Bioindicators in marine waters: Benthic diatoms as a tool to assess water quality from eutrophic to oligotrophic coastal ecosystems

Marine waters in general and coastal areas in particular suffer globally from stressing events. These events can be largely attributed to higher levels of human-induced eutrophication, contamination by pollutant, and siltation. The European Union and other countries have adopted legislation to guide monitoring efforts and set goals for good quality coastal environments. Few biological quality elements were identified by legislators for their ability to detect and identify sources of ecosystem perturbations. Research programs have developed indices calculation methods for benthic invertebrate fauna, phytoplankton, macroalgae and angiosperms. Most of these indices are multimetric, based on the richness, abundance and biomass of the community. Such indices determine the quality at one site after observational comparison with a reference situation. Functional indices, however, are based on species ecological preferenda and autoecology. Since species are generally response-specific, functional index allows the discernment of the stressing factor involved in the ecosystem perturbation. The final index value provides a direct appreciation of the ecological status of the study site. At the present time, there are few functional indices that can be used on a routine basis for monitoring coastal waters. Because biological quality elements cited below are not available in all areas, the range of monitoring is restricted to particular environments. This review paper suggests that marine benthic diatoms (Bacillariophyceae) may result in a functional index. Arguments for the use of these organisms as bio-indicators include knowledge from the past and present use of diatoms in freshwater environmental analysis. Marine benthic diatoms are abundant and ubiquitous in terms of ecosystems and water chemistry and are sensitive to nutrient variation, as revealed by two studies conducted in mesotrophic and oligotrophic areas. This review is focused on benthic diatoms and reveals that no study has yet been conducted on the use of these organisms as bio-indicators. Aspects of practical sampling and sample treatment for marine diatoms are considered. As the bio-indication role of diatoms is little explored in marine environments, the dataset on ecological indicator values of species is scarce. Future work that relates to taxonomic determination and physico-chemical description of sites are needed to better define species ecological preferenda, prior to the development of a marine diatom biotic index.