Response of different biotic indices to gradients of organic enrichment in Mediterranean coastal waters: Implications of non-monotonic responses of diversity measures

According to the Water Framework Directive (Directive 2000/60/CE), assessment methods for the biological quality element benthic invertebrates must include diversity, abundance and proportion of pollution sensitive/indicator taxa as indicative parameters. By one hand, the use of pollution sensitive/indicator taxa (indicator taxa indices) is criticized due to the lack of a reliable methodology to know the level at which indicator species can be well represented in unaffected communities. By the other hand, it is often remarked in the literature that the response of diversity measures may be biased by several methodological constraints. In the last few years, several multimetrics, combining both types of indices, have been proposed with the aim of providing a better picture of the response of benthic communities to disturbance gradients. In order to understand how different responses of diversity measures may affect the responses of multimetric indices, several biotic indices, including diversity measures, indicator taxa indices and multimetrics, were calculated for a set of Mediterranean coastal ecosystems affected by different ranges of organic matter content. Diversity measures did not show monotonic patterns of response to the gradient of organic content, particularly at the low end of its range, while strong correlations were found between indicator taxa indices and this pressure indicator gradient. The multimetric used in the study (M-AMBI) was more correlated with its diversity components (H′ and S) than with its indicator taxa component (AMBI) and, consequently, M-AMBI was always less correlated with the gradient of organic content than AMBI. In Mediterranean coastal water ecosystems naturally poor in sediment organic matter content, indicator taxa indices, such as MEDOCC, BOPA, AMBI or BENTIX, seem to give a more reliable picture of the response of benthic communities to moderate increments of organic content than diversity indices.