PLANNING & RESPONSE CONSIDERATIONS

National Oceanic and Atmospheric Administration • NOAA Ocean Service • Office of Response and Restoration

Oil Spills in 

Mangroves

NAT OI NAL OCEANIC AND ATMOSPHERIC ADMINISTRAT OI N

U.S. DEPARTMENT OF COMMERCE

January 2002

Rebecca Hoff, Editor1

Philippe Hensel2 Edward C. Proffi tt Edward C. Proffitt 2 3 Edward C. Proffi tt and Patricia Delgado4 (Ecology)

Gary Shigenaka (Toxicity) 1

Ruth Yender (Response)1

Rebecca Hoff (Recovery and Restoration)1

Alan J. Mearns (Case Studies)1

1) Office of Response and Restoration, NOAA Ocean Service, 

National Oceanic and Atmospheric Administration, Seattle, Washington

2) Johnson Controls World Services, National Wetlands Research Center, Lafayette, Louisiana

3) U.S. Geological Survey, National Wetlands Research Center, Lafayette, Louisiana

4) University of Louisiana at Lafayette, Lafayette, Louisiana

PLANNING & RESPONSE CONSIDERATIONS

National Oceanic and Atmospheric Administration • NOAA Ocean Service • Office of Response and Restoration

Oil Spills in 

Mangroves

NAT OI NAL OCEANIC AND ATMOSPHERIC ADMINISTRAT OI N

U.S. DEPARTMENT OF COMMERCE

5

Table of Contents

Introduction

Chapters

1. Mangrove Ecology 9

2. Oil Toxicity 23

3. Response 36

4. Recovery and Restoration 48

5. Case Studies 56

Glossary 69

Figures

1.1 World map showing mangrove distribution zones 9 

1.2 Mangrove distribution in the U.S. Gulf Coast 10

1. 3 Three species of mangroves

a. Conocarpus 10

b. Laguncularia 10

c. Avicennia 11

1.4. Rhizophora trees in Florida, with propagules 12

1.5 Mangrove leaf showing salt crystals 13

1. 6 Rhizophora tree showing prop roots 14

1.7 Mangrove forest with hurricane damage in Honduras 17

2.1 Roosevelt Roads, Puerto Rico jet fuel spill 27

2.2 Oil in mangroves with dead bird 28

3.1 Oil impacts to different types of mangrove forests 37

3.2 Oil stranded in mangrove islets in Tampa Bay 38

3.3 Worker removing heavy oil by vacuuming 43

4.1 Planting Rhizophora harrisonii propagules in Ecuador propagules in Ecuador 49

5.1 Oiled crab and snail on red mangrove trunk at the Peck Slip spill 59

Tables

1.1 Common mangrove species 10

2.1 Responses of mangrove forests to oil spills 28

3.1 Recommendations for response techniques in oiled mangroves 44

4.1 Mangrove impacts and recovery at eight oil spills 51

7

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 fi lter 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 specifi c 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 

defi nes 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 specifi c 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. 
8
9

CHAPTER 1. Mangrove Ecology

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 disturbances.

• Mangroves maintain water quality by trapping sediments and taking up excess nutrients from the water.

What is a Mangrove?

Ecologically, mangroves are defi ned 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 fl ooding and salinity stress conditions imposed by the coastal environment. Therefore, mangroves are 

defi ned 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 25o

N 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 grassdominated marshes in middle and high latitudes fi ll a similar ecological niche.

The global distribution of mangroves is divided into two hemispheres: 

the Atlantic East Pacifi c and the Indo West Pacifi c . The Atlantic East Pacifi c 

has fewer species than the Indo West Pacifi c (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. 

Figure 1.1 World map showing 

mangrove distribution zones. 

Dark lines show coastal areas 

where mangroves occur (N. 

C. Duke, American Geophysical 

Union).

Mangrove - a tree or 

shrub that has evolved 

the adaptations for 

growing in the intertidal zone (specifi cally, 

adaptations to salinity 

and fl ooded conditions).
10

Virgin Islands, Hawaii, and the Pacifi c 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). 

Table 1.1 Common mangrove species with common and scientifi c names and general distribution.

Scientific name Common name Distribution

Acrostichum aureum Mangrove fern Both hemispheres

Rhizophora mangle Red mangrove Caribbean

Avicennia marina* Grey mangrove Australia

Avicennia germinans* Black mangrove Caribbean, FL,TX, LA, MS, American Pacific Coast

Laguncularia racemosa* White mangrove Caribbean, American Pacific Coast

Conocarpus erectus Button-mangrove or Buttonwood Caribbean

* shown in Fig. 1.3a, b, c.

The California Current, which limits the northern extent of mangroves along the 

Pacifi c 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 Pacifi c 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 1986). 

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 defi ne 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 fringe forest borders protected shorelines, canals, and borders protected shorelines, canals, and 

Figure 1.2 Mangrove distribution 

in the U.S. Gulf Coast (USGS).

Figure 1.3b Laguncularia

Avicennia (C.E. Proffi tt).

Figure 1.3a Conocarpus (C.E. 

Proffi tt).
11

lagoons, and is inundated by daily tides. A riverine forest fl anks the estuarine reaches of a 

river channel and is periodically fl ooded by nutrient-rich fresh and brackish water. Behind 

the fringe, interior areas of mangroves harbor basin forests, characterized by stagnant or 

slow-fl owing 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 selfpollinated (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 fl ooding 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 fl oat 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 fl ooding, 

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 diffi cult, but 

fl owering individuals have been recorded as young as 1.5 years old. Tree growth, Figure 1.3c Avicennia. 

(C.E. Proffi tt).

Hermaphroditic - 

Both sexes present in an 

individual organism. 

Vivipary – The condition in which the 

embryo (the young 

plant within the seed) 

germinates while still 

attached to the parent 

plant (synonymous with 

viviparity).

Propagule - Seedling 

growing out of a fruit; 

this process begins 

while the fruit is still 

attached to the tree. 

For some species of 

mangroves, propagules 

represent the normal, 

tidally dispersed means 

of reproduction. 
12

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 fl ooding 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 infl uence 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 fl ow. Thus, salinity can fl uctuate 

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 ultrafi lters 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 fl ooded, with the frequency and magnitude 

of fl ooding determined by local topography combined with tidal action, river fl ow, rainfall, surface runoff, groundwater, and evapotranspiration. As with salinity, hydrology in 

Figure 1.4 Rhizophora Rhizophora trees in trees in

Florida, with propagules (C. E. 

Proffi tt).

Evapotranspiration - 

The transfer of water 

from the soil, through a 

plant, and to the atmosphere through the 

combined processes of 

evaporation and transpiration. Evaporation 

is a function of surface 

area, temperature, and 

wind. Transpiration is 

a process of water loss 

through leaf stomatal 

openings, and is related 

to gas exchange and 

water transport within 

a plant. When the stomates open, a large 

pressure differential in 

water vapor across the 

leaf surfaces causes the 

loss of water from the 

leaves. 
13

mangrove ecosystems varies greatly in time and space, and mangrove species differ in 

their ability to tolerate fl ooding.

At the intertidal scale, the magnitude and frequency of fl ooding 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 fl ooded more than 45 times a month. The middle intertidal is inundated 

by normal high tides and it is generally fl ooded from 20 to 45 times a month. The 

upper intertidal zone represents areas fl ooded 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 sulfi des 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 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 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 sulfi de 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 fl ooding (Day et al. 1987). Less ideal conditions, such as hypersalinity or permanent fl ooding, 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. 

Figure 1.5 Close-up of mangrove 

leaf showing salt crystals

(C.E. Proffi tt; Gulf of Fonseca, 

Honduras).

Aerial roots- Roots 

that are formed in and 

exposed to air. In 

mangrove species (e.g., 

Rhizophora spp.), aerial 

roots develop into stilt 

roots (prop roots and 

drop roots) that anchor 

into the sediment, offering mechanical support 

and nutrient absorption. 

Pneumatophore - A 

vertical extension of an 

underground root, with 

lenticels and aerenchyma to allow for gas 

exchange. Pneumatophores are characteristic of trees adapted 

to fl ooded conditions 

(such as Avicennia spp.). 

Lenticel – A small, 

elliptical pore in the 

periderm that is a 

means of gaseous 

exchange.
14

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 densitydependent. 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 infl ict 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, aboveground 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 fi ne 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.

Figure 1.6 Rhizophora tree (with 

man in branches) showing prop 

roots (C.E. Proffi tt).

Defoliation - The 

removal of the foliar tissues of a plant, resulting from mechanical 

(e.g., hurricanes), biological (herbivore), or 

chemical agents (e.g., 

plant hormones).
15

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 fl oods 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 specifi c 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 signifi cant role in mangrove ecosystems. As 

mentioned earlier, some species of crabs, recognized as propagule or seedling predators, 

can infl uence 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 fi sheries resource, fi nd 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 fi sh, which fi nd 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 

Detritus – Non-living 

organic matter that is 

so decomposed that it 

is impossible to identify 

the original parent 

material.
16

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 denitrifi cation 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 

quantifi ed in economic terms. Mangrove products can be obtained directly from the 

forest (wood) or from a derivative, such as crabs, shrimp, and fi sh. The most common 

uses of mangrove wood are as a source of fuel, either charcoal or fi rewood, 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 fi shing fl oats; 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 fi bers 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
17

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 fi sheries by providing nursery habitat, refuge 

from predators, and food to important species of fi sh and 

shrimp. Demonstrating a statistical relationship between 

mangroves and fi shery yields has proven diffi cult, however, 

because mangroves, seagrasses, and other nearshore habitats are closely linked, and all provide nursery habitat and 

food for fi sh (Pauly and Ingles 1999).

Mangrove ecotourism is not yet a widely developed 

practice, but seems to be gaining popularity as a nondestructive 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 noncohesive nature of the forest soils. The effect of storms and hurricanes varies, depending 

on factors such as wind fi elds 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 signifi cant 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 

Figure 1.7 Aerial photo of mangrove forest showing hurricane 

damage in Guanaja, Honduras, 

taken 14 months after Hurricane 

Mitch (D.R. Cahoon and T.C. 

Michot). 
18

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 fl ooding 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 benefi cial 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. 

Eustatic sea level 

rise - The worldwide 

rise in sea level elevation due mostly to the 

thermal expansion of 

seawater and the melting of glaciers.

RSLR – relative sea 

level rise - The net 

effect of eustatic sea 

level rise and local 

geomorphoplogical 

changes in elevation. 

Local subsidence can 

make apparent RSLR 

much greater than 

eustatic rise.

Microtidal – A tidal 

range of less than one 

meter.

Deposition - The 

accumulation of material on a substrate. In 

mangrove systems this 

term is typically used 

in relation to accumulation of surface sediment. 
19

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 fl oods 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 outfl ows), 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 fi sh larvae, 

and heavy metals cause physiological stress and affect crab reproduction.

Runoff from agricultural fi elds 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 fi sh, 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). 

Chlorosis/chlorotic – 

abnormal condition 

characterized by the 

absence of green pigments in plants, causing 

yellowing of normally 

green leaves. 

Anoxic - Without free 

oxygen. Aerobic metabolism (e.g., bacterial respiration) can consume 

dissolved free oxygen 

in water and soils, 

resulting in anoxic 

conditions that are 

detrimental to oxygenbreathing organisms. 

(p.19)

Bioaccumulate – 

Uptake of dissolved 

chemicals from water 

and uptake from 

ingested food and sediment residues. (p. 19)
20

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 fi rewood 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 fi sheries, 

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 infi lling 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 fl ora and fauna. The cost of their removal has been reported to vary 

from $108,000 to $377,000 per hectare (Allen 1998).

For Further Reading

Allen, J.A. 1998. Mangrove as alien species: the case of Hawaii. Global Ecology and Biogeography Letters 7: 

61-71.

Augustinus, P. G. E. F. 1995. Geomorphology and sedimentology of mangroves. In: Perillo G.M.E. (ed.), 

Geomorphology and Sedimentology of Estuaries. Developments in Sedimentology 53. Amsterdam: Elsevier 

Science. 

Anchialine ponds – 

A rare Hawaiian ecosystem, consisting of pools 

with no surface connection to the ocean, but 

affected by tides. These 

pools have a characteristic water quality 

and support an assemblage of animals and 

plants, many of which 

are endangered.
21

Bandaranayake, W.M. 1998. Traditional and medicinal uses of mangroves. Mangroves and Salt Marshes 2:

133-148.

Carlton, J. M. 1974. Land-building and stabilization by mangroves. Environmental Conservation 1(4):285-294

Craighead, F.C. 1971. The Trees of South Florida. Coral Gables: University of Miami Press. 212 pp.

Duke, N.C. 1992. Mangrove fl oristics and biogeography. In: Robertson, A.I. and Alongi, D.M. (eds.). Coastal and 

Estuarine Studies. Tropical Mangrove Ecosystems. Washington, D.C.: American Geophysical Union. pp. 63-100.

Ewel, K.C., R.R Twilley, and J. E. Ong. 1998. Different kinds of mangrove forests provide different goods and 

services. Global Ecology and Biogeography Letters 7: 83-94.

Ellison, A. M. 2000. Mangrove restoration: Do we know enough? Restoration Ecology 8: 219-229.

Ellison, J.C. 1998. Impacts of sediment burial on mangroves. Marine Pollution Bulletin 37: 420-426.

Gornitz, V. 1995. A comparison of differences between recent and late Holocene sea-level trends from eastern 

North America and other selected regions. Journal of Coastal Research 17: 287-297.

Hartshorn G.S. 1978. Tree fall and tropical forest dynamics. In: P.B. Tomlinson. and M.H. Zimmerman (eds.). 

Tropical Trees as Living Systems. London: Cambridge University Press. pp. 617-38.

Hogarth, P.J. 1999. The Biology of Mangroves. Oxford: Oxford University Press. 228 pp.

Jimenez, J.A. and A. E. Lugo. 1985. Tree mortality in mangrove forests. Biotropica 17(3): 177-185.

Kathiresan, K. and B.L. Bingham. 2001. Biology of mangroves and mangrove ecosystems. Advances in Marine 

Biology 40:81-251.

Lowenfeld, R. and E. J. Klekowski. 1992. Mangrove genetics. I. Mating system and mutation rates of Rhizophora 

mangle in Florida and San Salvador Island, Bahamas. International Journal of Plant Science153:394-399.

Lugo, A.E., S. Brown,. and M.M. Brinson. 1990. Concepts in wetland ecology. In: A.E. Lugo, M. Brinson, and 

S. Brown (eds.). Ecosystems of the World. 15. Forested Wetlands. Amsterdam: Elsevier Science. pp. 53-85.

Lugo, A.E. and S.C. Snedaker. 1974. The ecology of mangroves. Annual Review of Ecology and Systematics 

5: 39-64.

Markley, J.L., C. McMillan, and G.A. Thompson, Jr. 1982. Latitudinal differentiation in response to chilling 

temperatures among populations of three mangroves, Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle from the western tropical Atlantic and Pacifi c Panama. Canadian Journal of Botany 60: 

2704-2715.

Mazda, Y., M. Magi, M. Kogo, and P.N. Hong. 1997. Mangroves as a coastal protection from waves in the Tong 

King delta, Vietnam. Mangroves and Salt Marshes 1: 127-135.

McKee, K. L. and P. Faulkner. 2000. Restoration of biogeochemical function in mangrove forests. Restoration 

Ecology 8: 247-259.

McMillan, C. 1971. Environmental factors affecting seedling establishment of the black mangrove on the 

central Texas Coast. Ecology 52(5): 927-930.

Mook, D. 1986. Absorption effi ciencies of the intertidal mangrove dwelling mollusk Melampus coffeus Linne

and the rocky intertidal mollusk Acanthopleura granulata Gemlin. Marine Ecology 7:105-113.

Pauly, D. and J. Ingles, 1999. The relationship between shrimp yields and intertidal vegetation (mangrove) 

areas: a reassessment. In: Yanez-Arancibia, A. and A.L. Lara-Dominguez (eds). Mangrove Ecosystems in Tropical 

America. Instituto de Ecologia, A.C. Xalapa, Mexico; UICN/ORMA Costa Rica; NOAA/NMFS Silver Spring, 

Maryland. pp. 311-316.

Proffi tt, C.E., K.M. Johns, C.B. Cochrane, D.J. Devlin, T.A. Reynolds, D.L. Payne, S. Jeppesen, D.W. Peel, and 

D.D. Linden. 1993. Field and laboratory experiments on the consumption of mangrove leaf litter by the 

macrodetritivore Melampus coffeus L. (Gastropoda: Pulmoneta). Florida Scientist 56: 211-222.

Rützler, K. and I.C. Feller. 1996. Caribbean mangrove swamps. Scientifi c American, March 1996:94-99.
22

Sherrod, C.L., D.L. Hockaday and C. McMillan. 1986. Survival of red mangrove Rhizophora mangle, on the Gulf of 

Mexico coast of Texas. Contributions in Marine Science 29:27-36.

Smith, T.J. 1992. Forest structure. In: Robertson, A.I. and Alongi, D.M. (eds), Coastal and Estuarine Studies. 

Tropical Mangrove Ecosystems. Washington, D.C. : American Geophysical Union. pp. 101-136.

Smith, T.J. III. 1987. Seed predation in relation to tree dominance and distribution in mangrove forests. 

Ecology 68: 266-273.

Terrados, J., U. Thampanya, N. Srichai, P. Kheowvongsri, O. Geertz-Hansen, S. Boromthanarath, N. Panapitukkul, 

and C.M. Duarte. 1997. The effect of increased sediment accretion on the survival and growth of Rhizophora 

apiculata seedlings. Estuarine, Coastal and Shelf Science 45: 697-701.

Tomlinson, P.B. 1986. The Botany of Mangroves. New York: Cambridge University Press. 433pp.

Turner, R.E. 1977. Intertidal vegetation and commercial yields of penaeid shrimp. Transactions of the American 

Fisheries Society 106(5):411-416.

Twilley, R.R. 1989. Impacts of shrimp mariculture practices on the ecology of coastal ecosystems in Ecuador. A 

sustainable shrimp mariculture industry for Ecuador. pp. 91-120. In: S. Olsen and L. Arriaga (eds.), International 

Coastal Resources Management Project. Technical Report Series TR-E-6. Providence: University of Rhode Island.

Twilley, R.R. 1998. Mangrove wetlands. In: M.G. Messina and W.H. Conner (eds.). Southern Forested Wetlands. 

Ecology and Management. Boca Raton, Florida: Lewis Publishers. pp. 445-473.

Yañez-Arancibia, A., A. L. Lara-Domínguez, J.L. Rojas-Galaviz, P. Sánchez-Gil, J.W. Day and C.J. Madden. 1988. 

Seasonal biomass and diversity of estuarine fi shes coupled with tropical habitat heterogeneity (southern Gulf 

of Mexico). Journal of Fisheries Biology 33 (Suppl. A):191-200. 
23

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 dispersants, reduce the resultant toxicity as well. Tradeoffs include potential increased 

toxicity to adjacent communities, and increased penetration of dispersed oil to mangrove 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 defi ning 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 fi rst 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 diffi cult 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 

Weathering - 

Changes in the physical 

and chemical properties 

of oil due to natural processes, including evaporation, emulsifi cation, 

dissolution, photo-oxidation, and biodegradation.

Canopy – topmost 

layer of leaves, twigs, 

and branches of forest 

trees or other woody 

plants. 

Sublethal effect- An 

effect that does not 

directly cause death but 

does affect behavior, 

biochemical or 

physiological functions, 

or tissue integrity. 
24

literature on the effects of oil to mangroves. This work includes monitoring of mangrove 

areas oiled during actual spills, fi eld 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 fi eld 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 (Proffi tt 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, refi ning 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 fi xed 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 infl uences.

Much like “oil,” the term “mangrove” is also a broadly encompassing and sometimes vague category that defi es strict defi nition (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 diffi culties 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: fi rst, 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 
25

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 infl uenced (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 infl uence 

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 fi lters 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 identifi ed 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 signifi cant 

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 defi cient 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.

PAH - polynuclear aromatic hydrocarbon; also 

called polycyclic aromatic hydrocarbon, a 

component of oil. PAHs 

are associated with 

demonstrated toxic 

effects. 

Genotype - Genetic 

makeup of an individual 

organism.
26

Acute Effects

The acute toxicity of oil to mangroves has been clearly shown in laboratory 

and fi eld experiments, as well as observed after actual spills. Seedlings and saplings, in 

particular, are susceptible to oil exposure: in fi eld 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 fi eld 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 signifi cantly higher mortality 

than controls and the Bunker C treatment;

• Addition of chemical dispersant to the crude signifi cantly reduced the toxicity but 

not to control levels;

• Most tree deaths occurred in the fi rst 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 fi rst 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 fi rst month post-spill determined 

that 46 percent of mangrove trees, saplings, and seedlings along a transect in the most 

impacted basin area were stressed (defi ned 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 confi rmed the visual observations. 

Analysis of the infrared photographs of the affected mangrove area shown in Figure 2.1 

Infrared photography – Photography 

using fi lms sensitive to 

both visible light and 

infrared radiation. Live 

vegetation is particularly highlighted with 

infrared fi lms and so is 

a useful tool for aerial 

surveys of live and 

dead plants. 
27

indicated that two weeks after the release, 82 percent of the total mangrove area was 

classifi ed as “impacted” relative to pre-spill conditions.

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 fi ve 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 defi ned at 30 days; others may shift the border one way 

or the other.

Figure 2.1 Aerial view of 

Roosevelt Roads, Puerto Rico jet 

fuel spill in 1999 showing dead 

mangroves (Dan L. Wilkinson, 

Geo-Marine, Inc). 
28

Table 2.1. Generalized responses of mangrove forests to oil spills. From Lewis (1983).

STAGE OBSERVED IMPACT

Acute

0 - 15 days Deaths of birds, fish, invertebrates 

15 - 30 days Defoliation and death of small (<1 m) mangroves

Loss of aerial root community 

Chronic

30 days - 1 year Defoliation and death of medium (<3 m) mangroves

Tissue damage to aerial roots

1 year – 5 years Death of larger (>3 m) mangroves

Loss of aerial roots

Regrowth of roots (sometimes deformed)

Recolonization of oiled areas by new seedlings

1 year – 10 years? Reduction in litter fall

Reduced reproduction

Reduced seedling survival

Death or reduced growth of recolonizing trees?

Increased insect damage?

10 – 50 years? Complete recovery

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 modifi cations may 

require expending additional energy, which in turn, could 

reduce the mangroves’ ability to withstand other non-spillrelated 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 diffi cult 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 shortFigure 2.2 Close up of oil in 

mangroves with dead bird (C.E. 

Proffi tt).
29

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 diffi cult 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-specifi c 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 pneumatophores. 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 

confi rmed 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 signifi cant 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 fi fteen years suggest that chronic effects of such events can be measured over 

long time periods, potentially a decade or decades. They also indicate the diffi culties 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.
30

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 artifi cial to consider only the impacts of oil on the mangroves 

themselves.

Studies of the Bahía las Minas spill in Panama concluded that signifi cant longterm impacts occurred to mangrove communities. Both the habitat itself and the epibiotic community changed in oiled areas. After fi ve 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 signifi cant 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 diffi cult 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 

signifi cantly 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 

(Dodge et al. 1995).

Endpoint- A 

measured response of 

a natural resource to 

exposure to a contaminant, such as oil, in the 

fi eld or laboratory. 
31

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 fi ndings 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 signifi cantly 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 fi sh populations around oiled mangroves found no measurable assemblage differences between 

groups inside and outside oiled zones, although juveniles of several species were signifi -

cantly 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 identifi ed. 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 fi ve years 

post-spill, the tree remnants had protected young seedlings, but when the roots fi nally 

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 

signifi cant 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 fl ooding 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 
32

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 signifi cantly 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, lessweathered 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 
33

affected than well-exposed, “outer fringe” mangrove areas. A physico-biological consideration that also can be signifi cant 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., openwater countermeasures such as burning or dispersants, shoreline countermeasures such 

as chemical cleaners or fl ushing). The long-term character of many of the mangrove 

impacts that have been observed argues for serious consideration of such strategies.

For Further Reading

Böer, B. 1993. Anomalous pneumatophores and adventitious roots of Avicennia marina (Forssk.) Vierh. 

Mangroves two years after the 1991 Gulf War oil spill in Saudi Arabia. Marine Pollution Bulletin 27:207-211.

Burns, K.A., S.D. Garrity, and S.C. Levings. 1993. How many years until mangrove ecosystems recover from 

catastrophic spills? Marine Pollution Bulletin 26(5):239-248.

Connolly, R.M. and G.K. Jones. 1996. Determining effects of an oil spill on fi sh communities in a mangroveseagrass ecosystem in southern Australia. Australasian Journal of Ecotoxicology 2:3-15.

Dodge, R.E., B.J. Baca, A.H. Knap, S.C. Snedaker, and T.D. Sleeter. 1995. The effects of oil and chemically 

dispersed oil in tropical ecosystems: 10 years of monitoring experimental sites. MSRC Technical Report Series 

95-104. Washington, D.C.: Marine Spill Response Corporation. 82 pp. + appendices.

Duke, N.C., Z.S. Pinzón, and M.C. Prada. 1993. Mangrove forests recovering from two large oil spills in Bahía 

Las Minas, Panama, in 1992. In Long-Term Assessment of the 1986 Oil Spill at Bahía Las Minas, Panama. MSRC 

Technical Report Series 93-019. Washington, D.C.: Marine Spill Response Corporation. pp. 39-87.
34

Duke, N.C., K.A. Burns, R.P.J. Swannell, O. Dalhaus, and R.J. Rupp. 2000. Dispersant use and a bioremediation 

strategy as alternate means of reducing impacts of large oil spills on mangroves: The Gladstone fi eld trials. 

Marine Pollution Bulletin 41(7-12):403-412.

Geo-Marine, Inc. 2000. Natural resource damage assessment for a JP-5 fuel spill at Naval Station Roosevelt 

Roads, Puerto Rico. Draft fi nal report. Norfolk: Commander, Atlantic Division, Naval Facilities Engineering 

Command. 58 pp.

Getter, C.D., G.I. Scott, and J. Michel. 1981. The effects of oil spills on mangrove forests: A comparison of 

fi ve oil spill sites in the Gulf of Mexico and the Caribbean Sea. In Proceedings of the International Oil Spill 

Conference, pp. 535-540.

Grant, D.L., P.J. Clarke and W.G. Allaway. 1993. The response of grey mangrove ( Grant, D.L., P.J. Clarke and W.G. Allaway. 1993. The response of grey mangrove (Avicennia marina Avicennia marina (Forsk.) Vierh.) 

seedlings to spills of crude oil. Journal of Experimental Marine Biology and Ecology 171:273-295.

Jackson, J., J. Cubit, B. Keller, V. Batista, K. Burns, H. Caffey, R. Caldwell, S. Garrity, C. Getter, C. Gonzalez, H. Guzmán, 

K. Kaufmann, A. Knap, S. Levings, M. Marshall, R. Steger, R. Thompson, and E. Weil. 1989. Ecological effects of a 

major oil spill on Panamanian coastal marine communities. Science 243:37-44.

Klekowski, E.J. Jr., J.E. Corredor, J.M. Morell, and C.A. del Castillo. 1994a. Petroleum pollution and mutation in 

mangroves. Marine Pollution Bulletin 28(3):166-169.

Klekowski, E.J. Jr., J.E. Corredor, R. Lowenfeld, E.H. Klekowski, and J.M. Morell. 1994b. Using mangroves to screen 

for mutagens in tropical marine environments. Marine Pollution Bulletin 28(6):346-350.

Lai, H.C. 1986. Effects of oil on mangrove organisms. In: Maclean, J.L., L.B. Dizon, and L.V. Hosillos (eds.). 

Proceedings of the First Asian Fisheries Forum, pp. 285-288.

Lai, H.C., H.J. Teas, F. Pannier, and J.M. Baker. 1993. Biological impacts of oil pollution: Mangroves. IPIECA 

Report Series, Volume Four. London: International Petroleum Industry Environmental Conservation Association. 20 pp.

Lewis, R.R. III. 1983. Impact of oil spills on mangrove forests. In: Tasks for Vegetation Science, Vol. 8 (Biology and 

Ecology of Mangroves), H.J. Teas, ed. The Hague: Dr W. Junk Publishers. pp. 171-183.

Mackey, A.P. and M. Hodgkinson. 1996. Assessment of the impact of naphthalene contamination on mangrove fauna using behavioral bioassays. Bulletin of Environmental Contamination and Toxicology 56:279-286.

McGuinness, K.A. 1990. Effects of oil spills on macro-invertebrates of saltmarshes and mangrove forests in 

Botany Bay, New South Wales, Australia. Journal of Experimental Marine Biology and Ecology 142:121-135.

Page, D.S., E.S. Gilfi llan, J.C. Foster, J.R. Hotham, and L. Gonzalez. 1985. Mangrove leaf tissue sodium and 

potassium ion concentrations as sublethal indicators of oil stress in mangrove trees. In Proceedings of the 1985 

International Oil Spill Conference, pp. 391-393.

Proffi tt, C.E. (ed.). 1997. Managing oil spills in mangrove ecosystems: effects, remediation, restoration, and 

modeling. OCS Study MMS 97-0003. New Orleans: U.S. Department of the Interior, Minerals Management 

Service, Gulf of Mexico OCS Region. 76 pp.

Snedaker, S. C., P. D. Biber, and R. J. Aravjo. 1997. Oil spills and mangroves: an overview. In: Proffi tt, C.E. (ed.). 

Managing oil spills in mangrove ecosystems: effects, remediation, restoration, and modeling. OCS Study MMS 

97-0003. New Orleans: U.S. Department of the Interior, Minerals Management Service, Gulf of Mexico OCS 

Region. pp. 1-18.

Wardrop, J.A., B. Wagstaff, P. Pfennig, J. Leeder, and R. Connolly. 1996. The distribution, persistence and 

effects of petroleum hydrocarbons in mangroves impacted by the “Era” oil spill (September, 1992): Final 

Phase One report. Adelaide: Offi ce of the Environment Protection Authority, South Australian Department of 

Environment and Natural Resources.
35

Wilkinson, D.L., C. Moore, M. Lopez, and M. Figueroa. 2001. Natural resource damage assessment for a JP-5 fuel 

spill at Naval Station Roosevelt Roads, Puerto Rico. Pre-fi nal report. Norfolk: Atlantic Division, Naval Facilities 

Engineering Command.
36

CHAPTER 3. Response

Key Points

• Mangroves are highly sensitive to oil and often are priority areas for protection.

• Winds and tides carry spilled oil into mangrove forests, where oil coats the soil 

surface, aerial roots, and propagules.

• Dispersing or burning oil offshore can prevent or lessen impacts to mangroves.

• Spill containment and cleanup techniques should minimize any additional impacts 

to mangroves and other natural resources at risk. 

As detailed in the previous chapter, mangroves are particularly sensitive to oil 

and, where they are native, often are priority areas for protection. The objective of spill 

response in mangroves, as in any habitat, is to minimize the damage caused by the 

accident and released oil. Spill containment and cleanup techniques should minimize 

any additional impacts to mangroves. Mangrove forests are a biogenically structured 

habitat—the trees themselves create the habitat. Death of the trees, the structuring 

organism, causes loss of habitat, with corresponding impact on the suite of associated 

species dependent upon them, including offshore resources such as coral reefs. Potential 

response strategies should be evaluated to determine whether the ultimate benefi ts from 

the response action outweigh any environmental costs to the mangrove forests and 

associated sensitive habitats at risk.

Variables such as oil type, weather, location, and availability of response equipment will determine initial spill response options. In the best-case scenario, oil is prevented from moving into and contaminating mangrove areas. Promising, on-water 

response techniques that can help prevent oil from reaching mangrove forests include 

chemical dispersion and in-situ burning.

On-Water Response Options to Prevent Mangrove Oiling

Mechanical Recovery Offshore

Mechanical containment and collection of spilled oil on water using equipment 

such as booms and skimmers are primary initial cleanup methods used at many spills. 

Experience has shown, though, that mechanical recovery alone usually cannot adequately 

deal with very large spills offshore. Weather and sea conditions, the nature of the oil, and 

other factors may limit the effectiveness of mechanical recovery. In such cases, alternative 

open-water response techniques, such as dispersant application or in-situ burning of oil 

Biogenic - In mangroves, the trees themselves create the 

habitat. Biogenic also 

means “resulting from 

the actions of living 

organisms.” 
37

on water, may signifi cantly reduce the risk that oil will 

reach shore and impact mangroves and other sensitive 

intertidal and shoreline habitats. 

Offshore Dispersant Application 

Chemical dispersants are products applied to oil 

on the water surface to enhance formation of fi ne oil 

droplets, which mix into the water column and are dispersed by currents. Most oils physically disperse naturally 

to some degree due to agitation created by wave action 

and ocean turbulence. Chemical dispersants enhance 

and speed up this natural dispersion process. Dispersing 

oil soon after release minimizes impacts to wildlife at 

the water surface (e.g., birds and marine mammals) and 

reduces the amount of fl oating oil that reaches sensitive 

nearshore and shoreline habitats. If applied appropriately 

offshore, chemical dispersants can be an effective tool 

for protecting mangrove forests and the habitat they provide. Tradeoffs among other resources at risk, such as 

potential effects of temporarily higher concentrations of 

oil in the water column on pelagic organisms and coral 

reefs, should be considered before dispersant use. When 

applied appropriately in suffi ciently deep water, impacts 

to corals are expected to be minimal. 

Offshore In-situ Burning

In-situ burning is a response technique in which 

spilled oil is burned in-place. When used appropriately, 

in-situ burning can remove large quantities of oil quickly 

and effi ciently with minimal logistical support. Like dispersants, in-situ burning can help minimize impacts to 

wildlife at the water surface and reduce the amount of oil 

that reaches sensitive nearshore and shoreline habitats, 

including mangroves. A potential disadvantage of open-water in-situ burning is that a 

small percentage of the original oil volume may remain as a taffy-like residue after the 

burn. Floating residue can be collected but residues that sink or escape collection and 

move inshore could potentially contaminate mangroves. 

Figure 3.1 Schematic showing 

possible impacts to different types 

of mangrove forests from oiling 

(Research Planning Inc.).
38

It is important to note that, in contrast to open-water burning, in-situ burning 

should not be conducted within mangrove forests, as explained below under “Response 

Techniques Inappropriate for Mangroves.”

Oil Behavior in Mangroves

Mangroves grow in low-energy depositional areas, which also tend to be the sites 

where oil accumulates (Figure 3.1). Spilled oil is carried into mangrove forests by winds 

and tidal currents. Oil slicks generally move into mangrove forests when the tide is high, 

depositing on the soil surface and on aerial roots and propagules when the tide recedes. 

The resulting distribution of deposited oil is typically patchy due to the variability in 

tidal heights within the forest. If there is a berm or shoreline, oil tends to concentrate 

and penetrate into the berm or accumulated detrital wrack. The oil can penetrate into 

the soil, particularly through crustacean burrows and other voids like those formed by 

dead mangrove roots. Lighter oils tend to penetrate more deeply into mangrove forests 

than heavier and more weathered oils, but will not persist unless they mix into the soil. 

However, crude oils and heavier refi ned products can pool onto sediment surfaces and 

are highly persistent. These heavy oils and emulsifi ed oil can be trapped in thickets of 

red mangrove prop roots and black mangrove pneumatophores and are likely to adhere 

to and coat these surfaces, as well as other organic materials, such as seagrass wrack. 

Re-oiling from resuspended oil, particularly as tides rise and fall, may further injure plants 

over time. Where oil persists, sheens may be generated for months or years (Figure 3.2).

Assessing the extent and distribution of stranded oil can be diffi cult, 

particularly in dense forests, because the forest interior sometimes can be oiled 

even if the mangrove fringe is not, due to its lower 

tidal height. Access to interior areas of forests usually 

must be limited in order to minimize damage. Also, 

the tree canopy may hide oil on the ground during oilobservation overfl ights. Affected areas may become more 

apparent from the air as trees die or defoliate. Oiled trees 

may start to show evidence of effects, such as leaf-yellowing, 

within weeks after oiling. Trees may take months to die, 

especially with heavy oils.

Cleanup of oiled interior mangroves can be particularly diffi cult because some mangrove forests are nearly 

impenetrable. Intrusive cleanup operations may signifi -

cantly damage roots and seedlings, and also trample oil 

deeper into sediments, where it is slower to break down. Consequently, access to interior 

areas of mangrove forests should be limited and highly supervised. During later, lesssupervised stages of mangrove cleanup on Eleanor Island at the 1993 Bouchard B-155

Figure 3.2 Oil stranded in 

and around mangrove islets in 

Tampa Bay (Bouchard Barge 

B-155 spill, 1993; NOAA OR&R).

Wrack – Organic 

material, usually from 

dead seagrass or algae 

that wash up on shorelines.
39

Bunker oil spill in Tampa, Florida, cleanup workers reportedly spread oil from the mangrove fringe to the roots of previously unoiled mangrove plants in the mangrove interior 

as they moved back and forth removing surface sediment contamination. In spills of 

relatively fresh, lighter oil, such as diesel or crude, sediment penetration and toxic damage 

can occur very rapidly and the oil can break down relatively quickly. In such cases, 

cleanup operations are not expected to save many mangrove trees or effectively remove 

much oil, and any benefi ts are probably outweighed by the potential additional damage 

from access for cleanup. 

Natural processes will eventually remove remaining oil. Tidal action and precipitation can help physically fl ush stranded oil out of contaminated mangrove areas. 

Weathering processes degrade the oil, gradually reducing quantity and toxicity. Oiled 

substrate may not be able to support mangrove growth while toxicity levels remain high. 

Oil can degrade quickly in warm tropical environments, but more slowly if degradation is 

inhibited by anaerobic soil conditions. Oil may persist for very long periods in the peaty 

or muddy sediment where mangroves are most often found. Heavier oils can persist in 

mangrove sediment for decades after a spill. 

Cleanup Options for Oiled Mangroves

If mangrove forest shorelines are oiled, extreme caution must be exercised in 

selecting cleanup activities. Potential benefi ts of oil removal must be weighed against 

the risks of potential additional harmful impacts from the cleanup technique.

No Action/Natural Recovery

There are several circumstances under which it is appropriate to do nothing. 

The foremost of these situations is when cleanup would cause more harm than benefi t 

to mangroves or other associated habitats, or when shorelines are inaccessible. When 

no cleanup is conducted, oil will slowly degrade and be removed naturally, assisted by 

natural and storm-generated fl ushing. (See Era spill case study, Chapter 5.)

Spills of light oils, which will naturally evaporate and break down very rapidly, do 

not require cleanup. Such light oils are usually gone within days. Furthermore, light fuel 

oils such as gasoline and jet fuels typically impart their toxic impacts immediately, and 

cleanup can do little to reduce the damage. The only light refi ned product that might 

warrant some cleanup is diesel (No. 2 fuel oil) if sediment could be contaminated. It is 

important to recognize, though, that even where no cleanup is advisable, light oils can 

cause signifi cant injury and contaminated mangrove habitats may require many years 

to recover. 

Cleanup also is not recommended for small accumulations of oil, regardless of 

product type. Impacts caused by light accumulations generally do not warrant the tradAnaerobic – Occurring with little or no 

oxygen. 
40

eoffs associated with cleanup activity. Even for major spills, there may be cases for which 

it is best to take no action, depending on the nature of the oiling and the characteristics 

of the mangrove forest affected. Generally, cleanup should not be conducted in interior 

areas of mangrove forests because of the risk of damaging mangrove roots and seedlings, 

trampling oil into the sediment where it will degrade much more slowly, and spreading 

oil into previously unoiled areas. Exceptions may be made if access is possible from 

upland areas or if vegetation is sparse enough to permit access without injury to pneumatophores and prop roots. If cleanup is attempted in interior mangroves, experienced 

personnel must constantly oversee cleanup crews to prevent further injury. 

In any case, attempts should be made to control the movement and spread of 

any mobile oil within the mangroves to prevent contamination of adjacent areas. Several 

response techniques described below, including barriers, passive collection, and fl ushing 

can be used to help control and contain mobile oil. 

Barrier Methods

Several forms of barriers can defl ect or contain oil, including booms, sediment 

berms, dams, and fi lter fences. Barriers can be used along mangrove shorelines and inlets 

to prevent oil entry. Proper strategic boom deployment in sheltered lagoon areas may 

be highly effective in trapping large quantities of mobile oil and reducing oil impact to 

interior mangroves. To be effective, barriers must be deployed immediately after a spill 

before oil moves into mangrove areas. This means that appropriate types and suffi cient 

amounts of barrier materials must be stockpiled and available at the time of the spill, 

and that strategies for boom placement and deployment have already been established 

and tested. 

Because of the soft substrate and sensitivity of prop roots and pneumatophores, 

barrier methods should be deployed carefully and maintained vigilantly to prevent physical damage during installation and removal. Untended boom that breaks loose can 

become entangled in the mangrove fringe, breaking off pneumatophores, prop roots, and 

juvenile plants. Boom deployed under inappropriate conditions or improperly deployed 

can cause additional harm, so caution must be exercised in planning where, when, and 

how boom will be used. 

There are some shorelines where barriers will be ineffective due to physical characteristics, such as current strength and water depth. Where barrier methods are not an 

option, mangrove forests will remain vulnerable to contamination. For example, booms 

generally cannot be deployed successfully along mangrove shorelines with strong currents or along sections of mangrove shorelines behind shallow fl ats. Also, boom usually is 

not effective with light oils because they can readily mix into the water column and pass 

under fl oating boom. Heavier oils are more likely to remain at the water surface and so 
41

are more easily controlled with booms, although very heavy oils can sometimes become 

negatively buoyant and pass under boom.

Manual Oil Removal

Manual removal, using hand tools and manual labor, is often conducted to 

remove bulk oiling by heavier oils, such as crude oil or Bunker C oil, stranded in mangroves. Manual removal can help prevent other areas from becoming contaminated as 

the oil moves around, and helps limit long-term sediment contamination. Consideration 

should be given, however, to the trade-off between these benefi ts of manual removal 

and the mechanical damage to the mangroves that often accompanies manual cleanup. 

It is nearly impossible to reach the tangle of prop roots and pneumatophores of most 

mangroves without causing physical damage. Trampling of oil deeper into the sediment 

from foot traffi c can be another harmful consequence of manual cleanup. Garrity and 

Levings (1996) observed that black mangrove pneumatophores along paths used by 

cleanup workers were signifi cantly more likely to be killed than those in areas accessed 

by one or a few workers. Where pneumatophores had been dense at the time of the spill, 

paths often were bare substrate by 15 months post-spill as broken pneumatophores died 

and rotted away. (See Bahía las Minas case study.)

If manual removal is conducted in mangroves, and particularly in interior areas, 

consideration should be given to ways to minimize foot traffi c and other impacts. Conducting activities from boats, when possible, is advisable. Close supervision of cleanup 

crews is essential. 

Passive Collection with Sorbents

Sorbent boom or other sorbent materials can be placed at the fringe of oiled 

mangrove forests to passively recover any mobile oil, including sheens. Sorbents are 

oleophilic and either absorb or adsorb oil. They can be composed of either synthetic 

or natural materials, and they come in a variety of forms, including sausage boom, “pompom” or snare boom, sheets, rolls, pellets, and loose particulates. Sorbents vary in their 

effectiveness depending upon oil type, degree of oil weathering, and sorbent absorption 

or adsorption capacity. Sorbent materials must be placed and removed carefully to 

minimize disturbance of sediments and injury to mangrove roots. Sorbent materials 

must be closely monitored to ensure they do not move and damage mangrove roots, and 

must be removed when they become saturated or are no longer needed. 

Sorbents have been used to wipe heavy oil coating from mangrove surfaces. 

Before using sorbents in this way, consideration should be given to associated physical 

damage. This activity is best conducted under close supervision and only in areas where 

substrate is fi rm enough to prevent oil mixing into it.
42

Vacuuming 

Vacuuming can remove pooled oil or thick oil accumulations from the sediment 

surface, depressions, and channels. Vacuum equipment ranges from small units to large 

suction devices mounted on dredges, usually used outside vegetated areas. Generally, 

vacuuming should be conducted only at the outer fringe of mangrove forests; it is most 

feasible and least damaging where vegetation is not very dense, enabling easy access. 

Vacuuming can be used effectively on heavier and medium oils, providing they are still 

reasonably fl uid. Lighter, more fl ammable petroleum products such as jet fuel and diesel 

generally should not be vacuumed. 

As shown in Figure 3.3, vacuuming was used effectively to remove thick mats of 

Bunker C oil that stranded in mangroves during the 1993 Tampa Bay oil spill response (see 

Case Studies for more details). Vacuuming worked particularly well where oil stranded 

on sand substrate at the mangrove fringe. The technique was less effective over fi ne 

sediment and oyster beds. In order to minimize cleanup damage, care was taken to place 

the vacuum barge over fi rm sand substrate, where there were no seagrass beds. 

Ambient Water Flooding (Deluge) and Low-Pressure Ambient Water Flushing

Low-pressure fl ushing with ambient seawater can wash fl uid, loosely adhered oil 

from the sediment surface and mangrove vegetation into areas where it can be collected, 

as long as it can be done without resulting in signifi cant physical disturbance of the sediment. Generally, fl ushing is most feasible at the outer fringe, but can sometimes be used 

to remove oil trapped within the mangrove forest. Flushing at water levels high enough 

to submerge sediments may help minimize impact to the substrate. If substrate mixing is 

likely or unavoidable, responders should allow the oil to weather naturally. Flushing is not 

effective with heavy oils, such as Bunker C, or highly weathered oils. Oil should be fl ushed 

only during ebbing tides to move it out where it can be collected. 

Flushing can be a useful technique to help control the movement and spread of 

mobile oil in mangrove areas to prevent contamination of adjacent areas. When fl ushing 

free-fl oating oil, care should be taken to minimize emulsifi cation. 

Chemical Shoreline Cleaners

Chemical shoreline cleaners are products sprayed on oil-coated surfaces to 

“loosen” the oil so that it can be fl ushed off with ambient water. Tidal waters or water 

sprays alone cannot effectively wash away heavy oil. Shoreline cleaning products vary 

in their toxicity and recoverability of the treated, mobilized oil. Chemical shoreline cleaners loosen or dissolve heavy oil deposited over the lenticels on coated prop roots or 

pneumatophores so the residue can be washed away and lenticel functioning restored. 

Functioning of the lenticels, which enable delivery of oxygen to the subsurface roots, is 

critical to survival of the trees. 
43

Some experimental studies (Teas et al. 1987, 1993) have reported promising 

results using chemical shoreline cleaners on mangrove trees coated with oil. A shoreline 

cleaner (Corexit 9580) applied to oiled red mangroves coated with Bunker C oil and 

then washed with seawater (within 7 days of oiling) reportedly effectively reduced oil 

adhesion and exposed the lenticels, restoring their air permeability. The study concluded 

that mangrove trees can be saved with shoreline cleaners if the interval between oiling 

and cleaning is no longer than about a week. Another 

study (Quilici et al. 1995) reported harmful effects on 

mangrove trees treated with shoreline cleaner without 

fl ushing. Results likely depend on the particular product 

used and application technique. Further testing and 

more experience with the effectiveness and effects of 

using shoreline cleaners on mangroves are needed to 

determine whether their use is advisable. 

Nutrient Addition/Bioremediation

Nutrient addition can enhance biodegradation 

of oil under nutrient-limited conditions. Microbes and 

essential nutrients for oil degradation generally are not 

limited in mangrove habitats, so nutrient enrichment 

may not offer much benefi t. Studies conducted by Teas et al. (1991) and Quilici et al. 

(1995) concluded that adding fertilizer does not signifi cantly enhance biodegradation 

of oil in mangrove sediment. Another study (Scherrer and Mille 1989) reported that 

oleophilic fertilizer enhanced the oil biodegradation process in peaty mangrove sediment, though the fertilizer in this experiment was added to the oil before the mangrove 

vegetation was contaminated. In any case, applied nutrients would be diffi cult to keep 

in place as tides fl ood through mangrove forests. There is also some risk that nutrient 

application might cause localized eutrophication and acute toxicity, particularly from 

ammonia, due to low mixing rates and shallow waters. 

Burns et al. (1999) concluded that aeration of contaminated sediments may be 

effective in enhancing biodegradation of oil in mangrove sediments, since mangrove 

sediments are usually anaerobic below surface layers. The researchers suggest a bioremediation strategy that employs selective aeration to promote the survival of the trees 

vital to maintaining the structural integrity of the mangrove forest. The trees also provide 

the habitat necessary for the return of burrowing animals to impacted sediments. 

Burns et al. (1999) point out that aeration is not necessarily a strategy to be used over 

large areas. Reports on trial experiments to test this strategy are not yet available. More 

testing of this potential response technique is needed. 

Figure 3.3 Cleanup worker 

removing heavy oil by vacuuming among mangrove prop roots 

in Tampa Bay during 1993 spill 

(NOAA OR&R).
44

Removal of Oiled Wrack and Debris

Heavily oiled wrack and debris should be removed if it can be done without 

signifi cantly damaging prop roots, pneumatophores, and seedlings or trampling oil into 

the sediment. However, oiled wrack should not be removed until the threat of oiling has 

passed, since wrack and leaf litter can act as a sort of natural barrier sorbent and actually 

protect the trees from direct oil contact. Unoiled and lightly oiled wrack and leaf litter 

should not be removed because they provide habitat and contribute to the ecosystem.

Table 3.1 Chart summarizing recommendations for various response techniques in oiled mangrove forests. (From Characteristic Coastal Habitats: 

Choosing Spill Response Alternatives Choosing Spill Response Alternatives, NOAA OR&R 2000.) , NOAA OR&R 2000.)

Response Techniques Inappropriate for Mangroves

Under no circumstances should live mangrove vegetation be cut or burned. Both 

techniques will destroy trees and mangrove habitat. Mangrove trees are slow-growing 

and take decades to be replaced by mature vegetation. The loss of a large number 

of trees may compromise the forest structure, making it unlikely to recover naturally. 

Other cleanup techniques used at some oil spills but inappropriate in mangroves include 

mechanical oil removal, high-pressure or hot-water fl ushing, steam-cleaning, slurry sand 

blasting, trenching, and sediment reworking, tilling, or removal. All these methods would 

severely damage or destroy mangrove forests and associated organisms and habitats. 

Techniques such as pressure washing and sand blasting risk causing severe erosion. 
45

For Further Reading

Allen, A. A. and R.J. Ferek. 1993. Advantages and disadvantages of burning spilled oil. In: Proceedings of the 

1993 International Oil Spill Conference, pp. 765-772. 

Ballou, T.G., S.C. Hess, R.E. Dodge, A.H. Knap, and T.D. Sleeter. 1989. Effects of untreated and chemically 

dispersed oil on tropical marine communities: A long-term fi eld experiment. 1989. In: Proceedings of the 1989 

International Oil Spill Conference, pp. 447-454.

Burns, K.A., S.D. Garrity and S.C. Levings. 1993. How many years until mangrove ecosystems recover from 

catastrophic oil spills? Marine Pollution Bulletin 26(5): 239-248. 

Burns, K.A, S.D. Garrity, D. Jorrisen, J. MacPherson, M. Stoelting, J, Tierney and L. Yelle-Simmons. 1994. The 

Galeta oil spill. II. Unexpected persistence of oil trapped in mangrove sediments. Estuarine, Coastal and 

Shelf Science 38:349-364.

Burns, K.A., S. Codi, C. Pratt, R.J.P Swannell, and N.C. Duke. 1999. Assessing the oil degradation potential of 

endogenous micro-organisms in tropical marine wetlands. Mangroves and Salt Marshes 3: 67-83.

Burns, K.A., S. Codi, C. Pratt, and N.C. Duke. 1999. Weathering of hydrocarbons in mangrove sediments: testing 

the effects of using dispersants to treat oil spills. Organic Geochemistry 30: 1273-1286. 

Chaw, LH, H.J. Teas, F. Pannier, and J.M. Baker. 1993. Biological impacts of oil pollution: mangroves. 

International Petroleum Industry Environmental Conservation Association (IPIECA) Report Series Volume 4. 

20 pp.

Duke, N.C., Burns, K.A, S. Codi, O. Dalhaus, J.C. Ellison, C. Pratt and R.J. Rupp. 1999. Fate and effects of oil 

and dispersed oil on mangrove ecosystems in Australia. Final Report to the Australian Petroleum Production 

Exploration Association. June 12, 1999. Queensland: Australian Institute of Marine Science and CRC Reef 

Research Centre. 

Cintron-Molero. 1992. Restoring mangrove ecosystems. In G. Thayer (ed.), Restoring The Nation’s Marine 

Environment Environment, College Park, Maryland: Maryland Sea Grant College. pp. 223-277. , College Park, Maryland: Maryland Sea Grant College. pp. 223-277.

Corredor, J.E., J.M. Morell, and C.E. Del Castillo. 1990. Persistence of spilled crude oil in a tropical intertidal 

environment. Marine Pollution Bulletin 2(8):385-388.

Daykin, M., G. Sergy, D. Aurand, G. Shigenaka, Z. Wang, and A. Tang. Aquatic toxicity resulting from in-situ

burning of oil-on-water. In: Proceedings of the Seventeenth Arctic and Marine Oil Spill Program (AMOP) Technical 

Seminar 2:1165-1193.

Fingas, M.F., F. Ackerman, K. Li, P. Lambert, Z. Wang, M.C. Bissonnette, P.R. Campagna, P. Boileau, N. Laroche, P. 

Jokuty, R. Nelson, R. D. Turpin, M.J. Trespalacios, G. Halley, J. Belanger, J. Pare, N. Vanderkooy, E.J. Tennyson, D. 

Aurand, and R. Hiltabrand. 1994a. The Newfoundland Offshore Burn Experiment - NOBE: Preliminary results 

of emissions measurement. In: Proceedings of the Seventeenth Arctic and Marine Oil Spill Program (AMOP) 

Technical Seminar 2:1099-1164. 

Fingas, M.F., G. Halley, F. Ackerman, N. Vanderkooy, R. Nelson, M.C. Bissonnette, N. Laroche, P. Lambert, P. Jokuty, 

K. Li, G. Halley, G. Warbanski, P.R. Campagna, R. D. Turpin, M.J. Trespalacios, D. Dickens, E.J. Tennyson, D. Aurand, 

and R. Hiltabrand. 1994b. The Newfoundland Offshore Burn Experiment - NOBE: Experimental design and 

overview. In Proceedings of the Seventeenth Arctic and Marine Oil Spill Program (AMOP) Technical Seminar

2:1053-1163.

Garrity, S.D. and S.C. Levings. 1993. Chronic oiling and long-term effects of the 1986 Galeta spill on fringing 

mangroves. In: Proceedings of the 1993 International Oil Spill Conference, pp. 319-324.

Garrity, S.D. and S.C. Levings. 1993. Effects of an oil spill on some organisms living on mangrove (Rhizophora 

mangle L.) roots in low wave-energy habitats in Caribbean Panama. Marine Environmental Research 35: 

251-271.
46

Garrity, S.D. and S.C. Levings. 1994. The 10 August 1993 Tampa Bay oil spill: Injury assessment for the 

mangrove keys inside John’s Pass. Final Report, Findings through June 1994. 140 pp. Silver Spring, Maryland: 

Damage Assessment Center, National Oceanic and Atmospheric Administration.

Garrity, S.D., S.C. Levings, and K.A. Burns. 1994. The Galeta oil spill. I. long-term effects on the physical structure 

of the mangrove fringe. Estuarine, Coastal and Shelf Science 38: 327-348. 

Getter, C.D., T.G. Ballou, and C.B. Koons. 1985. Effects of dispersed oil on mangroves: synthesis of a seven-year 

study. Marine Pollution Bulletin 16:318-324.

Getter, C.D., T.G. Ballou, and J.A. Dahlin. 1983. Preliminary results of laboratory testing of oil and dispersants on 

mangroves. In: Proceedings of the 1983 International Oil Spill Conference, pp. 533-538.

Ibáñez, M. 1995. Mangrove restoration: Cartagena, Colombia, coastal oil spill case study. In: Proceedings of the 

1995 International Oil Spill Conference, pp. 990-991.

Jackson, J.B., J.D. Cubit, B.D. Keller, V. Batista, K. Burns, M. Caffey, R.L. Caldwell, S.D. Garrity, D.C. Getter, C. Gonzalez, 

H.M Guzmman, K.W. Kaufmann, A.H. Knap, S.C. Levings, M.J. Marshall, R. Steger, R.C. Thompson, and E. Weil. 1989. 

Ecological effects of a major oil spill on Panamanian coastal marine communities. Science 243: 37-44.

Levings, S.C., S.D. Garrity, and K.A. Burns. 1994. The Galeta Oil Spill. III. Chronic reoiling, long-term toxicity 

of hydrocarbon residues and effects on epibiota in the mangrove fringe. Estuarine, Coastal and Shelf Science

38:365-395. 

Levings, S.C. and S.D. Garrity. 1994. Effects of oil spills on fringing red mangroves (Rhizophora Mangle): losses 

of mobile species associated with submerged prop roots. Bulletin of Marine Science 54: 782-794. 

Levings, S.C. and S.D. Garrity. 1995. Oiling of mangrove keys in the 1993 Tampa Bay oil spill. In: Proceedings of 

the 1995 International Oil Spill Conference, pp. 421-428.

Levings, S.C. and S.D. Garrity. 1996. The 10 August 1993 Tampa Bay oil spill: Injury assessment for the 

mangrove keys inside John’s Pass: Final Report, Findings through January 1996. Silver Spring, Maryland: 

Damage Assessment Center, National Oceanic and Atmospheric Administration. 193 pp. 

NOAA. 2000. Characteristic coastal habitats: choosing spill response alternatives. Seattle: Offi ce of Response 

and Restoration, National Oceanic and Atmospheric Administration. 87 pp.

Proffi tt, C.E., D.J. Devlin, and M. Lindsey. 1995. Effects of oil on mangrove seedlings grown under different 

environmental conditions. Marine Pollution Bulletin 30(12):788-793.

Proffi tt, E and P.F. Roscigno (eds.). 1996. Symposium Proceedings: Gulf of Mexico and Caribbean Oil Spills in 

Coastal Ecosystems: Assessing Effects, Natural Recovery, and Progress in remediation Research. OCS Study/MMS 

95-0063. New Orleans: U.S. Department of Interior, Minerals Management Service, Gulf of Mexico OCS Region. 

245 pp. 

Proffi tt, E. (ed.). 1997. Managing oil spills in mangrove ecosystems: effects, remediation, restoration, and 

modeling. OCS Study/MMS 97-0003. New Orleans: U.S. Department of Interior, Minerals Management Service, 

Gulf of Mexico OCS Region. 76 pp. 

Quilici, A., C. Infante, J. Rodriguez-Grau, J.A. La Schiazza, H. Briceno, and N. Pereira. 1995. Mitigation strategies at 

an estuarine mangrove area affected by an oil spill. In: Proceedings of the 1995 International Oil Spill Conference, 

pp. 429-433.

Rycroft, R.J., P. Matthiessen, and J.E. Portmann. 1994. MAFF Review of the UK Oil Dispersant and Approval 

Scheme. Burnham-on-Crouch, Essex, United Kingdom: Ministry of Agriculture, Fisheries and Food Directorate 

of Fisheries Research. pp. 14-18. 

Scherrer, P. and G. Mille. 1989. Biodegradation of crude oil in an experimentally polluted peaty mangrove soil. 

Marine Pollution Bulletin 20:430-432. 
47

Scientifi c and Environmental Associates, Inc. 1995. Workshop Proceedings: The Use of Chemical Countermeasures Product Data for Oil Spill Planning and Response, Volume I, April 4-6, 1995. Leesburg, Virginia: SEA. 

83 pp. 

Teas, H.J., E.O. Duerr, and J.R. Wilcox. 1987. Effects of South Louisiana crude oil and dispersants on Rhizophora

mangroves. Marine Pollution Bulletin 18:122-124.

Teas, H.J., M.E. De Diego, E. Luque, and A. H. Lasday. 1991. Upland soil and fertilizer in Rhizophora mangrove 

growth on oiled soil. In: Proceedings of the 1991 International Oil Spill Conference, pp. 477-481.

Teas, H.K, R.R. Lessard, G.P. Canevari, C.D. Brown, and R. Glenn. 1993. Saving oiled mangroves using a new 

non-dispersing shoreline cleaner. In: Proceedings of the 1993 International Oil Spill Conference, pp.147-151.
48

CHAPTER 4. Mangrove Recovery and Restoration

Key Points

• Mangroves can take more than 30 years to recover from severe oil spill impacts.

• Adequate tidal exchange is critical to restoration success.

• Mangrove seedling and tree density and health are the only widely measured recovery indicators at many spills.

• Restoration that works with natural recovery processes to reestablish mangrove habitat is the best course of action over the long term.

Mangrove ecosystems around the world suffer degradation from logging, coastal 

development, spraying of herbicides, conversion to fi sh ponds, and from oil spills and 

other pollutants. The continued loss of mangrove forests worldwide underscores the 

importance of projects focusing on restoration of forest structure and functions.

Since mangroves take 20–30+ years to recover from severe oil spill impacts, 

restoration projects attempt to speed up this recovery process. Adequate tidal exchange 

is most critical to restoration success. Mangrove restoration projects in Florida and the 

Caribbean often involve re-establishing natural hydrologic and tidal regimes, planting 

mangrove propagules, and/or planting marsh plants to provide a “nurse” habitat that can 

be colonized more easily than bare areas by mangrove trees. 

An oil spill alone rarely changes the basic geophysical appearance and shape 

of the mangrove ecosystem; this is left for hurricanes, clear-cutting, and development. 

For this reason, restoration after an oil spill may be easier than after an event that substantially changed tidal elevation or hydrology or decimated mangrove trees. However, an 

oil spill may come as an additional impact on a mangrove ecosystem already degraded 

by human and industrial development, such as near refi neries (Bahía las Minas), ports, or 

airfi elds (Roosevelt Roads). Cumulative or chronic impacts may decrease the resiliency of 

the mangrove ecosystem and increase the time it takes the system to recover or make it 

more diffi cult for the system to recover at all. 

As with other marsh ecosystems adversely impacted by oil spills, we have learned 

valuable lessons from past mangrove restoration projects, including those that failed. 

Restoration projects need a clear goal from the outset that is based on understanding the 

mangrove ecosystem’s natural ability to recover. The most effective role for restoration 

projects is to correct or assist when natural recruitment mechanisms are impeded or no 

longer functioning.
49

Recovery

Recovery of any impacted ecosystem following a perturbation such as an oil 

spill is interpreted by many to mean a return to the system in place at the time of the 

spill. Mangroves’ specialized niche is in a unique, changeable zone, subject to sediment 

fl ow that accretes and erodes, varying amounts of fresh water, impacts from storms and 

hurricanes, invasion by foreign species, and predation. Thus, even if we had a precise 

description of ecosystem conditions just before the spill, we still might not be able to 

return it to its pre-spill state. 

A more practical way to measure recovery is to compare the impacted system 

with an unimpacted one (hopefully, nearby), using metrics such as tree height, density, 

canopy cover, above-ground biomass, and abundance and diversity of associated invertebrates, fi sh, and plants. Since compromised ecosystems can be more vulnerable to 

stresses such as disease or predation, the recovering habitat must also show the resilience 

of a functioning ecosystem.

Sadly, it is rare to fi nd long-term, follow-up studies on mangroves beyond 1-2 

years post-spill. It is even rarer to fi nd studies that measure associated communities 

of invertebrates or other components of the mangal (mangrove forest habitat) besides 

the mangrove trees themselves. Even when mangrove trees appear to have recovered, 

restored mangal may differ from unimpacted mangal in its functioning and ecosystem 

complexity. Even with its limitations, mangrove tree density and health are the only 

widely measured recovery indicators at many spills, so we are using mangrove tree 

recovery to compare between spills shown in Table 4.1. Keep in mind that the recovery 

times indicated would probably be even longer if more comprehensive and ecological 

recovery measures were used.

Table 4.1 summarizes impacts and recovery times for mangrove trees at eight oil 

spills impacting fi ve regions. Mangroves in the Bahía las Minas region of Panama were 

oiled by the Witwater Witwater spill in 1968 and again in 1986 by a refi nery spill. Mangroves at spill in 1968 and again in 1986 by a refinery spill. Mangroves at 

Roosevelt Roads Naval Air Station in southeastern Puerto Rico were impacted by spills in 

1986 and again in 1999, though different sections of mangroves were oiled at each spill. 

Because of the short duration of the follow-up studies, no cases were able to document 

recovery, except for fringe mangroves at the Witwater spill. In most of these studies, spill. In most of these studies,

mangroves were regrowing in the oil-impacted areas but tree height, percent area of 

open canopy, and other parameters remained different from controls. 

Da Silva et al. (1997) diagrammed generalized mangrove impact and recovery 

from an oil spill in four stages. These timeframes are approximate and will likely vary in 

different systems. See also Table 2.1 in Chapter 2 for additional details on timeframes for 

oil impacts to mangroves. 

Figure 4.1 Restoration project 

showing forestry technicians 

planting planting Rhizophora harrisonii Rhizophora harrisonii

propagules in the Congal Biological Station, Esmeraldas Province, 

Ecuador (Arlo H. Hemphill).

Mangal - a mangrove 

forest and its associated 

microbes, fungi, plants, 

and animals. 
50

§ Initial impact ~ 1 year 

propagules and young plants are most likely to die during this time

§ Structural damage ~ 2 1/2 years

trees begin to die

§ Stabilization ~ 5 or more years 

deterioration of mangroves ceases, but no improvement noticeable 

§ Recovery ~ timeframe unknown 

system improves via colonization, increased density, etc.

Additional impacts such as from hurricanes, or other natural or human-caused 

disturbances could signifi cantly delay these recovery processes.

Mangrove Restoration

Restoration success has rarely been studied quantitatively, but we know restored 

mangrove ecosystems often do not equate with natural ones. Shirley (1992) found that 

plant diversity was similar in restored and natural forests one year after restoration, but 

that environmental conditions were different and a number of fi sh and invertebrate 

species were absent from the restored site. McKee and Faulkner (2000) found that 

development of structure and biogeochemical functions differed in two restored mangrove stands because of different hydrological and soil conditions. Tree production and 

stand development was less where tidal exchange was restricted, and some waterlogging 

occurred due to uneven topography. Other assessments of restoration success, in terms 

of initial survival and percent cover after one or several years, have been mixed. Cintron 

(1992) reviewed a number of these projects.

These experiences emphasize the need for developing clear restoration goals 

that incorporate the mangrove ecosystem and its functions, as well as the growth and 

health of the trees themselves. Once the goal is defi ned, the project is designed and 

implemented, followed by monitoring to ensure that restoration is proceeding as anticipated. Projects should be monitored for 10 or more years to adequately assess long-term 

survival, resiliency, and complexity of the restored system (Field 1998). Depending on 

the type of impact and the state of the impacted mangal, restoration may take several 

approaches:

• Replant mangroves

• Remediate soils

• Encourage natural regeneration through improved site conditions

• Restore an alternate site to provide similar habitat (in-kind restoration)
51

Replant Mangroves

There is an extensive body of technical information on replanting mangroves. 

Specifi c details on elevation, use of fertilizer, planting density, species selection, etc. can 

be found in Snedaker and Biber (1996) and Field (1996, 1998). Today, restoration projects 

have moved away from broad use of planting except in those cases where natural 

processes are inadequate to naturally repopulate the area with recruits from surviving 

trees or more distant sources. Examples include mangrove forests where hydrology has 

been substantially altered, or where physical barriers such as dead trees, debris, or berms 

restrict circulation such that propagules have no access to denuded areas.

If planting is chosen as the best course, seedlings will survive best when they 

are planted in a sheltered location and at appropriate tidal elevation levels for each 

species. Planted seedlings are lost primarily because of erosion, predation, death from 

natural causes, planting at incorrect elevations, and residual oil toxicity (Getter et al. 

1984). Planting one- to three-year old trees (usually supplied from nurseries) costs more 

but results in much better survival rates, especially in locations exposed to higher wave 

energy. Seedlings and propagules can survive even when planted in soils with residual 

oil contamination, though generally only after oil has weathered for 9-12 months. 

Table 4.1. Impacts and recovery 

times for mangrove trees at eight 

oil spills impacting fi ve regions.
52

Red mangrove seedlings (R. mangle) survived when planted in areas with one-year old 

residual oil at Bahía las Minas. A restoration planting project at St. Croix in the U.S. Virgin 

Islands planted seedlings 8 years after heavy oiling from the Santa Augusta spill, with 40% 

survival after two years (Lewis 1989). 

Planting is still used to establish new mangrove forests in areas where they 

have not previously existed (such as in newly accreted shorelines or along human-built 

structures), or to replant in forests that have been logged. Survival of planted mangroves 

ranges from 0% to as high as 80% after one year. Lowest rates are often in areas with 

high wave energy where propagules are simply washed away. A planting technique that 

successfully increases survival rates of planted mangroves in exposed areas is called the 

Riley encasement method. Seedlings are planted inside PVC tubes (bamboo can also be 

used) to anchor and protect the seedlings until they become established (Rothenberger 

1999). 

Survival rates drop as the time after planting increases (e.g., one to two years or 

more). Even when plantings survive and grow, densities of planted trees may be lower 

than those naturally recruited, as found at the Bahía las Minas spill. Five years post-spill, 

replanted R. mangle survived well (especially in sheltered areas), but trees were less dense 

than in areas that recolonized naturally (Duke 1996). Restoration that enhances natural 

recovery processes to reestablish mangrove habitat has proven to be the best course of 

action over the long term.

Remediate Soils

Residual oil that has contaminated soils in mangrove forests degrades very slowly, 

since these soils are anaerobic below the top 1-2 mm (Burns et al. 2000). Experiments and 

fi eld studies examining the possibility of accelerating oil degradation through addition 

of nutrients or increased aeration have shown little advantage to these methods. During 

the fi rst year after a spill, biodegradation occurs at very low levels, and the main routes 

of oil removal are dissolution and evaporation. Thus, it is critical during spill response to 

attempt to keep oil from penetrating into sediments. Some restoration-planting projects 

surround seedlings with clean, fertilizer-augmented soil so the new trees can establish 

themselves and develop root structures in uncontaminated soils, before having to contend with possible toxic effects from residual oil. 

Erosion of soils in mangrove forests following a disturbance can impede future reestablishment of new trees, since mangroves thrive only at specifi c tidal elevations. Since 

mangrove root mass comprises 40-60% of the total forest biomass, any substantial die-off 

of adult trees, as may occur after an oil spill, could cause subsidence of soils and erosion 

as a secondary impact. In such cases, augmenting soils, or assisting processes of sediment 

accretion may be a necessary part of restoration activities.
53

Encourage Natural Regeneration

Restore hydrology

Adequate hydrology is tagged as the most important parameter for mangrove 

recruitment (Lewis and Streever 2000). When tidal connections have been cut off or 

altered, as is common along developed coasts, re-establishing these connections can 

promote natural recruitment and improve the overall health and functioning of the 

mangrove ecosystem. Roosevelt Roads NAS is an example where impounded mangroves 

were impacted by a jet fuel spill in 1999. These mangroves suffered both from 

toxic fuel impacts and from extended submersion of roots when tidal conduits were 

closed to contain the spill during response. Facilitating or increasing tidal exchange to 

these impounded mangrove forests could be a promising restoration activity. In-kind 

restoration conducted after the Tampa Bay spill involved, in part, restoring tidal circulation at a previous dredge disposal site where mangroves had been impounded by dikes. 

Plant “nurse” habitat

Since mangrove propagules and seedlings grow best in sheltered conditions, one 

strategy for more exposed areas is to plant indigenous marsh plants such as Spartina 

alternifl ora to create a nurse habitat. These plants grow quickly (one to two years), 

trap and hold sediments (which decreases erosion), and create a more sheltered habitat 

where young mangroves can establish themselves. This staged approach is modeled 

after natural successional patterns and boosts natural recruitment of mangroves (Mauseth et al. 2001).

Propagules may be available only during certain times of the year or may not 

distribute far from the parent tree due to poor circulation or blocking by debris. Removing fl oating debris that may block channels enables propagules to reach and recolonize 

denuded areas naturally. 

Restore in-kind resources

Increasingly, in-kind restoration is used for projects in the United States, especially 

for resource damage settlements after oil spills. In-kind restoration restores habitat in a 

different location in the same ecosystem and is meant to contribute to the overall habitat 

function of the region.

A recent example of in-kind restoration is Tampa Bay, Florida, where several 

mangrove islets were heavily oiled during a spill in 1993. Restoration efforts purchased a 

former dredge disposal site within Tampa Bay that included degraded mangrove forest. 

Tidal connections were restored, marsh grasses were planted along the shoreline, and the 

land was deeded to the County to function as wildlife habitat and provide water fi ltering 

functions for the waters of Tampa Bay (see Case Studies for more detail). 
54

For Further Reading

Ballou, T. G. and R. R. Lewis III. 1989. Environmental assessment and restoration recommendations for a 

mangrove forest affected by jet fuel. In: Proceedings of the 1989 International Oil Spill Conference, pp. 407-412.

Burns, K. A., S. Codi, and N.C. Duke. 2000. Gladstone, Australia fi eld studies: weathering and degradation of 

hydrocarbons in oiled mangrove and salt marsh sediments with and without the application of an experimental bioremediation protocol. Marine Pollution Bulletin 41: 392-402.

Burns, K. A., S.D. Garrity, and S.C. Levings. 1993. How many years until mangrove ecosystems recover from 

catastrophic oil spills? Marine Pollution Bulletin 26:239-248.

Cintrón, G. 1992. Restoring mangrove systems. In: G. W. Thayer (ed.). Restoring the Nation’s Marine Environment. 

College Park, Maryland: Maryland Sea Grant College. pp. 223-277.

Da Silva, E. M., M.C. Peso-Aguiar, M.F.T. Navarro, C. De Barros, and A. Chastinet. 1997. Impact of petroleum 

pollution on aquatic coastal ecosystems in Brazil. Environmental Toxicology and Chemistry 16: 112-118.

Duke, N. 1996. Mangrove reforestation in Panama, an evaluation of planting in areas deforested by a large 

oil spill. In: C. Field (ed.). Restoration of Mangrove Ecosystems. Okinawa: The International Society for Mangrove 

Ecosystems. pp. 209-232.

Duke, N.C., Z.S. Pinzon, and M.C. Prada T. 1997. Large-scale damage to mangrove forests following two large 

oil spills in Panama. Biotropica 29:2-14.

Ellison, A. M. 2000. Mangrove restoration: do we know enough? Restoration Ecology 8: 219-229.

Field, C. 1996. General guide for the restoration of mangrove ecosystems. In: C. Field (ed.), Restoration of 

Mangrove Ecosystems. Okinawa: The International Society for Mangrove Ecosystems. pp. 233-250.

Field, C. 1998. Rehabilitation of mangrove ecosystems: an overview. Marine Pollution Bulletin 37:383-392.

Garrity, S.D., S.C. Levings, and K.A. Burns. 1994. The Galeta oil spill: I. Long-term effects on the physical structure 

of the mangrove fringe. Estuarine, Coastal and Shelf Science 38:327-348.

Getter, C. D., G. Cintron, B. Dicks, R.R. Lewis III, and E. D. Seneca. 1984. The recovery and restoration of salt 

marshes and mangroves following an oil spill. In: J.J. Cairns, Jr. and A.L. Buikema, Jr. (eds.). Restoration of 

Habitats Impacted by Oil Spills. Boston: Butterworth Publishers. pp. 65-113.

Gilfi lan, E.S., D.S. Page, R.P. Gerber, S. Hansen, J. Cooley and J. Hothman. 1981. Fate of the Zoe Colocotronis

oil spill and its effects on infaunal communities associated with mangroves. In: Proceedings of the 1981

International Oil Spill Conference, pp. 360.

Levings, S.C., S.D. Garrity, E.S. VanVleet, and D.L. Wetzel. 1997. Sublethal injury to red mangroves two years after 

oiling. In Proceedings of the 1997 International Oil Spill Conference, pp. 1040-41

Levings, S.C. and S.D. Garrity. 1995. Oiling of mangrove keys in the 1993 Tampa Bay oil spill. In: Proceedings of 

the 1995 International Oil Spill Conference, pp. 421-428.

Lewis, R. R., and B. Streever. 2000. Restoration of mangrove habitat. WRP Technical Notes Collection (ERDC 

TN-WRP-VN-RSW-3.2). Vicksburg, Mississippi: U.S. Army Engineer Research and Development Center. 7 pp. 

Available: www.wes.army.mil/el/wrp.

Lewis, R. R. 1990. Creation and restoration of coastal plain wetlands in Florida. P. 73-101. In: J. A. Kusler and M. E. 

Kentula (eds.), Wetland Creation and Restoration. Washington, D.C.: Island Press.

Lewis, R. R. 1979. Large scale mangrove restoration on St. Croix, U.S. Virgin Islands. In: Proceedings of the Sixth 

Annual Conference on the Restoration and Creation of Wetlands, pp. 231-241.

Mauseth, G. S., J.S. Urquhart-Donnelly, and R. R. Lewis. 2001. Compensatory restoration of mangrove habitat 

following the Tampa Bay oil spill. In: Proceedings of the 2001 International Oil Spill Conference, pp.761-767.

McKee, K. L. and P. Faulkner. 2000. Restoration of biogeochemical function in mangrove forests. Restoration 

Ecology 8: 247-259.
55

Nadeau, R.J. and E.T. Bergquist, 1977. Effects of the March 18, 1973 oil spill near Cabo Rojo, Puerto Rico on 

tropical marine communities. In: Proceedings of the 1977 International Oil Spill Conference, pp. 535-538.

Rothenberger, P. 1999. Utilization of encasement technology in restoration of mangrove forest on St. Croix, 

U.S. Virgin Islands. Reef Research 9 (3). Great Barrier Reef Marine Park Authority, Townsville, Australia. 2pp. 

Available: http://www.gbrmpa.gov.au/corp_site/info_services/publications/reef_research/

Shirley, M. A. 1992. Recolonization of a restored red mangrove habitat by fi sh and macroinvertebrates. In:

F. J. Webb (ed.). Proceedings of the 19th Annual Conference on Wetlands Restoration and Creation. pp. 159-173.

Snedaker, S. C. and P. D. Biber. 1996. Restoration of mangroves in the United States of America. In: C. Field 

(ed.). Restoration of Mangrove Ecosystems. Okinawa: The International Society for Mangrove Ecosystems. pp. 

170-188.

Wardrop, J.A., B. Wagstaff, P. Pfennig, J. Leeder, and R. Connolly. 1997. The distribution, persistence and effects 

of petroleum hydrocarbons in mangroves impacted by the “Era” oil spill (September, 1992). Final Phase One 

report (1996). Report ERAREP/96. Adelaide, South Australia: Offi ce of the Environmental Protection Authority, 

S.A. Department of Environment and Natural Resources.

Wilkinson, D. L., C. Moore, M. Lopez, and M. Figueroa. 2001. Natural resource damage assessment for a JP-5 

fuel spill at Naval Station Roosevelt Roads, Puerto Rico. Pre-Final Report. Norfolk: Atlantic Division, Naval 

Facilities Engineering Command. 96 pp.
56

CHAPTER 5. Mangrove Case Studies

Introduction

Mangroves around the world have been exposed to oil both from individual 

spills and from chronic pollution from refi nery and storage tank discharges. Welldocumented oil spills in mangrove areas provide us with a good idea of some of the 

complexities and variability of the impacts and response options. We have highlighted 

techniques (learned from fi eld trials, toxicology, and laboratory studies) to measure the 

health of mangroves. With help from NOAA’s IncidentNews.gov database and from 

colleagues around the world we searched for case studies of oil spills impacting—or 

potentially impacting—mangroves. We kept our focus on individual incidents and did 

not include cases involving long-term pollution. However, we know that some spills 

occurred at sites that had been impacted by spills in the past (Bahía las Minas, Panama 

and Roosevelt Roads, Puerto Rico). We also focused more on the direct and indirect 

effects of oiling and cleanup on the mangroves themselves, less on associated fauna and 

fl ora. The incidents include a wide range of documentation and a wide range of oil 

types. From these we identifi ed several case histories that provided information about 

the incident, response methods, and long-term impacts and recovery. These are briefl y 

reviewed below in chronological order. 

One lesson that is quite clear from even a few of the cases is that the full extent 

of damage to mangroves is not apparent for many months or years after an incident, 

regardless of the fuel type and extent of response (other than full protection). Many 

questions remain about most studies. The most important is, How long does recovery 

actually take? Although a number of post-spill studies were conducted for as long as 10 

to 20 years, we were able to fi nd only a few reports where monitoring continued long 

enough to confi rm full recovery.

Zoe Colocotronis, La Parguera, Puerto Rico, 1973

On March 18, 1973, the Zoe Colocotronis ran aground on a reef 3.5 miles off 

the La Parguera tourist area on the southwest coast of Puerto Rico. The master intentionally released 37,579 barrels (1.58 million gallons) of Venezuelan (Tijuana) crude oil. An 

estimated 24,000 barrels (1.01 million gallons) stranded on the beaches of Cabo Rojo. 

Three separate pools of black oil 6-8 inches thick oiled the shore of Cabo Rojo on the 

Bahía Sucia side. On March 21, a large number of sea cucumbers, conchs, prawns, sea 

urchins, and polychaete annelids washed ashore. Organisms died in the Thalassia seagrass beds and oil moved into mangrove forests composed of white, red, and black 

mangrove trees (Nadeau and Berquist 1977).
57

Response

Cleanup efforts were conducted outside the mangrove areas and involved booming, digging sumps, and pumping the collected oil into tank trucks. On March 23, before 

the oil in the mangroves could be recovered, an unexpected wind shift drove patches 

of oil out of the mangroves and into other areas and beaches. By March 24, 604,000 

gallons of nearly pure oil had been removed from other areas using sumps, skimmers, and 

vacuum trucks. Steam cleaning was not used because there was no accessible source of 

fresh water. No cleanup was conducted in the mangroves.

Impacts

EPA scientists surveyed the mangrove areas for a week beginning 24 hours after 

the spill. Detailed surveys were conducted of all oiled areas during the second week after 

the spill and again during the thirteenth week. Additional EPA site visits were made in 

January 1974 (10 months later) and January 1976 (34 months later) providing some idea 

of long-term effects. In one well-studied area, one hectare of red and black mangrove 

trees was defoliated and died during the three years following the spill. However, the EPA 

scientists also noted that much of the associated invertebrate life had recovered (Nadeau 

and Bergquist 1977).

In November 1973, eight months following the spill, oil chemists from Bowdoin 

College in Maine visited several oiled sites and noted a re-emergence of young trees. 

Although sediment oil concentrations remained high, the oil was heavily weathered and 

degraded. These observations suggested that the toxic components were gone in about 

half a year. This team had also visited oiled black mangrove sites four times between April 

1979 and April 1981, 6 to 8 years after the spill. The scientists measured ratios of sodium 

and potassium in some plants, supporting the idea that oil injured the trees by disrupting 

salt and water balance and that such disruption might have been alleviated by directed 

cleanup. However, they made no comment on the visible health of the mangroves at that 

time (Page et al. 1979; Gilfi llan et al. 1981). 

Eleven years after the spill other chemists took sediment cores from several previously oiled mangrove sites and found concentrations ranging from 10,000 to 100,000 

ppm (dry weight, total unresolved hydrocarbons) in a layer 6 cm below the relatively 

clean surface sediments. In addition, they found oil, possibly from the 1962 Argea Prima

spill, 14-16 cm below the surface. These last researchers did not report the status of the 

mangrove trees themselves (Corredor et al. 1990).

For Further Reading

Nadeau, R.J. and E.T. Bergquist, 1977. Effects of the March 18, 1973 oil spill near Cabo Rojo, Puerto Rico on 

tropical marine communities. In: Proceedings of the 1977 International Oil Spill Conference, pp. 535-538. 
58

Page, D.S., D.W. Mayo, J.F. Cooley, E. Sorenson, E.S. Gilfi llan, and S.A. Hanson. 1979. Hydrocarbon distribution and 

weathering characteristics at a tropical oil spill site. In: Proceedings of the 1979 International Oil Spill Conference, 

pp. 709-712.

Gilfi llan, E.S., D.S. Page, R.P. Gerber, S. Hansen, J. Cooley, and J. Hothman. 1981. Fate of the Zoe Colocotronis

oil spill and its effects on infaunal communities associated with mangroves. In: Proceedings of the 1981 

International Oil Spill Conference, p. 360.

Page, D.S., E.S, Gilfi lan, C.C. Foster, J.R. Hotham, and L. Gonzales. 1985. Mangrove leaf tissue sodium and 

potassium ion concentrations as sublethal indicators of oil stress in mangroves. In: Proceedings of the 1985 

International Oil Spill Conference, pp. 391-393. 

Corredor, J.E., J.M. Morell, and C.E. Del Castillo. 1990. Persistence of spilled crude oil in a tropical intertidal 

environment. Marine Pollution Bulletin 21:385-388.

Peck Slip, Eastern Puerto Rico, 1978

On December 19, 1978 the Peck Slip released between 440,000 and 450,000 

gallons of Bunker C oil into open waters offshore of eastern Puerto Rico. Within two days 

oil had stranded in segments along 26 km of eastern Puerto Rico shorelines, mostly sand 

beach. However, some oil entered outer and inner fringing mangroves in three areas, and 

inner basin mangroves in one of these areas. 

Response

No cleanup actions were undertaken although observers noted fl oating absorbent pads at one site. Surveys of mangroves were conducted shortly after the spill 

(December-early January 1979; Robinson 1979), about three months later (Gundlach et al. 

1979), 10 months later, and 18 months later (Getter et al. 1981). 

Impacts

Mangroves on a small island (Isla de Ramos) were lightly impacted (prop roots 

had a 15-cm band of oil 50 to 60 cm above the substrate) and apparently did not suffer 

long-term injury. Near Punta Medio Mundo, about 2.6 acres of inner fringe and inner 

basin mangrove roots were heavily oiled (prop roots with up to a one-meter band of oil) 

and two acres moderately oiled (0.3 to 0.45-m band of oil; Robinson, 1979). An estimated 

3.5 tons of oil coated the mangrove roots. Algae growing on the prop roots absorbed the 

oil. Another two acres of mangroves at Pasaje Medio Mundo were moderately oiled with 

an estimated 1.3 tons of oil (prop roots oiled by a 0.2-meter band on oil). 

Within two to three months the heavily oiled inner fringing and basin mangroves 

at the Punta Medio Mundo forest were defoliated. Prop-root oiling had widened to a 

band of over two vertical meters, possibly from oiled climbing crabs. Later site visits 
59

confi rmed that mangroves with the most heavily oiled prop roots remained defoliated 10 

and 18 months later (Getter et al. 1981). 

This was one of fi ve sites studied by Getter et al. (1981). From these studies the 

authors urged that inner fringing and inner basin mangroves receive highest priority for 

protection from oil spills. 

Restoration

No restoration activities were undertaken at this spill.

For Further Reading

Robinson, J.H. (ed). 1979. The Peck Slip oil spill: a preliminary scientifi c 

report. Boulder: Offi ce of Marine Pollution Assessment, National Oceanic 

and Atmospheric Administration. Unpublished report.

Gundlach, E.R., J. Michel, G.I. Scott, M.O. Hayes, C.D. Getter, and W.P. Davis. 

1979. Ecological assessment of the Peck Slip (19 December 1978) oil 

spill in eastern Puerto Rico. In: Proceedings, Ecological Damage Assessment 

Conference, Society of Petroleum Industry Biologists, pp. 303-317.

Getter, C.D., G.I. Scott, and J. Michel. 1981. The effects of oil spills on 

mangrove forests: A comparison of fi ve oil spill sites in the Gulf of Mexico 

and the Caribbean Sea. In: Proceedings of the 1981 Oil Spill Conference, pp. 

535-540.

JP-5 Jet Fuel Spills, Roosevelt Roads, Puerto Rico (1968 and 1999)

In 1986 and again in 1999, Roosevelt Roads Naval Air Station storage tanks 

released JP5 jet fuel into a cove in eastern Puerto Rico. Before the 1986 and 1999 JP-5 

spills, the area had been contaminated by oils from several past spills: a Bunker C spill in 

1958 and a diesel spill in 1978, both from onshore storage tanks, and a 210,000-gallon 

diesel spill in 1981 from a tanker. All of these spills contaminated mangrove areas but 

effects of the earlier spills are unknown. In both recent cases, mangrove forests were 

contaminated, though response strategies differed markedly. Effects on mangroves were 

monitored at both spills. 

On November 27, 1986, 59,000 gallons of JP-5 fuel washed down a catchment 

stream (tidal creek) and into Ensenada Honda. Two mangrove forest areas were contaminated, one in the tidal creek and the other at the head of the saltwater bay. 

On October 20, 1999, 112,000 gallons of JP-5 fuel spilled from a day-tank at the 

U.S. Navy Base. The oil fl owed into an underground drainage pipe, which runs under 

Figure 5.1 Oiled crab and snail 

on red mangrove trunk at the 

Peck Slip spill in 1979. (OR&R)
60

a runway and several roads for several hundred yards. The pipe empties into an open 

drainage ditch, which drains to a 12-hectare mangrove forest. This forest drains through 

a culvert into Ensenada Honda Bay. 

Response

No cleanup actions were mentioned in reports dealing with the 1986 incident, 

presumably because of the high evaporation rate of JP-5 jet fuel in open conditions.

In the 1999 incident the Navy’s primary environmental concern was the bay. In 

the face of an approaching hurricane, USN Construction Battalion (Sea Bees) personnel 

constructed a dam to plug the culvert between the fi rst impacted mangrove (later named 

“mangrove A”) and the mangrove adjacent to the bay (later named “mangrove C”). This 

dam trapped the water in mangrove area A. The fi nal reports should be consulted for 

specifi cs as there were many details to the fl ow diversion response. Fuel was recovered, 

where practical, using under fl ow dams, skimmers, vacuum trucks, and sorbent materials. 

Attempts to manually remove oil with sorbents proved both ineffective and a human 

health risk for responders from inhalation of jet fuel fumes. It was estimated that 15 to 

20% of the product was recovered, over 70 percent evaporated, and some 10 to 15% 

(approximately 11,200 - 16,800 gallons) remains unaccounted for; presumably stranded in 

the mangroves or in the sediments near the spill site. 

The fuel fl owed through the mangroves and some portion of the oil changed 

color from almost clear with a slight yellow tint to brown/black, similar to a light crude oil. 

It is unknown as to whether this was as a result of tannins from the mangroves dissolving 

into the oil or the JP-5, liberating heavier product remaining from previous spills. 

Impacts

1986 Spill. 

In the 1986 incident two mangrove areas were contaminated by JP-5 fuel: 

(1) the northernmost red mangroves drained by the tidal creek, and (2) the mixed species 

mangroves adjacent to the Coast Guard pier in Ensenada Honda. Local responders noted 

visible effects on adult trees within 10 days of oiling. Follow-on surveys were conducted 

in the second area 17 months later and again 23 months later. During these surveys 10 

x 10-meter grids along transects documented tree height, canopy, tree death, percent 

open canopy, seedling counts, and invertebrate biota. There were three transects in oiled 

areas plus two in unoiled areas. In June 1987 false-color aerial photos were taken of the 

impacted forest. 

Detailed surveys fi ve months later found most adult trees in the oiled areas dead 

and/or defoliated. However, there were live seedlings with highest densities along the 

forest front. Furthermore, sediment oil concentrations were extremely low (less than 1 
61

ppm) and similar to concentrations in unoiled areas. Because of the low impact on 

seedlings and the near-absence of fuel oil six months later, researchers concluded that 

there was no smothering effect from the jet fuel. Adult tree defoliation and mortality was 

likely caused by initial direct toxicity of the fuel to root structures. 

Apparently these mangroves recovered suffi ciently from the 1986 JP-5 spill to 

merit no comment from personnel responding to the 1999 spill, other than that they 

were protected by the response itself. Given the location of the 1999 contamination (tidal 

creek mangroves), very little cleanup was possible. However, the series of water diversion 

activities resulted in preventing oiling of the mangrove (C) in Ensenada Honda. 

1999 Spill. 

Tidal creek mangroves (areas A and B) were clearly damaged from the 1999 

incident, due either to fuel toxicity or extended fl ooding, or both. Follow-up studies 

through October 2001 indicated that there was some recovery in the fl ooded area A two 

years after the incident, with new propagules and new shoots on injured trees. However, 

there were no signs of recovery in area B. Of a total of 50 acres of injured mangrove 

forest, about 30 acres showed no signs of recovery two years later (Csulak 2001).

For Further Reading

Ballou, T.G. and R.R. Lewis III. 1989. Environmental assessment and restoration recommendations for a 

mangrove forest affected by jet fuel. 2 In: Proceedings of the 1989 International Oil Spill Conference, pp. 407-412.

Lehman, S., F. Lopez, and F. Csulak. 2001. Case study: spill of JP5 fuel at Roosevelt Roads Naval Air Station, 

Puerto Rico, into a basin mangrove. In: Proceedings of the 2001 International Oil Spill Conference, pp. 197-201.

Vesta Bella Oiling and Cleanup of U.S.Virgin Islands Mangroves, 1991

On March 6, 1991, the barge Vesta Bella sank southeast of Trinidad, releasing an 

unknown amount of high aromatic No. 6 fuel oil. The barge continued to leak for more 

than 20 days. Some oil moved north, eventually stranding on several beaches on the 

north side of St. John, in the U.S. Virgin Islands. Beach surveys began there on March 

23. Red mangrove oiling was not extensive: one-meter prop roots of individual or small 

groups of mangrove trees were oiled 30 to 35 cm above the substrate. However, the short 

(15 cm) prop roots of supratidal white mangroves were heavily coated. These trees were 

also stressed before the spill due to beach erosion. 

Response

A modest level of cleaning was attempted with a planned revisit to the site a 

year later. Roots were carefully wiped by a select group of workers, and then snare boom 
62

was strung and allowed to scrub roots with the rise and fall of the tide. Snare boom 

was removed after 24 hours. One year after the spill the mangroves were revisited and 

measured for a variety of plant health indicators. 

Impacts

The white mangroves at one site were heavily defoliated but also showed extensive new growth on both oiled and unoiled trees, growth that apparently began six to 

twelve months post spill. There was some sign of chlorosis and no signs of oil on roots. 

Close inspection of formerly oiled fringing red mangroves indicated these trees were 

healthy—fully foliated, with no signs of chlorosis. Only one tree was severely oiled and 

cleaned at the time of the spill: measurements indicated this tree was in good health. 

Unfortunately, no oiled mangroves were left uncleaned, to serve as a reference, 

so it is diffi cult to ascribe the good condition of the trees one year later, to the cleaning. 

However, it is clear that this level of cleaning did not cause any mortality to the trees. The 

authors caution that this cleanup method was done in areas with a fi rm substrate. Finally, 

they confi rmed that there was very little contamination of the substrate.

Restoration

No restoration activities were undertaken at this spill.

For Further Reading

Dahlin, J.A., J. Michel, and C. Henry. 1994. Recovery of mangrove habitats at the Vesta Bella oil spill site. 

HAZMAT Report 95-3. Seattle: Hazardous Materials Response and Assessment Division, National Oceanic and 

Atmospheric Administration. 30 pp.

T/V Era, Spencer Gulf, South Australia, 1992

On August 30, 1992, the tanker Era released an estimated 296 tonnes (974,000 

gallons) of heavy Bunker oil (a blend of diesel and heavy residual) at a jetty near the 

head of Spencer Gulf, South Australia. On the night of September 1-2, an estimated 20 

tonnes (5,500 gallons) stranded along 10-15 km of mangrove (Avicennia Avicennia) forest south of 

Port Pirie, S.A. However, subsequent surveys estimated that the actual quantity stranded 

in the mangroves was 57 tonnes (15,600 gallons).

Response

Within two to three hours of the release, the oil slick was treated from vessels 

spraying dispersants Corexit 9527 and 7667; the following day, aircraft also sprayed slicks 
63

with Ardrox dispersant. Responders were advised that cleanup within the mangrove 

forest was not feasible and would likely increase damage to adjacent, unimpacted areas. 

Thus, all subsequent activity in the mangrove forest was restricted to detailed and longterm monitoring.

Impacts

Oiled mangroves were monitored for four years after the spill. This is perhaps 

one of the most well documented accounts available of the fate and effects of oil in 

a mangrove forest. Only a brief, highly simplifi ed account can be given here and the 

reader is advised to consult the report for important details and qualifi cations (Wardrop 

et al. 1997).

Due to an extremely high tide, oil penetrated far into the mangrove forest 

(50 m) coating leaves as well as stems, trunks, and sediment. Oil concentrations and 

visible damage to mangrove trees were recorded over four years. About 75-100 hectares 

were oiled: 4.2 heavily, 7.3 moderately, and 38.0 lightly. In 1992 heavy oiling of canopy 

and extensive mats of oiled sea-grass debris characterized heavily oiled areas. By 

November 1992 mangroves over a total area of 2.3 hectares suffered extensive defoliation; the area expanded slightly to 3.2 hectares by 1995 and then stopped increasing. 

Trees that were totally defoliated did not recover during the four-year period. Defoliation 

and degree of sediment oiling were correlated: heavily oiled areas were completely 

defoliated and moderately oiled areas were “severely” defoliated. In lightly oiled areas 

trees had less leaf damage and recovered rapidly. “Overall the extent of damage in each 

of the studied locations, and the speed with which it occurred, has correlated to the oiling 

classifi cation assigned in the fi rst survey” (Wardrop et al. 1997). Finally, the veracity of the 

original recommendation of ”no cleanup“ was supported: injury to mangrove trees was 

restricted to those initially impacted by moderate to heavy oiling. 

For Further Reading

Wardrop, J.A., B. Wagstaff, P. Pfennig, J. Leeder, and R. Connolly. 1997. The distribution, persistence and effects 

of petroleum hydrocarbons in mangroves impacted by the “Era” oil spill (September, 1992). Final Phase One 

report (1996). Report ERAREP/96. Adelaide, South Australia: Offi ce of the Environmental Protection Authority, 

S.A. Department of Environment and Natural Resources. 

Witwater and Texaco Storage Tank Spills Texaco Storage Tank Spills exaco Storage Tank Spills, Bahía Las Minas, Panama, , Bahía Las Minas, Panama, 

1968 and 1986

 Two large oil spills, 18 years apart, resulted in long-term injury and recovery to a 

portion of the 1,200 ha of mangroves of the Bahía Las Minas area of Panama.
64

Witwater. On December 13, 1968, the oil tanker Witwater Witwater broke up in heavy seas broke up in heavy seas 

off the Atlantic coast of Panama, spilling 14,000 barrels (588,000 gallons) of Bunker C and 

diesel oil into the water 5 miles from Galeta Island. Strong seasonal winds pushed the 

slick towards the island, oiling sand beaches, rocky coasts, and mangroves. 

Texaco Storage Tank. On April 27, 1986, a Texaco storage tank at a refi nery on Isla 

Payardi, Panama, ruptured, releasing approximately 240,000 barrels (10.1 million gallons) 

of medium-weight crude oil. Approximately 140,000 barrels (5.9 million gallons) of oil 

fl ooded through a dike and overfl owed separators and a retaining lagoon and fl owed into 

Bahía Cativá, an arm of Bahía las Minas. 

Responses

Witwater. Several thousand barrels were pumped from the waters surrounding 

Galeta Island, and approximately 5,000 barrels (210,000 gallons) were ignited and burned 

along shorelines in the bay. By December 17, pumping and shoreline burning cleaned up 

approximately half of the spilled oil.

Texaco Storage Tank. Refi nery personnel reported that 60,000 barrels 

(2.52 million gallons) of oil were recovered. It is not known how much of this recovered 

oil was from the sea. Dispersants were applied in Bahía Cativá, Islas Naranjos, offshore of 

Bahía Las Minas, near Portobelo, and along the northern breakwater at the mouth of the 

Panama Canal. Although dispersants appeared to be ineffective due to the weathered 

state of the oil and the calm seas, skimmers recovered some fl oating oil. Vacuum trucks 

were used as part of the shore-based cleanup effort. Several channels were dug through 

the mangroves to drain the oil. These channels appeared, instead, to have helped move 

the oil inshore. Increased disturbance due to the construction of the channels may have 

also contributed to subsequent erosion. Oiled rocks and debris were manually removed 

along the more accessible shorelines. Seawater was sprayed on some sandy areas to 

aid oil removal. Pumping to recover fl oating oil appeared to be the most effective oil 

recovery method. The shallow waters and mangroves rendered many oil spill cleanup 

techniques impractical.

Impacts

Archived aerial photographs (1966, 1973, 1979, and 1990) and ground surveys 

were keys to understanding the effects of these two spills on mangrove forests.

Witwater. Despite the cleanup, both red and black mangrove trees were severely 

oiled, and the majority of the red mangrove seedlings were killed. Oil also damaged many 

of the mangrove forest inhabitants. Initial reports did not indicate that adult trees had 

suffered. Aerial survey photos from 1966 and 1973 were used to assess deforestation, oil 

gaps, and open canopy. About 49 hectares of mangrove forest (representing 4 percent 

of the total mangrove forest) had been completely deforested in 1973 (fi ve years after 
65

the spill). Most deforested areas had new recruits by 1979 (eleven years after the spill) 

but 3 ha were lost to sea-margin encroachment. Observable differences (oil gaps, and 

canopy height and structure) and oiled sediment persisted into 1992, 23 years after the 

Witwater spill. spill. 

Texaco Storage Tank Spill. The distribution of oil was surveyed from aircraft 

for two months following the release. A total of 51 miles of shoreline was heavily 

oiled, including some mangroves recovering from the Witwater Witwater spill. In a central embay- spill. In a central embayment (Bahía Cativá), approximately half the surrounding forested area (and halfway up 

the intertidal zone) was killed. Oiled habitats within this distance included extensive 

mangroves, intertidal reef fl ats, seagrass beds, and subtidal coral reefs. Re-oiling of the 

shoreline and mangroves was a continuing problem. Oil slicks were regularly observed 

within Bahía las Minas for at least four years following the spill with oil coming predominantly from areas of fringing mangroves. As the oiled red mangrove trees decayed, it was 

believed that eroding, underlying sediments released trapped oil. 

An affected reef fl at habitat was the site of an ongoing study at the Smithsonian 

Tropical Research Institute’s fi eld station at Punta Galeta. A detailed study of mangrove 

trees revealed that one- to two-year-old seedlings appeared to survive whereas the surrounding adults died. It was believed that, somehow, young seedling structure (perhaps 

lack of prop roots) enabled the young trees to tolerate periods of oil immersion. It was 

suggested that the disruption of the substrate before replanting may remove such survivors, hampering forest recovery. Oil persisted in the mangroves through May 1989. Initial 

oiling of the trees produced measurable amounts of oil on 100% of all the roots that were 

sampled. Through May 1989, the mangrove roots in the open coast and channel areas 

showed 70% oiling, while the oiled proportion in the stream mangroves remained 100% 

oiled. The decrease in oil coverage resulted from weathering, microbial degradation, and 

loss of oiled bark or encrusting organisms. Root mortality was greater in oiled areas.

Subsequent aerial and ground surveys indicated “recovery of the 1986 spill was 

well-advanced by 1992” (Duke et al. 1997) due, in part, to extensive restoration. However, 

about 5 hectares of fringing forest were lost to sea-margin encroachment and there 

remained important differences between sheltered and exposed areas.

Although ten times more oil was spilled in 1986 than in 1968, this did not result 

in ten times more damage to mangroves. Calm winds, lower tides, different oil type, and 

longer weathering time before impact may have resulted in less toxicity. 

Restoration

Because of extensive mangrove mortality, several replanting projects were conducted at Bahía las Minas, in hopes of speeding mangrove forest recovery, which was at 

the time estimated to take 20 years or longer (Teas et al. 1989). 
66

Experiments to determine whether propagules could survive if planted directly in 

oiled sediment found 100% mortality up until six months post spill. By nine months postspill, propagules survived at rates similar to those at unoiled sites. Beginning 12 months 

after oiling, red mangrove seedlings that had been raised in a separate nursery area were 

planted (with added fertilizer) in areas of the damaged mangrove forest. A total of 42,000 

nursery plants and 44,000 propagules were planted. 

Studies conducted in 1989 (33 months post-spill) looked at the effectiveness 

of the plantings conducted in 1987, by comparing mangrove densities in areas that 

had recruited naturally with those that were replanted. Though planted seedlings had 

survived in all areas studied, naturally recruited plants were most dense. Thus, natural 

recruitment was more effective at recolonizing oil-damaged areas and, over time, natural 

recruits out-competed planted seedlings. Researchers also noted detrimental collateral 

impacts from planting, including cutting and removing dead timber for boat access 

(which removed shelter for seedlings), trampling sediments, digging holes (which accelerated erosion), and damaging existing seedlings (Duke 1996). Overall, planting did not 

result in a net benefi t to the mangrove forest. However, since recolonization of mangroves was lowest in exposed areas, Duke suggests that an effective restoration activity 

could be to protect very exposed areas until mangrove trees are well established. 

For Further Reading

Duke, N. 1996. Mangrove reforestation in Panama, an evaluation of planting in areas deforested by a large 

oil spill. In: C. Field (ed.). Restoration of Mangrove Ecosystems. Okinawa: The International Society for Mangrove 

Ecosystems. pp. 209-232.

Duke, N.C., Z.S. Pinzon, and M.C. Prada T. 1997. Large-scale damage to mangrove forests following two large 

oil spills in Panama. Biotropica 29:2-14.

Garrity, S.D., S.C. Levings, and K.A. Burns. 1994. The Galeta oil spill: I. Long-term effects on the physical structure 

of the mangrove fringe. Estuarine, Coastal and Shelf Science 38:327-348.

Jackson, J.B.C., J.D. Cubit, B.D. Keller, V. Batista, K. Burns, H.M. Caffey, R.L. Caldwell, S.D. Garrity, C.D. Getter, C. 

Gonzalez, H.M. Guzman, K.W. Kaufmann, A.H. Knap, S.C. Levings, M.J. Marshall, R. Steger, R.C. Thompson, and 

E. Weil. 1989. Ecological effects of a major oil spill on Panamanian coastal marine communities. Science 

243:37-44.

Teas, H. J., Lasday, A. H., Luque L., Elias, Morales, R. A. De Diego, M. E. and J. M. Baker. 1989. Mangrove restoration 

after the 1986 refi neria Panama oil spill. In: Proceedings of the 1989 International Oil Spill Conference, San 

Antonio, February 13-16, 1989, pp. 433-437.

Bouchard Barge B-155, Tampa Bay, August 1993

On August 10, 1993, the freighter Balsa 37, the barge , the barge Ocean 255, and the barge 

Bouchard 155 collided in the shipping channel west of the Skyway Sunshine Bridge and 

south of Mullet Key in Tampa Bay, Florida. The collision caused three separate emergen-
67

cies: (1) the Balsa 37 Balsa 37 was listing, threatening to spill phosphate rock; (2) the jet fuel, was listing, threatening to spill phosphate rock; (2) the jet fuel,

gasoline, and diesel caught fi re on the Ocean 255; and (3) the Bouchard 155 was holed 

at the port bow, spilling approximately 8,000 barrels (338,000 gallons) of No. 6 fuel oil 

into Tampa Bay. By August 15 most of the fl oating fuel oil had come ashore and heavily 

coated sand beaches, several mangrove islands, and seawalls within Boca Ciega Bay. 

By August 16 very little fl oating oil was seen offshore. In the shallow, low-energy areas 

along the mangrove islands inside Johns Pass and at a few locations in the surf zone, oil 

had mixed with beach sand and shallow sediments to form underwater tarmats, some of 

which came ashore on the mangrove keys.

Response 

The No. 6 fuel from the barge is the only material known to have been released 

from this incident. Countermeasures used during this spill were mechanical or manual. 

Skimming operations were used to collect free-fl oating oil. Effi ciency and effectiveness 

of skimming operations were extremely high. Oil in and around mangrove islands was 

removed by vacuuming. Areas were left oiled when it was felt that cleanup methods 

would cause greater impact than leaving the oil in place. Some of the submerged oil in 

very shallow areas was removed using buckets and shovels. Oiled seagrass beds were 

cleaned by gently lifting oil out of them by hand. ”How clean is clean“ inspections for 

mangroves, seagrass beds, and other sensitive areas were judged on a case-by-case basis 

by the inspection committee. 

Impacts

Tarmats formed when sediment was mixed with oil along the shallow fl ats surrounding the islands. Large, thick mats coated mangrove roots, oyster and seagrass beds, 

and tidal mud fl ats. Much of this oil was vacuumed out using vacuum transfer units on 

grounded barges staged around the islands and shallow areas. 

Scientists visited oiled and unoiled mangrove keys quarterly between November 

1994 and April 1996. Individual trees, pneumatophores, and prop roots were tagged 

to enumerate trends in defoliation, leaf health, shoot number and length, and mortality 

of juvenile and adult plants or their structures. Visual oiling trends were documented 

through late 1995 and sediment samples for wet chemistry collected in 1996. Adult red 

mangrove trees at the most heavily oiled site (outer Eleanor Island) deteriorated over 

this time period, with moderate to heavy defoliation and soft, rotting prop roots. “Of 

marked trees, 20% were totally defoliated and appeared dead by June 1994” (Levings and 

Garrity 1995). Nine-month mortality of juvenile red and black mangrove plants was 5% 

at unoiled reference sites, 35% in heavily oiled areas on the protected side of the island 

and 50% in heavily oiled areas on the exposed side of Eleanor Island. It was predicted 

additional mortality would continue to occur.
68

The researchers also measured for signs of sublethal stress in adult trees: one 

to two years after the spill and cleanup, surviving red mangroves experienced graded 

negative responses in four measures of shoot growth and production, suggesting that 

sublethal long-term effects may be common in oiled mangroves. Sediments around trees 

experiencing these responses contained greater than 500 ppm total hydrocarbons (dry 

weight).

More follow-up observations are needed at these sites, but we are not aware of 

any extending beyond three years after the spill and cleanup.

Restoration

Trustees from state and Federal agencies and the responsible party developed 

a restoration plan for mangroves and associated habitats damaged in the spill. A compensatory plan provided mangrove and associated wetland habitat for fi sh, birds, and 

epibenthic communities at a site in the same watershed but not necessarily actually 

impacted by the spill. 

The responsible party purchased a former dredge disposal site in Boca Ciega Bay 

and deeded it into public ownership. This site contained degraded mangrove forest that 

was restored through increased tidal exchanges and removal of exotic plants and debris. 

On the bayward edge of the mangrove forest, smooth cordgrass (Spartina alternifl ora) 

was planted to create a fringing saltmarsh buffer that could eventually provide habitat 

for mangrove seedlings. A monitoring program was established with specifi c “success” 

criteria outlined, including vegetative cover and height of mangroves, absence of exotic 

species, and functional tidal exchanges. 

For Further Reading

Levings, S.C. and S.D. Garrity. 1995. Oiling of mangrove keys in the 1993 Tampa Bay oil spill. In: Proceedings of 

the 1995 International Oil Spill Conference, pp. 421-428.

Levings, S.C., S.D. Garrity, E.S. VanVleet, and D.L. Wetzel. 1997. Sublethal injury to red mangroves two years after 

oiling. In: Proceedings of the 1997 International Oil Spill Conference, pp. 1040-41.

Mauseth, G. S., J.S. Urquhart-Donnelly, and R. R. Lewis. 2001. Compensatory restoration of mangrove habitat 

following the Tampa Bay oil spill. In: Proceedings of the 2001 International Oil Spill Conference, pp.761-767.
69

Mangrove Glossary

Aerial roots- Roots that are formed in and exposed to air. In mangrove species (e.g., Rhizophora spp.), 

aerial roots develop into stilt roots (prop roots and drop roots) that anchor into the sediment, offering 

mechanical support and nutrient absorption. 

Anaerobic – Occurring with little or no oxygen.

Anchialine ponds – A rare Hawaiian ecosystem, consisting of pools with no surface connection to 

the ocean, but affected by tides. These pools have a characteristic water quality and support an 

assemblage of animals and plants, many of which are endangered. 

Anoxic - Without free oxygen. Aerobic metabolism (e.g., bacterial respiration) can consume dissolved 

free oxygen in water and soils, resulting in anoxic conditions that are detrimental to oxygen-breathing 

organisms. 

Bioaccumulate – Uptake of dissolved chemicals from water and uptake from ingested food and sediment residues. 

Biogenic - In mangroves, the trees themselves create the habitat. Biogenic also means “resulting from 

the actions of living organisms.” 

Canopy – topmost layer of leaves, twigs, and branches of forest trees or other woody plants. 

Chlorosis – abnormal condition characterized by the absence of green pigments in plants, causing 

yellowing of normally green leaves. 

Defoliation - The removal of the foliar tissues of a plant, resulting from mechanical (e.g., hurricanes), 

biological (herbivore), or chemical agents (e.g., plant hormones). 

Deposition - The accumulation of material on a substrate. In mangrove systems this term is typically 

used in relation to accumulation of surface sediment. 

Detritus – Non-living organic matter that is so decomposed that it is impossible to identify the original 

parent material. 

Drop roots- Roots that develop on a branch and begin as aerial roots but eventually grow into a 

substrate; these roots can provide mechanical support (e.g., Rhizophora spp.). 

Endpoint- A measured response of a natural resource to exposure to a contaminant, such as oil, in the 

fi eld or laboratory.

Eustatic sea level rise - The worldwide rise in sea level elevation due mostly to the thermal expansion 

of seawater and the melting of glaciers.

Evapotranspiration - The transfer of water from the soil, through a plant, and to the atmosphere 

through the combined processes of evaporation and transpiration. Evaporation is a function of surface area, temperature, and wind. Transpiration is a process of water loss through leaf stomatal openings, and is related to gas exchange and water transport within a plant. When the stomates open, a 

large pressure differential in water vapor across the leaf surfaces causes the loss of water from the 

leaves. 

Genotype - Genetic makeup of an individual organism. 

Hermaphroditic - Both sexes present in an individual organism. 
70

Infrared photography – Photography using fi lms sensitive to both visible light and infrared radiation. 

Live vegetation is particularly highlighted with infrared fi lms and so is a useful tool for aerial surveys 

of live and dead plants. 

Lenticel – A small, elliptical pore in the periderm that is a means of gaseous exchange. 

Mangal - a mangrove forest and its associated microbes, fungi, plants, and animals. 

Mangrove - a tree or shrub that has evolved the adaptations for growing in the intertidal zone (specifi -

cally, adaptations to salinity and fl ooded conditions). 

Microtidal – A tidal range of less than one meter. 

PAH - polynuclear aromatic hydrocarbon; also called polycyclic aromatic hydrocarbon, a component of 

oil. PAHs are associated with demonstrated toxic effects. 

Pneumatophore - A vertical extension of an underground root, with lenticels and aerenchyma to allow 

for gas exchange. Pneumatophores are characteristic of trees adapted to fl ooded conditions (such as 

Avicennia spp.) 

Prop roots - Roots that develop on a trunk and begin as aerial roots but eventually grow into a 

substrate; these roots can provide mechanical support (e.g. Rhizophora spp.), sometimes called “stilt 

roots.”

Propagule - Seedling growing out of a fruit; this process begins while the fruit is still attached to the 

tree. For some species of mangroves, propagules represent the normal, tidally dispersed means of 

reproduction. 

RSLR – relative sea level rise - The net effect of eustatic sea level rise and local geomorphoplogical 

changes in elevation. Local subsidence can make apparent RSLR much greater than eustatic rise. 

Sublethal effect- An effect that does not directly cause death but does affect behavior, biochemical or 

physiological functions, or tissue integrity. 

Vivipary – The condition in which the embryo (the young plant within the seed) germinates while still 

attached to the parent plant (synonymous with viviparity)

Weathering - Changes in the physical and chemical properties of oil due to natural processes, including 

evaporation, emulsifi cation, dissolution, photo-oxidation, and biodegradation. 

Wrack – Organic material, usually from dead seagrass or algae that wash up on shorelines. 

U.S. Department of Commerce

National Oceanic and Atmospheric Administration

NOAA Ocean Service

Donald L. Evans

Secretary, U.S. Department of Commerce

Vice Admiral Conrad C. Lautenbacher, Jr., USN (Ret.)

Under Secretary for Oceans and Atmosphere and NOAA Administrator

Margaret A. Davidson

Acting Assistant Administrator for

Ocean Services and Coastal Zone Management,

NOAA Ocean Service

January 2002