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Mhlathuze Estuary constitutes one of the ecological most important estuaries in southern Africa and is regarded as an estuary of high conservation importance. The ongoing expansion of the adjacent industrialized Richards Bay Harbour increases the risk of metal pollution to the estuary. This study provides insight into the extent and sources of trace metal contamination using pollution indices and sediment quality guidelines and the effect on macrobenthic habitat quality. Sediment samples for sediment metal and macrobenthic analysis were collected quarterly during 2016-2017 at five sites in the estuary using a marine-grade Zabalocki grab. Metal concentrations were determined using an ICP-OES. Sediment metal concentrations were consistently highest in the subtidal mudflats and lowest in marine sand at the mouth of the estuary. Concentrations of all metals displayed significant differences between sites (P < 0.05). Pollution indices indicated moderate enrichment of Cr at all sites, although the mean pollution load index showed the estuary to be unpolluted. Comparison with sediment quality guidelines revealed that concentrations of Ni and Cr were potentially toxic to biota. Using multivariate analysis, metal concentrations appeared not to significantly affect the macrobenthic community. The multi-metric biotic index M-AMBI proved to be a robust tool in the habitat quality assessment of the estuary. The continuing use of M-AMBI as a biomonitoring tool for ecological management of the estuary is advocated.

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Bioaccumulation of metals in an endemic ocypodid burrowing mud crab, Paratylodiplax blephariskios, was investigated in contaminated mudflats of the subtropical Richards Bay Harbour (RBH), South Africa. Following sampling of water, sediment and mud crabs from three sites (Bhizolo, Mzingazi and Richards Bay Coal Terminal) in RBH during 2016/2017, tissue and sediment samples were oven dried for 48 hours, weighed and digested in an advanced microwave digester. Samples were then analyzed in triplicate for Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn using inductively coupled plasma optical emission spectrometry (ICP-OES). Highest tissue concentrations of Co, Cu, Mn, Ni, Pb and Zn were recorded at Bhizolo. High Biota Sediment Accumulation Factor (BSAF) values for Cd, Cu, Mn, Ni and Zn at all sites confirmed that crabs bioaccumulated these metals. The high Cr concentration in sediment (Bhizolo: 104 ± 18.2 µg g −1 ; Richards Bay Coal Terminal (RBCT): 104 ± 29.2 µg g −1 ; Mzingazi: 94.1 ± 38.7 µg g −1 ) was not reflected in the biota-sediment accumulation factor (BSAF) value, suggesting low bioavailability of Cr. High BSAF values for Cd, Cu, Mn, Ni and Zn suggest that the threshold of regulation in P. blephariskios was exceeded, resulting in net bioaccumulation.

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