Metallothioneins as biomarkers of metal pollution in estuaries on the south coast of England

Abstract

Contamination of aquatic environments via anthropogenic release of metals is an increasing global environmental concern. The greatest concerns exist within estuarine and harbour environments, where point and non-point metal sources are prevalent. Historically, monitoring programs were initially (and largely) based on chemical analyses; however, the identification of a number of shortcomings in this approach resulted in a shift towards the complementary use of biological monitoring (biomonitoring). The aim of this study is to investigate and compare the suitability of selected aquatic invertebrate taxa biomarker responses (by production of metallothionein- MT) for assessment of aquatic environmental conditions in three coastal estuaries in Southern England: Poole Harbour, the Fal estuary and the Solent. This was delivered through a critical review of the current status of metal contamination in biota, water and sediment within these estuaries; by investigation of the suitability of selected species’ metallothioneins to act as biomarkers by assessing their correlation with metal pollution; by assessment of the effectiveness of these organisms as bioindicators in biomonitoring studies; and though investigation of the potential use of MT concentrations in these species as biomarkers of metal exposure in monitoring programmes.

Legislation has been established to facilitate the protection, and enhance the quality, of all water bodies, including coastal waters and estuaries to prevent their deterioration and to ensure they achieve ‘good’ ecological health; but examination of water and sediment samples showed that historical metal pollution has not been dispersed and still affects environmental quality. In Poole Harbour, metal contamination was detected in all environmental compartments. The enclosed nature of the harbour and its secondary embayments make it vulnerable to the effects of these metal sources which disperse extremely slowly due to restricted tidal exchange; and a significant part of the pollution load remains in coastal areas close to land based contamination sources. In Poole Harbour, water was polluted with As (ranged from 29.6 to 212.8 ?g/l) and Hg (ranged from 0.368 ?g/l to 11.06 ?g/l) which are above the Environmental Quality Standard (EQS) set by the European Union dangerous substances directive (EUDSD). Sediment metals were mostly within “the possible effect range” at which adverse effects occasionally occur, according to the Canadian Sediment Quality Guidelines (CSQG). Although cockles (Cerastoderma edule) had higher tissue concentrations of Ni, Ag and Hg in areas close to pollution sources, and sponges (Haliclona oculata) had accumulated Cu and Zn with a very high magnitude, the absence of EQS for metals in living organisms’ tissues makes it difficult to specify whether the metal concentrations reached a dangerous level or not in these organisms. The results confirmed the presence of a marked metal pollution gradient in the Fal Estuary between the ‘clean’ sites in the north and east of the estuary and the ‘polluted’ sites in the west of the estuary which were historically polluted by water from mines. Moreover, the concentrations of metals in almost all of the contaminated sites in the Fal and Solent estuaries have not changed significantly over the past few decades.

This study investigated the potential of MT as biomarker of metal pollution in the following wild species: two well-studied bioindicators- common cockles (C. edule) from Poole Harbour, and mussels (Mytilus edulis) from the Fal estuary and the Solent; and tested the following novel bioindicators: sponges (H. oculata) from Poole Harbour, ragworms (Hediste diversicolor) From the Fal estuary, and limpets (Patella vulgata) from the Solent. The spectrophotometric method was used to measure MT in these species as it has been reported to be a sensitive, time saving, and low-cost technique able to detect MT content in the tissues of aquatic organisms. The results showed that MT concentration in these estuarine wild invertebrates vary between sites; however, these variations were not exclusively associated with metal concentrations and that other environmental factors may explain some of the MTs variability. The many uncertainties surrounding MT in wild organisms indicate that it may have limited potential as a biomarker in estuarine environments. It is also apparent that the importance of biotic and abiotic factors at polluted sites may limit its application.

In this study, transplanted mussels (M. edulis) and Manila clams (Tapes philippinarum) were used as active biomonitoring tools in Poole Harbour. It was found that under extreme conditions of metal pollution, the relationship between metal concentrations and MT deviates from linearity: a threshold appears to be reached beyond which the physiological capacity of organisms introduced to polluted areas is exceeded. In the above transplanted animals, MT induction increased to peak levels with increased metal concentration, and then declined with further increases in the concentration.

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ORCID for Ian D. Williams: orcid.org/0000-0002-0121-1219

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