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

The phylum-level meta-analysis approach has been proposed as a way of compa ring geographically different areas along a common scale of disturbance. Ho wever, the training data set which establishes the scale of disturbance and all subsequent studies using the approach have been exposed to some sort o f 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 d isturbed communities. The positioning of the original data set along a prim ary axis of disturbance was maintained; however, a second significant separ ation, along the vertical axis, distinguishes between macrobenthic assembla ges from southern Africa and the NE Atlantic Shelf. The southern African sa mples are characterised by a larger proportion of Crustacea, and in the cas e of mined samples Mollusca, whereas the NE Atlantic data contain relativel y more Echinodermata in the unpolluted samples and are dominated by Annelid a in the organically enriched areas. The proportion of annelids decreased b y about 50% in mined areas compared to the non-mined areas and the NE Atlan tic samples. Conversely, bivalves and gastropods exhibited a notable increa se in proportion in the mined patches compared to the adjacent non-mined ar eas, possibly as a result of their preferential ability to survive the mini ng process or their better ability to recolonise after mining, or an interp lay of both factors. The mining activity may result in the selection of spe cies for their physical robustness and tolerance to mining rather than thei r resistance to pollution in the conventional sense. The failure of the met a-analysis to ordinate the mined samples along the primary horizontal axis of disturbance, as defined by Warwick & Clarke's original study (1993, 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 d etermined by the opportunistic species characteristic of organically enrich ed areas. It appears that phylum-level meta-analysis is better suited to as sessing the impact of organic and chemical pollution on an ocean-basin scal e than it is to physical disturbance caused by offshore mining.