The application of active and passive bioaccumulation methods to measure metallic and organic pollutants along the Namibian coastline

Abstract

Human activities and natural weathering processes are responsible for the input of elements and organic pollutants into marine environments. There is currently no ongoing marine pollution monitoring program in Namibia and there are limited studies on marine pollution along the Namibian coastline. Since the 1970s, mussels have been used as bioindicators because they are widely distributed, plentiful, and stationary. The main aim of this study was to use active and passive biomonitoring tools to determine the element and organic contaminant concentrations present in the coastal areas of Swakopmund and Walvis Bay Harbour in Namibia. Artificial mussels (AMs), semi-permeable membrane devices (SPMDs), resident- and transplanted mussel bioindicators (Choromytilus meriolionalis) were deployed at two exposure sites (Walvis Bay Harbour & Swakopmund) for six weeks. Element concentrations (Al, As, Cd, Co, Cu, Fe, Mn, Mo, Ni, Pb, Se, U, and Zn) were determined using inductively coupled plasma mass spectrometry (ICP-MS) techniques. Chromium concentration was determined using an atomic absorption spectrometer (AAS) and mercury (Hg) concentrations with a flow injection mercury system (FIMS). The concentrations of organochlorine pesticides (OCPs) and polychlorinated biphenyl (PCB) were analysed in both SPMD and mussels following QuECHERS extraction methods and using a GC-20230 Plus coupled with a GCMS-TQ8050 NX triple quadruple mass spectrometer. The element levels in all AMs were significantly lower (p<0.05) when compared to mussels and did not show a significant increase in uptake during the deployment period. Transplantation studies are very effective as a biomonitoring tool, but for more comprehensive data interpretation, resident mussels should be assessed in conjunction with transplanted mussels. The transplanted and resident mussels from Walvis Bay Harbour had significantly (p<0.05) higher Cu concentrations than the transplanted and resident mussels collected at Swakopmund. Walvis Bay Harbour’s resident mussels had significantly lower element concentrations for the most part when compared to Swakopmund’s resident mussels. This was attributed to the adaptation of resident mussels to their environment through the regulation of elements. The differences are also attributed to constant exposure since resident mussels remain submerged and therefore constantly exposed to metals compared to the transplanted mussels that are subjected to tidal influences and exposed to the air during low tide. There were notable fluctuations in the element levels in both the AMs and the transplanted mussels during the deployment period for both study sites. The results indicated that there are clear trends in the spatial and bioaccumulation indicators for elements in mussels at the two study sites. There were also clear differences between the uptake patterns of OCPs in the mussels and the SPMDs. Although analyses for PCBs were done, they were not detected in the SPMDs or the mussels. When comparing the results of this study in Namibia to element concentrations in mussels from South African harbours, the levels for most of the elements were in the same range. However, passive biomonitoring devices (AMs and SPMD) displayed much lower levels than those found in other studies. For monitoring purposes, passive (AM and SPMD) should be used in combination with the active biomonitoring organisms (transplanted and resident mussels) to acquire an accurate representation of contaminant exposure in the marine environment. This study could potentially contribute to the development of a Namibian coastal biomonitoring plan.