BACKGROUND OF THE STUDY

Studies on heavy metals in rivers, lakes, biota and sediments have been a major environment focus especially in the last decades. Sediments are important sinks for various pollutants like pesticides and heavy metals and also play a significant role in the remobilization of contaminants in aquatic systems under favourable conditions in interactions between water and sediment. (Klavins, Briede and Kokrite, 2000; Grosheva, Veronskaya and Pastakhove, 2000).

Knowledge of concentration of heavy metals is desirable for the estimation of metal concentration in lake’s water, sediment and biota. Metal accumulated in benthic organisms may be further bioaccumulative in food webs. Barnacles have been shown to fulfill many of these characteristics and used to assess the bioavailability of metals in the coastal waters of many parts of the world (White and Walker, 1981; Powell and White, 1990). In addition, the characteristics described make mussels useful indicators of the abundance and spatial distribution of metals in aquatic ecosystems (Doherty, Evans and Neuhauser., 1993; Oertel, 1998).

The biomonitoring of pollutants using accumulator species is based on the capacity which has some plant and animal taxa to accumulate relatively large amounts of certain pollutants, even from much diluted solutions without obvious noxious effects. The use of this type of monitoring is widespread in marine and freshwater environments also because the measuring of the pollutant content in the organisms is the only way of evaluating the bioavailability of a pollutant present in the environment. This technique makes it possible to measure trace element concentrations even when their amounts in the natural environment are lower than the detection limits of the methods commonly used. In addition, the pollutant concentrations in the organism are the result of the past as well as the recent pollution level of the environment in which the organisms lives, while the pollutant concentrations in the water only indicate the situation at the time of sampling (Ravera, Beone, Dantas and Lodigmani, 2003).

Although, Calabar River attracts attention of many authors because of its historical and scientifically importance to study its unique ecosystem, the studies dealing with the accumulation of heavy metals in different ecosystem components are still scarce. In fact, additional information is needed to provide a database for the ecological status of Calabar River that helps the policy makers to take effective decision for proper management of the river.

Moreover, the disposal of materials into the environment by man has characterized his activities.  The physical processes in the world around us cause continual change by removing, recycling and redistribution of enormous volume and variety of materials.  For instance, volcanic eruption discharges gases, rain erodes and transports soil particles and a wide variety of other particles, plants remove nutrients from the soil, recycle them through food webs and return them in different concentrations.  The physical world is in fact dynamic, constantly changing by rearranging and therefore constantly polluting and depolluting or apolluting itself.  It thus has a natural purifying system.

However, man as a social being has created a new order of products, which have increased in volume faster than population growth, and these have been concentrated by urbanization.  This disproportional increase has resulted in increasing contamination of the environment to a point where the natural purifying process can no longer cope, resulting in severe contamination known as pollution.  Pollution could, therefore be summarized as the degradation of an ecosystem to a level that could be detrimental to organisms including plants and animals. However, excessive pollution normally interferes with the health and happiness of a people residing in the area.

Aquatic pollution is a major hazard facing the world today and has been increasing significantly over the years, posing a greater environmental challenge on many developing countries.

Bioaccumulation of pollutants, especially trace metals by benthic organisms is a serious environmental problem because of the probability of biomagnification in which man is at greater health risk than any other organism within the regime of the food chain (Smith, Hofmans and Cook, 1996; Connolly, 1992). Bioaccumulation refers to the accumulation of the pollutant by direct uptake from the environment (water or sediment) into the organism which may be invertebrate, fish or algal species, etc., while biomagnification refers to increase in pollutant concentration in the tissues of successive organisms along a food chain. Thus some alarm has been raised concerning possible health implications of some heavy metals in seafoods, e.g. shellfish.

The serious consequences of trace metal pollution have been reported (Clark and Frid, 1997). Toxicity may result from intake of high levels of trace metals through seafoods as has been reported in the incidence of mercury poisoning in Minamata, Japan, where local inhabitants had neurological illness after consuming sea fish and shellfish contaminated with methyl-mercury (Woodwell, Graig and Johnson 1971). Because of the ability of metals to be concentrated from one trophic level to the next, it is necessary to examine organisms at all levels of the food chain, especially the higher organisms such as shellfish and fish.  Studies of metal accumulation have been stimulated by the possibility of using aquatic organisms as pollution indicators.

Although the discovery of a high concentration of extraneous substances may provide a warning signal, unless it is the result of human activities and it is damaging to the environment, it does not constitute pollution.  Clark and Frid (1997) regard pollution in practical terms as an example of one set of human interest such as the use of leaded petrol in cars, burning of coal, the transportation of petroleum products, the generation of electricity, the disposal of waste products etc.  They also view it in terms of healthy human amenities, tourism, recreation, aesthetics, scientific values, etc.  Environmental manipulations and other human activities are major causes of stress on natural ecosystems.

In recent times, there has been a growing concern world wide that near shore marine and fluvial ecosystems are being affected by man-made pollutants leading to environmental stress (Clark and Frid, 1997; Michel and Zengel, 1998; Asuquo, 1999).  For example, a single oil spill in Puerto Rico resulted in the contamination of extensive area, impacting both biotic and abiotic resources along more than 48km of shore and killing thousands of marine organisms.  Furthermore, the decomposition of dead wild life from this incidence aggravated the situation by generating obnoxious odour and exerting high biochemical oxygen demand (BOD).

Environmental pollution is clearly evident by the present degradation of rivers in relatively high-industrialized regions.  Most of the surface waters are endangered and some have already reached a stage in which higher organisms can no longer survive.  This situation is due to a strong increase in the quantity of industrial wastes, domestic sewage and agricultural run-off which accumulate in the very limited river and discharge into the oceans.

According to Smith, Hofmans and Cook (1996), urban effluents always contain Trace metals and some hydrocarbon from waste-oil.  Their concentrations in water are related to the sources of water and activities in the urban environment.  Trace metals are widely used in industrial processes and so are hydrocarbons, especially as fuel oils.  The discharges of untreated effluents from industries and municipalities as a whole are generally the case in this study area.  The lack of waste treatment facilities in Calabar Municipality, the main urban areas here is probably responsible for (possible) increase in the levels of toxicants in rivers and eventually the marine environment.  Some of the important potential heavy metal pollutants from industrial and municipal sources are Cd, Cr, Co, Hg, Pb, Ni, Mo, Sn, B and Zn.

These pollutants are transported through runoff, streams and rivers to the estuary.  Besides, the atmospheric fall outs introduce metals into estuary and sea.  Other sources of pollution include direct discharge into the rivers and the estuaries of municipal wastes.  The singular most important source of heavy metals is petroleum.  This study area is located in the region of offshore petroleum activities which pollute the waters with trace metals through oil spills.

All these sources introduce large quantities of heavy metals into the estuarine water.  River-borne metals tend to settle to the bottom of the coastal water/estuaries or to be incorporated into living matter before eventual transportation into the open sea (UNEP, 1992.  The estuarine and coastal areas have been polluted with heavy metals by sedimentation of the pollutants while some fractions of the trace metals are transformed, dissolved or settle in sediments.

Also, through the same sources of urban water discharges, estuaries have been known to be polluted with pathogens.  Incidence of gastroenteritis associated with the consumption of shellfish harvested from estuarine water is actually most likely to happen or occur.

As aquatic and estuarine pollution affects water resources by the introduction of potentially toxic chemicals (such as trace metals) into the aquatic ecosystem, the inherent pollution problems may include toxicity to aquatic flora and fauna with benthic organisms at greater reception end.  Uptake of these toxins in the tissue of organisms especially benthic organisms which can easily bio accumulate and eventually biomagnify may create health hazards at higher levels to predators including man that consumes them (Smith, Hofmans and Cook 1996; Connolly, 1992).

From a chemical point of view, an estuary is the mixing zone of river and sea water characterized by gradients in ionic strength and composition.  Because of their locations, estuaries pose a particular problem globally since there are often conflicting interests such as industrial development, shipping and associated harbor development, fishing, tourism and the need for conservation.

Presently, in Cross River State and Nigeria as a whole, much concern has been shown by environmentalists, non Governmental organizations (NGOs) and government over the constant impingement of the coasts by incessant oil spills.  For instance, a major oil spill occurred in January of 1998 when Mobil Producing Nigeria oil pipe ruptured and released over 40,000 bbls of crude oil into the Atlantic coastal waters.  This evoked concerns and gave impetus for a government investigating team to study the impact of the spill on the coastline, estuaries and fluvial system.  This study confirmed the fear of oil spill impact on the marine ecosystem. Since most of the oil companies are located at the Niger Delta area of Nigeria, it leads to the conclusion that several of such oil spills may occur regularly in the area.

Generally, water along the coasts (including rivers and estuaries) contains a variety of marine resources namely fishes, shellfish and seaweeds which attract fishing activities.  Riverine communities of Cross River exploit these fishery resources not only for local consumption but also for economic activities.  In addition, rivers, estuaries and other coastal ecosystems are very delicate because they serve as nursery grounds for most species in the sea.

1.2       STATEMENT OF PROBLEM

Due to dense population and industrialization, the riverine, estuarine and general coastal environments of the world (including south eastern coast of Nigeria) are faced with an ever-increasing impact of all kinds of human activities.  Trace metal pollution is one of the most serious and widely studied marine environmental problems in many parts of the world.  Moreover, current health risks associated with seafood originate in the estuarine environment. For example, some alarm has been raised concerning possible health implications of some Trace metals in sea foods such as residues of some shellfishes (Han, Teng & Hung, 1994).  In effect therefore, much of the trace metals introduced into the aquatic environment are found to be associated with the bottom sediments where they constitute a potential danger to both autotrophic and heterotrophic benthic organisms.  Therefore, there is a need to investigate our coastal water and sediments for heavy metal levels and possible implications of their distribution.

This growing concern about the environmental quality of the marine/estuarine ecosystem world wide especially the estuarine and coastal environment because of their impact by anthropogenic pollutants which may lead to environmental stress.  Whereas extensive studies have been carried out in many tropical coastal and fluvial ecosystems on the levels of heavy metals contaminate (Ekpo and Ibok, 1999, Ekpo and Ibok, 1998; Asuquo, 1999), there is a dearth of information on such in the Calabar River.

There is therefore urgent need to provide information on the level, distribution and possible sources and impacts of heavy metals in this area since it is one of the sources of seafood in south eastern coast of Nigeria.  Such information would be needed for environmental planning, quality control and recommendations that may achieve efficient economic utilization of the marine and other aquatic resources from this area.  The study will also provide base line data for future environmental quality assessment of the area.

1.3       RESEARCH QUESTION

Do benthic organism (e.g. clams, lobster, periwinkle) from Calabar river estuary bioaccumulate trace metal (e.g. Cu, Zn, Co, Hg and Ni) and how much?

1.4       OBJECTIVE OF THE STUDY

The specific objective therefore is to estimate the degree of bioaccumulation of trace metals by some benthic organisms in the study area. This objective shall be achieved by;

1. determining trace metal levels in water, sediment and the benthic organisms
2. determining the concentration and distribution of trace metals in water and sediments from Calabar River Estuary, and using the results to do the following.
   1. determining the bioconcentration factor to establish the location between exposure and the amount of trace metals accumulated from the surface sediment by benthic organism.
   2. using standard statistical model to predict the concentration and the aquatic pollution state and comparing such predictions with experimental data. E.g. clams, lobsters and periwinkles.

• To determine the seasonal variation of the concentration of trace metals in the river.

1. Making recommendations based on the findings from this study for pollution abatement and control.

1.5       SCOPE AND LIMITATION OF THE STUDY

This study was on the bioaccumulation of trace metals by some benthic organisms in Calabar river estuary e.g clam, lobster, and periwinkle). The consumers of these seafood varies from highly learned person to these who are not which may include civil servant, doctors, farmers, engineer, chemist etc. samples where collected randomly along the stretch of the river in about five different locations in the period of six (6) months from January, 2015 – June,2015  to reflect dry and wet season. This study was limited by financial constraint which made it impossible to wider the scope.          

1.6   THE STUDY AREA

The Calabar River in Cross River State, Nigeria flows from the North past the city of Calabar, joining the larger Cross River about 8 kilometres to the south. The river of Calabar forms a natural harbor deep enough for vessels with a draft of 6 metres.

The tropical rainforest in the Calabar River basin is rapidly being destroyed, and pollution is decreasing fish and shrimp catches in the estuary. Those that are caught have unsafe level of contaminants.

Given its coastal area location and as a major tributary of a large estuarine complex with a strong tidal influence, the Calabar River exhibits the main characteristics of an estuary, with all its environmental factors and general implication to water qualities in the marine environment.  The Calabar River originates from at the Oban Hills, Cross River State, Nigeria.  It flows through black shakes and siltstone, clay, sand and silt and an alluvial deposit (Asuquo 1998) it also flows through several villages, Calabar Municipality and farmlands (plantation) before emptying into the Cross River Estuary at Atlantic Coast.  This part of the Cross River estuarine complex is located between the coast of Calabar Municipality and Bakassi Peninsula as shown in figure 1.1.  A few establishments (including a Unicem, Naval base, JT, NNPC-substation) and fishing ports are located along the river.  A major swamp that receives the main drainage channel carrying runoff water from Calabar Municipality is located along the river, implying that materials from the Municipal run off eventually enter the river during high tides.  A major drainage channel is being opened into the river from the municipality; a water works complex is being sited upstream at this river, besides major sand mining and quarrying activities that are carried out upstream.  Besides, the area has nearby off shore location of petroleum industry in the Atlantic Ocean.

Furthermore, the Calabar River drains part of the Oban Hills in the Cross River national Park. The geology of the river basin includes the pre-Cambrian Oban massif, cretaceous sediments of the Calabar flank and the recent Niger Delta sedimentary basin. The basin is about 43km (27mi) wide & 62 km (39mi) long with an area of 1,514 square kilometers (585sqmi). At one time it was entirely covered by tropical rainforest.

The region has rainy season from April until October and dry from November until March, during which 80% of the annual rain falls, with peak in June and September. Annual rainfall averages 1,830 millimeters (72 in). Average temperature ranges from 240C (750F) in August to 300C (860F) in February. Relative humidity is high between 80% and 100%. The basin has 223 streams with a total length of 516 killometres (321mi). This is a small number given the size of the basin. Drainage is poor, so the basin is subject to flooding, gully erosion and landslides. A 2010 study said that flooding had increased in recent years, “Calabar (Encyclopedia, Retrieved 2015-04-09).

In 1862 the geological society of London received a description of a new crocodile named crocodiles frontatus that had been taken from the old Calabar River, with a much broader head in crocodiles vulgaris. A new bat called sphyrocephalus labrosus was also reported. The river system formed by the Cross River Calabar, Great kwa and other tributaries forms extreme flood plains and wetlands that empty into the Cross River estuary. The system has an estimated area of 54,000 square kilometres (21,000 sq mi). As of 2000 about 8,000 tonnes of fish and 20,000 tonnes of shrimp were caught annually since shrimp provide a relatively cheap form of protein to the people of Calabar.

The creeks and the estuarine environment of south Eastern Nigeria including the Calabar River are becoming increasingly polluted as a result of man’s activities, this is largely due to indiscriminate disposal of agro-allied, industrial, mining and municipal wastes into these coastal environments.  These sometimes heavy pollutant-laden wastes, when continually discharged into the marine ecosystems, can increase pollutant concentrations to toxic levels thereby rendering the ecosystems potentially harmful to man and marine life.  This is more so in this study area with a lot of human activities. Marine activities such as cargo transportation, loading and off-loading of petroleum products at the jetties, conveyance of transit passengers between the riverine communities by moterised engine boats, sand mining and some of the routine operations carried out directly in this study area have aggravated the effects of offshore oil industry on the environment.  Environmental pollution emanating from each of these operations is yet to be fully investigated.

Also, this river, in fact the entire coastal areas of south Eastern Nigeria is influenced by semi-diurnal tides that may go upstream to about 40km (Asuquo Ntekim, Bassey and Nya 1997).  Dynamics of pollutants in the coastal waters and particularly the Cross River estuarine complex are enhanced by the two principal ocean currents – northwards rising Benguela current, and eastward flowing Guinea current.  These currents flow through offshore areas where crude oil prospecting and production are located (Mobil Report, 1998, Asuquo, Ntekim, Bassey and Nya 1997).  Accidental oil spills in this zone are transported by prevailing winds and waves into the Cross River estuarine complex (which the Calabar River is a part), the associated mangrove swamps, tidal flats, creeks and other adjoining tributaries.  These are some of the potential sources of hydrocarbon and heavy metals in this coastal region (Asuquo, Ntekim, Bassey and Nya 1997).  The impacts of oil activities are aggravated by the fact that the shoreline adjoining this region is a low energy, shallow water environment and highly vulnerable and sensitive to the frequent oil spills in the area.  The zone is also one of the busiest fishing grounds in the Atlantic coast of West Africa.

The general objective of this study therefore is to assess the levels of heavy metals and their distribution in water and sediments from the Calabar River Estuary with a view to determining the pollution status of the estuary.

1.6.1    SOURCES OF POLLUTANTS IN ESTUARY

Estuaries are complex systems, which receive chemical inputs from a variety of sources. Unfortunately, the same factors that make them productive also make them vulnerable to pollution. The clay particles that concentrate nutrients adsorb heavy metals as do the filter feeders. The ions of heavy metals, sinking rapidly into denser salt layers may be remobilized into an estuary rather than to the sea. Due to this assumed regular removal of wastes by tides, any efforts to control pollutants are either not made or could be initiated so late that any recovery is extremely expensive.

Estuaries, because they are usually drowned river mouth form natural harbours, protected from the oceans and yet connecting the ocean and rivers-drained hinter land. These vital connections have made them a focus of transportation, and many great cities have been built on or near them. Consequently, estuaries have witnessed enormous anthropogenic activities that are sources of pollution of the coast and sea. This has been exacerbated by hinterland based pollution sources which may include industrial activities, other municipal activities and other point and non point sources (Table 2.7 and 7.2). Sources in the marine environment is shown in.

The Calabar River estuary is under increasing threat of contamination through the discharge of increasing quantities of effluents from urban areas containing toxicants such as heavy metals, and biological species.

1.6.2  SOME ANTHROPOGENIC FACTORS THAT MAY HAVE POTENTIAL EFFECTS ON CALABAR RIVER ESTUARY

Exploration, exploitation and transportation of oil are prevalent off coast in the vicinity of this study area.  The attendant oil spills and pollution of the area, through ocean current and tides have affected the coast, fluvial water and the mangrove swamps.

The main channelization from the municipality when completed will transport untreated wastes from most parts of the Calabar urban areas through surface run-off to the Calabar  River.  The sand mining activities upstream may release trace metals and other toxicants into the water while run-off from mining/quarrying area upstream is a potential source of pollutants to the Calabar River Estuary.  Inland water transportation, which involves outboard engines may spill petroleum products in a significant quantity.  The proposal by NPA to build oil jetties on the shores of Calabar River to serve as oil depot for ships docking at the Calabar Free Trade Zone (FTZ) is a diversion that is intended to decongest the FTZ during peak activities with a great potential for degradation of this aquatic system.

There are fishing activities at the coastal area where the Calabar River adjoins and forms part of a major estuarine complex of Cross River at the Atlantic coast.  In this area is found one of the busiest fishing grounds in the west coast of Africa.  Such high level of human activities is bound to affect the environmental quality of the coastal area of this river.