Charles D. Getter, * Geoffrey I. Scott, t and Jacqueline Michel. Research Planning Institute, Inc. 806 Pavillion Avenue. Columbia, South Carolina 29205.
THE EFFECTS OF OIL SPILLS ON MANGROVE FORESTS: A COMPARISON OF FIVE OIL SPILL SITES IN THE GULF OF MEXICO AND THE CARIBBEAN SEA Charles D. Getter, * Geoffrey I. Scott, t and Jacqueline Michel Research Planning Institute, Inc. 806 Pavillion Avenue Columbia, South Carolina 29205
ABSTRACT: Recent field studies at five oil spill sites where mangroves were affected provide a broad base of information on the response of mangrove communities to oiling. Three study sites in Florida (two in the Florida Keys, one in Tampa Bay) and two in eastern Puerto Rico were visted in 1978, 1979, and 1980. At each site, impacts on mangroves were assessed by the compartmental method, which uses statistical comparisons of ecological parameters between impacted and comparison stations and produces an array of biological and geomorphic data sets that allows spill sites to be compared. Despite many differences in the size of the spills and the spill sites, the responses of the oiled-mangrove communities were similar in terms of tree mortality; leaf defoliation, deformation, and stunting; seedling deformation and mortality; lenticel expansion; adventitious growth of pneumatophores; and changes in the density and distribution of plants and animals. Each spill site differed mainly in the magnitude of the stress response. Observations of the spills showed that differences in the physical environment, such as the degree of exposure to waves and currents and geomorphic features like the terrain, greatly influence the distribution and persistence of oil within different mangrove forest types. From these studies, mangrove forest types can be ranked by their predicted sensitivity to oil. This differentiation in ranking increases the value of the Environmental Sensitivity Index, especially where it is desirable to assign priorities in a campaign to protect oil-sensitive habitats from oil spills along mangrovedominated coastlines.
Figure 1. Mangroves of the Gulf of Mexico and the Caribbean that received small to medium amounts of spilled oil (solid circles) during the 1970s. The insets show the areas that were the hardest hit: southern Florida and Puerto Rico.
Introduction Mangroves in the United States are several species of plants that may form a productive coastal community consisting of four dominant flowering plants: the red mangrove (Rhizophora mangle), the black mangrove (Avicennia germinans), the white
mangrove (Laguncularia racemosa), and the buttonwood (Conocarpus erectus). Although mangroves are widely distributed throughout the Gulf of Mexico and the Caribbean Sea, welldeveloped mangrove communities in the United States are distributed mainly in the southern part of Florida and in Puerto Rico. Mangroves are salt-tolerant plants (halophytes) with welldeveloped, stabilizing root systems. Their ability to tolerate salt
* Also affiliated with the Belle W. Baruch Institute for Marine Biology and Coastal Research, University of South Carolina, Columbia, South Carolina 29208 t Also affiliated with the School of Public Health, University of South Carolina, Columbia, South Carolina 29208.
535
536 and to stay planted in surf and currents is related to this unique root system. The roots are partially submerged and are therefore the most vulnerable target of floating oil reaching mangrove communities. Damage to roots may affect the respiratory and osmoregulatory capability of the plant, which may lead to subsequent death. In addition to providing lumber, mangroves have direct value as spawning and nursery grounds for fish as well as habitats for recreational fisheries. They are useful in the control of erosion and in protection from storm hazards. Their indirect value is based on high productivity, wildlife habitat, and such biochemical contributions as capacity to recycle nutrients, high storage of organic matter, and the capacity to sequester heavy metals and other toxic materials. In the 1970s, with increased oil tanker traffic in the Gulf of Mexico and the Caribbean Sea, numerous small to moderate spills (10,000 to 460,000 gallons (gal) ) of oil reached U.S. mangroves. These spills affected mainly the mangroves of Florida and Puerto Rico (Figure 1). Without a doubt, an intensive monitoring program in the Gulf and the Caribbean would find hundreds of small to moderate oil spills.
areas were selected during the flights for subsequent ground visits. Figures 2, 3, and 4 give the localities and designated names of all sites visited during this study.
TAMPA BAY
Objectives and approach The study reported here had three objectives: to examine the effects of medium to small oil spills on U.S. mangrove communities, to establish the factors that appear to control the loss of environmental value (damages) after oiling, and to provide a base of information concerning trends in the response of mangrove communities to oiling; the latter is necessary in establishing priorities for protecting mangrove forest types during oil spills. To accomplish these objectives, visits were made between December 1978 and May 1980 to five localities where oil spills had occurred in mangroves. Table 1 gives the dates of these visits and the details of each oil spill. Each spill site and surrounding comparison areas were examined in aerial surveys and photographed at low altitude to locate areas of defoliation in coastal mangroves. Areas of significant defoliation and comparison
Figure 2. In October 1978, the ship Howard Star released approximately 40,000 gal of Bunker C and lubricating oils onto at least 20 kilometers (km) of mangrove-dominated shoreline (broken line). Five sites with significant defoliation (solid circles) appeared by the following winter. Studies at these sites revealed damage to the plants and animals of the mangrove community (Gundlach, Scott, and Davis, 1979; Getter, Snedaker, and Brown, 1980).
Table 1. Sites Visited in the Mangrove Forest Study: Locations, Dates, and Data on Oil Spills Spill source and locality
Dates visited
Howard Star Tampa Bay, Florida
December 1978 July 1979 January 1980 March 1980
Gar bis Lower Keys, Florida Peck Slip Medio Mundo, Puerto Rico
May 1980
Ensenada Honda, Puerto Rico Keys Marsh, Florida a
December 1978 March-April 1979 October 1979 October 1979 May 1980
Sites studied
Date of spill
Product spilled
Amount spilled, IO3 gal 3
References
October 1978
Bunker C Lubricating Oils
40 (10)
Getter, Snedaker, and Brown (1980)
July 1975
Crude emulsion
63-126(40)
Chan (1977) Getter et al. (1980b)
Machos Creek Comparison M Pasaj e Comparison P Ensenada Honda
December 1978
Bunker C
440-460 (26)
Getterei al. (1980a)
Unknown
Unknown
Unknown
This report
Keys Marsh
Unknown
Motor oil
(5 quarts)
This report
Port Sutton The Kitchen Wolf Creek NE Park Simmons Park Comparison Sugar loaf Comparison
The numbers in parentheses indicate the quantity of oil (in thousands of gallons) estimated to have reached the shore.
537 LOWER K E Y S
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SADDLEBUNChK
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KEY
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10 KM STRAITS
OF FLORIDA
Figure 3. In July 1975, the ship Garbis discharged 63,000 to 126,000 gal of crude oil onto at least 20 km of mangrove-dominated shoreline (broken line). Several patches of defoliated fringing mangroves appeared subsequently along Boca Chica, Saddlebunch, and Sugarloaf Keys. Keys Marsh was the site of an unrelated small release of motor oil in a high marsh area.
Figure 4. In December 1978, the barge Peck Slip released 440,000 to 460,000 gal of Bunker C oil onto at least 10 km of mangrovedominated shoreline. Medio Mundo was the most heavily damaged area, especially sites at Machos Creek and Pasaje (solid circles). Significant damage to the plants and animals of the mangrove community was observed at these sites (Gundlach et al., 1979; Getter et al., 1980a). In October 1979, a heavily defoliated mangrove forest was studied in Ensenada Honda.
Methods
Results and discussion
After aerial surveys, ground stations were visited and a rapid assessment method was implemented to measure various ecological components or compartments (i.e., herbivores on prop roots) in the mangrove forest community. This method, termed the comparirnental method, was used to assess biological changes by testing the null hypothesis that "no significant biological changes were induced at defoliated areas by an oil spill." Impacts were assessed by comparing ecological parameters at impacted and comparison stations. The examination of these parameters allowed hypothesis testing. Establishing that the presence of oil was the only difference between impacted and comparison stations allowed observed differences in various biological parameters to be attributed to oil. Significant parameters were then examined along a degree of oiling gradient. Transects were selected to cross from the outer edge of mangrove forests and over the interior berm or high-tide swashline. The intersection of single trees along the transect line determined the location of stations. Thus, stations consisted of single trees, including the canopy and trunk as well as all prop roots and sediment surface within 1.78-meter (m) radius of the station center. This resulted in an area of 10 square meters (m2) around the station center within which 24 parameters were measured (Figure 5). A detailed description of methods is given in another report (Getter et al., 1980a).
Natural variability (parameters 1 through 6 in Figure 5). Pairs of impacted and comparison sites at Medio Mundo and Tampa Bay proved to lie in forests that were statistically of the same height and the same density, and growing along slightly sloping shorelines. These sites are exposed to open bays that receive light to moderate wave energy (waves smaller than 10 centimeters), are restricted in tidal water movement, and experience low-flow velocities; large detrital debris was not observed to be flushed into adjacent waters on outgoing tides. These characteristics indicate that both impacted and comparison stations lie within forests typical of the fringe mangrove forest type (Snedaker and Pool, 1973). These parameters were noted on a tape recorder and film at the other three spills and showed at least an intuitively significant similarity between impacted and comparison pairs. Distribution of oiling (parameters 7 through 10 in Figure 5). Oiling at pairs of controlled and impacted sites at Medio Mundo and Tampa Bay were statistically tested for significance. Tests show a significantly greater extent of oiling of all compartments, including oiled adult trees, oiled substrate, oiled seedlings, and oiled canopy-dwelling animals. These parameters were noted on a tape recorder and film at the other three spills and showed at least an intuitively significant difference between impacted and comparison pairs. The oil had
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Spill Sites, Sources, and Localities
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Tested Parameters
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1 INNER FRINGE IMPACT
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To Control for Natural Variability 1. 2. 3. 4. 5. 6.
Height of forest Density of forest Slope of shoreline Exposure to wave activity Tidal water flow The movement of detrital debris
(No significant differences betwee n impacted-comparison site pairs)
Ì
7. 8. 9. 10.
Adult tree oiling Substrate oiling Seedling oiling Consumer oiling
Tests for Biological Changes 11. 12. 13. 14. 15. 16. 17. 18. 19 20. 21. 22. 23. 24
Mangrove mortality Air hole counts Defoliation Root mortality Leaf length and weight Seedling mortality Leaf deformities Epiphytic density Epiphytic desiccation Epiphytic organic content Epiphytic carbon/nitrogen ratios Tree snail density Tree crab density Infaunal richness, density, and diversity
Key to symbols —,
Heavy damage
[7]
Light to moderate damage
LI
No damage observed or measured
• •• *
• •• •
• •
• • •• • •
• • • •• • ••
• • • • •
1
L1
*^ S u b s t r a t e v^^ Oiling
^
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To Determine the Presence and Extent of Oiling
--—
-
. •
11-75% 11-50%
• . • ••
1l·25 %
-
Figure 5. Parameters examined in tests for biological damage at the five oil spill and comparison site pairs.
Figure 6. In the inner fringe impact (A), oil is pushed into a forest and deposited on an inner berm. Such an impact is seen in a ground view of the Machos Creek site (B), resulting in defoliation (C).
weathered significantly at the Garbis site, making comparisons difficult. Tests for oil-induced damage (parameters 11 through 24 in Figure 5). Since the impacted and comparison sites share similar independent variables (major producer species, forest types, and other characteristics), the only major difference between impacted and comparison areas is the presence of oil. Therefore, significant differences between dependent variables at impacted and comparison stations reflect biological damage related to oiling. Figure 5 lists significant effects on plants and animals. The heaviest defoliation of trees, mortalities of seedlings, and mortalities of canopy-dwelling animals were found where the heaviest oiling occurred. The distribution of heavily oiled areas is controlled to a great extent by geomorphic features within the forest. Three types of impact were observed: inner fringe impact, outer fringe impact, and inner basin impact. An inner fringe impact was observed at Medio Mundo (Machos Creek). Defoliation is concentrated on the inner mangroves, which are on the inner berm (Figure 6A). The Machos Creek site became heavily defoliated within 2 months of oiling (personal communication, Gilberto Cintron, Puerto Rico Department of Natural Resources) and remained defoliated 18 months later. The substrate and prop roots remained oiled even after Huf ricane David in 1979. Figures 6B and 6C show the extent of defoliation at the Machos Creek site. Such defoliation allows more light to penetrate to the forest floor, elevating temperatures and salinities significantly. Under such conditions numerous seedlings may appear to thrive in oiled substrates, but closer examination indicates lowered survival and leaf deformities to be common. Commonly, dead plants and animals will be found in the area of an inner fringe impact. In fact, the degree of oiling is often related to the degree of substrate oiling. An outer fringe impact was observed at Tampa Bay (NE Park and Simmons Park). Defoliation is concentrated in the outer mangroves (Figure 7A), and the extent of mortality is apparently related to two factors. The first is exposure to waves and currents;
the second is the penetration of oil into the substrate, aided greatly by burrowing crabs. In exposed areas with few burrows, oil was removed within a few weeks at Tampa Bay (Simmons Park) and Medio Mundo (Pasaje). Little or no impact was observed at these specific sites. However, at sites that are sheltered and have extensive burrows (Ensenada Honda), the impact in the outer fringe was extensive (Figure 7B). Prop roots are the principal
lOUTER FRINGE IMPACT
ATI
^^^TSub e t r a t e ì
I I · 76 % [1-50%
Lost Productivity/
/
1 |
I I · 25 %
LJiJLeieeiBee
Figure 7. In the outer fringe impact (A), oil is trapped on and in front of a steep coastal berm. This concentrates the spilled oil in a relatively small area, resulting in defoliation along the shoreline, as observed at Ensenada Honda (B). Where oil has impacted red mangrove prop roots along a more sheltered fringing forest on a steep coastal berm, extensive damage may occur to roots, as at NE Park Site of Tampa Bay (C).
539 INNER BASIN IMPACT
Figure 8. In the inner basin impact (A), oil is carried over the coastal berm or through tidal flats into a sheltered, mangrove forest basin. The oil is more dissipated since it is not trapped against a berm, and damage is more scattered. Stress symptoms characteristic of impacts like these include damage to black mangrove pneumatophores (B) and the induction of leaf deformities (C).
N.
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Oil spill vulnerability
Forest type
/
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º Low potential biological damage Riverine forest Overwash forest
•
.
Moderate potential biological damage Dwarf forest Outer fringe forest
•
•
. •
• •
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High potential biological damage Inner fringe forest Basin forest
.
Key to symbols
IS High ÎModerate ú Low to None
Figure 9. The predicted, relative effects of spilled oil on various mangrove forest types ranked by susceptibility, persistence, and sensitivity. target in outer fringe impact, with potential losses of substrate for epiphytes and attached filter feeders (Figure 7C). An inner basin impact was observed at Tampa Bay (Simmons Park), Lower Keys (Keys Marsh), and Medio Mundo (Machos Creek). Oil, during high tides, had moved up and over the coastal berms and into shallow basins behind them. This caused oil to be spread over a wide area (Figure 8A) and the impact to be less well defined. The target of such an impact is often black mangrove
roots (Figures 8B and 8C). In fact, many stress symptoms that indicate chronic (sublethal) stress are not readily observed. Other mangrove forest types (described by Snedaker and Pool, 1973) appear to be less sensitive to oil-induced damage. Overwash mangrove forests have been observed oiled in Medio Mundo and at Tampa Bay. These forests grow in small, nearshore islands that are subjected to moderate to heavy waves and currents. Oil would not persist in these environments. Since overwash forests often have no intertidal substrate, the potential for substrate oiling is greatly reduced. Riverine mangroves, subject to freshwater runoff, have not been observed oiled. Indeed, during the Garbis and Peck Slip spills extensive mangrove areas in Florida and Puerto Rico appear to have been protected from oil spilled in coastal waters by both freshwater runoff and ebb-tidal flow. Microtidal flooding would not be expected to carry oil far inland except under severe tidal and climatic conditions. It is conceivable, however, that massive amounts of oil could reach far inland after an oil spill during a hurricane or tropical storm. Figure 9 lists the predicted effects of oil spills on six different mangrove forest types. These effects are based on the potential susceptibility to oil impact, the duration or persistence of oil, and the sensitivity of the forest type. Figure 9 indicates that riverine and overwash mangrove forest types have the lowest potential for biological damage because their high exposure to waves and tides decreases the duration of exposure to oil. Dwarf and outer fringe mangrove forests have a moderate potential for biological damage. This is due to the low susceptibility of dwarf forests to oil spill impacts and to the high exposure of outer fringe forests to waves and tides, which decreases the duration of exposure to oil. Inner fringe and basin mangrove forest types have the highest potential for biological damage. This is attributed to their low exposure to waves and tides, which increases the duration of exposure to oil. The long exposure to oil results in chronic stress and disruptions in normal physiological, photosynthetic, and osmoregulatory pathways, which may result in tree mortality.
Conclusions and applications Despite many differences between spill sites, there were many parallel responses by mangrove communities. The major variation in response appeared in the extent and distribution of oil-induced damage. This variation in oil impact response appears to be related to geomorphic features and kinetic energy levels (waves and currents), which control the distribution and persistence of spilled oil. This, in turn, controls the duration and intensity of the "toxicity test" for oiled mangroves. On one extreme, exposed mangroves are subject to acute (shore-term), higher concentration exposures. Between these extremes, other forest and impact types seem to be "sorted out" by potential severity of response to spilled oil. In preparing contingency plans for oil spills, it is necessary to establish priorities for protecting sensitive areas (Hayes et al, 1980). On shorelines that are predominantly mangroves, it is ineffective to classify all mangroves as the most sensitive areas (i.e., the whole shoreline becomes the most sensitive area). In the past, some efforts were made to assign priorities for protection on the basis of mangrove forest types. Different mangrove forest types were delineated in a recent survey of oil-sensitive environments in southeastern Florida (Getter et al., 1980b). Coastal berms
540 within mangroves were located to allow the use of these berms in protecting sensitive environments. The most effective application of these concepts will follow a careful delineation of mangrove forest types in oil-threatened U.S. waters.
Acknowledgments Field work for this report was funded by the Office of Marine Pollution Assessment, National Oceanic and Atmospheric Administration (NOAA); the Florida Department of Natural Resources; and the South Florida Regional Planning Council through funding from NOAA's Coastal Energy Impact Program. The artwork was done by Charlotte Johnson, editing by Diana Gaines, and typing by Phyllis Carter-Frick. We are indebted for the many opportunities we have had to visit and discuss oiled mangroves with Drs. Samuel C. Snedaker (Rosenstiel School of Marine and Atmospheric Sciences, University of Miami), Aerial Lugo (Institute of Tropical Forestry, San Juan), and Gilberto Cintron (Puerto Rico Department of Natural Resources). Much of the conceptual information presented in this report resulted from these visits and discussions.
References Chan, E. I. 1977. "Oil Pollution and Tropical Littoral Communities: Biological Effects of the 1975 Florida Keys Oil Spill. In Proceedings of
the 1977 Oil Spill Conference, American Petroleum Institute, Washington, D.C., pp. 187-192. Getter, C. D., J. M. Nussman, E. R. Gundlach, and G. I. Scott. 1980a. Biological changes of mangrove and sand beach communities at the Peck Slip oil spill site, eastern Puerto Rico. Report to the Office of Marine Pollution Assessment, National Oceanic and Atmospheric Administration. Getter, C. D., J. Michel, G. I. Scott, and J. L. Sadd. 1980b. The Sensitivity of the Coastal Environments and Wildlife to Spilled Oil in Southeastern Florida. Report to South Florida Regional Planning Council. Getter, C. D., S. C. Snedaker, and M. S. Brown. 1980. Assessment of biological damages at the Howard Staro// spill site. Hillsborough Bay and Tampa Bay, Florida. Department of Natural Resources. Gundlach et al. (E. R., J. Michel, G. I. Scott, M. O. Hayes, C. D. Getter, and W. P. Davis). 1979. "Ecological assessment of the Peck Slip Oil Spill in Eastern Puerto Rico." In Proceedings of the Ecological Damage Assessment Conference, Society of Petroleum Industry Biologists, p. 303. Gundlach, E. R., G. I. Scott, and W. P. Davis. 1979. Preliminary Assessment of the Howard Star Oil Spill Site, Tampa Bay, Florida. Report to the Regional Response Team. Hayes, M.O., E. R. Gundlach, and C. D. Getter. 1980. "Sensitivity Ranking of Energy Port Shorelines." In Proceedings of the Specialty Conference on Ports—1980. American Society of Coastal Engineers, pp. 697-708. Snedaker, S. C , and D. Pool, 1973. The Role of Mangrove Ecosystems in the Maintenance of Environmental Quality and a High Productivity of Desirable Fisheries. Report to the Bureau of Sport Fisheries and Wildlife.
