (dashed line) nutrient areas. Differences between sites were significant by t-tests (df = 10) for dates where *(p < .05) or **(p < .01) appear in the upper part of the figure (Reprinted with permission from the Ecological Society of America)

Fig. 16

Simplified conceptual diagram depicting the interaction between top-down and bottom-up forces influencing mangroves on islands that are used as rookeries

The plant-animal interactions in this community become more complex when other community members, such as insects, are considered. Leaf production on trees in rookeries is not always augmented as much as might be expected based on the amount nutrient supply from guano. This is largely due to higher herbivory pressure on rookery islands – insects prefer the guano-enriched leaves, and herbivory can be up to four times higher than on non-rookery islands. Ultimately, increased mangrove productivity from nutrient enrichment is mitigated by nutrient-induced herbivory. This case study shows how complex interactions between bottom-up (resource availability, e.g., nutrient supply) and top-down (consumption, e.g., herbivory) forces can structure plant communities (Fig. 16).

Future Directions: The Salt Marsh-Mangrove Ecotone: A Developing Field

Mangroves are not tolerant of freezing temperatures; this temperature sensitivity limits mangroves to tropical and subtropical latitudes (Fig. 10). Many families of salt marsh species can tolerate a wide range of weather conditions, but on tropical coastlines, smaller salt marsh species are outcompeted by dense, tall mangrove canopies. In some subtropical areas, there is a transition zone – an ecotone – between marsh and mangrove habitats. These ecotones occur in temperate areas of Australia, New Zealand, and the southern continental United States. Mangrove-marsh ecotones are dynamic habitats – mangroves often expand into salt marshes during periods with warm winters and contract during periods with hard freezes. This dynamic is primarily driven by temperature, but many other factors influence mangrove-marsh distribution as well, including rainfall, salinity, sea level, propagule supply, and interspecific competition. For example, Spartina alterniflora can outcompete newly sprouted black mangrove propagules (McKee and Rooth 2008), but if the mangrove seedlings survive through a few growing seasons, the established tree will begin to displace the surrounding marsh grasses and forbs.

Current research suggests that mangrove distributions may continue to expand in response to climate change. For example, models predict that an increase in winter minimum temperatures of 2–4 °C may lead to black mangroves replacing salt marsh on nearly all of the Texas and Louisiana coastlines by the year 2100 (Fig. 17; Osland et al. 2013). Other climate-related factors that may increase mangrove expansion rates include rising sea level due to glacial melting and thermal expansion. As little as 10 cm of sea level rise over the next 100 years will likely result in substantial mangrove expansion in all Gulf of Mexico states; sea level rise may cause mangroves to displace over 10,000 ha of coastal marsh in both Florida and Louisiana (Doyle et al. 2010). Climate change scenarios that include increasing atmospheric carbon dioxide concentration and changing herbivore populations will also likely influence mangrove-marsh dynamics, though these interactions are complex. Elevated CO₂ alone may not be sufficient for mangrove seedlings to outcompete marsh plants, but if there is also low herbivory pressure and sufficient nitrogen supply, then elevated CO₂ may accelerate mangrove growth (McKee and Rooth 2008). The exact role of each of these factors, and how they interact with each other, is a rapidly growing field of study in coastal plant ecology. Furthermore, appropriate management of coastal resources depends on our understanding of the ecological implications of this shift in plant communities. Will this change in plant species composition alter the ecosystem services that wetlands provide, such as fishery nurseries, erosion control, or water quality improvement? Key ecosystem services of coastal plant habitats are described in the next section.

Fig. 17

Excerpted from Fig. 6 in Osland et al. (2013). Predictions of mangrove forest relative abundance (i.e., percentage of tidal saline wetlands dominated by mangrove forests) under alternative future (2070–2100) winter climate projections: left panel, mangrove forest relative abundance with an ensemble B1 scenario climate; right panel, mangrove forest relative abundance with an ensemble A2 scenario climate. Note that these predictions apply just to the tidal saline wetland habitat within each cell and not the entire cell. Climate scenarios are defined by the Intergovernmental Panel on Climate Change (Reprinted with permission from Blackwell Publishing Ltd.)

Ecosystem Functions and Services

Coastal plant communities provide a unique suite of ecosystem functions and associated ecosystem services. Ecosystem functions are characteristic exchanges or processes within an ecosystem, such as primary productivity, energy flow, or nutrient cycling. Ecosystem services are ecosystem functions that benefit humankind. Human valuation of ecosystem functions is complex and based on many factors, including the provision of food for sustenance, monetary gain, aesthetic value, and clean air and water. Several ecosystem functions of coastal plant communities that are particularly valued by humankind are highlighted in the following section.

Water Quality

Coastal plant communities are widely recognized for their capacity to improve nearshore water quality. This plant-mediated improvement of water quality is termed phytoremediation. Coastal wetlands are not stagnant water bodies – many have slow but directional water flow from inland sources to nearshore habitat. Some wetlands are specifically constructed to manage water flow between terrestrial and marine ecosystems – these are called treatment wetlands. The plants in natural and treatment wetlands provide frictional resistance, slowing down water flow, thus facilitating the removal of nutrients, bacteria, and other pollutants through a variety of mechanisms. When wetland plants lower water velocity, this facilitates the settlement of suspended solids and adhered contaminants. Settlement is the primary mechanism for removal of organic solids (i.e., sewage waste) from water moving through coastal wetlands. Many nutrients, especially ammonium, nitrate, and phosphate, can be removed from the water through direct uptake by plants and bacteria, which then use these nutrients for metabolic processes. Bacteria in wetland soils can transform ammonium into nitrate (nitrification) and then into N₂ gas (denitrification). Nitrogen gas can then volatilize (evaporate or diffuse) from the water into the atmosphere. Some nutrients, particularly inorganic forms of phosphorus, can become tightly bound to clay particles in a process called adsorption. These phosphorus-clay complexes are largely biologically inert, and as the clay particles settle to the benthos, the phosphorus is functionally removed from the water column.

Nutrient Cycling and Storage

Coastal wetlands play critical roles in many global nutrient cycles; nitrogen and carbon cycles are among the most important (Vitousek et al. 1997). The anoxic soils in coastal wetlands harbor nitrogen-fixing bacteria that convert atmospheric nitrogen (nitrogen gas, N₂) to organic forms such as ammonium. Nitrogen fixation is the primary mechanism by which inert pools of atmospheric nitrogen become biologically available for plant uptake. Other bacteria in wetlands facilitate denitrification, the conversion of nitrate to nitrogen gas (N₂). Denitrification is important for controlling the export of organic nitrogen out of wetlands – without denitrification, nitrogen will stay in biologically available organic forms. Excess nitrogen will eventually be exported out of wetlands through rivers and streams to nearshore ecosystems, potentially exacerbating eutrophication (see “Management Issues and Strategies” section).

Coastal wetlands also play an important role in the global carbon cycle, particularly given their potential for carbon sequestration. Carbon sequestration occurs when carbon assimilation is greater than carbon loss in an ecosystem. In marine environments, including coastal wetlands, sequestered carbon is referred to as blue carbon. Mechanisms of carbon assimilation in wetlands include photosynthesis, soil microbe assimilation, and the decomposition and burial of plant tissue. Carbon is lost from wetlands through microbial and plant respiration and through the decomposition and export of plant tissue into adjacent waterways. Natural and anthropogenic (human-caused) wetland loss can accelerate carbon loss and reduce sequestration potential. Changes in wetland vegetation – such as the shift from marsh- to mangrove-dominated systems – may also alter the blue carbon storage potential in wetlands; the nature of these potential changes is a currently growing field of study.

Erosion Control and Surge Buffer

Many regions of the world are prone to large, powerful hurricanes. The east and Gulf coasts of the United States have been hit by several particularly damaging hurricanes over the last 10 years. Storm damage to coastal urban communities can potentially be lessened, to a degree, by the fringing coastal marsh and dune ecosystems. Coastal plant communities can attenuate (reduce) storm surge through wave energy dispersal, where the physical structure of the plant assemblage breaks up wave energy. In addition, wetlands can store large amounts of water, subsequently reducing the amount of water that travels inland to urban communities. For example, the storm surge from Hurricane Katrina, which struck New Orleans, Louisiana in 2005, entered the coastal salt marshes in less than 24 h, but after the storm, it took more than 4 days for all of the surge waters to drain back out into the Gulf of Mexico.

A commonly used rule of thumb is that each 2.7 miles of marsh (from the coastline extending inland) attenuates storm surge by 1 ft. The actual attenuation rate and inland extent of the storm surge varies among and even within storms and is influenced by the geometry of the shoreline, vegetation type, slope of ocean floor, and, perhaps most importantly, the size, speed, direction, and duration of storm. Hurricane Rita, which struck the Louisiana-Texas border as a Category 3 storm in 2005, is an excellent example of how variable real-world attenuation rates can be. Due to the track of the storm, western Louisiana experienced among the highest wind speeds (up to 120 mph), but high winds persisted for a relatively short duration. The attenuation of storm surge in the area was close to the 2.7 miles: 1 f. prediction. Eastern Louisiana, however, was exposed to the powerful winds in the northeast quadrant of the storm for nearly a full day. Even though the maximum wind speed was lower (about 90 mph), the duration of the exposure to hurricane-force winds was much longer than in West Louisiana. Coastal marshes in this area became inundated, and their attenuation capacity was essentially nullified.

Wave attenuation in mangrove forests is similarly variable. Although it is currently popular to promote mangroves as “bioshields” against storm surges and tsunamis, their role in actually reducing human casualties in the face of natural catastrophes remains somewhat controversial. Mangrove trees can undoubtedly reduce wave intensity through friction and wave disruption, but the effect is probably limited to relatively small waves. For catastrophic wave inundation events associated with tsunamis or large cyclones, most quantitative studies suggest that the risk of damage to a coastal settlement is more closely linked to distance from the shoreline than to the presence or absence of a mangrove forest (e.g., Gedan et al. 2011).

In addition to providing occasional protection against catastrophic erosion and flooding, coastal wetlands also provide day-to-day protection to the built environment on smaller spatial scales. Urban areas with higher levels of permitted wetland alteration have more frequent flooding following precipitation events. In fact, the number of permitted alterations may be a stronger predictor of flooding risk than watershed characteristics like area, slope, or population density (Brody et al. 2007).

Nursery Habitat

A wide range of commercially and recreationally important fish and invertebrate species rely on coastal wetlands, especially salt marshes, for part or all of their life cycle. In fact, over 75 % of commercially and recreationally targeted fishery species spend at least part of their life cycle in estuarine wetlands. For example, red drum (Sciaenops ocellatus) is a popular sport fish on the Atlantic and Gulf coasts of the United States. This fish spawns in nearshore habitats. Larvae and juveniles reside in estuaries, foraging on small shrimp, crabs, and other larval fish in salt marshes at high tide. Shrimp fisheries are also dependent on salt marshes. In the Gulf of Mexico, brown (Farfantepenaeus aztecus) and white shrimp (Litopenaeus setiferus) spawn at sea but inhabit Spartina alterniflora or Juncus spp. marshes in the postlarval (non-planktonic) stage. These shrimp fisheries are most productive in areas with extensive estuarine marshes, like the Mississippi Delta.

Recreation

Wetland plants provide habitat for many species of animals beyond those that directly contribute to commercial fisheries. Many recreationally fished species also rely on coastal wetlands. In the Gulf of Mexico, for example, over 80 % of recreationally targeted species spend at least some of their life in estuarine wetlands. Coastal wetlands also provide critical stopover and wintering habitat for migratory birds: on a typical winter day in any given coastal wetland in Baja California, 5,000 or more migratory shorebirds may be spotted. Some coastal wetlands provide essential habitat for endangered species, such as the whooping crane (Grus americana), which forages exclusively in salt marshes in Texas in the winter. While enjoying these diverse and abundant wildlife populations, recreational fishers and birders contribute billions of dollars to coastal economies each year. In 2006, a typical year, birders alone contributed to $82 billion in total industry output to the United States economy, primarily through purchases of lodging, transportation, food, and equipment (Carver 2009).

Management Issues and Strategies

Coastal wetlands have been substantially reduced in area over the past 200 years, and many remaining wetlands are impacted or degraded. In the continental United States, almost every state has lost at least a quarter of its historical wetland area; much of this loss has occurred in coastal wetlands. This section will include a discussion of the major mechanisms of wetland loss and impacts on remaining wetlands and will conclude with a brief discussion of the dynamic and sometimes controversial legal policies that protect coastal wetlands.

Development

Modern civilization, and accompanying urban and agricultural development, has dramatically altered coastal ecosystem landscapes. Some wetlands are filled for urban development; other developments occur in upland habitats directly adjacent to wetlands. The higher elevations of mangrove swamps are sometimes cleared to create room for urban growth or resort communities. Other mangrove swamps are cleared and excavated to create room for mariculture ponds to grow shrimp or other farmed seafood resources. Coastal marshes have been diked or drained to create agriculture fields or livestock grazing habitat; other marshes are flooded for rice farming.

In addition to directly causing habitat loss, development also increases groundwater use, which can accelerate subsidence. Subsidence is the gradual lowering of the sediment surface through mechanisms such as sediment compaction. Natural subsidence occurs slowly and is usually mitigated by the accumulation of sediment that enters estuaries from rivers. However, subsidence rates can be greatly exacerbated by anthropogenic activities, especially the withdrawal of groundwater. A particularly striking example of anthropogenic subsidence was documented around Houston, Texas, in the 1970s. A booming oil industry spurred population growth in the area, driving up the industrial and residential demand for groundwater. Rapid withdrawal of groundwater accelerated subsidence, and over a period of less than 10 years, many neighborhoods sunk more than half a meter. Some localized spots sank even more – up to 3 m (Fig. 18). This rapid subsidence permanently inundated tidal marshes, causing over 95 % marsh loss in a very short time period. Entire neighborhoods had to be abandoned due to chronic flooding problems. By the mid-1980s, a wider municipal and public appreciation of the anthropogenic subsidence issue led to better management of groundwater, and subsidence rates slowed dramatically.

Fig. 18

Google Earth images of Armand Bayou (near Houston, TX) in 1953 and 2012. In the 1953 image, note the tidal marshes in Horsepen Bayou and at marker #1 and the narrow tidal channel at marker #2. By 2012, subsidence had flooded most of those features. Ongoing restoration work in Horsepen Bayou is reestablishing some of the tidal marsh features

Sea Level Rise

Although anthropogenic subsidence rates in many estuaries in the United States have slowed, coastal wetlands are also subject to inundation from sea level rise, which is partly driven by climate change. Relative sea level rise in any one particular place is determined by both eustatic (global) and regional changes in sea level. Eustatic changes are related to climate change, including thermal expansion and glacial melting. Regional changes in sea level are linked to local dynamics like subsidence and riverine sediment supply. Most conservative estimates, as synthesized by the Intergovernmental Panel on Climate Change, suggest that at least 50 cm of relative sea level rise will occur in many coastal regions by 2100 (IPCC 2007).

Prior to human development of the coastline, wetlands could respond to sea level rise by migrating, albeit slowly, upland. However, many coastlines are now heavily developed or “hardened”; roads, bulkheads, and other built structures, as well as natural topographic features, prevent landward migration. As a result, many wetlands are experiencing a “coastal squeeze,” where wetland area shrinks as rising seas and anthropogenic barriers to upland migration limit the area of elevation suitable for wetland plant growth.

Freshwater Diversion

Recall the concept of an estuary: a body of water where fresh and salt water mixes. Many estuaries are parts of heavily developed watersheds, which are the areas encompassing all the lakes and rivers that eventually drain into a large water body. Demands for fresh water from urban and agricultural developments ultimately reduce freshwater input to the estuaries. What happens to an estuary when freshwater inflows decrease? The most acute impact, arguably, is an increase in salinity. These increases in salinity are likely to be exacerbated by extreme environmental events phenomena like droughts (Fig. 1). Long-term effects of high salinity could include plant or animal die-offs or shifts toward more marine species assemblages.

Eutrophication

Plant productivity in most ecosystems is limited by particular nutrients – those nutrients that are in shortest supply relative to others, and will therefore limit organism growth. In pristine coastal habitats, nitrogen and phosphorus are typically the most limiting. There are many anthropogenic sources of these limiting nutrients, including fertilizer runoff, sewage, and livestock waste. Moderate input of anthropogenic nutrients can increase ecosystem productivity, but excessive nutrient input can cause anthropogenic eutrophication: the rapid buildup of organic matter. In salt marshes, plants respond to excess nutrients by accelerating aboveground production: this produces the excess organic matter that is characteristic of eutrophic conditions. However, increased aboveground production is typically matched by a decrease in belowground production (Deegan et al. 2012). Lower root biomass is linked to decreased sediment stability, which eventually results in marsh erosion and habitat loss.

Policy

Wetlands are currently the only ecosystem with an international agreement focused on conservation and sustainable utilization. This agreement, the Ramsar Convention, was formed in 1971 by conservation groups in Europe that recognized the ecological and economic implications of widespread wetland loss. Currently, at least 163 nations are members of the convention. Central to the Ramsar Convention is the “wise use” concept: wetlands should be conserved and sustainably used for the benefit of humankind. Although the Ramsar Convention has no regulatory power, it has helped nations identify conservation priorities and define management strategies.

In the spirit of the Ramsar Convention, George H.W. Bush adopted a “No Net Loss” policy for the United States in 1989. The essence of this policy is that for every one acre of wetlands that is lost, at least one acre must be created or restored in its place. This policy applies specifically to jurisdictional wetlands, which are generally defined as those wetlands that fall under federal or local protection, based on the 1977 Section 404 amendment to the Clean Water Act (Kruczynski 1990). The definition of jurisdictional wetlands has narrowed and widened at times in response to sometimes contentious disputes among landowners, developers, environmental groups, and federal management agencies. These legal scuffles are complex and ongoing, but at this time, most coastal wetlands, including salt marshes and mangroves, are protected by the No Net Loss policy.

Restoration

The No Net Loss policy stipulates that if development impacts jurisdictional wetlands, then an equivalent area of wetland needs to be restored as compensation for the impact. The process of wetland restoration is simple in concept, but challenging in practice. In concept, wetland restoration first involves creating (by excavating, filling, or leveling) an appropriate elevation for the targeted plant species. Then, plants are allowed to establish naturally or are transplanted into the site – an undertaking that often involves large groups of volunteers, who then develop a stewardship of the new habitat (Fig. 19). Once plants are established, the “Field of Dreams” hypothesis is usually implicitly or explicitly invoked: “If you build it, they will come” (Palmer et al. 1997). In this context, “they” refers to the animals and ecosystem processes that are characteristic of reference marshes. Although it may take decades for restored wetlands to develop a full set of target conditions, and not all restoration projects are fully successful, the study of wetland restoration can better inform future projects, helping to ensure future successes.

Fig. 19

A young volunteer helps the Galveston Bay Foundation plant smooth cordgrass (Spartina alterniflora) in a restored marsh in Galveston Bay, TX (Photo credit A.R. Armitage)

Future Directions: Integrating Science and Restoration

On a global scale, coastal wetlands have been substantially reduced in area over the past 100 years, primarily due to urban and agricultural development, hydrological alterations, and subsidence due to natural events (soil consolidation and faults) and extraction of groundwater and minerals. Although the rate of loss has slowed in recent years, coastal wetlands continue to be vulnerable to disturbance from development, storm events, and offshore oil spills. Near- and long-term management priorities focus on conserving and restoring ecosystem functions of coastal wetlands, as they provide substantial support for local and state economies. To address these management priorities, wetland restoration projects, ranging from large (>100 ha) to small (<1 ha), have been implemented in many parts of the world.

In practice, restoration usually focuses on permit stipulations, which often emphasize vegetation cover characteristics and cover ecologically short time scales (3–5 years). Vegetation cover in restored sites can be linked to some specific ecosystem functions (e.g., nutrient uptake). However, metrics that are more closely linked to long-term ecosystem health, such as nutrient storage and belowground plant biomass, rarely achieve natural levels, even decades after restoration (Craft et al. 1999). This highlights a major challenge: is there a way to improve ecosystem functions and long-term sustainability, without making restoration markedly more expensive or labor intensive? For example, will increasing the number of plant species or genetic diversity improve ecosystem functions? Can facilitative interactions among plants, animals, and microbial communities be used to augment restoration success? Will the integration of higher elevations into restoration design improve ecosystem resilience in the face of near-term sea level rise? The answers to these types of questions will vary among and even within sites and regions. That heterogeneity presents a substantial challenge for restoration practitioners: ecologically successful restoration requires an in-depth understanding of local wetland ecology. Incorporating that understanding into a restoration plan that includes both near- and long-term ecological measures of success is the ultimate goal of those who study and those who practice coastal wetland restoration.