Strengths and weaknesses of damage assessment programs: the IXTOC-I and Burmah Agate oil spills and the benthic macroinfauna of the Texas continental shelf
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The uncontrolled oil leak and fire at Ixtoc-I, a well in the Gulf of Campeche, Mexico, released more than 475,000 metric tons of crude oil into the ocean in 1979-80. While Ixtoc-I was still flowing, a collision and fire near Galveston Bay, Texas, released an additional 7000-9000 metric tons of crude oil into the ocean from the oil tanker Burmah Agate. Currents transported residues from both spills through an area of the Texas continental shelf that had been thoroughly studied in previous years by researches from the South Texas Outer Continental Shelf (STOCS program). The paper discusses the subtidal macroinfaunal communities at 12 STOCS stations sampled in 1979 and 1980, utilizing data from the earlier baseline study. In addition, communities are described at 26 new stations within the STOCS study area, a station at the Burmah Agate spill site, and another station 40 kilometers down current from the wreck, all sampled in 1980. Temporal changes at the 12 STOCS sites and areal differences between the 38 stations sampled in 1980 within the STOCS area were not due to either spill, because no Ixtoc-I or Burmah Agate residues were found in benthic samples. Major biological findings at the STOCS sites included: (1) reductions in numbers of individuals and numbers of taxa, and restriction in depth range of taxa in 1979 and 1980 samples, compared to 1975-1977 samples; (2) positive correlation between sediment grain size and both abundance and numbers of taxa; (3) minimal temporal changes in sediment grain size; (4) three clusters of stations arrayed parallel to shore; (5) a distinctive set of species associated with coarser sediment; and (6) relatively constant proportions through time of many numerically dominant taxa and groups of taxa, with polychaets most abundant, followed by amphipods, molluscs, sipunculids, and nemerteans. The 1980 samples from all 38 stations within the STOCS area revealed the (1) more individuals and taxa at shallow, sandy stations than at deeper stations with finer sediment; (2) four clusters of stations parallel to shore; and (3) similarities in large groups of taxa, with polychaetes and gastropods most abundant in all four clusters. Oil from the Burmah Agate was found in benthic samples adjacent to the wreck, and tentatively identified at the station 40 kilometers away. Two of the 38 uncontaminated stations sampled in 1980 served as a posteriori control sites for community comparisons. However, because the accident occurred in a highly disturbed area, differences between these four stations cannot be attributed unequivocally to the Burmah Agate spill. Major biological findings included: (1) polychaetes and sipunculids most abundant at both sets of stations; (2) the lowest numbers of individuals and taxa near the spill; and (3) few taxa common to both sets of stations. A number of problems in the damage assessment program are addressed in this paper. Even if oil had affected the STOCS stations, this program might not have been able to assign temporal changes to their proper causes. The new stations within the STOCS area might have been useful in mapping the extent of spills, and several of them were valuable as posterior controls. Recommendations for improving oil spill damage assessment programs include increasing the frequency of sampling; formation of permanent, accessible voucher collections; acquiring life history and toxicological information; and sequencing sample analysis so that chemical results are available before biological sample analysis begins.