Abstract

Fin and shellfish species inhabiting a polluted environment are exposed to heavy metal contaminations. The Lagos Lagoon is a polluted aquatic habitat that receives a huge amount of untreated industrial and municipal effluents from Lagos city and adjoining rivers. The aim of this study is to determine the dynamics, concentration and bioaccumulation potential of four heavy metals (Pb, Zn, Cu and Cr) in four economic important fin and shellfish species (Sardinella maderensis (SM), Pseudotholithus elongatus (PE), and Chrysichthys nigrodigitatus (CN) and shrimp (Peaenus monodon (PM)) from three sampling stations: Apapa, Makoko and Ibeshe). The metal concentrations in different sampling stations in this study differed significantly in the order of Zn>Cr>Pb>Cu, Cr>Cu>Zn>Pb and Cr>Zn>Cu>Pb at Apapa, Makoko and Ibeshe respectively. The study also demonstrates that Zn and Pb were the most and least accumulated metals, respectively. Heavy metal concentrations in fish muscles were high and above recommended standard for aquatic life. Generally, the distribution of metal concentrations in species is as follows P. monodon>C. nigrodigitatus>P. elongatus>S. maderensis. Heavy metal concentrations varied significantly depending on fish species and highest concentrations were found P. monodon followed by C. nigrodigitatus. This suggests that PM and CN accumulated more metals than other species and which can be related to their feeding habits. This study shows that essential metals such has zinc and chromium have a higher accumulation potential in fish and shrimp in this habitat. This reflects the role these metals play in promoting immune and adaptive mechanisms against environmental pollution.

1. Introduction

Aquatic contamination with heavy metals has become a major concern over the years due to their toxicity and non-biodegrade nature ^{1, 2}. Heavy metals constitute a major class of pollutants with ecological concern. They are found at naturally at low concentrations in aquatic habitat, but their concentrations have progressively increased due to increased human activities such as urban developments and industrialization especially in developing countries ³. Large quantities of heavy metals can accumulate and biomagnified along water, sediment and food chain, leading to adverse effect on resident biota ⁴.

Aquatic organisms such as fin and shellfish species occupy higher trophic level in the food chain, as such represents common bioindicators for pollutants ^{4, 5}. The concentrations of heavy metals in fish and shrimp could be used to reflect anthropogenic impacts on aquatic ecosystem. Generally, bioaccumulation and biomagnification occur over the years due to longstanding anthropogenic activities within a coastal ecosystem ⁴. The levels of metals that are bioaccumulated with aquatic organisms are regulated by some certain environmental parameters, including temperature, pH, alkalinity of the habitat complexity, feeding habits, sampling station and pollutant type ³.

Fish and shrimp are mostly used in metal accumulation investigations because they are higher trophic level organisms and such investigations reflect the metal levels in water and sediments ^{3, 6}.

The Lagos Lagoon estuary is the biggest among the four lagoons within the Gulf of Guinea ⁷. This estuary is polluted by various types of human activities such as shipping, petrochemicals, industrial effluents, wood waste, sawdust, textile effluents. Some economically important species of fish and shrimp species in this habitat, transforms it into an important region for fisheries ⁸. At present, the ecosystem is receiving untreated industrial and municipal effluents.

Early reports on the estuary include investigation of some aspects of the ecology of the lagoon ⁷, fisheries resources ⁹, plankton distribution and diversity ¹⁰. There has been a progressional shift to studies centered on pollution load ⁸, sources ¹¹ and effects of pollutants in the lagoon ¹² but limited empirical data on the bioaccumulation capacity and dynamic of heavy metals on fish and shrimp from Lagos Lagoon.

The aim of this study is to determine the concentrations and bioaccumulation potential of four heavy metals (Pb, Zn, Cu and Cr) in four economic important fish species (Sardinella maderensis (SM), Pseudotholithus elongatus (PE), and Chrysichthys nigrodigitatus (CN) and shrimp (Peaenus monodon (PM)) from Lagos lagoon.

2. Materials and Methods

The Lagos Lagoon estuary is the biggest among the four lagoons within the Gulf of Guinea ⁷. Osun, Yewa, Ona and Ogun Rivers are the major rivers that drain into the lagoon (Fig. 1). The lagoon is divided into three major wedges: Lagos Harbour, the Metropolitan and Epe division ¹³. The Lagos section is the largest with a harbour, which represents the entrance of fresh water from the lagoon when it rains ¹³. The major body of the lagoon lies within E3º 23' and 3º 40' and N6º 22' and 6º 38'. The incidents that are associated with this brackish water is the coastline dynamics and distribution of contaminants from the hinterlands and shores of the lagoon. Three sampling stations were selected based on major anthropogenic activity located there. Apapa (shipping and marina operations), Makoko (wood industry) and Ibeshe (textile factory).

Samples were collected from sampling stations with the service of fishermen and a cast net of 50-55mm mesh size was used. Four economic important fish species: one hundred and eighty-two samples were collected: Sardinella maderensis (SM), (n=64), Pseudotholithus elongatus (PE), (n=40), Chrysichthys nigrodigitatus (CN) (n= 58), and Peaenus monodon (PM)) (n=20) were collected from three sampling stations in Lagos Lagoon bimonthly from April 2017-October 2017. One gram of muscle, from each sample of S. maderensis, P. elongatus, C. nigrodigitatus and P. monodon were dissected for analysis. The dissected specimen was placed in a Teflon beaker and digested in an acid solution (HNO₃) in preparation for heavy metal analysis (Kenstar closed vessel microwave digestion) using the microwave digestion procedure ¹⁴. The specimens were digested with 5 millilitres of HNO₃ (65%) and filtered with the Whatman Filter Paper no 41 into a 25 millilitres volumetric flask and made up to mark with distilled water ¹⁴. All digested specimens were analysed in triplicates for the following metals: lead, chromium, copper and zinc using an Atomic Absorption Spectrophotometer (Perkin-Elmer AA 700). The equipment was graded with standard solutions formulated from economically available chemicals purchased from Merck KGaA, Germany ¹⁵.

Data were given as mean±standard error. All statistical analyses were performed using Origin 8 software, (Originlab software, USA). A one-way ANOVA test was used to assess significant difference (p<0.05) of metals among species.

3. Results

The concentrations of heavy metals in muscle tissues of S. maderensis, P. elongatus, C. nigrodigitatus and P. monodon are given in Table 1, Table 2 and Table 3. The distribution pattern of heavy metals in fish species was Zn>Cr>>Pb>Cu, Cr>Cu>Zn>Pb and Cr>Zn>Cu>Pb in Apapa, Makoko and Ibeshe respectively. Generally, the levels of heavy metals in each fish species varied significantly. The concentration of zinc and chromium were significantly (p<0.05) higher than copper and lead in all the fish species. Also, metal concentrations in species were found to be PM>CN>PE>SM. Heavy metal concentrations in P. monodon and C. nigrodigitatus differed significantly and metal concentrations were higher than in P. elongatus and S. maderensis (Figure 2, Figure 3 and Figure 4).

4. Discussion

Fish and shrimp inhabiting polluted habitat can accumulate heavy metals and this can have adverse effect on the physiology of the fish species ^{2, 3}.

Heavy metals are taken up from food and water in fish and shrimp, distributed by circulation and later bioaccumulate in tissues of resident biota ⁴. In this study, essential heavy metals (Zn, Cu, Cr) were accumulated in amounts higher than non-essential metal (Pb). Zn, Cu, Cr and Pb were detected in all the muscles studied and levels were above recommended standard for aquatic life ¹⁶. In all the species, levels of Zn and Cr have been found to be higher than Pb, and could be explained because of these metals play a role in the enzymatic and respiratory processes in fish and shrimp ³. Non-essential metals do not have any function in fish and shrimp. In general, zinc has the highest muscle concentrations in all the species. Zinc is an essential metal that plays important role in metabolic activities of fish and shrimp, but its deficiency can to many activities such as loss of appetite, growth and immunological disturbances ¹⁷. Copper is also an essential metal for healthy life of organisms, but excess amounts of intake can be detrimental to organisms ¹⁷. The concentrations of Pb in fish and shrimp can represent a marker of the environmental levels of this metal ¹⁸. This study shows significant variations in the mean concentrations of the metals in all the species investigated in all the sampling stations. Over the years, the concentrations of heavy metals entering the Lagos Lagoon have rapidly increased due to industrial and urban development ². Apapa sampling stations has shipping and marina activities, Makoko has sawdust industry and Ibeshe has textile factory located there. All these industrial activities discharge different levels of heavy metal into the lagoon ^{2, 3}. As such the concentrations of metals determined by fish and shrimp in this study reflects the intensities of urban and municipal developments ³.

There were significant differences in metal concentrations among species. The highest metal levels were found in P. monodon. Although different species showed different metal levels. The species order as follows P. monodon> C. nigrodigitatus>P. elongatus>S. maderensis. These differences in metal levels of the species may be due to different metabolic processes, feeding habits and biotype ¹⁹. Among the species, P. monodon and C. nigrodigitatus are benthic species and have accumulated higher metal concentrations due to the greater exposure to bottom sediment ¹⁸. Hence, heavy metal levels are largely regulated by the habitat, eating habits, species and metal accumulation potential ^{6, 18, 19}.

5. Conclusion

In the ecosystem of Lagos Lagoon, the four species with different ecological needs determined different metal concentrations in their muscle tissues. The result showed that the concentrations of heavy metals varied among species. The metal concentrations in different sampling station in this study differed in the order of Zn>Cr>Pb>Cu, Cr>Cu>Zn>Pb and Cr>Zn>Cu>Pb at Apapa, Makoko and Ibeshe respectively. Generally, the distribution of metal concentrations in species is as follows P. monodon>C. nigrodigitatus>P. elongatus>S. maderensis. The highest concentrations of heavy metals in fish and shrimp species were found in muscles of P. monodon followed by C. nigrodigitatus, this indicates that these two species are usually in contact with bottom sediment. The study also demonstrates that Zn and Pb were the most and least accumulated metals, respectively in fish and shrimp muscle tissues.

This study shows that essential metals such has zinc and chromium have a higher accumulation potential in fish and shrimp in this habitat. This reflects the role these metals will play in promoting immune and adaptive mechanisms against environmental pollution.

Acknowledgments

The author would like to thank Prof Abiodun Sule (Executive Director of Nigerian Institute for Oceanography and Marine Research) for helpful suggestions and thankful to my colleagues at Department of Biological Oceanography and Marine Biology.

References