Copper and zinc in water, sediment and gastropods in the harbours of the Cape Town Metropole, South Africa

Abstract

The harbours in Cape Town are an important hub for development and socio-economic activities, some of which include shipping traffic, ship repair and maintenance, commercial and artisan fishing, construction (dredging and reclamation) and recreational vessel activities. These harbour related activities have contributed enormously to the influx of contaminants such as metals into the coastal environment. The semi-enclosed nature of the harbours associated with limited water exchange is conducive for water pollution. Thus, harbours are increasingly becoming a hotspot for metal loading into coastal ecosystems. Copper and zinc are metal-based biocides in present-day antifouling (AF) paints. Concerns have been raised over the effect of these metal ions in the marine environment with high levels being detected in areas of intense vessel activities such as harbour. The aim of the study was to determine concentration levels of copper and zinc in seawater, sediment and gastropods (Burnupena spp. and Nucella spp.) from selected harbours and reference sites in the Cape Town metropole. Also, to determine the suitability of the two gastropods for use as biomonitors of metal contamination as well as whether there was a causal relationship between copper and zinc content in the gastropods and the concentrations in water and sediment from the harbours. Samples were collected once-off seasonally in March (dry season) and September (wet season) 2016 from sampling point(s) in the harbours and reference sites at spring low tides. Samples of seawater, sediment and gastropods (soft tissue and shell) were acid digested and metal concentrations analysed in quintuplicate using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Statistical analyses were conducted using the SigmaPlot 13 software. Statistically significant differences in copper and zinc concentrations between sampling points in harbours and the reference sites were evaluated using a Kruskal–Wallis One-Way ANOVA on Ranks and Student Newman Kuels Method for post hoc tests as datasets were non-parametric. Dunn’s Method was used for Post hoc test after the ranked based ANOVA to evaluate significant differences in copper and zinc concentrations between harbours (unequal datasets). The Mann Whitney Rank Sum Test was used for comparisons in copper and zinc concentrations between the two seasons per sampling point, per sampling sites and between soft tissue and shell per sampling point. The Spearman’s Rank Order Correlation was used to determine if there was a relationship between the metal concentrations in the ambient samples (seawater and sediment) and soft tissue and shells of the gastropods. The results showed that the mean copper and zinc concentrations (mg/L) in seawater ranged between not detected (ND) to 0.0818±0.0494 and ND to1.7679±0.639, respectively. The corresponding mean concentrations (mg/kg dry weight) of copper and zinc in sediment were ND to 3432.16±2306.68 and 1.20±1.53 to 2380.43±1456.79, respectively. The highest mean copper and zinc concentrations (mg/kg dry weight) were found in Nucella soft tissue with a range of 19.84±6.43 to 2211.61±3168.07 and 77.20±15.14 to 5045.44±2447.15, respectively. The mean copper and zinc concentrations in seawater, sediment and gastropods (soft tissue and shell) were found to be generally higher in the dry season than the wet season. Generally, the mean copper and zinc concentrations in the soft tissue of the gastropods were higher than in the shells. The findings indicated that variations in copper and zinc concentrations in seawater, sediment and gastropods (soft tissue and shells) at sampling points in the harbours could be attributed to proximity to contamination sources, the rate of water exchange, metal handling strategies of gastropods as well as changes in environmental factors. The highest mean copper and zinc concentrations were found at sampling points close to areas of intense vessel-related activities in the harbours. Therefore, it could be suggested that AF paints are a predominant source of copper and zinc in seawater, sediment and the gastropods in the harbours. The correlation analyses revealed that there were generally no significant correlations between copper and zinc contents in the soft tissue or shell of the gastropods (Nucella spp.) and the ambient environmental concentrations in the harbours and reference sites although some distinct trends were observed. It was shown that copper and zinc concentrations in the soft tissue or shell of Nucella spp. may not be directly affected by those of the ambient seawater and sediment. It may, therefore, be presumed that the changes in copper and zinc loading in seawater and sediment were not the only factors that influenced the level of bioavailability of these metals to the Nucella spp. It is possible that the bioaccumulation of copper and zinc in the soft tissue or shell of Nucella spp. may have been influenced by many physicochemical and biological parameters. By comparing the data with water and sediment quality guidelines, it was observed that mean copper and zinc concentrations in seawater from some of the sampling points in the harbours exceeded the South African Water Quality Guidelines (SAWQGs). Likewise, the mean copper and zinc concentrations in sediment from some sampling points in the harbours were moderately or seriously polluted based on the Benguela Current Large Marine Ecosystem Sediment Quality Guidelines for southern Africa (BCLME-SQGs). It is, therefore, strongly suggested that source identification and continuous monitoring of copper and zinc in water, sediment and biota in the harbours is imperative.