The Galeta Oil Spill. III. Chronic Reoiling, Long-term Toxicity of Hydrocarbon Residues and Effects on Epibiota in the Mangrove Fringe

In April 1986, 75 000-100 000 barrels of medium-weight crude oil ( 10 000-13 500 metric tons) spilled into Bahía las Minas, a large mangrove-lined bay on the Caribbean coast of Panamá. Between 1986 and 1991, biological and chemical effects of this spill were studied. The epibiota of fringing mangroves (Rhizophora mangle L.) were examined in three habitats: (1) the shoreward margins of reef flats that fronted the open sea, (2) the edges of channels and lagoons, and (3) the banks of streams and man-made cuts that drained interior mangroves or uplands into lagoons. Chemical analyses of bivalves collected from submerged prop roots (oysters and false mussels) and records of slicks and tarry deposits on artificial roots documented chronic reoiling. Each habitat was repeatedly oiled between 1986 and 1991, with petroleum residues identified as the oil spilled in 1986. There was a decline in the release of tarry oils recorded as slicks and on roots over time, but not in tissue burdens of hydrocarbons in bivalves. This suggested that the processes that released these different types of oil residues were at least partially independent and that toxic hydrocarbons were likely to be released from sediments over the long term. The submerged prop roots of fringing mangroves in each habitat had a characteristic epibiota. On the open coast, roots were covered with a diverse assemblage of sessile invertebrates and algae. In channels, the most abundant species on roots was the edible oyster Crassostrea virginica (rhizophorae morph). In streams, the false mussel Mytilopsis sallei covered the most space on roots. Cover of sessile invertebrates was significantly reduced at oiled compared with unoiled sites on the open coast for 4 years after oiling, while oysters and false mussels were reduced in cover at oiled sites in channels and streams through at least 1991, when observations ended. False mussels transplanted from an unoiled stream to oiled and unoiled streams were significantly more likely to die in oiled than unoiled sites 4-5 years post-spill and their settlement was significantly depressed in oiled streams during the same period. Prop roots that entered the water 15-18 months post-spill were more likely to die at oiled than unoiled sites in channels and streams, but not on the open coast. Five years after the spill, oiled sites were typified by (a) persistent reductions in epibiota in two of the three habitats, and (b) overall reductions in the surface area of submerged prop roots available for epibiotic settlement. Within Bahía las Minas, we estimated that the net result 5 years post-spill was a 33% reduction in the standing crop of sessile invertebrates on the open coast, 65% of oysters in channels and 99% of false mussels in streams.