Page images
PDF
EPUB

Caribbean, Gulf of Mexico, and southeastern United States (10). Thus our observations are relevant to assessment of potential biological effects of pollution in several areas where extraction or refining of oil is ongoing on planned (11).

On 27 April 1986 at least 8 million liters of medium-weight crude oil (12) spilled from a ruptured storage tank into the sea on the Caribbean coast of Panama (6) (Fig. 2). This is the greatest amount of oil spilled directly into a sheltered coastal habitat in the tropical Americas (3, 13). For 6 days, onshore winds held the oil within Bahia Cativa adjacent to the refinery (Fig. 2), but runoff from rains and shifting winds then flushed the oil out to sea. At this time dispersant was sprayed on slicks in the mouth of Bahia las Minas and Spatial Distribution of Oil Among Habitats offshore (14). By 15 May, the oil had swept across fringing reefs and entered mangrove forests, small estuaries, and sand beaches within 10 km of the refinery.

Table 1. Comparison of visual assessment of amount of oiling on four coral reefs (ranked heavy (H), moderate (M), and none (N)] with results of gas chromatographic (GC) analyses of saturated hydrocarbons in coral tissues and by ultraviolet fluorescence (UVF) analyses of aromatic hydrocarbon fractions. Oil content by GC was conservatively estimated as the unresolved hydrocarbons in the elution range for alkanes containing 12 to 36 carbon atoms. Oil units by UVF were determined by comparison with the spilled oil. Values are expressed as micrograms per milligram of coral tissue (lipid or protein) and are the mean and standard deviation of triplicate analyses. Reefs are listed from west to east (Fig. 2), Payardi West is adjacent to the refinery and Palina West is just west of Isla Grande.

Reef

Galeta Channel
Payardi West
Naranjos South

Palina West

Measure

Orling
Density/m2

Individuals

240 mm (%)

Injured animals ≥35 mm (%)

18

Growth†† (%)

[blocks in formation]

Table 2. Population changes of gonoactylid stomatopods on four reef flats. Data for all years are from August to September. Densities and growth are given

as the means ± SE.

Year

1986

1986

1980

1981

1982

1983

1986

Lipid

by GC

1979-1983
1986

2.7 ± 1.1 4.5 ± 0.8 1.4 ± 0.1 1.0 ± 0.6

Lipid by UVF

5.0 ± 3.4 25.5 ± 6.0 1.3 ± 0.3 0.1 ± 0.1

Isla
Margarita

Light

9.9 ± 1.55

9

25

31

38

Plants and animals died wherever they came in contact with oil However, the types and magnitude of effects varied greatly with coastal topography and location, and among habitats and taxa. Such complex effects are similar to those of powerful hurricanes on Jamaican and Australian coral reefs (15). We first consider the spacial pattern of the spill and its effects in different environments, inclading variations in effects on different taxa. We then describe the patterns of responses of different organisms that discuss prospeca for recovery.

The distribution of oil within 2 months of the spill was visual assessed by aerial surveys between Rio Chagres, 27 km west of the refinery, and Punta San Blas, 98 km to the east, and by boat and foor from Rio Chagres to Nombre de Dios (16) (Fig. 2). No oil was observed on shorelines east of Isla Grande, and only a few patches were observed east of Maria Chiquita and west of the Panama Canal entrance. Heavy oiling occurred along most of the coast between Isla Margarita and Islas Naranjos (Fig. 2). The straight-line distance between these two points is 11 km, but the labyrinthine shoreline is at least 82 km long and contains about 16 km2 of mangroves and 8 km2 of intertidal reef flats and subtidal reefs (17). Within this area, the only large unoiled areas 2 months after the spill were two mangrove lagoons that are isolated from open water by narrow channels (hatched areas in Fig. 2). Oil slicks ranging from a metallic sheen to brown patches are still common between Punta Muerto and Galeta, especially after heavy rains flush oil from mangroves and from the landfill beneath the refinery (18).

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][ocr errors][merged small][merged small]

Abundant oil on and in sediments and on adjacent mangroves. Thalas leaves shed, but

*No od evident on surface or in sediments and Thalassia appeared healthy. appeared to be alive. Entire Thalassia bed is gone, no leaves are present and the rhizomes are dead. $Mean density here prior to the spill was 960 539, no compa data on densities are available for the other sites. Significantly different from 1981 to 1986. G test, P < 0.01. Sigraficantly different from previensa reg. Ĉ zen P < 0.01 **Significantly different from 1980, C test, P < 0.05 **Values pooled for the two heavily oiled and light oiled sites. Analysis of covariance showed car sacges of percent growth against body length were not significantly different among the four groups of data (P > 0.05), but that mean growth adjusted for elevation did differ sigrubkamer (P<0.001) There was a significant increase in adjusted mean growth at the heavily oiled sites after the spill compared to growth at these wires before the spill, there were wa ngul cant differences in adjusted mean growth at the lightiv oled sites before and after the spill, and between the lightly and heavily oiled sites prior to the spull

SCIENCE, VOL 241

and much less at Islas Naranjos and Isla Margarita (19). Moreover, similar differences occurred over just a few hundred meters between shores directly exposed to or sheltered from the wind-driven oil (20). On an even smaller scale, extreme low tides between 10 and 19 May (6, 21) caused oil to accumulate along the seaward borders of reef flats, whereas just shoreward much less oil contacted the substratum. In general, intertidal habitats just above mean low water were the most heavily oiled, including mangrove roots and sedi

Fig. 2. Region of the Republic of Panama affected by the 27 April 1986 oil spill, shown as increasing enlargements (A to C). (A) Location within Panama, just east of the Caribbean entrance to the Panama Canal. (B) The boxed area includes the most heavily oiled coastal habitats. Punta Galeta, inside the boxed area, is 9°24′N, 79°52'W. Lightly oiled and unoiled study sites are cast of Bahia las Minas, near Portobelo and Isla Grande. (C) Detail of the most heavily oiled area and location of study sites. Encircled "R" on Isla Pavardi marks the refinery where the oil spill occurred. Horizontal hatching denotes embayments where little oil penetrated. Symbols for types of study sites (open symbols, unoiled or lightly oiled sites; filled symbols, oiled sites): A, mangrove root;, seagrass bed; O, subtidal coral reef, data collected only after the oil spill; 0, subtidal coral reef, data collected both before and after the oil spill: the four sites near Portobelo and Isla Grande were not oiled, the site at Isla Margarita was moderately oiled, and the site at Punta Galeta was heavily oiled; Ŏ, reef-flat stomatopods; multirayed star, reef flat community, Punta Galeta; six-rayed symbol, mangrove forest.

Fig. 3. Organisms and habitats affected by the April 1986 oil spill at Bahia las Minas, Panama (all photographs by C. Hansen except (D) by S. D. Garnty) (A) Oil accumulated along the seaward edge of the Galeta reef flat at low tide (seen as the dark border in the cover photo), directly coating and killing plants and animals, including the zoanthid Palythoa sp. (lighter patches in foreground) and the hydrocoral Millepora sp. (projecting through oil in background). (B) At high tide o accumulated along sand beaches, where it soaked into the sand and settled onto the shoreward reef flat at low tide, killing seagrasses, algae, and invertebrates. The rectangular objects in the foreground are polyurethane mattresses used by cleanup crews to absorb oil. (C) Underwater view of the coral S. siderea partially killed by oil (horizontal length in the photograph, 12 cm). Live assue forms the dark reddish area at the bottom. The central, light-colored area is skeleton showing through partially dead tissue, which is also being colonized by algae. The lightest area at the top is bare skeleton covered by a film of microalgac. (D) Oil-covered intertidal surfaces of prop roots of the red mangrove, R. mangle, killing oysters and other epibiota on the roots. Relieved of the weight of their leaves, defoliated branches flexed upward, lifting the roots out of the water and thus killing subtidal epibiota that previously escaped direct contact with floating oil. (E) Dead mangrove trees form a band about 8 to 100 m wide (February 1987), marking the area where oil accumulated as it entered the mangrove forests (horizontal distances: foreground -0.4 km and background-1.3 km). A band of defoliated trees was apparent within 2 months after the spill and widened thereafter.

6 JANUARY 1989

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors]
[graphic]

ARTICLES 39

[ocr errors]
[merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][subsumed][subsumed]

906 1906 1 2 3 4 607 81-82 86 87

Open

Fig. 5. Abundance or biomass per sample of eight major infaunal taxa in three oiled () and four control (O) seagrass beds 5, 7, and 9 months after the oil spill (30). Means are plotted 1 SE, backtransformed from in (x + 1); some error bars lie within the plotting symbols. "Burrowing shrimp" include alpheids (most abundant), processids, callianassids, and upogebids. NS, P>0.05; P < 0.05 by repeated measures analysis of variance. Number per sample is shown at two scales. Data for polychaetes are shown as grams per sample.

Oil

...

Leaty

Sessile animals

[ocr errors]

81-82 86-07

01-82 84-87

Fig. 4. Percent cover of formerly abundant taxa and oil on mangrove roots in riverine, channel, and open coast habitats before and after the oil spill. O, Prespill data; O, unoiled sites; and ●, oiled sites. Means are plotted±1 SE, converted from arcsine transformations. Some error bars lie within the plotting symbols, except for sampling time 3, when = 1. ND, no data. "Leafy algae" include Polysiphoma, Acanthophora, and Ceramium as common genera; the most abundant "sessile animals" include hydroids and sponges. Sampling dates: 1 September to October 1981; 2 = January 1982; 3-June 1982; 4 = July to August 1986; 5 = October to November 1986; 6 February 1987, and 7 May 1987. Results of repeated measures analysis of variance for oiled and unoiled sites after the spill are shown on each graph, NS, P > 0.05; *, P < 0.05; **, P < 0.01; ***, P < 0.001.

100

sis of aromatic hydrocarbon fractions (22). Preliminary generally parallel classification of sites based on visual inspection, as shown here for the concentration of saturated hydrocarbons in tissues of the coral Siderastrea sidera (Table 1).

Biological Effects

Consequences of the spill were assessed differently, depending on the types of data available from before the oil spill (23). Ideally, biological parameters should have been measured at oiled and unoiled sites before and after the spill. This condition was sanshed for biota of mangrove roots and subtidal corals. Extensive prespal data are available for the reef flat at Galeta, but there are no comm sites and effects must be inferred from temporal change and from th spatial distribution of oiling on the reef flat. In contrast, there are little or no appropriate prespill data for subtidal seagrass commens ties. In this case, comparisons were made between oiled and unoued sites after the spill, confounding the treatment (oiling) and geogra phy (Bahia las Minas verus the region between Portobelo and Isla Grande) (Fig. 2). The value of this approach is strengthened by information suggesting that faunas in seagrass beds were similar un the two regions before the spill (24), but confidence in assessment of oiling effects in this habitat is still more limited than in other habitas studied.

Mangrove communities. Red mangrove, Rhizophora mangie, forms nearly all of the fringing forest along this coast (17). By September 1986, a band of dead or dying trees marked the zone where ou washed ashore between Punta Galeta and Islas Naranjos (Fig. 3), no such band appears in photographs taken just as the oil was coming ashore. By November 1987 dead mangroves occurred along an estimated 27 km of the coast (25). Seedlings transplanted to heavi oiled sites did not produce new leaves, in contrast to transplants at an unoiled site (26).

60

SUM

20

The prop roots of R. mangle are overgrown by algae and invertebrates that vary in species composition with exposure to the sea (Fig. 3) (6, 27). The epibiota on roots in three different habitan were sampled before and after the spill (Fig. 2) (28). Before the spill (Fig. 4), roots of trees directly facing the open ocean were covered with foliose algae and sessile invertebrates such as sponges, hv droids, and ascidians. In mangrove channels, the edible over Crassostrea rhizophorae, and a barnacle, Balanus improvisus, were most abundant on roots. Roots in small rivers were dominated by the false mussel Mytilopsis domingensis and B. improvisus. Certain groups were more abundant a few years before the oil spill (1981 to 1982than in 1986 to 1987 at unoiled sites (Fig. 4), possibly because of natural fluctuations in abundance. After the spill, the cover of all major groups was very greatly reduced in each oiled habitar (Fig. 4 There has been patchy recovery in the open habitat of foliose algae and sessile invertebrates, although not of the same relative abun dance of species. In the channels, cover of both the oyster and

SCIENCE, VOL 244

barnacle dropped greatly and have since recovered little. False mussels and barnacles disappeared in oiled rivers and were not recorded again until 15 months after the spill.

The greatest cover on roots in all oiled habitats after the spill was oil (Fig. 4). In addition, most roots sampled for epibiota (28) were dead, broken, or rotting by August 1987 (54% on open coasts, 56% in channels, and 66% in rivers at oiled sites versus 8%, 1%, and 3%, respectively, at unoiled sites). Thus the mangrove-fringe root habitat has been largely destroyed at oiled sites and will not be restored unless new trees grow.

Seagrasses. Extensive meadows of the seagrass Thalassia testudinum cover much of the intertidal reef platforms and subtidal floors of shallow embayments and lagoons along this coast (24). Entire beds of intertidal Thalassia were killed on some heavily oiled reef flats, as shown by abundant oil-covered dead leaves washed ashore and dead but intact root-rhizome mats (for example, the site at Largo Remo North) (Table 2). In contrast, subtidal Thalassia survived everywhere after the spill, although leaves became brown and heavily fouled by algae for several months in heavily oiled areas.

Thalassia provides food, habitat, and refuge for dense populations of invertebrates and fishes (24, 29). Before the spill, amphipods, small decapods, ophiuroids and other echinoderms, bivalves, gastropods, polychaetes, and small fishes were all abundant in seagrass within the study area (24) and still are in unoiled beds (Fig. 5). Sampling of infauna began September 1986 at three oiled and four unoiled sites (30) (Fig. 2). Four taxa of invertebrates were significandy less abundant in oiled grassbeds after the spill, but four others showed no such difference (Fig. 5). Abundances of most taxa increased between September and January; preliminary analyses of body size indicate that recruitment caused these increases. Only the abundance of hermit crabs increased in oiled relative to unoiled areas (7.3 versus 3.4 crabs per sample, respectively, in January 1987, P<0.05, repeated measures analysis of variance), perhaps because of a surplus of shells of recently dead snails (31).

Intertidal reef flats. Platforms of fringing reefs from extensive shallow to intertidal flats throughout the study region (Fig. 3) (7, 9, 17, 32). Populations on the reef flat at Punta Galeta have been monitored for up to 15 years (9) (Fig. 2). Damage was most extensive at the seaward border, where the oil accumulated at low tide. Immediately after the spill a bloom of microalgae covered recently vacated substratum (33) (Fig. 6). This area had been dominated by perennial macroalgae, particularly the fleshy red alga Laurencia papillosa, crustose corallines, and the articulated calcareous green alga Halimeda opuntia. Cover of all these plants was reduced to. levels well below those observed previously, but had regained or exceeded typical abundance within 12 to 18 months, concurrent with a reduction of the microalgae. The most common sessile animals before the spill were zoanthids (Zoanthus sociatus and Palythos spp.), hydrocorals (Millepora spp.), and scleractinian corals (Porites spp.). At the seaward border of the reef flat, populations of all these animals were severely reduced, and only Zoanthus had returned to typical abundance after 18 months (Fig. 6). Densities of sea urchins on the reef flat varied over three orders of magnitude before the spill (Fig. 6) (9). At the seaward edge, the most abundant

Percent cover

Urchins per 10 m2

[ocr errors][subsumed][subsumed][merged small][merged small][merged small][merged small][merged small][ocr errors]

1000

100

1

10

[ocr errors]

84

Crustoss
corallines

85

Halimeda

Zoanthus

Fig. 6. Pre- and postspill comparisons of the abundance of algae and sessile animals (top seven panels) and the sea urchin Echinometre hunter (bottom panel) on the seaward portion of the Galeta reef flat (33). Filled symbols denotes the heavily oiled aone closest to the water's edge at low tide (Fig. 3). Open symbols and the smooth line in the E. lucunter graph indicate less oiled (more landward) habitats; and + mark postspill surveys of percent cover that were significantly higher or lower than any prespill survey at P ≤ 0.05 and P ≤ 0.01, respectively. In all cases, differences were significant only in the heavily oiled zone (). Cover was compared survey by survey with the use of paired t tests matched by individual transects within surveys. Abundances of E. lucunter were compared with the use of residuals from regressions (34).

6 JANUARY 1989

ARTICLES 41

86 Year

[ocr errors][merged small][merged small][merged small][merged small][merged small]

87 Oil Spill

urchin Echinometra lucunter was reduced by about 80% within a few days of oiling, and the reef flat was littered with its skeletons. Although this decrease was small in an absolute sense relative to changes in abundance among years, it nevertheless departed significantly from the average trend in abundance between months of May and June over the last 8 years (34). Farther inshore, no such reductions occurred.

Gonodactylid stomatopods (mantis shrimp) are abundant in intertidal Thalassia beds from Isla Margarita to Isla Largo Remo, where they live in and aggressively defend cavities in coral rubble at densities up to 20 per square meter (35). They are prey for shorebirds, fish, and crabs, and in turn consume large numbers of hermit crabs, snails, and other animals (36). Population data are available for stomatopods from intertidal seagrass beds on four reef flats, including certain types of data collected at various times before the spill (Table 2 and Fig. 2). Two of the sites were heavily oiled, and large amounts of oil were still present on mangrove roots and in sandy sediment when sampling first began 3 months after the spill; the other two flats were at most lightly oiled. Densities of stomatopods were less on the heavily oiled flats, particularly for animals greater than 40 mm long (37). Loss of these large animals apparently made larger cavities available to the survivors, which consequently fought less and suffered fewer injuries (38) compared to the same sites before the spill and compared to lightly oiled flats afterwards (Table 2). Growth of larger survivors also increased on heavily oiled flats (39), probably because of decreased competition for cavities and an apparent population explosion of hermit crabs for food (35, 40). These effects have persisted in diminished form through our last census in September 1987.

Percent cover after oil spill

42

[ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][ocr errors][subsumed][merged small][merged small][merged small][merged small][ocr errors]

Fig. 7. Percent cover of corals before the oil spill plotted against their cover afterward. The diagonal lines represents no change in coral cover, points below this line indicate a decrease after the spill, and points above the line show an increase. The points are means ± 1 SE; some error bars lie within the plotting symbol. Öiling significantly affected coral cover only at the shallowest depth (42). Abbreviations: 3, 0- to 3-m depth; 6. 3- to 6-m depth; and 12, 6 to 12-m depth. JUG and PALW are located just west of Isla Grande, DMA and DÓNR are northeast of Portobelo, GAL is at Punta Galeta, and MAR3 is at Isla Margarita (Fig. 2, half-filled circles).

Subtidal reefs. Populations of subtidal sessile organisms were surveyed on six fringing reefs between Isla Margarita and la Grande within 1 year before and 4 months after the oil spill (Fig. 2, (41). These include the heavily oiled reef at Punta Galeta, a lightly oiled reef at Isla Margarita, and four unoiled reefs cast of Portobele Abundance of most common scleractinian coral genera in depats ≤3 m decreased at Galera by 51 to 96%, and total coral cover decreased by 76%; even at 9 to 12 m the drop was 45% (Fig. 7 Reductions were less at Isla Margarita and generally absent on the unoiled reefs, except at one site northeast of Portobelo (Fig. 2) for no apparent reason. This relation between amount of oiling on the six reefs and decrease in coral cover was significant for 0 to 3 ms but not deeper (42).

Sublethal effects were also substantial. Within Bahis las Munas, most of the scleractinians still alive in depths less than 3 m showed signs of recent stress including bleaching or swelling of issues conspicuous production of mucus, recently dead areas devoid of coral tissue, and globules of oil (Fig. 3) (43). In some cases, bleached or dead areas were surrounded by a black halo characteristic of bacterial infection (44). Both the frequency and size of recently dead lesions on the commonest massive corals increased markedly with the amount of oiling at each reef and decreased with water depth: (Fig. 8) (45, 46). These effects were also species specific. In the case of S. siderea, which suffered most, new partial mortality was still disproportionately common on heavily oiled reefs 1 year after the spill (46).

The oil spill also affected other organisms, including snails on the reef flat and intertidal zone at Punta Galeta and mobile epifauna. particularly shrimps above subtidal seagrasses. In summary, the spuil harmed prominent organisms in all intertidal and subtidal environ ments examined, infauna and epifauna, and members of all trophe. levels including primary producers, herbivores, carnivores, and detritivores.

Patterns of Responses and Their Significance

The severe damage to intertidal biotas and their response, such the flush of ephemeral microalgae just after the spill, are similar to those documented previously for other oiled intertidal communities (47-50), including those inferred for Galeta after the Witwater spill (4). Likewise, biological effects of spills are usually greatest in low energy environments, where oil tends to accumulate and be retained in fine sediments, than in high energy environments where it is soon washed away (50). This pattern matches the greater and more persistent disturbance to riverine and channel mangrove root com munities than to those along open coasts and the rapid recovery u abundance of many organisms at the seaward edge of the reet flat (Fig. 6).

In contrast, other results were not expected, and in some cases contradict widely held views about the effects of oil spills and the ways they are studied. First, extensive mortality of subtidal corala and infauna of seagrasses had not been demonstrated before (1. J) and contradicts undocumented assertions that these orgamume are not affected by oil spills. In part, subtidal effects may have bee caused by the use of dispersant (51) but it seems unlikely that tha was the only reason, given the regional breadth of the impact and the relatively small amount of dispersant employed (14).

Second, the magnitude of subtidal coral mortality and unary within Bahia las Minas is in striking contrast to findings of no lasting change in coral condition or growth after exposure to oil, with or without chemical dispersant, in small-scale experiments (51 52). Such discrepancies underlie the importance of detailed long term ecological studies, and the dangers inherent in extrapolation to

SCIENCE, VOL. 341

« ՆախորդըՇարունակել »