Oil Spill in Mangroves

Scientific Planning against the Oil Pollution on Mangroves

 

Spot Selection:

 'Elephanta Cave, Coastal regions of Uran, Mud island, Aqsa beach, Mud island-Mumbai,

 

 

Safar Mohammad Khan

MSc.(Botany-Environmental Pollution),MBA-Marketing

Green Apple Environmental Technologies

287-Patpadganj Industrial Area, Delhi-110 092(India)

 

 

 

 

 

 

 

 

 

 

 

Table of Contents

 

Introduction

 

Chapters

1. Mangrove Ecology

 

2. Oil Toxicity

3. Response

4. Recovery and Restoration

5. Case Studies

Glossary

Tables

1.1 Common mangrove species

2.1 Responses of mangrove forests to oil spills

3.1 Recommendations for response techniques in oiled mangroves

4.1 Mangrove impacts and recovery at eight oil spills

Introduction

This report is intended to assist those who work in spill response and planning in regions where mangrove ecosystems are an important part of the coastline. By under­standing the basics of the ecology of these forests and learning from past oil spills in mangroves, we can better plan for, protect, and respond to spills that may threaten them. Mangroves often border coastlines where coral reefs live offshore, and these two ecosys­tems are closely linked. Mangroves filter and trap excess sediment that could harm coral, and coral reefs protect shorelines where mangroves grow from excessive wave energy. Both habitats can be adversely impacted by oil spills, and spill responders must often consider tradeoffs between land-based and offshore resources during a response. This guide is a companion to Oil Spills in Coral Reefs: Planning and Response Considerations

 

This is not intended to be a specific guide for choosing cleanup methods, as many comprehensive versions of these exist already. Rather, we summarize current research on mangroves from the perspective of those who may need to make decisions about response in mangroves and present the information in an accessible format for people with some science or response background. Experienced responders unfamiliar with mangroves may want background on mangrove ecology, while biologists may want an overview of oil toxicity and mangroves and response and cleanup applied to man­grove ecosystems. We have organized the topics by chapters, each of which can be read as a standalone, with additional references provided at the end of each chapter. A glos­sary defines specialized terms.

Chapter 1, mangrove ecology, provides an overview of mangrove forests, their associated communities, and how they respond to various natural and human stresses. Chapter 2, oil toxicity to mangroves, reviews the research available on oil toxicity and impacts to mangroves. In Chapter 3, we discuss general guidance for responding to spills in mangroves and provide specific considerations for cleanup measures. Chapter 4 discusses long-term recovery of mangroves from oil spill impacts and restoration tech­niques and approaches. Lastly, in Chapter 5 we have compiled several case studies that illustrate a range of issues from oil spills impacting various regions.

 

Though mangrove forests are in many ways very adaptable ecosystems, and are inherently able to respond to physical changes in their environment, they are highly vulnerable to oil toxicity and can be further damaged by many types of cleanup activities. Thus, we must approach any type of response or restoration activities in mangroves with knowledge and caution. The information in this document will, we hope, help to mini­mize environmental impacts in mangroves when oil spills threaten them.

Chapter 1. Mangrove Ecolology

 

Key Points

• Mangroves worldwide cover an approximate area of 240 000 square kilometers of sheltered coastlines in the tropics and subtropics.

• Four of the most common ecotypes include fringe, riverine, basin, and scrub forests.

• Mangroves are restricted to the intertidal zone.

• Mangroves in general have a great capacity to recover from major natural distur-bances.

• Mangroves maintain water quality by trapping sediments and taking up excess nutri-ents from the water.

 

What is a Mangrove?

Ecologically, mangroves are defined as an assemblage of tropical trees and shrubs that inhabit the coastal intertidal zone. A mangrove community is composed of plant species whose special adaptations allow them to survive the variable flooding and salinity stress conditions imposed by the coastal environment. Therefore, mangroves are defined by their ecology rather than their taxonomy. From a total of approximately 20 plant families containing mangrove species worldwide, only two, Pellicieraceae and Avicenniaceae, are comprised exclusively of mangroves. In the family Rhizophoraceae, for example, only four of its sixteen genera live in mangrove ecosystems (Duke 1992)

Where are Mangroves and What do They Look Like?

Mangroves worldwide cover an approximate area of 240 000 km2 of sheltered coastlines (Lugo et al. 1990). They are distributed within the tropics and subtropics, reach­ing their maximum development between 25oN and 25oS (Figure 1.1). Their latitudinal distribution is mainly restricted by temperature since perennial mangrove species gener­ally cannot withstand freezing conditions. As a result, mangroves and grass-dominated marshes in middle and high latitudes fill a similar ecological niche.

The global distribution of mangroves is divided into two hemispheres: the Atlantic East Pacific and the Indo West Pacific. The Atlantic East Pacific has fewer species than the Indo West Pacific (12 compared to 58 species, respec­tively). Species composition is also very different between the two hemispheres. Out of a total of approximately 70 mangrove species, only one, the mangrove fern, is common to both hemispheres.

In the continental United States, mangroves are mainly distributed along the Atlantic and Gulf coasts of Florida (Figure 1.2). They also occur in Puerto Rico, the U.S

Virgin Islands, Hawaii, and the Pacific Trust Territories. Craighead (1971) estimated a coverage of approximately 1,750 km2 of mangroves along the Florida coast, with the highest development along the southwest coast. The Gulf of Mexico and Caribbean regions are characterized by low spe­cies richness, with only four dominant species: Rhizophora mangle (red mangrove), Avicennia germinans (black man­grove), Laguncularia racemosa (white mangrove), and Cono­carpus erectus (button-mangrove or buttonwood). Black mangroves, however, can be found as far north as Texas, Louisiana, and Mississippi , indicating this species’ greater tolerance to low temperatures and its ability to recover from freeze damage (Markley et al. 1982; Sherrod et al. 1986

The California Current, which limits the northern extent of mangroves along the Pacific coast of the Americas, brings cold water as far south as Baja California. At the southern tip of this peninsula, mangroves are represented by an occasional, scrubby black or white mangrove. The mangroves of the Pacific Islands are represented by a very differ­ent assemblage of species belonging to the Australasian group. Some of the more characteristic genera include Bruguiera, Rhizophora, Avicen­nia, Sonneratia, and Ceriops (Tomlinson 1988

 

Mangrove Ecotypes

Mangroves colonize protected areas along the coast such as deltas, estuaries, lagoons, and islands. Topographic and hydrologi­cal characteristics within each of these settings define a number of different mangrove ecotypes. Four of the most common ecotypes include fringe, riverine, basin, and scrub forests (Lugo and Snedaker 1974; Twilley 1998). A fringe forest borders protected shorelines, canals and lagoons, and is inundated by daily tides. A riverine forest flanks the estuarine reaches of a river channel and is periodically flooded by nutrient-rich fresh and brackish water. Behind the fringe, interior areas of mangroves harbor basin forests, characterized by stagnant or slow-flowing water. Scrub or dwarf forests grow in areas where hydrology is restricted, resulting in conditions of high evaporation, high salinity, low temperature, or low nutrient status. Such stressful environmental conditions stunt mangrove growth.

Each of these mangrove ecotypes is characterized by different patterns of forest structure, productivity, and biogeochemistry, all of which are controlled by a combina­tion of factors such as hydrology (tides, freshwater discharge, rainfall), soil characteristics, biological interactions, and the effects of storms and other disturbances.

Life History

Mangrove Reproduction and Growth

Most mangroves are hermaphroditic (both sexes are present in an individual organism). Mangroves are pollinated almost exclusively by animals (bees, small insects, moths, bats, and birds), except for Rhizophora, which is primarily self-polli­nated (Lowenfeld and Klekowski 1992). In most mangroves, germination takes place while the embryo is still attached to the parent tree (a condi­tion called vivipary). The embryo has no dormant stage, but grows out of the seed coat and the fruit before detaching from the plant. Because of this, mangrove propagules are actually seedlings, not seeds (Figure 1.4).

Vivipary as a life history strategy helps mangroves cope with the varying salinities and frequent flooding of their intertidal environ­ments, and increases the likelihood that seedlings will survive. Since most non-viviparous plants disperse their offspring in the dormant seed stage, vivipary presents a potential problem for dispersal. Most species of mangroves solve this problem by producing propagules containing substantial nutrient reserves that can float for an extended period. In this way, the propagule can survive for a relatively long time before establish­ing itself in a suitable location (McMillan 1971; Tomlinson 1988).

Buoyancy, currents, and tides disperse mangrove propagules and deposit them in the intertidal zone. Once established, the numer­ous seedlings face not only the stresses of salinity and variable flooding, but also competition for light (Smith 1992). These, in addition to other sources of mortality, cause very low survival rates for seedlings and saplings. Determining the age of mangroves is difficult, but flowering individuals have been recorded as young as 1.5 years old. Tree growth survival, and the ensuing forest structure are determined by the mangrove forests’ ecotype. There are few estimates of mangrove forest turnover (the time required for the forest to replace itself). Despite a precarious existence in the intertidal zone, Smith (1992) esti­mates mangrove turnover at 150-170 years. For comparison, estimates for turnover in lowland tropical rainforests is about 118 years (Hartshorn 1978)

 

Adaptations To Salinity

Mangroves can establish and grow under a relatively wide range of flooding and salinity conditions but are generally restricted to the intertidal zone where there is less competition with freshwater plants. Mangroves have developed a series of physiological and morphological adaptations that have allowed them to suc­cessfully colonize these environments.

Mangroves do not require salt water to survive, but because of poor competi­tion with freshwater vegetation and unique adaptations to the intertidal zone, they are generally found under the influence of salt water. Salinity is mainly determined by local hydrology, where input of salt water comes from the periodic tides and fresh water comes from rivers, rainfall, groundwater, and runoff. High evapotranspiration (water loss through the soil and plant leaves) in the tropics and subtropics can increase salinity considerably, especially under environments with restricted water flow. Thus, salinity can fluctuate widely within mangrove forests, both over time and space.

Mangroves have evolved different mechanisms to tolerate high salinities: salt exclusion, salt secretion, and tolerance of high salt concentrations within plant tissues are the main strategies. Most mangroves have developed all three mechanisms, although to varying extents. Rhizophora, Bruguiera, and Ceriops have root ultrafilters that exclude salt while extracting water from soils (Rutzler and Feller 1996). In salt secretion, special organs or glands remove salts from plant tissues. For example, Avicennia and Laguncularia have special, salt-secreting glands that cause salt crystals to form on the leaf surfaces (Figure 1.5). These crystals then can be blown away or easily washed away by the rain. Leaf fall is another mechanism for eliminating excess salt in mangroves (Kathiresan and Bingham 2001)

Adaptations To Flooding

Mangrove forests are periodically flooded, with the frequency and magnitude of flooding determined by local topography combined with tidal action, river flow, rainfall, surface runoff, groundwater, and evapotranspiration. As with salinity, hydrology in mangrove ecosystems varies greatly in time and space, and mangrove species differ in their ability to tolerate flooding.

At the intertidal scale, the magnitude and frequency of flooding decreases in a landward direction. Mangrove species often show a distinctive distribution across this gradient, which is the basis for classifying mangroves by lower, middle, and upper inter­tidal zones. The lower intertidal zone represents an area inundated by medium-high tides and is flooded more than 45 times a month. The middle intertidal is inundated by normal high tides and it is generally flooded from 20 to 45 times a month. The upper intertidal zone represents areas flooded less than 20 times a month (Robertson and Alongi 1992).

Flooded conditions can decrease soil oxygen, impact­ing root tissues that need oxygen to metabolize, and toxic substances such as sulfides can accumulate. Mangroves have evolved special morphological adaptations to cope with this lack of oxygen. First, mangroves have shallow root systems to avoid the lack of oxygen in deeper soils. As a result, most of the root biomass is found above 70-cm soil depth (Jimenez 1992). In some species (Avicennia, Laguncularia), roots form an extensive network close to the soil surface. Other species (Rhizophora) form extensive aerial roots (prop roots and drop roots) that help stabilize the tree in unconsolidated sediments (Figure 1.6). Second, above-ground root tissue such as aerial roots (Rhizophora) and pneumatophores (Avicennia, Laguncu­laria) transport oxygen from the atmosphere to the root system.

These specialized roots contain spongy tissue connected to the exterior of the root via small pores called lenticels. During low tide, when lenticels are exposed to the atmosphere, oxygen is absorbed from the air and transported to and even diffused out of the roots below ground. This diffusion of oxygen maintains an oxygenated microlayer around the roots that enhances nutrient uptake. The microlayer also avoids toxicity of compounds such as hydrogen sulfide that otherwise accumulate under such conditions.

Despite the harsh conditions under which mangrove forests develop, they can form highly diverse and productive communities. Riverine mangrove forests are rec­ognized among the most productive ecosystems in the world, due in large part to low salinities, high nutrient supply, and regular flooding (Day et al. 1987). Less ideal condi­tions, such as hypersalinity or permanent flooding, severely limit mangrove growth and productivity; extreme conditions, such as restricted hydrology due to impounding, can kill many mangroves. Growth and productivity of mangroves thus ranges widely depending on the conditions under which they grow.

Mangrove Mortality

Mangrove mortality from biological sources includes competition, disease, herbivory predation, and natural tree senescence. All developmental stages are affected, including propagules, seedlings, saplings, and trees. However, mangroves in early stages of development experience higher mortality rates and mortality is generally density-dependent. At the tree stage, smaller trees are at higher risk due to competition with larger trees for light and/or nutrients.

Mangrove diseases include impacts from fungi that defoliate and kill black and red mangroves in Australia and Florida. Insects such as scales and caterpillars cause defoliation and, in Puerto Rico, beetles and other boring insects are known to kill mangroves. Rhizophora seedlings are especially vulnerable to mortality caused by the boring beetle. Crabs are important predators of propagules and are a major source of mortality at this stage. Differences in predation rates on seedlings of different mangrove species may eventually alter species dominance in the adult trees (Smith 1987). Overall, these various biotic disturbances have a relatively minor impact on the mangrove forest when compared with larger-scale environmental impacts.

In contrast with purely biological causes, severe environmental dis­turbances can inflict larger-scale mortality on mangrove forests. These disturbances include periodic frosts, and hurricanes and other storms, which bring heavy sedimentation (Jiménez and Lugo 1984). In spite of the drastic consequences of massive tree mortality, mangrove forests are generally able to recover.

Habitat Function

Shoreline Stabilization and Protection

Located along the coastline, mangroves play a very important role in soil forma­tion, shoreline protection, and stabilization. The mangrove forest’s extensive, above-ground root structures (prop roots, drop roots, and pneumatophores) act as a sieve, reducing current velocities and shear, and enhancing sedimentation and sediment reten­tion (Carlton 1974; Augustinus 1995). The intricate matrix of fine roots within the soil also binds sediments together. Not only do mangroves trap sediments—they also produce sediment through accumulated, mangrove-derived organic matter. Mangrove leaves and roots help maintain soil elevation, which is especially important in areas of low sediment delivery, such as the southern coast of Florida. By enhancing sedimentation, sediment retention, and soil formation, mangroves stabilize soils, which reduces the risk of erosion, especially under high-energy conditions such as tropical storms.

Coastal protection is also related to the location of mangroves in the intertidal zone. Mangroves are able to absorb and reduce the impacts of the strong winds, tidal waves, and floods that accompany tropical storms, thereby protecting uplands from more severe damage (Tomlinson 1986; Mazda et al. 1997). Even though some of these forces can devastate the mangrove forest, mangroves in general have a great capacity to recover after major disturbances. Mangroves produce abundant propagules, their seedlings grow quickly, and they reach sexual maturity early—characteristics that accel­erate their natural ability to regenerate. The speed of recovery, however, depends on the type of forest affected, the nature, persistence, and recurrence of the disturbance, and the availability of propagules.

Animal Habitat and Food Source

Mangroves provide both habitat and a source of food for a diverse animal com­munity that inhabits both the forest interior and the adjacent coastal waters. Some animals depend on the mangrove environment during their entire lives while others uti­lize mangroves only during specific life stages, usually reproductive and juvenile stages (Yañez-Arancibia et al. 1988).

Mangroves’ intricate aerial root system, which is most highly developed within the lower intertidal zone, provides a substrate for colonization by algae, wood borers, and fouling organisms such as barnacles, oysters, mollusks, and sponges. From the diverse group of invertebrates found in mangroves, arthropods, crustaceans, and mollusks are among the most abundant and have a significant role in mangrove ecosystems. As men­tioned earlier, some species of crabs, recognized as propagule or seedling predators, can influence mangrove forest structure (Smith 1987), as may seedling predation by beetles or other insects. Crabs and snails, important components of the detritus food chain, help break down leaf litter through grazing.

Shrimp, an important fisheries resource, find food and shelter in mangrove forests. Likewise, commercially important bivalves such as oysters, mussels, and clams are commonly found in and around mangrove roots. Mangroves are also recognized as essential nursery habitat for a diverse community of fish, which find protection and abundant food in these environments, especially during juvenile stages.

Many animals found within mangroves are semi-aquatic or derived from terrestrial environments. Numerous insect species are found in mangrove forests; some play critical roles as mangrove pollinators, herbivores, predators, and as a food source for other animals (Hogarth 1999). Amphibians and reptiles such as frogs, snakes, lizards, and crocodiles also inhabit mangrove forests. Birds use mangroves for refuge, nesting, and feeding. In Florida and Australia, up to 200 species of birds have been reported around mangrove communities (Ewel et al. 1998). Most of these birds do not depend completely on mangroves, and use these habitats only during part of their seasonal cycles, or during particular stages of the tide. Mammals living in mangrove forests include raccoons, wild pigs, rodents, deer, monkeys, and bats. Finally, turtles, manatees, dolphins, and porpoises can be occasional visitors to mangrove-dominated estuaries.

Water Quality Improvement

Mangrove habitats maintain water quality. By trapping sediments in the man­grove root system, these and other solids are kept from offshore waters, thereby pro­tecting other coastal ecosystems such as oyster beds, seagrasses, and coral reefs from excessive sedimentation. This process can also remove agrochemical and heavy-metal pollutants from the water, since these contaminants adhere to sediment particles.

Mangroves also improve water quality by removing organic and inorganic nutri­ents from the water column. Through denitrification and soil-nutrient burial, mangroves lower nitrate and phosphorus concentrations in contaminated water, preventing down­stream and coastal eutrophication (Ewel et al. 1998). However, the potential of mangroves to “clean” water is limited and depends on the nature of the inputs, and the surface area and nutrient biochemistry of the mangrove forest.

Mangroves have also been used as a tertiary wastewater treatment (Twilley 1998). Even though this practice may increase mangrove productivity by providing nutrients, it should be conducted under carefully designed and monitored conditions. This will reduce negative impacts, such as contamination of adjacent waterways or introduction of invasive species.

Mangrove Economic Value and Uses

There are many mangrove products and services, not all of which are easily quan­tified in economic terms. Mangrove products can be obtained directly from the forest (wood) or from a derivative, such as crabs, shrimp, and fish. The most common uses of mangrove wood are as a source of fuel, either charcoal or firewood, and as the primary material for the construction of boats, houses, furniture, etc. Given these uses, commer­cial mangrove production (especially of Rhizophora spp.) is common around the world, primarily in Asia (Bandaranayake 1998).

Besides wood, other mangrove products have been exploited commercially. Mangrove bark has traditionally been used as a source of tannins, which are used as a dye and to preserve leather. The pneumatophores of different mangrove species are used in making corks and fishing floats; some are also used in perfumes and condiments. The ash of Avicennia and Rhizophora mangle is used as a soap substitute. Other mangrove extracts are used to produce synthetic fibers and cosmetics. Mangroves are also used as a source of food (mangrove-derived honey, vinegar, salt, and cooking oil) and drink (alcohol, wine). For example, the tender leaves, fruits, seeds, and seedlings of Avicennia

marina and vegetative parts of other species are traded and consumed as vegetables (Bandaranayake 1998).

Mangroves have great potential for medicinal uses. Materials from different species can treat toothache, sore throat, constipation, fungal infections, bleeding, fever, kidney stone, rheumatism, dysentery, and malaria. Mangroves also contain toxic sub­stances that have been used for their antifungal, antibacterial, and pesticidal properties (Bandaranayake 1998).

Mangrove forests have been widely recognized for their role in maintaining commercial fisheries by providing nursery habitat, refuge from predators, and food to important species of fish and shrimp. Demonstrating a statistical relationship between mangroves and fishery yields has proven difficult, however, because mangroves, seagrasses, and other nearshore habi­tats are closely linked, and all provide nursery habitat and food for fish (Pauly and Ingles 1999).Mangrove ecotourism is not yet a widely developed practice, but seems to be gaining popularity as a non-destructive alternative to other coastal economic activities. Mangroves are attractive to tourists mostly because of the fauna that inhabit these forests, especially birds and reptiles such as crocodiles.

 

 

 

Anthropogenic and Naturally Occurring Impacts

Storms and Hurricanes

Mangroves are particularly sensitive to storms and hurricanes because of their exposed location within the intertidal zone, their shallow root systems, and the non-cohesive nature of the forest soils. The effect of storms and hurricanes varies, depending on factors such as wind fields and water levels. Small storms generally kill trees by light­ning or wind-induced tree falling, creating forest gaps—an important mechanism for natural forest regeneration. Coastal sedimentation resulting from storms can also lead to mangrove forest expansion.

In contrast, high-energy storms (hurricanes and typhoons) can devastate man­grove forests. Entire mangrove populations can be destroyed, with significant long-term effects to the ecosystem (Figure 1.7; Jiménez and Lugo 1985). Mangrove forests that are frequently impacted by hurricanes show uniform tree height, reduced structural devel­opment and, sometimes, changes in species composition. However, mangrove forests can recover despite such impacts. How fast a forest recovers depends on the severity of mangrove damage and mortality, mangrove species composition, the degree of sediment disturbance and propagule availability.

Sea Level Rise

In response to global climate change, a gradual increase in sea level rise has been documented since the late Holocene (7000 YBP) and continues to the present. Estimated global rates of sea level rise (eustatic) have been estimated between 1 and 1.8 mm/yr-1 (Gornitz 1995). Local subsidence, uplift, or other geomorphological changes can cause relative sea level rise (RSLR) to be greater or less than eustatic rise. Along the Atlantic Coast of the United States, for example, an estimated RSLR of 2-4 mm/yr-1 has been calcu­lated for a period spanning the last 50 years. In contrast, some areas along the Louisiana coast are experiencing a RSLR of 10 mm/yr-1.

Changes in sea level affect all coastal ecosystems. Changes in hydrology will result as the duration and extent of flooding increases. How well mangrove ecosystems will adapt to this hydrological change will depend on the magnitude of the change and the ability of mangroves to either 1) increase mangrove sediment elevation through vertical accretion, or 2) migrate in a landward direction. The mangrove sediment surface itself is in dynamic equilibrium with sea level, since a local loss of elevation will result in faster sediment accumulation. The problem with accelerated sea level rise is that the rate of rise might be faster than the ability of mangrove forests to accumulate and sta­bilize sediments. Mangroves can migrate back into previous uplands, but only if there is enough space to accommodate the mangroves at the new intertidal level. Local eleva­tion gradients may make this regression impossible.

Mangroves colonizing macrotidal environments and receiving land-based and/or marine sediments (i.e., riverine mangroves) are generally less vulnerable to changes in sea level rise than are mangroves in microtidal environments, such as in Florida and the Yucatan, or mangroves with restricted hydrology. Land-based and marine sediments increase vertical accretion through direct deposition on mangrove soils. Nutrient and freshwater supply tend to enhance mangrove productivity, which contributes to verti­cal accretion through the production and deposition of organic matter and root growth. Mangroves under restricted hydrology depend mostly on in-situ organic matter produc­tion to attain vertical accretion. Different mangrove ecotypes will therefore have differing sensitivities to increases in RSLR.

Sedimentation

Even though mangroves colonize sedimentary environments, excessive sedi­ment deposits can damage them. Moderate sedimentation is beneficial to mangroves as a source of nutrients and to keep up with predicted increases in eustatic sea level rise. When excessive, sudden sedimentation can reduce growth or even kill mangroves. Complete burial of mangrove root structures (aerial roots, pneumatophores) interrupts gas exchange, killing root tissue and trees. For example, Avicennia trees will die after 10 cm of root burial (Ellison 1998). Seedlings are especially sensitive to excessive sedi­mentation. Under experimental conditions, Rhizophora apiculata seedlings had reduced growth and increased mortality after 8 cm of sediment burial (Terrados et al. 1997). Excessive sedimentation can result from natural phenomena such as river floods and hur­ricanes, but also from human alterations to the ecosystem. Road and dam construction, mining, and dredge spoil have buried and killed mangroves.

Mangrove Pollution

Human-caused pollution in mangrove ecosystems includes thermal pollution (hot-water outflows), heavy metals, agrochemicals, nutrient pollution (including sewage), and oil spills. Oil spill toxicity is discussed in detail in Chapter 2. Thermal pollution is not common in the tropics but, when present, reduces leaf area and causes chlorotic leaves, partial defoliation, and dwarfed seedlings. Seedlings are more sensitive than trees, show­ing 100% mortality with a water temperature rise between 7 and 9 ºC (Hogarth 1999).

Mining and industrial wastes are the main sources for heavy metal pollution (especially mercury, lead, cadmium, zinc, and copper). When heavy metals reach a man­grove environment, most are already bound onto suspended particulates (sediments) and in general do not represent an ecological threat. Although the accumulation of heavy metals in mangrove soils has not been studied in detail, they may decrease growth and respiration rates of mangroves, and will also negatively impact associated animals. Concentrations of mercury, cadmium, and zinc are toxic to invertebrate and fish larvae, and heavy metals cause physiological stress and affect crab reproduction.

Runoff from agricultural fields represents the main source of organic chemical contamination in mangrove ecosystems. Little is known about the effects of pesticides in mangroves and associated fauna, although chronic effects are likely. As with heavy metals, many of these compounds are absorbed onto sediment particles and degrade very slowly under anoxic conditions. Despite the possibility of burial, heavy metals and pesticides may bioaccumulate in animals that use mangroves (especially those closely associated with mangrove sediments), such as fish, shrimp, and mollusks.

Nutrient pollution in mangroves can have various effects. Sewage disposal under carefully managed conditions can enhance tree growth and productivity as a result of added nutrients, especially nitrogen and phosphorus (Twilley 1998). However, if the rate of disposal is greater than the uptake rate (a function of forest size and mangrove ecotype), excessively high nutrient concentrations will result. This causes excessive algal growth, which can obstruct mangrove pneumatophores and reduce oxygen exchange. Algal mats can also hinder growth of mangrove seedlings (Hogarth 1999).

Excessive microbial activity accompanies high levels of nutrients, and depletes oxygen in the water, which is harmful for mangrove-associated aquatic fauna.

Development and Forest Clearing

Despite the ecological and economic importance of mangroves, deforestation has been widespread. Deforestation has mostly been related to firewood and timber harvesting, land reclamation for human establishment, agriculture, pasture, salt produc­tion, and mariculture. Tropical countries have sustainably harvested mangrove wood for generations, but increasing populations have led to unsustainable practices. Human activities have had varying degrees of impact: a residential project in Florida destroyed approximately 24% of mangrove cover (Twilley 1998). In Ecuador, the leading exporter of farm-raised shrimp, approximately 45-63% of mangrove habitat in the El Oro River has been lost due to mariculture pond construction (Twilley 1989).

Despite laws established for mangrove protection in many different countries, unregulated exploitation and deforestation continues. In the Philippines, approximately 60% of the original mangrove area has disappeared. In Thailand, 55% of the mangrove cover has been lost over about 25 years. Eventually, the overexploitation of mangrove for­ests will degrade and, ultimately, lose habitat, increase shoreline erosion, damage fisherier, and lose services derived from these ecosystems.

Invasive Species

Mangroves have been successfully introduced in several tropical islands where they did not occur naturally, and may thus be considered an invasive species. Hawaii is an example of such a case, where the proliferation of Rhizophora mangle has deteriorated habitat for some endemic waterbirds and has damaged sensitive archaeological sites. The proliferation of mangroves has also been linked to the premature infilling of a unique Hawaiian aquatic ecosystem called anchialine ponds. Despite providing useful environ­mental services (e.g., shoreline protection, organic matter production, and water quality), the mangroves may proliferate in these foreign environments and seriously impact the native flora and fauna. The cost of their removal has been reported to vary from $108,000 to $377,000 per hectare (Allen 1998).

 

 

 

 

 

CHAPTER 2. Oil Toxicity

Key Points

• Mangroves are highly susceptible to oil exposure; oiling may kill them within a few weeks to several months.

• Lighter oils are more acutely toxic to mangroves than are heavier oils. Increased weathering generally lowers oil toxicity.

• Oil-impacted mangroves may suffer yellowed leaves, defoliation, and tree death. More subtle responses include branching of pneumatophores, germination failure, decreased canopy cover, increased rate of mutation, and increased sensitivity to other stresses.

• Response techniques that reduce oil contact with mangroves, such as chemical dis-persants, reduce the resultant toxicity as well. Tradeoffs include potential increased toxicity to adjacent communities, and increased penetration of dispersed oil to man-grove sediments.

• The amount of oil reaching the mangroves and the length of time spilled oil remains near the mangroves are key variables in determining the severity of effect.

• Mangrove-associated invertebrates and plants recover more quickly from oiling than do the mangroves themselves, because of the longer time for mangroves to reach maturity.

Introduction

In many tropical regions, mangrove forests are the defining feature of the coastal environment. Mangrove habitats represent the interface between land and sea and, as such, are one of the principal places where spilled oil and associated impacts converge. The diversity and abundance of the biological communities associated with mangroves are evident with the first visit to a healthy mangrove stand.

Observations from many spill events around the world have shown that man­groves suffer both lethal and sublethal effects from oil exposure. Past experience has also taught us that such forests are particularly difficult to protect and clean up once a spill has occurred because they are physically intricate, relatively hard to access, and inhospitable to humans. Each of these considerations contributes to the overall assess­ment that mangrove forests are a habitat at risk from oil spills. In the rankings of coastal areas in NOAA’s Environmental Sensitivity Indices, commonly used as a tool for spill con­tingency planning around the world, mangrove forests are ranked as the most sensitive of tropical habitats.

In this chapter we discuss the toxicity of oil to the broad class of trees called man­groves. In contrast to other habitats, tropical or otherwise, there is a fairly robust litera­ture on the effects of oil to mangroves. This work includes monitoring of mangrove areas oiled during actual spills, field studies of oil impacts on mangroves, and laboratory studies that attempt to control some of the variables that may otherwise complicate the inter­pretation of research results. Predictably, the body of results is not unanimous in type of impact or the severity of those documented, but there are some consistencies that can serve as the starting point for spill response guidance.

Mechanisms of Oil Toxicity to Mangroves

It is clear from spills, and field and laboratory studies, that—at least in many circumstances—oil harms or kills mangroves. What is less obvious is how that harm occurs and the mechanism of toxicity. Although there is some consensus that oil causes physical suffocation and toxicological/physiological impacts, researchers disagree as to the relative contributions of each mechanism, which may vary with type of oil and time since the spill (Proffitt et al. 1997).

One of the universal challenges faced by resource managers and spill responders when dealing with oil impacts is the fact that “oil” is a complex mixture of many kinds of chemicals. The oil spilled in one incident is almost certainly different from that spilled in another. In addition, oils within broad categories like “crude oil” or “diesel” can be vastly different, depending on the geological source of the original material, refining processes, and additives incorporated for transportation in barges or tankers. Even if we could somehow stipulate that all spilled oil was to be of a single fixed chemical formulation, petroleum products released into the environment are subjected to differential processes of weathering that immediately begin altering its original physical and chemical charac­teristics. As a result, samples of oil from exactly the same source can be very different in composition after being subjected to a differing mix of environmental influences.

Much like “oil,” the term “mangrove” is also a broadly encompassing and some­times vague category that defies strict definition (see Chapter 1). Mangroves are designed for life on the margin—literally. Because the generic term brings together many plant groups, it is easy to imagine the difficulties in forming generalities about the effects of any contaminant—much less an amorphous one like “oil.” Nevertheless, we will try to do so.

Similar to the oil toxicity situation for many other intertidal environments, the mangrove-related biological resources at risk in a spill situation can be affected in at least two principal ways: first, from physical effects; second, the true toxicological effects of the petroleum.

Many oil products are highly viscous. In particular, crude oils and heavy fuel oils can be deposited on shorelines and shoreline resources in thick, sticky layers that may either disrupt or completely prevent normal biological processes of exchange with the environment. Even if a petroleum product is not especially toxic in its own right, when oil physically covers plants and animals, they may die from suffocation, starvation, or other physical interference with normal physiological function.

Mangroves have developed a complex series of physiological mechanisms to enable them to survive in a low-oxygen, high-salinity world. A major point to remember in terms of physical effects of oil spills on mangroves is that many, if not most, of these adaptations depend on unimpeded exchange with either water or air. Pneumatophores and their lenticels tend to be located in the same portions of the intertidal most heavily impacted by stranded oil. While coatings of oil can also interfere with salt exchange, the leaves and submerged roots of the mangrove responsible for mediation of salts are often located away from the tidally influenced (and most likely to be oiled) portions of the plant.

These physical impacts of oil are linked to adaptive physiology of the mangrove plants, but are independent of any inherent chemical toxicity in the oil itself. The addi­tional impact from acute or chronic toxicity of the oil would exacerbate the influence of physical smothering. Although many studies and reviews of mangroves and oil indicate that physical mechanisms are the primary means by which oil adversely affects man­groves, other reviewers and mangrove experts discount this weighting. See, for example, Snedaker et al. (1997). They suggest that at least some species can tolerate or accommo­date exposure to moderate amounts of oil on breathing roots.

The lighter, or lower molecular weight, aromatic hydrocarbons that often are major components of oil mixtures are also known to damage the cellular membranes in subsurface roots; this, in turn, could impair salt exclusion in those mangroves that have the root filters described in Chapter 1- adaptations to salinity. Disruption of ion transport mechanisms in mangrove roots, as indicated by sodium to potassium ion ratios in leaves, was identified as the cause of oil-induced stress to mangroves in the 1973 Zoe Coloco­tronis spill in Puerto Rico (Page et al. 1985). Mangroves oiled by the 1991 Gulf War spill in Saudi Arabia showed tissue death on pneumatophores and a response by the plants in which new, branched pneumatophores grew from lenticels—an apparently compensa­tory mechanism to provide gaseous exchange (Böer 1993).

Genetic damage is a more subtle effect of oil exposure, but can cause significant impact at the population level. For example, researchers have linked the presence of polynuclear aromatic hydrocarbons (PAHs) in soil to an increased incidence of a man­grove mutation in which chlorophyll is deficient or absent (mangroves such as Rhizophora mangle are viviparous and can self-fertilize, so they are well-suited for genetic screening studies such as those examining the frequency of mutations under different conditions; Klekowski et al. 1994a, 1994b). The presence or absence of pigmentation allows for easy visual recognition of genotype in the trees. The correlation between sedi­ment PAH concentration and frequency of mutation was a strong one, raising the possi­bility that a spill can impact the genetic mix of exposed mangroves.

Acute Effects

The acute toxicity of oil to mangroves has been clearly shown in laboratory and field experiments, as well as observed after actual spills. Seedlings and saplings, in particular, are susceptible to oil exposure: in field studies with Avicennia marina, greater than 96% of seedlings exposed to a weathered crude oil died, compared to no deaths among the unoiled controls (Grant et al. 1993). Other studies found that mangrove seedlings could survive in oiled sediments up to the point where food reserves stored in propagules were exhausted, whereupon the plants died.

The Avicennia study cited above also found that fresh crude oil was more toxic than weathered crude. Based on laboratory and field oiling experiments conducted in Australia, the authors cautioned against readily extrapolating results from the laboratory to what could be expected during an actual spill. Container size and adherence of oil to container walls were thought to be important factors that may have skewed laboratory toxicity results by lowering actual exposure concentrations (Grant et al. 1993).

Another set of Australian studies investigated the toxicity of two oil types, a light crude and a Bunker C, to mature mangroves (Rhizophora stylosa) over a period of two years (Duke et al. 2000). A number of interesting results were obtained from this study, including:

• Unoiled control mortality was low over the two-year study period;

• Plots oiled with Bunker C showed no difference in mangrove mortality relative to unoiled controls;

• Mangroves treated with the light crude oil showed a significantly higher mortality

than controls and the Bunker C treatment;

• Addition of chemical dispersant to the crude significantly reduced the toxicity but not to control levels;

• Most tree deaths occurred in the first six months after treatment.

The last observation is consistent with conditions observed at several oil spills in mangrove areas. In fact, obvious signs of mangrove stress often begin occurring within the first two weeks of a spill event, and these can range from chlorosis to defoliation to tree death. In the 1999 Roosevelt Roads Naval Air Station (Puerto Rico) spill of JP-5 jet fuel, an initial damage assessment survey conducted in the first month post-spill deter­mined that 46 percent of mangrove trees, saplings, and seedlings along a transect in the most impacted basin area were stressed (defined as showing yellowed, or chlorotic, leaf color). This compared to 0 percent along the unoiled reference transect (Geo-Marine, Inc. 2000). Figure 2.1 shows the most heavily impacted area about nine months after the initial release with many of the initially stressed trees dead. Color infrared, aerial pho­tography taken at regular intervals through 19 months post-spill confirmed the visual observations. Analysis of the infrared photographs of the affected mangrove area shown.

Under more controlled conditions, studies using fresh crude oils have suggested that defoliation, when it occurs, should reach a maximum between 4-12 weeks post-spill.

A monitoring study conducted in Australia after the Era spill in 1992 found a consistent set of mangrove responses including leaf staining, chlorosis, leaf death, and complete defoliation. Within three months after the oil washed ashore, extensive defolia­tion of mangrove trees had begun and many appeared to be dead. The degree to which mangroves were damaged and the extent that they recovered from spill damage were correlated to extent of oiling (Wardrop et al. 1996).

In the 1986 Bahía las Minas (Panama) spill, scientists monitoring the effects of the oil on mangroves recorded a band of dead and dying trees where oil had washed ashore five months previously. A year and a half after the spill, dead mangroves were found along 27 km of the coast. Photographs taken just before the spill showed no evidence of tree mortality (Jackson et al. 1989).

Chronic Effects

The line between acute and chronic impacts can be a little blurry at times. In the case of mangroves, visible response to oiling may be almost immediate, with leaves curl­ing or yellowing, as at the Era and Bahía las Minas spills. The tree, however, may survive for a time only to succumb weeks or months later. Alternatively, depending on the nature of exposure, it may recover to produce new leaf growth.

At least one researcher has summarized acute and chronic effects of oil to man­groves in tabular form, reproduced below (Lewis 1983). In this case, the line between acute and chronic effect was defined at 30 days; others may shift the border one way or other.

Mangroves can be chronically impacted by oil in several ways. Stressed man­groves could show differences in growth rates or alter reproductive timing or strategy. They may also develop morphological adaptations to help them survive either the physi­cal or chemical consequences of residual oil contamination. Such modifications may require expending additional energy, which in turn, could reduce the mangroves’ ability to withstand other non-spill-related stresses they may encounter.

One consequence of the complex physical structure and habitat created by mangrove trees is that oil spilled into the environment is very difficult to clean up. The chal­lenge and cost of doing so, and the remote locations of many mangrove forests, often results in unrecovered oil in mangrove areas affected by spills. This, in turn, may expose the trees and other components of the mangrove com­munity to chronic releases of petroleum as the oil slowly leaches from the substrate, particularly where organic-rich soils are heavily oiled.

Researchers who have compared oil spill impacts at several different spill sites have found similar types of impacts that differ primarily in the magnitude of effect. The degree of impact appears to be related to the physical factors that control oil persistence on the shoreline and exposure to waves and currents. Interestingly, the presence and density of burrowing animals like crabs also affects the persistence of oil in mangrove areas and can determine whether an exposure is short- or long-term, because of oil penetration via the burrows into an otherwise impermeable sediment.

In many parts of the world, mangrove stands co-occur with industrial facilities and thus may be subjected to chronic contamination from petroleum compounds, other organic chemicals, and heavy metals. As a result, it can be difficult to determine the addi­tional stress imposed by a spill event vs. existing stress. Newer assessment tools, such as molecular biomarkers, can isolate sources of stress more readily than non-specific but commonly used methodologies, and show promise for distinguishing spill impacts from other pollution sources.

• Follow-up studies of mangroves oiled during the 1991 Gulf War spill indicated that oiled pneumatophores that survived tended to develop branched secondary pneu-matophores. These were observed two years after the spill in areas that were known to have been oiled, and were interpreted to be a response to impairment of normal respiration (Böer 1993)

• Studies of the 1986 Bahía las Minas (Galeta) oil spill in Panama concluded that its impact was “catastrophic.” Five years after the incident, researchers suggested that oil remaining in mangrove sediments adversely affected root survival, canopy condi­tion, and growth rates of mangrove seedlings in oil-deforested gaps. Six years after the spill, surviving forests fringing deforested areas showed continued deterioration of canopy leaf biomass (Burns et al. 1993).

• The follow-up study of the 1992 Era spill in Australia also noted a lack of recovery four years after the initial release—although effects themselves had appeared to have peaked, no strong signs of recovery were recorded in the affected mangrove areas (Wardrop et al. 1996).

• The experimental (i.e., intentional and controlled) 1984 TROPICS spill in Panama confirmed long-term impacts to oiled mangroves, termed “devastating” by the original researchers who returned to the study sites ten years later. They found a total mortality of nearly half of the affected trees and a significant subsidence of the underlying sediment. This was compared to a 17-percent mortality at seven months post-oiling, a level that appeared to be stable after 20 months (Dodge et al. 1995).

These results from the more intensively studied spills that have occurred in the last fifteen years suggest that chronic effects of such events can be measured over long time periods, potentially a decade or decades. They also indicate the difficulties in mea­suring longer-term impacts due to the time frames involved—and, hence, the value of longer-term monitoring of mangrove status following an oil spill.

Mangrove Community Impacts

With the realization that mangrove stands provide key habitat and nursery areas for many plants and animals in the tropical coastal environment, many researchers have included the associated biological communities in their assessments of oil impacts. Of course, this considerably broadens the scope of spill-related studies, but realistically, it would be arbitrary and artificial to consider only the impacts of oil on the mangroves themselves.

Studies of the Bahía las Minas spill in Panama concluded that significant long-term impacts occurred to mangrove communities. Both the habitat itself and the epibi­otic community changed in oiled areas. After five years, the length of shoreline fringed by mangroves had decreased in oiled areas relative to unoiled areas, and this translated to a decrease in available surface area ranging from 33 to 74 percent, depending on habitat type. In addition, defoliation increased the amount of light reaching the lower portions of the mangrove forest (Burns et al. 1993).

In the Bahía las Minas spill, a massive die-off of plants and animals attached to the mangrove roots followed the initial release. Five years after the spill, the cover of epibi­otic bivalves was reduced in oiled areas relative to unoiled reference areas. Open-coast study sites recovered more quickly, although differences in cover of sessile invertebrates remained significant through four years.

More controlled experimental oiling experiments have been less conclusive. One such study in New South Wales, Australia found that invertebrate populations were highly variable with differences attributable to oiling treatment difficult to discern. Though snails were less dense shortly after oiling treatments, they recovered by the end of the study period several months later (McGuinness 1990).

Another experiment in Australia focused on the effect of one toxic component of oil, naphthalene, on a gastropod snail common in the mangroves of eastern Australia. The sublethal endpoint used for impact assessment was the crawling rate of the snails. Two responses were elicited in short- and long-term exposures to naphthalene. An increased level of activity in the short-term exposure was interpreted as an avoidance response, while the decreased crawling rate induced by the longer-term exposure suggested a physiological consequence of the toxicant. The measurable differences in response attributed to the hydrocarbon implied that normal behavior patterns of the snails would be significantly disrupted by oil exposure (Mackey and Hodgkinson 1996).

The TROPICS experimental spill follow-up found no short- or long-term effects to three species of mangrove oysters studied in the experiment. In fact, populations at oiled sites showed the most substantial increases over time that was speculatively attributed to breakdown and mobilization of petroleum hydrocarbons as additional food sources.

One area of focus in interpreting mangrove community impacts in the context of oil spill response has been comparing the toxicity of undispersed and dispersed oil to the mangroves themselves and to the associated invertebrate community. The limited find­ings are somewhat equivocal: one study found that dispersing oil appears to reduce the inherent toxicity of the oil to mangroves, but increases the impacts to exposed inverte­brates (Lai 1986). Another assessment concluded no difference in toxicity to crustaceans from dispersed and undispersed crude oil (Duke et al. 2000). However, the same study also evaluated toxicity of Bunker C fuel oil and found that the crude oil was significantly more acutely toxic than the Bunker. The authors attributed this to the physical and chemical differences between the oil types.

The TROPICS study in Panama found a notable lack of mortality to mangrove trees at the oil/dispersant-treated site, in contrast to a measurable and seemingly increas­ing mortality at the oil-only treatment site.

Australian researchers studying the effects of the 1992 Era spill on fish popula­tions around oiled mangroves found no measurable assemblage differences between groups inside and outside oiled zones, although juveniles of several species were signifi­cantly smaller in oiled creeks than in unoiled creeks (Connolly and Jones 1996).

Indirect Impacts

As is the case with most, if not all, spill-affected resources, some indirect impacts on mangroves have been identified. For example, residual oil remaining on the surface of mangrove sediments oiled during the Gulf War spill in Saudi Arabia increased the ambi­ent soil temperatures to the point where germination and growth of intertidal plants was adversely affected (Böer 1993).

In Panama, the breakdown of protective structure provided by roots of dead mangroves caused a secondary impact from the oil spill at Bahía las Minas. For five years post-spill, the tree remnants had protected young seedlings, but when the roots finally gave way, drift logs crushed the recovering mangrove stand and essentially destroyed that part of the mangrove fringe (Duke et al. 1993).

Decomposition of the mangrove root mass following large-scale mortality causes significant erosion and even subsidence of the land where the forest was located. In the experimental TROPICS oiling, approximately 8 cm of surface elevation loss was noted by researchers who returned to the study site 10 years after the oiling (Dodge et al. 1995).

Prolonged flooding of diked mangrove areas due to cleanup operations is a pos­sible indirect spill impact that would be limited to those areas where hydrologic condi­tions are easily controlled. This was suggested as a factor in the 1999 jet fuel spill at Naval Station Roosevelt Roads in Puerto Rico. In that spill, culverts providing water exchange with coastal waters were closed both to facilitate oil recovery and to prevent the spread.

of oil to other areas. However, in doing so, the water levels in some basin mangrove for­ests were held at much higher levels (> 1 meter) than the norm for periods of more than a week. It has been suggested that this action either contributed to or was a major source of mortality to mangroves in the weeks that followed (Wilkinson et al. 2000).

Even though a sublethal exposure to oil may not kill a mangrove stand outright, several post-spill, follow-up studies have suggested that oil can significantly weaken mangroves to the point where they may succumb to other natural stresses they ordinarily would survive. Examples of these stresses include cold weather and hypersalinity (Snedaker et al. 1997).

Summary and Response Implications

The body of literature available for the toxicity of oil to mangroves presents a range of results from which we can extract some points for spill response guidance.

• Mangroves are highly susceptible to oil exposure. Acute effects of oil (mortality) occur within six months of exposure and usually within a much shorter time frame (a few weeks). Commonly observed mangrove responses to oil include yellowing of leaves, defoliation, and tree death. More subtle responses include branching of pneu­matophores, germination failure, decreased canopy cover, increased rate of mutation, and increased sensitivity to other stresses.

• Different oil types confer different toxicity effects. While this is a universal truth in spill response, for mangroves the lighter oils are more acutely toxic than heavier oils (for example, light crude oil is more toxic than a Bunker-type fuel oil). Similarly, less- weathered oil is more toxic to mangroves than the same oil that has been subjected to longer or more intense weathering.

• The physical effects of oiling (e.g., covering or blocking of specialized tissues for respiration or salt management) can be as damaging to mangroves as the inherent toxicity of the oil. Although some studies indicate that mangroves can tolerate some coating without apparent damage, many others identify physical effects of oiling as the most serious.

• Response techniques that reduce oil contact with mangroves reduce the resultant toxicity as well. For example, chemical dispersants seem to reduce oil toxicity to mangroves. In this case, the tradeoff is the possibility of increased toxicity to adjacent and associated communities, such as offshore coral reefs, and increased penetration of dispersed oil that may reach mangrove sediments.

• Comparing spill impacts at several mangrove sites indicates that variable effects are related to geomorphology and hydrologic kinetics of the mangrove ecosystem that, in turn, control whether oil persists in the mangrove habitat. Oiled mangrove forests that are sheltered from wave and current exposure are likely to be more severely affected than well-exposed, “outer fringe” mangrove areas. A physico-biological consideration that also can be significant is the density of burrows from associated organisms such as crabs, which can increase the penetration and persistence of oil with depth into sediments. Berms can protect inner areas or concentrate oil in front of them.

• Mangrove communities are complex and, as might be expected, the impacts of oil to the associated plants and animals vary. The available information suggests that, while oil spills undoubtedly affect such communities, they appear to recover more quickly than the mangroves themselves. Because of this, longer-term effects are likely to be related to death of the mangroves and loss of the habitat that supports and protects the community.

As we have noted, the toxicity implications from an oil spill in a mangrove area depend on a wide variety of different factors. Generally, the amount of oil reaching the mangroves and the length of time spilled oil remains near the mangroves are key vari­ables in determining the severity of effect. Although it is stating the obvious to a spill responder that prevention is the best tool for minimizing the environmental impacts of an incident, for mangroves this is especially true. Reducing the amount of oil reaching the mangroves not only reduces the short- and long-term toxicological effects but also reduces cleanup impacts and the potential for chronic contamination. In a response, these considerations may translate into increased protection for mangroves at risk from exposure and possible use of response measures that reduce that exposure (e.g., open-water countermeasures such as burning or dispersants, shoreline countermeasures such as chemical cleaners or flushing). The long-term character of many of the mangrove impacts that have been observed argues for serious consideration of such strategies.