Petroleum hydrocarbons and trace metal loads in the mangrove oyster (Crassostrea rhizophorae) from the Qua Iboe Estuary and adjoining creeks in Nigeria

Abstract

The mangrove oyster Crassostrea rhizophorae was harvested from clutches associated with mangroves in the Qua Iboe Estuary and adjoining creeks to determine the concentrations of total petroleum hydrocarbons (TPH) and trace metals (Fe, Ni, Pb, Cr, V, Cu and Zn) and to understand their bioaccumulation status. The mean concentrations of trace metals in C. rhizophorae were 36.06 mg·kg− 1 for Fe, 0.58 mg·kg− 1 for Ni, 0.05 mg·kg− 1 for Pb, 0.66 mg·kg− 1 for Cr, 0.32 mg·kg− 1 for V, 6.70 mg·kg− 1 for Cu and 3.93 mg·kg− 1 for Zn. The oysters demonstrated relative affinity for accumulation of Fe, Cu and Zn and the relative status of the metals accumulated, in comparison with the background trace metal load, revealed bio-concentration factors of 0.024, 0.016, 0.004, 0.213, 0.037, 0.302 and 0.036 for Fe, Ni, Pb, Cr, V, Cu and Zn, respectively. The concentrations of TPH in the oyster were highly variable and ranged between 6.37 and 8.44 mg·kg− 1 dry weight of C. rhizophorae. Although the correlation between TPH levels in intertidal (epipelic) sediment and oyster specimens was positive (r = 0.097), it was not significant (p>0.05). However, the relationship between TPH and trace metal levels in C. rhizophorae revealed well-fitting linear models that indicated that the trace metals originated from petrogenic sources. Although the levels of TPH and trace metals in the oyster were within permissible levels, they might affect calcium metabolism and the general productivity of the shellfish. The recorded levels are also indicative of long-term and chronic accumulation of hydrocarbons in the estuarine ecosystem and potential risk to human health.