Studies on the Short-Term Effects of the Mobil Idoho Oil Spill on the Littoral Biota of Southeastern Nigeria

Quantitative surveys of the intertidal macro-fauna were conducted during September-October 1998 along transects established at various locations along the Nigeria coastline, following the rupture of a 24-inch pipeline at Idoho, off the Gulf of Guinea, southeastern Nigeria on 12 January 1998. Samples were taken within impacted areas and at control unpolluted sites approximately 5 km to the east of the Idoho off-shore platform. Spilled oil moved rapidly ashore and into river mouths, and estuaries and their mangals shortly after the spills. Biomass of macrofauna in the impacted areas tended to decrease with level of oiling, as the mean abundance decreased rapidly to about 50% of that found on the control unpolluted sites. Edible gastropod, mainly species of Tympanotomus fuscatus, and the brachyuran decapod, Uca tangeri, typically consumed by coastal inhabitants, had reduction in mean densities (up to 62%) in the oiled Bonny, Brass, Lagos and Forcados than in the non-oiled areas of Imo, Andoni and Cross River, showing partial recovery of the environment from the debacle after 9 months. The ecological implications of these findings are discussed. Introduction A 24-inch oil pipeline ruptured at Mobil-Idoho platform on the inner shelf of the Atlantic coast of southeastern Nigeria on 12 January 1998 releasing over 40,000 bbls (approxi-mately 6,000 tones) of Qua Iboe light crude oil into the marine environment over the next three days. More than 700 km of the Nigerian coastline including estuaries were impacted. Within the impacted zone were a number of macro-benthic communities, the dominant species of which included the edible gastropod, Tympanotomus fuscatus, the Ocypodid brachyuran, Uca tangeri and Ocypode cursor, and several species of bivalves and polychaetes. Some of the estuaries that were oiled are known to be subject to pollution from a number of sources including hydrocarbon, sewage, metal, etc., showing that the spill was not in a pristine environment. It was, therefore, essential to include as many factors as possible in the sampling programme to look at the spill effects. In September 1998, 9 months after the oil came ashore, a rapid assessment survey was conducted at nearly all the estuarine/river mouths along the Nigerian coastline both within the oil impacted estuaries and at the clean control site, 5 km east of the ruptured point. This included measurement of physical and chemical parameters of sea water, and presence and absence of typical macrobiota, together with estimates of overall abundance at each site. Analysis of biota and physical characteristics demon-strated that a full range of Nigeria coastline habitat was present within the impacted area (Ewa-Oboho, 1998). As separation and classification of intertidal habitats were possible on the basis of macrobiota alone, this survey demonstrated that earlier predictions of massive species elimination were unfounded, and the result provided the basis for selection of permanent sites for longer-term studies published elsewhere (Ewa-Oboho, in press). Nine months after the spill, a number of species (diversity) and individual species abundance were still lower at oiled areas in comparison with similar habitat clean control site. While normal diversity appeared to be the case at low tide levels of most impacted estuaries, the mid and the high tidal diversities ranged from 20 to 45% of the diversity on control sites. One major problem plaguing marine biologists is the identification of short-term impacts of spill petroleum hydrocarbons on off-shore marine environments due to the open, complex and very dynamic nature of the system (Boesch & Rosenberg, 1981). This is because effects are usually subtle, requires much funding and the relevant experimentation great. In most cases, natural variability in space and time often overshadow impact effects or confounds the resolution of such effects (Mcintyre & Pearce, 1980; Lewis 1982; Clark, 1982; Koons & Gould, 1998; EwaOboho, 1988, 1994). Limits of detection which are dependent upon the sampling design could plague identification also. Insensitive methods using design with poor powers are capable of detecting only the gross effect, and have little to contribute to determination of potential subtle effects (Boesch et al.,1986). Effects of other human activities may complicate assessment, as practical difficulties are encountered in the comparisons of various uses and assignments of cause of observed alterations among users. Besides, ecosystem compounds are complexly interralated and variation or alteration in one biotic components may have subtle repercussions in another, thus, contributing to the lingering uncertainty about adequate understanding of the effects of off-shore oiling. Recovery gaps do exist about the process and rates of recovery of living resources and ecosystem after oil perturbation. Thus, time required for a system to recover, however defined, can be use as a measure of significance of the spill effect (Oviatt et al. 1992, 1994). As Sell et al. (1995) has demonstrated, the natural recovery of some tropical oiled intertidal habitats elsewhere may take at least 5 years while McGlade & Price (1993) predicted 6–10 years. This paper summarizes results from quantitative surveys of macrobiata 9 months after oiling using standard techniques, designed to be capable of detecting the degree of environmental change which could be considered unacceptable. Materials and methods Sampling methods and locations The sampling area stretched from a control location, 5 km east of the pipe break point in Cross river estuary (4032’:06" N and 08 04":46" E) to Takwa bay (Lagos) 06 47" N and 03 32’:51" E) (Fig. 1) approx. 700 km east of the spill spot. Five heavily oiled estuaries (R. Bonny, R. Brass, R. Forcados, R. Escravos and Lagos Lagon), together with four unoilded ones (R. Imo, Cross river, Qua river and R. Nicholas) (Fig. 1) were chosen for sampling. Also, Bonny, Sangana, New Calabar, Andoni and Nun rivers were sampled at random. In total 104 stations were sampled during the period 15 September–30 October 1998. Two creeks were sampled per river, two transects per creek and two stations per transect. Fig. 1. Shoreline oiling map from aerial video survey on 16, 17 and 28 January 1998 showing oiled areas along the