Biomonitoring of Heavy Metals Level in Wetland Plants of Lagos Lagoon, Nigeria

Abstract

The purpose of this research was to monitor the distribution of Cd, Cr, Cu, Ni, Pb and Zn in plants of Lagos lagoon wetlands in Nigeria. Water, soil and dominant plants were collected from 46 sampling points for a period of 1 year and analysed using ICP-AES. The order of heavy metals presence in soil samples was as follow: Zn>Cr>Cu>Pb>Cd>Ni. The Zn concentration was the highest whereas the lowest concentration was Ni. All water samples showed varying degrees of contamination across all the sampling points in these wetlands. A greater percentage of all metals concentration for Pb, Cd, Cr and Ni were higher than the permissible limit set by World Health Organisation. Pb ranged from 0.01±0.00 to 0.91±0.04 mg/L, Cd from 0.01±0.00 to 0.31±0.02 mg/L, Cr from 0.05±0.00 to 1.15±0.01 mg/L, Ni from 0.01±0.00 to 0.52±0.03, Cu from 0.21±0.01 to 1.11±0.01 mg/L and Zn ranged from 0.15±0.00 to 10.28±0.02 mg/L. The median values of each metal that the shoots and roots of individual plants accumulated metals in the order: Zn>Cu>Pb>Cr>Ni>Cd. Ipomea aquatica had the highest concentration of Pb in its shoot (1.12 mg/kg) while Ludwigia adscendens had the least (1.12 mg/kg) in its shoot. Pb level in the roots was highest in Eichonia crassipes (5.69 mg/kg). The highest level of Cr in shoot (2.23 mg/kg) and root (5.41 mg/kg) was observed in Commelina benghalensis while Cr concentration is lowest in the shoot (0.04 mg/kg) and root (0.18 mg/kg) of Althernathera philoxerrides. Ludwigia adscendens had the lowest concentration of Ni in its root (0.01 mg/kg). The highest shoot concentration of Cu (4.21 mg/kg) was observed in Eichonia crassipes while Ipomea aquatica had the lowest concentration in its shoot (0.23 mg/kg). Paspalum vaginatum’s root had the highest Cu concentration (12.32 mg/kg) while lowest concentration of Cu was observed in the root of Sagittaria sagittifolia (0.69 mg/kg). Transfer factors for most of the plants species were less than 1, indicating that metals accumulated by these plants were largely retained in the roots. A. philoxerrides had translocation factor greater than one for Ni (10.30), while for Cr was 1.25 and 1.40. This present findings indicate that, despite ecological similarities, the different plant species tend to respond differently to exposure to heavy metals and also in their ability to accumulate the various metals. Thus, heavy metals sequestration from the soil to these plants characterized them as metals pollution indicators.

Keywords: wetland plants, heavy metals, biomonitoring, bioaccumulation capacity, translocation factor