Comparative meta-analysis of the impact of offshore marine mining on macrobenthic communities versus organic pollution studies

Abstract

The phylum-level meta-analysis approach has been proposed as a way of comparing geographically different areas along a common scale of disturbance. However, the training data set which establishes the scale of disturbance and all subsequent studies using the approach have been exposed to some sort of organic or inorganic pollution. Using macrobenthic communities subjected to a short-lived but intense physical disturbance from offshore mining off the west coast of Southern Africa, we tested the applicability of the meta-analysis approach for assessing the severity of disturbance in physically disturbed communities. The positioning of the original data set along a primary axis of disturbance was maintained; however, a second significant separation, along the vertical axis, distinguishes between macrobenthic assemblages from southern Africa and the NE Atlantic Shelf. The southern African samples are characterised by a larger proportion of Crustacea, and in the case of mined samples Mollusca, whereas the NE Atlantic data contain relatively more Echinodermata in the unpolluted samples and are dominated by Annelida in the organically enriched areas. The proportion of annelids decreased by about 50% in mined areas compared to the non-mined areas and the NE Atlantic samples. Conversely, bivalves and gastropods exhibited a notable increase in proportion in the mined patches compared to the adjacent non-mined areas, possibly as a result of their preferential ability to survive the mining process or their better ability to recolonise after mining, or an interplay of both factors. The mining activity may result in the selection of species for their physical robustness and tolerance to mining rather than their resistance to pollution in the conventional sense. The failure of the meta-analysis to ordinate the mined samples along the primary horizontal axis of disturbance, as defined by Warwick & Clarke¹s original study (Mar Ecol Prog Ser 92:221-231), does not reflect a failure of the meta-analysis to detect disturbance, but rather shows that the primary axis is strongly determined by the opportunistic species characteristic of organically enriched areas. It appears that phylum-level meta-analysis is better suited to assessing the impact of organic and chemical pollution on an ocean-basin scale than it is to physical disturbance caused by offshore mining.