The use of benthic biodiversity indicators: pros and cons

Indicators are the scientific translation of governmental needs for reliable information on the condition or so-called ‘status’ of an ecosystem. Biodiversity is an important component within status assessments and a high-level objective of marine policy (E.g. Marine Strategy Framework Directive [MSFD], Water Framework Directive [WFD]). Consequently, diversity is incorporated into most of the indicators for the assessment of benthic ecosystem status. However, diversity is not always considered in their algorithm or in various forms, and that lead to a lower comparability between indicators (cf. WFD intercalibration exercise). The experts excluding diversity in their indicator, justifying this by the fact that diversity does not show a monotonic trend along gradients of pollution (cf Pearson Rosenberg model). And also when considering physical disturbances relationships between impact and diversity are not always linear, which was also the case in Belgian waters as a result of aggregate extraction and dredge disposal activities. This non-linear response, however, does not hamper a consistent environmental assessment by benthic indicators with a biodiversity component, due to an adequate definition of the GES boundaries. In the near future, genetic based biodiversity indicators will be used alongside the classic ones, based on taxonomic species identifications. Genetic tools such as meta-barcoding in environmental monitoring, however, are not yet fully applicable, and currently lead to discrepancies with the regular taxonomical approaches (genetic diversity higher than regular, cryptic species). In this contribution, we discuss the consequences (pros and cons) of the use of various types of benthic diversity indicators.