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Assessment of marine benthic quality change in gradients of disturbance

Comparison of different Scandinavian multi-metric indices

image of Assessment of marine benthic quality change in gradients of disturbance

In this study we compared three Scandinavian multi-metric indices to assess marine coastal benthic ecological status in seven different gradients of disturbance related to pollution. The indices measure in different ways species diversity and species composition in terms of sensitivity. The impact factors in the gradients included organic load, hypoxia, sediment metal contamination, urban effluents and physical disturbance. The indices responded in a similar way, mostly as a threshold response, to the pressure factors irrespective if the factor was organic pollution, metal contamination or other pressures. Usually, the border between Good and Moderate Ecological Quality Status (EcoQS) according to the Water Framework Directive (WFD), the G/M border, is determined as some deviation from a reference, near-natural, situation. Reference data, however, are difficult to find. An alternative procedure is described here to estimate the G/M border, which does not require near-natural reference data. Threshold values, beyond which faunal structure deterioration commences, were identified from non-linear regressions between indices and impact factors. Index values from the less impacted side of the thresholds were suggested to come from environments of Good and High EcoQS, and the lower border of these data, estimated by the 5th percentile of data after bootstrapping, should form the G/M border. G/M borders for each coastal area (gradient) were calculated and compared with previous studies.

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Discussion

Since the end of the last century several marine scientists have presented different indices for the assessment of benthic habitat quality in European waters. Here we present comparative analyses of three indices from Denmark, Norway and Sweden applied on materials from coastal areas affected by various degrees and types of disturbance. The disturbances include combinations of organic enrichment, hypoxia, heavy metal contamination and physical disturbance by tailings deposition. Despite difference in impact factors, the response by the benthic fauna structure, measured by the indices, shows high similarities. Overall the values of the indices decrease with increasing values of the impact factors. Furthermore, it appears that the respons of indices along the various pressure gradients is typically non-linear. At some threshold value of the impact factor there is often a marked change in change rates of the indices, indicating an increased deterioration of the benthic fauna structure. When the pressure exceeds the resistance of the community, for instance considerable species loss will occur. Such threshold responses of benthic macrofauna has previously been described in relation to hypoxia (Diaz and Rosenberg, 1995; Levin and Gage, 1998), organic enrichment in general (Hyland et al., 2005), and sediment metal contamination (Burd, 2002; Josefson et al., 2008).

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