Abstract
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.
Highlights
Metals are introduced into estuarine and coastal environments largely due to industrialization and economic development (Feng et al 2002), and they constitute, amongst others, environmental contaminants in aquatic ecosystems
This led to the introduction of pollution indices based on geochemical normalization to ecologically evaluate sources and contamination status of pollutants in coastal sediments (Muller 1979; Tomlinson et al 1980)
Sediment quality guidelines have been employed in classifying metal toxicity and as potential signals of pollution effect to resident biota in aquatic ecosystems (Ho et al 2012; Veerasingam et al 2012; Xu et al 2014)
Summary
Metals are introduced into estuarine and coastal environments largely due to industrialization and economic development (Feng et al 2002), and they constitute, amongst others, environmental contaminants in aquatic ecosystems. The distinction between the sources of trace metal contamination in many coastal environments poses a challenge to coastal managers due to the availability of metals from both anthropogenic and natural sources (Takarina 2011) This led to the introduction of pollution indices based on geochemical normalization (e.g., geoaccumulation index, enrichment factor, and pollution load index) to ecologically evaluate sources and contamination status of pollutants in coastal sediments (Muller 1979; Tomlinson et al 1980). Sediment quality guidelines have been employed in classifying metal toxicity and as potential signals of pollution effect to resident biota in aquatic ecosystems (Ho et al 2012; Veerasingam et al 2012; Xu et al 2014)
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