This document is an excerpt from the executive summary of the final synthesis report of the oil spill. It includes parts of the introduction and conclusion. To download the complete executive summary click here.
Unplanned Environmental Assessments
Environmental assessments ideally consist of monitoring natural variation or of studying planned environmental modifications, such as the release of heated effluents from a power plant, and their effects. In both cases the kinds of ecological data to collect, degree of replication, frequency of sampling, and so on can be carefully designed. In contrast, unplanned environmental modifications, such as those caused by major oil spills, generally cannot be studied using optimal designs because pre-existing monitoring data are not available as balanced sampling at both affected and unaffected areas. In such cases all that can be done in designing an assessment is to make the best use of available data.
Even optimally designed environmental assessments lack the rigor of controlled experiments, and conclusions about cause and effect must consider factors other than the presumed one. For example, in comparing areas affected and unaffected by an oil spill, there may be additional area-specific differences not related to the spill that explain some of the observations. Because effects of major oil spills on benthic communities can be severe, studies employing suboptimal designs may detect postspill changes, as was the case after a major oil spill at Bahia Las Minas, Panama.
Objectives of the Study and Organization of the Report
The study has two main objectives:
- to monitor the long-term changes that may occur in the distribution and abundance of marine organisms as a result of the 1986 oil spill at Bahia Las Minas, and
- to understand the ecological processes causing any observed changes.
This summary is organized into sections about the spread and stranding of oil under different sets of environmental conditions, the concentration and characterization of the spilled oil in environmental and organismal samples, and effects of the spill on particular populations and communities.
Here, following a brief review of oil spills in tropical seas, we describe the Bahia Las Minas ecosystem and the 1986 oil spill, and relate its size to that of other major oil spills. The region of the spill has a history of development and industrialization, and was polluted by a major tanker spill of diesel oil and Bunker C in 1968. After reviewing this background, we briefly describe relevant biological studies that were conducted prior to the 1986 spill, and review how existing information was incorporated into the design of each element of the study. A listing and map of all study sites is provided in Volume II.
Oil Spills in Tropical Seas
At least 157 major oil spills, defined as more than 1,000 bbl, originated from ships and barges in the tropics between 1974 and 15 June 1990. Of these, 99 occurred in coastal or restricted waters, mostly near such potentially vulnerable ecosystems as coral reefs, seagrass reef flats, sand beaches, and mangrove forests. There were 24 tanker or barge spills near Caribbean coastlines, as well as spills from other sources. At least 19 refineries ring the Caribbean. A pipeline for transshipment of crude oil crosses Panama, and there are facilities in Bonaire, Curacao (closed in 1985), the Bahamas, and Grand Cayman for transfer of crude oil from supertankers to smaller tankers. Spills occurred recently at the Caribbean end of the pipeline across Panama and at two of the refineries, including the 1986 spill in Panama.
Despite the frequency of spills, there has been very little study of effects of oiling on nearshore tropical communities. There is general agreement on the biological and economic value of mangroves forests and on the vulnerability of mangroves and their biota to oiling. Similarly, reef flats are known to be highly productive, to serve as nursery and foraging areas for spiny lobsters, and to be vulnerable to oiling. However, knowledge of effects of oil on mangroves and associated species is limited, and reef flats have been neglected. Most studies of mangroves have been one-time efforts following spills of opportunity, and thus lack baseline, prespill data and seldom include long-term postspill monitoring.
In contrast to intertidal habitats, the vulnerability of subtidal coral reefs and seagrass beds to oil spills has been much more controversial, despite a general paucity of data.
The Bahia Las Minas Ecosystem
The pattern, magnitude, and persistence of effects of the 1986 Bahia Las Minas oil spill can only be understood in the context of the special characteristics of tropical embayments. Bahia Las Minas (Fig. 1) is a topographically complex, shallow-water, embayment whose margins were dominated at the time of the spill by extensive mangrove forests, seagrass beds, and coral reefs. These habitats characterize Caribbean shores and many other regions of the tropics worldwide. In almost every case, the physical structure of these environments is built, stabilized, and maintained by a few species of relatively large, long-lived, photosynthetic organisms. Together they buffer the coastal zone from freshwater runoff and erosion from the land, and wave energy from the open sea.
Mangroves, seagrasses, corals, and coralline algae produce enormous quantities of biogenic structural materials (wood, rhizomes, and limestone) whose very presence baffles water movements and promotes the deposition of sediments. The red mangrove Rhizophora mangle forms dense, anastomosing thickets of prop roots and trunks that extend outward from land. Mangroves protect the shore from the impacts of debris such as floating logs and waves, reduce water circulation, increase sedimentation, and provide deep shade and shelter at all but the most open coastal margins. Seagrasses, especially turtlegrass (Thalassia testudinum), form dense beds that may extend for hectares seaward of the mangrove fringe, depending on the local bottom profile, size of the embayment, and degree of protection by coral reefs. Seagrass beds are supported by dense root and rhizome mats up to half-a-meter thick that stabilize sediments against erosion. In addition, long seagrass leaves slow water movements and increase sedimentation. Coral reefs are built primarily by a few species of corals and crustose coralline algae that produce the limestone framework and cement that is filled by skeletal debris of associated organisms. The physical complexity of reefs depends on the growth form of dominant coral species; branching corals form dense thickets whereas massive species form a more open framework. Regardless, reefs are the outer defense of the land against the sea, with calm-water, sediment-trapping lagoons and reef flats behind.
Mangroves, seagrass beds, and coral reefs provide habitat for a great diversity of species that depend largely or entirely on biogenic characteristics of the habitat, much like the animals and herbaceous vegetation of a deep forest. Moreover, many of these organisms consume and thereby strongly influence the species composition and abundance of the habitat-structuring organisms on which they depend. The best known cases are grapsid crabs feeding on mangrove seeds, and sea urchins, schooling fishes, territorial damselfish, and snails feeding on seagrasses, reef corals, and fleshy macroalgae. The principal groups of associated consumers and other organisms studied in this report are listed in Table 1.
Nearly all the oiled study sites and many of the unoiled mangrove sites were in Bahia Las Minas. However, many unoiled sites were northeast of this embayment in a region referred to here as the Costa Arriba, from Maria Chiquita to Isla Grande (Fig. 1). These included unoiled open-coast mangroves, reef flats (plus one site west of the bay), seagrass beds, and coral reefs. Environmental conditions and other factors differ between the two areas. Implications of these differences to findings of the study are discussed in later sections.
The 1986 Oil Spill at Bahia Las Minas, Panama
In 1986 a major oil spill polluted Caribbean coastal environments of Panama, including a biological preserve at a marine laboratory of the Smithsonian Tropical Research Institute (STRI). For the reef flat at this site, baseline biological and environmental data for some parameters had been collected for more than 15 yr There had also been surveys and short-term studies of reef flat gastropods, reef flat stomatopod crustaceans, coral reefs, mangroves, the epibiota of fringing mangrove roots, and seagrass communities. These prespill studies provided a relatively comprehensive background for assessing biological effects of the spill. Furthermore, observations of effects of the spill began as oil was washing ashore. Such promptness is important because many ecological changes start immediately after such acute pollution, and direct observations of immediate postspill die offs may be important.
The oil spill occurred on 27 April 1986 at a petroleum refinery at Bahia Las Minas, Panama ( ). Approximately 38.3 million L (240,000 bbl) of mediumweight crude oil drained from a ruptured storage tank. The oil was 70% Venezuelan and 30% Mexican Isthmian, with a specific gravity of 27° at 15.6°C (American Petroleum Institute) or about 0.89 g/cc. Approximately 22.3 million L (140,000 bbl) flooded through the containment dike around the storage tank and overwhelmed separators and a retaining lagoon. In May 1986 a refinery official reported recovery of 9.6 million L (60 000 bbl) of oil from the sea. We do not know, however, how much oil was not recovered and can only surmise that the volume of oil that spilled into the sea from the grounds of the refinery was at least 9.6-16.0 million L (60,000100,000 bbl).
The volume of this spill was greater than that of any other oil spill reported near coral reefs and mangroves in the tropical Americas, such as the 1975 Epic Coloctronis and 1978 Peck Slip spills at Puerto Rico. Compared with recent major spills from oil tankers in other tropical areas, however, the Bahia Las Minas spill was moderate in size (among the top 20% by volume). Considering some other well studied oil spills, the Bahia Las Minas spill was much larger than the 1969 spill from the barge Florida near Woods Hole, Massachusetts, similar in size to the 1969 Santa Barbara spill, and much smaller than the spills from the tankers Torrey Canyon (1967),Amoco Cadiz (1978), and Exxon Valdez (1989).
During the first six days after the spill onshore winds held the spilled oil in Bahia Cativa, adjacent to the refinery (Fig. 1). Shifting winds and runoff from rains then pushed a large quantity of oil out to sea past a boom placed across the mouth of this embayment. Starting 6 May 1986, aircraft sprayed approximately 21,000 L of the dispersant Corexit 9527 (Exxon Chemicals) over oil slicks. A C-130 aircraft was observed spraying dispersant from a very low altitude (at or under tree level) at the mouth of Bahia Cativa, including areas near the coastlines of western Isla Largo Remo, northwestern Isla Payardi, and Punta Muerto. It was reported that more than 11,000 L of dispersant were used in this spraying alone. In addition, a small cropduster aircraft was observed spraying dispersant on slicks between Islas Naranjos and the mainland. Other such applications of dispersant were observed off the breakwater of the Panama Canal, offshore of Bahia Las Minas, and offshore of Portobelo (Fig. 1). Additional back-pack spraying of dispersant was used in some areas of mangroves.
No dispersant was sprayed near Punta Galeta. Application of the dispersant nine days after the spill instead of within the first 24 hours, as well as calm sea conditions, probably rendered chemical dispersion ineffective. Although some coastal areas were exposed to dispersant, particularly Bahia Cativa and areas near Islas Naranjos, many oiled areas, including Punta Galeta. were not directly exposed to this compound. The overall dosage of dispersant was low based on a 1:20 dispersant:oil ratio from laboratory studies. Such a ratio would have required a total of 480,000800,000 L of dispersant, an order of magnitude greater than the estimated total used. Localized effects of the dispersant may have occurred, particularly at sites between Isla Largo Remo and Punta Muerto, and in Bahia Cativa. However, the limited use of dispersant cannot explain the widespread subtidal biological effects reported later in this report.
By 15 May oil had spread along the coast and washed across fringing reefs into mangroves, small estuaries, and sand beaches within 10 km of the refinery. During the first two months after the spill, the distribution of oil was surveyed from low-flying aircraft between Rio Chagres, 27 km west of the refinery, and Punta San Blas, 98 km to the east. Surveys by helicopter, airplane, foot, and boat were conducted from Rio Chagres to Nombre de Dios. During these surveys visual assessments were made of the degree of oiling (heavy, moderate, light, or absent) and of the habitats and types of organisms obviously oiled or affected by oiling.
The shoreline deposition of extensive, black oil slicks was limited to the coast between Isla Margarita and Maria Chiquita with the exception of two partially isolated lagoons in Bahia Las Minas (Fig. 1; the lagoons east of Isla Margarita and southwest of Isla Largo Remo). The length of heavily oiled coastline was approximately 82 km (straight-line distance = 11 km) and included more than 1,000 ha of mangroves and extensive intertidal reef flats and subtidal reefs. Only a few patches of oil were observed to strand east of Maria Chiquita and west of the entrance to the Panama Canal. However, oily sheens were observed offshore from Isla Margarita to Nombre de Dios. Approximately one month after the spill, oily sheen was observed offshore of Punta San Blas, The offshore slicks appeared to be transported by the easterly coastal current, aided by an unusual period of southerly winds. Instances of strandings of black slicks occurred northeast of Portobelo where generally prevailing northeasterly winds were likely to deposit slicks coming from far offshore.
In similar habitats within the heavily polluted area apparent degrees of oiling were highly variable. Probable causes of this heterogeneity included distance from the refinery, directions of movement of the spilled oil, and water depth. The greatest amounts of oil in mangroves, reef flats, and seagrass beds occurred within a few kilometers of the refinery. There was obviously less oil in these habitats at Islas Naranjos and Isla Margarita (Fig. 1). Large differences in visible oiling also occurred on a much smaller scale of a few hundred meters, depending on coastal orientation. Much of the oil escaping from Bahia Cativa spread to the west. Accordingly, coasts that faced north to northeast were much more heavily oiled than coasts that faced west or south. Furthermore, seasonal low tides occurred between 10 and 19 May 1986, causing oil to accumulate along the seaward margins of reef flats. As a result, visible oiling was heaviest in intertidal habitats just above mean low water, such as mangrove roots and associated sediments, reef flat seagrass beds, coral rock, and beaches.
Chemical analyses of petroleum hydrocarbons in surface sediments generally verified these visual assessments of variability in degrees of oiling (Burns, Chap. 3). Samples collected five months after the spill contained concentrations of oil as high as 372,856 micro grams oil/g sediment and as low as 1,830 micrograms/g in heavily oiled mangrove surface sediments. Subtidally, surface sediments from heavily oiled seagrass beds ranged from 97 to 24,555 micrograms/g, and 19 to 715 micrograms/g at heavily oiled coral reefs. There was considerable variability in concentrations of oil among samples at a given site, confirming observations of small-scale patchiness as well.
Several different procedures were used to clean up the spilled oil. Some oil was removed from the sea using “skimmers” and shore-based pump trucks. As noted above, approximately 9.6 million L (60,000 bbl) of oil were recovered. Channels were dug through mangroves, apparently to drain oil from these areas. However, these channels appeared to increase the movement of oil beyond the seaward mangrove fringe to inner areas, as well. Disturbance from workers crushed windrows and may have increased subsequent erosion. In other areas oiled rocks, rubble, and debris were physically removed and seawater was sprayed onto sandy areas. Skimming and pumping floating oil appeared to be effective ways to recover oil from this kind of shallow-water spill. Shallow water and mangroves impeded many of the kinds of cleanup operations deployed after major oil spills, perhaps for the better, because some of these procedures can be environmentally or biologically destructive.
During the five years since the spill oil slicks have been regularly observed above coral reefs at Bahia Las Minas and along the mangrove fringe. The appearance of these slicks ranged from metallic sheens to brown patches. Slicks appeared to originate mainly from fringing mangroves, where much of the spilled oil washed ashore. As dead mangrove trees (Rhizophora mangle) decayed, the wooden physical structure disappeared, followed by erosion of oiled sediments. Rhizophoraseedlings (survivors, recruits, and planted individuals) apparently have not prevented this erosion. Some slicks also appeared to come from oiled landfill beneath the refinery.
The Bahia Las Minas Ecosystem
Mangrove forests, seagrassbeds, and coral reefs characterize Caribbean shores, such as Bahia Las Minas, and many other regions of the tropics worldwide. The physical structure of these habitats almost always is built, stabilized, and maintained by a few species of large, long-lived, photosynthetic organisms. Collectively, biogenic habitats buffer tropical coasts from freshwater runoff and erosion from the land, and wave energy from the open sea.
The red mangrove Rhrizophora mangle protects shores from impacts of debris and waves, reduce water circulation, increase sedimentation, and provide shade and shelter. Seagrasses, especially turtlegrass (Thalassia testudinum), may cover hectares seaward of the mangrove fringe, and are supported by dense root and rhizome mats that stabilize sediments against erosion. Coral reefs are built primarily by a few species of corals and crustose coralline algae that produce the limestone framework and cement that is filled by skeletal debris of associated organisms. Reefs are the outer defense of the land against the sea, with calm-water, sediment-trapping lagoons and reef flats behind.
Mangroves, seagrass beds, and coral reefs are habitats for a great diversity of species. Many of the associated animals consume and thereby strongly influence the species composition and abundance of the habitat-structuring organisms on which they depend. The best-known cases are grapsid crabs feeding on mangrove seeds, and sea urchins, schooling fishes, territorial damselfish, and snails feeding on seagrasses,reef corals, and fleshy macroalgae.
Fate of the Spilled Oil and Environmental Damage
Pattern of Oiling and Methods of Assessment
The fate of the spilled oil fell conveniently into three phases: (1) spillage and entrapment in Bahia Cativa (Fig. 1), (2) escape and dispersal into the larger Bahia Las Minas ecosystem and beyond, and (3) residual storage, chemical degradation, release, and redispersal thereafter. Details of the spill are described in Volume II and elsewhere. Most of Bahia Cativa was bathed in oil for 6 d, where biological effects were the most severe. The pattern of oiling afterward was more complex, and depended on many factors including distance from the refinery, directions of oil and water movement, adjacent topography, and sea level. Eventual deposition and storage of oil were greatest in low-energy environments, especially mangrove channels and streams, and seagrass sediments. Chemical dispersant was applied in some areas and may have contributed to localized toxic effects nearby. The restricted use of dispersant could not, however, have caused the widespread, subtidal reduction in living corals observed and other widespread biological damage. In particular, the extensive mortality of subtidal corals on the Galeta. reef was far removed from areas where dispersants were used.
The initial assessment of where the oil went, and in what quantities, was based on visual assessment from land, sea, and air. Sites along the coast in each habitat type were qualitatively ranked into three or four categories ranging from heavily oiled to unoiled. Subsequent chemical analyses of amounts and types of hydrocarbons present in samples of sediments, water, and organisms were generally in excellent agreement with ranks based only on visual assessment (Burns, Chap. 3). Moreover, there was also unprecedented agreement between results of different chemical analytical methods (ultraviolet fluorescence and gas chromatography) shown by highly significant correlations for very large numbers of samples of sediments from different habitats and of bivalve molluscs, spanning as much as 4 yr of sampling..
These results further justify the “experimental treatment” approach for assessment of biological effects by analysis of variance, which was the cornerstone of most of the biological studies. Nevertheless, levels of oiling sometimes varied greatly within treatments in the same habitat type, so that a “dose-response” approach (e.g., Sheehan 1984a, b) relating amount of oiling to degree of biological effects on a site-by-site basis provided superior resolution when adequate hydrocarbon data were available. Good examples are the correlation of amounts of hydrocarbons to coral injury and growth rate, mangrove-leaf longevity and biomass, and proportions of dead mangrove roots.
Characterization, Persistence, Degradation, and Release of Oil from the 1986 Spill
Chemical characterization of an unspilled sample of the original oil from the refinery allowed detailed study of subsequent chemical alteration in different environments and uptake by organisms. Moreover, comparisons of the oil spilled at Bahia Las Minas with other common crude oils of the Caribbean region suggest that the fate and effects of the oil we observed could be expected in similar circumstances elsewhere.
Oil persisted in greatest quantities after the spill within sediments, and both the amount and length of persistence were inversely correlated with prevailing energy conditions and sediment grain size in the environment, a pattern that has been documented many times before. Sediments from reefs classified visually as heavily oiled showed initially high concentrations of hydrocarbons comparable to those near large oil fields or depots, but amounts decreased by an order of magnitude within 2.5 yr, and to trace amounts thereafter. Heavily oiled seagrass sediments had 10 to 100 times more oil than reef sediments, but this level also decreased by an order of magnitude within 2.5 yr. Mangrove sediments contained the most oil (to 39% dry weight of sediments 5 mo after the spill), and oil was still present in large quantities 4 yr after the spill (as high as 25%).
Oil in reef and seagrass sediments was greatly weathered and degraded in the first samples collected only 5 mo after the spill, but could still be clearly identified as coming from the refinery. In striking contrast, mangrove sediments from a heavily oiled stream contained a fairly fresh oil residue with a full suite of n-alkanes preserved 5 yr postspill. Moreover, significant levels of low-molecular-weight aromatics were still leaching from disturbed sediments 5 yr after the spill. Similar chemical stability has been demonstrated for oil trapped in saltmarsh sediments 20 yr after the oil spill at West Falmouth, Massachusetts and in mangrove sediments in Puerto Rico. Oil is still being flushed out of mangrove sediments at Bahia Las Minas in large quantities, as demonstrated by its abundance on recently submerged mangrove roots and experimental substrata, and by the almost chronic occurrence of oil slicks in mangroves and over reefs during the rainy season.
In oil-contaminated ecosystems, bivalves preferentially accumulate more soluble, lower-molecular-weight hydrocarbons. Quarterly samples of the false mussel Mytilopsis sallei from streams and the oyster Crassostrea Virginica from channels and lagoons were analyzed to monitor amounts of lower-molecular-weight hydrocarbons being released in mangrove environments. Mussels had two times greater concentrations of these substances in their tissues than oysters. However, the significance of this result for estimating environmental levels is complicated by different uptake kinetics and possible tissue-saturation levels of the two species for animals exposed to the same environment.
Sentinel organisms used to monitor hydrocarbons must be, by definition, highly tolerant of hydrocarbons in the environment. Alternatively, one can also monitor the distribution and abundance of highly intolerant species as bioindicators of hydrocarbon pollution, especially if they can be easily and quickly counted in the field. Echinoderms may fulfill these requirements well. Sea urchins were greatly reduced or eliminated after the spill in the seaward edge of two heavily oiled reef flats, and were still absent from one of the flats 5 yr afterward. Likewise, ophiuroids, holothurians, and echinoids were rare on heavily oiled seagrass beds throughout 2.5 yr of sampling, and holothurians and echinoids were still rare 6 yr after the spill in the shallow areas sampled. In contrast, four species of sea urchins tended to be more abundant at oiled subtidal reefs than at unoiled reefs 4 yr after the spill. Identifying pollution-sensitive species that also play key roles in communities may be very helpful and requires further study.
Major Biological Effects and Their Persistence
The 1986 Bahia Las Minas oil spill had major biological effects in all environments examined including the principle habitat-structuring organisms of coral reefs, reef flats, mangroves, and seagrass beds. Moreover, initial effects of the spill displayed less taxonomic selectivity than observed after many natural disasters like hurricanes. There were widespread lethal and sublethal effects on both infaunal and epifaunal populations. All trophic levels were affected, including primary producers, herbivores, carnivores, and detritivores. Highly mobile animals, such as large fishes, may have escaped direct effects of the spill, but were not studied.
A Model of the Chain Reaction of Habitat Loss and Biological Effects
Initial effects of the oil spill in Bahia Las Minas have set off a chain reaction of events that continue to severely affect organisms in all habitats, even though they may no longer be exposed to oil from the spill (Fig. 40). Analysis of aerial photographs showed that 64 ha, or roughly 7% of the entire area of mangroves in Bahia Las Minas in 1986, were killed by the oil spill, and smaller but extensive areas of seagrass beds were also killed. Death and injury of these habitat-structuring organisms resulted in physical destruction of habitats. Dead trees rotted and fell, logs and storms battered the shore, seagrass rhizome mats entirely disappeared, and sediments in all these environments eroded at rates up to several centimeters per yr In some cases, like the seagrass bed at Isla Largo Remo North, 14 cm of sediment were removed.
The eroded sediments, and unknown amounts of varyingly degraded oil, were deposited in large amounts in neighboring environments, as measured by a more than doubling of resuspended sediments settling onto heavily oiled coral reefs between 1988 and 1991, while no increase occurred at unoiled reefs. There was also extensive deposition of sediments eroded from the seagrass bed at Isla Largo Remo North onto the adjacent bed at Isla Largo Remo West. Moreover, surviving mangroves and seagrasses, as well as associated organisms, are still repeatedly exposed to relatively fresh and toxic hydrocarbons, which further retards possibilities of recovery and decreases the productivity of these communities.
The secondary biological consequences of erosion and redeposition of oily sediments include greatly increased levels of injuries and decreased growth and sexual reproduction for surviving subtidal reef corals in Bahia Las Minas compared to reefs outside the bay. Other more speculative, but very plausible, effects are seen in the shift toward somewhat greater dominance of oiled reefs by fleshy macroalgae. which act as sediment traps. Also, changes occurred in food webs on reefs now dominated by damselfishes instead of larger and more voracious schooling fishes such as grunts that were present in considerable abundance at the Punta Galeta. reef before the spill.
The inevitable consequence is that the Bahia Las Minas ecosystem is more vulnerable to subsequent natural or anthropogenic disturbances. An example of this was the pattern of macroalgal mortality on reef flats due to extremely low tides in 1988. Macroalgae died back much more on previously oiled reefs, suggesting that re-establishing populations were more vulnerable to natural disturbances than those unaffected by the oil spill. Another example is the comparative failure of recruitment in Bahia Las Minas of corals that broadcast gametes into the sea, as compared with much higher recruitment on other reefs where the same species suffered apparently natural catastrophic mortality less than 2 yr after the oil spill.
The biology of the Punta Galeta. reef flat has been studied for 20 yr Natural variations in abundance are well documented for sessile organisms and sea urchins for most of that time, and extensive data exist for stomatopods covering nearly a decade. In addition, communities were intensively watched just before and during the time the oil came ashore, and for months afterward. Thus, in this case, observations of biological effects are not just based on statistical comparisons of conditions before and after the oil spill, because in many cases scientists watched organisms die as they were immersed in oil.
Macroalgae, crustose coralline algae, and sessile invertebrates at and near the seaward edge of the reef flat were directly exposed to oil and suffered heavy mortality, resulting in the lowest cover of these organisms measured in 20 yr. Elevation of the reef flat varies by only a few centimeters over wide areas, so the spatial pattern of damage was highly dependent on sea level and weather at the time of the spill. Apparent recovery (ignoring issues of resilience to future damage and a few particular species) was complete within a year, except for sessile invertebrates at the seaward edge, which declined everywhere after the lowest sea levels ever recorded at Punta Galeta. in 1988.
Effects on mobile animals were more variable, depending on their physiology and behavior. Sea urchins suffered an immediate, precipitous decline that could be distinguished statistically from normal variation despite the typically highly episodic fluctuations of sea urchin populations characteristic of this environment. Recovery was rapid at all but a single reef flat adjacent to the refinery.
Stomatopods were virtually eliminated from one site where the seagrass bed disappeared; these had not recovered after 5 yr. Seagrass beds disappeared from at least three additional sites. In all cases, reef flat topography caused oil to be trapped in the beds. Abundance and size decreased at the other oiled site as well, compared to two unoiled or slightly oiled sites. The consequence was more rapid growth, and decrease in aggression and competition for living cavities among survivors which actually enjoyed greater habitat quality and less injury than those on unoiled reef flats.
The small infauna of the foliaceous macroalga Laurencia suffered considerable mortality immediately after the spill, but populations were very similar among oiled and unoiled reef flats within 1 yr when detailed comparative investigations began.
Snails also died at heavily oiled sites on a reef flat near Punta Galeta. but not at one control site until the latter was affected by a small diesel fuel spill.
The Bahia Las Minas spill resulted in a band of community die-off and rapid recovery of most taxa, but this may not always be the case. A spill of a more toxic oil than weathered crude or a more prolonged and extensive exposure to oil could result in greater mortality and slower recovery on reef flats.
The cover, size, and diversity of live corals decreased greatly on two oiled reefs compared to their values before the oil spill (Fig. 17), while values initially increased on unoiled reefs. These differences persisted from 1988 through 1991, although diminished, even after the occurrence of precipitous, unexplained coral mortality at unoiled reefs between 1986 and 1988 (cover dropped from 28% to 12%; Fig. 17). In contrast, numbers of corals increased on oiled reefs as formerly large colonies were reduced to larger numbers of small, surviving fragments of live tissue. Likewise, the frequency of injuries to corals was much higher on heavily oiled reefs. For some species, these patterns were significantly correlated with both the amount of oil in reef sediments and subsequently increased amounts of resuspended sediments at heavily oiled reefs. The apparent sublethal consequences for corals included decreased growth, reproduction, and recruitment, which resulted in little prospect for rapid recovery.
The area, condition, and maturity (successional state) of mangrove forests in Bahia Las Minas were determined using a combination of multiple aerial photographic surveys of the region taken over the past 25 yr and ground surveys of present conditions to calibrate the images in the photographs and make additional observations. Deforestation after the 1986 oil spill amounted to 64 ha, mostly in an approximately 50-m wide coastal strip, and as wedges penetrating entrances of smaller streams. This is about 7% of the total area of mangroves in the bay. By comparison, the 1968 oil spill following the wreck of the tanker Witwaterresulted in the death of 46 ha, or roughly 72% of the 1986 value.
Measurements were made of the status of the canopy of surviving trees as an assay of forest condition. Factoring out salinity as a confounding variable, the numbers of leaves per shoot, leaf longevity, and leaf biomass per hectare all decreased significantly with the amount of oil in mangrove sediments. Thus, the oil spill affected far more than the area of forest that actually died.
Mangrove Fringe and the Epibiota of Mangrove Roots
The oil spill had immediate biological effects on the epibionts of mangrove prop roots on the open coast, in channels and lagoons, and in streams . Within 3 to 9 mo most of the common taxa were greatly reduced or eliminated. This was clearly evident in all
but coastal mangroves by piles of recently dead mollusc shells at oiled sites, as well as by statistical comparisons of community composition at a smaller number of sites before and after the spill. Community composition on roots had not recovered completely in any of the three environments after 5 years.
Open-coast roots in Bahia Las Minas were dominated by foliose macroalgae. before the spill, and the same was true at unoiled sites afterward. These populations had nearly recovered at the end of the study. Channel and lagoonal root community composition varied considerately with local differences in salinity, other environmental factors, and patterns of reoiling and new oiling. Nevertheless, there was a catastrophic decrease in abundance of oysters and other bivalves at oiled sites after the spill compared to unoiled mangroves, with little or no recovery evident after 5 yr (Fig. 33). The same was true in streams where roots had been dominated by false mussels, along with lesser populations of barnacles and foliose algae. None of these has recovered.
In addition to these effects on epibiota, there was substantial loss of mangrove roots as substrata. The area of mangrove fringe lost 5 yr after the spill totaled 33%, 38%, and 74% on the open coast, in channels, and in drainage streams, respectively This habitat loss probably affected associated mobile fauna, as well as the sessile epibiota.
The reef flat seagrass bed at Largo Remo North died and disappeared completely, and the area of shallow subtidal beds decreased, after the oil spill. Biomass of surviving parts of beds declined considerably, but such thinning occurred at unoiled beds as well. Numbers of infauna were much lower in oiled beds after the spill, and these differences persisted for more than 2 yr Amphipods, ophiuroids, sipunculids, and tanaids were the groups most affected, whereas hermit crabs increased, perhaps due to increased availability of shells. Epifauna showed generally similar patterns. The most lasting difference was in the total number of echinoderms (ophiuroids, echinoids, and holothurians) that were moderately abundant at unoiled sites but still virtually absent from oiled seagrass beds after 2.5 yr This pattern still held after 6 yr for large echinoids and holothurians (ophiuroids were not counted).
Processes of Repopulation (Recovery)
The re-establishment of populations to levels similar to those before an oil spill depends on at least six factors.
- Severity of initial damage: The amount of biological damage after the spill, relative to fluctuations due to natural processes characteristic of the species or region in question, provides a measure of severity independent of the typically large differences that exist between habitats and regions. By this criterion, effects of the 1986 oil spill were severe for all habitats, with the exception of many of the inhabitants of reef flats exposed to the open sea.
- Extent of habitat destruction: The extent of mortality of habitat-structuring species and the persistence of oil in the local environment together determine the magnitude of habitat loss.
- Frequency and severity of previous damage: The known biological history of the ecosystem may be of great importance in assessing re-establishment of populations.
- Life-history characteristics: Maximum potential recruitment and growth rates, and modes of reproduction and dispersal, limit the speed at which repopulation can occur.
- Multiple stable states: Interactions with other pre-existing or newly invading species, and the potential for nonlinear dynamics, threshold effects, and the development of alternative communities may prevent return to pre-existing conditions for indefinite periods.