Bio-monitoring of Mangroves Sediments and Tissues for Heavy Metal Accumulation in the Mangrove Forest of Cross River Estuary

Aims: The need to periodically monitor the mangroves of heavy metals pollution should not be overemphasized given the high premium placed on this ecosystem. The present study was aimed at evaluating mangrove sediments and tissues for heavy metals accumulation in the different tidal levels, which will be an indicator for pollution.
Methodology: Ten random samples of composite surface sediments (0-5 cm) were collected from low, mid and high tidal levels using a modified Van Veen (0.1 m2) grab sampler. Also samples of senescent leaves, barks and roots of randomly selected ten individual plants of Nypa fruticans (low tide), Rhizophora racemosa (mid tide) and Avicennia africana (high tide) were also collected for heavy metal analyses during the year 2011.
Results: Our results showed that there were significant variations (P < 0.05) in heavy metals deposition across the tidal levels. In the mangrove sediments, iron (Fe) content (415.8 mg/g dry weight) was highest in the high tidal level where A. africiana was predominantly sampled while N. fruticans (low tide) had the lowest deposition of the metal (304.4 mg/g dry weight). The accumulation of heavy metals in the mangrove tissues took the general trend of root > bark > leaves, the mangrove notwithstanding. It was also observed that cadmium deposition in sediments was comparatively higher than RSV indicating possible cadmium pollution.
Conclusion: Generally, accumulation of heavy metals in the mangrove forest of Cross River estuary was within permissible limits. Although the concentrations obtained for the heavy metals studied are generally below documented toxic levels, however, the increasing level of urbanization and industrialization in Calabar municipality and its environs calls for continuous vigilance, surveillance and monitoring of this sensitive and all important ecosystem to protect and ensure that heavy metal pollution is minimal.