Marine Biology 105, 51-57 (1990) Marine ................. Biology o~ ufeln 0o~.~ © Springer-Verlag 1990 Littoral infauna of a West African estuary: an oil pollution baseline survey R.J. Snowden * and I. K. E. Ekweozor Marine Biology Unit, Department of Biological Sciences, Rivers State University of Science and Technology, P.M.B. 5080, Port Harcourt, Nigeria Abstract Littoral infaunal density was recorded from December 1983 to October 1984 throughout the Bonny Estuary in the Niger delta, Nigeria, to provide a baseline for the monitoring of oil pollution. Results on the most abundant faunal elements, polychaetes and the fiddler crab Uca tangeri L., are present- ed. In the upper reaches, polychaete density peaked in Jan- uary - due to increases in the abundance of the principal species. In the lower reaches, euryhaline and stenohaline polychaetes exhibited maxima in April to July and lesser peaks earlier and later in the year. There were no wide fluctuations in the middle reaches, due to the lack of truly estuarine species - except U. tangeri which was most abun- dant between December and April. Features such as the salinity/substrate zones, faunistic components and reduc- tion of species upstream, are typical of estuaries elsewhere. Others, including the low abundance of all species in the middle reaches, and the occurrence of species minimum far downstream of 5%o S, are atypical, and pollution from the Okrika oil terminal, oil from outboard engines, and sub- strate disturbance by shipping, are suggested as causative factors. The wide variety of exploration, production and trans- portation facilities (Snowden and Ekweozor 1987) provide a constant threat of oil pollution, which has resulted in both small- and large-scale spillages. Generally, the effects of these are not monitored and many incidents are not even reported. A major problem is that the area has been little studied and there is an almost complete lack of baseline data. In view of this, data were collected for one year on the density of the littoral benthos, throughout one of the estuar- ies of the delta, to provide a baseline of existing conditions. Against this the impact of future pollution incidents can be assessed. Results are presented on the most abundant faunal elements: polychaetes and the fiddler crab Uca tangeri. The Bonny Estuary was selected because, in addition to being traversed by pipelines and surrounded by land-based, aquatic and offshore oilfields, production platforms and flow stations, it contains three tanker loading terminals, at Port Harcourt, Okrika and Bonny (Fig. 1). It is, therefore, an estuary where oil pollution is highly likely. It is also the major shipping route of the delta and to Port Harcourt, which contains the largest concentration of industry and population in the area. Introduction The River Niger drains into the Atlantic Ocean through a complex delta system along the south-eastern coast of Nige- ria. The delta consists of a 50 km wide estuarine mangrove zone, and a wider, inland zone of freshwater swamp forest, and covers 35 000 km 2. Over 90% of Nigeria's oil, some 100 million tons yr-i (UNEP 1982), originates from the delta, making it by far the major oil-producing region of West Africa. * Please address all correspondence and requests for reprints to Dr Snowden at his present address: Department of Zoology, Univer- sity of Kuwait, P.O. Box 5969, 13060 Safat, Kuwait Materials and Methods Macrobenthos Five transects were established across the littoral zone at ca 14 km intervals between the upper limit of saline influence north of Port Harcourt and the mouth of the estuary at Bonny (Fig. 1). Three stations were located along each tran- sect, at mean high tide level (MHTL), mid tide level (MTL) and mean low tide level (MLTL). The benthos was sampled at each station every 6 wk, between December 1983 and October 1984, during the low water of spring tides. Uca tangeri L. density was determined by counting the number of burrows within ten randomly-placed 0.25 m 2 quadrats. Preliminary studies (Ekweozor 1985) indicated