Bioindicators refer to any species/population/community, which has specific requirements in relation to an array of physical and chemical parameters. These organisms indicate by their presence/absence changes in the conditions of the water. Freshwater biota bears evolutive adaptations to certain environmental conditions, along with tolerance ranges to perturbations in their system. In this sense, unexpected changes in the composition and structure of the freshwater biota can be interpreted as strong signs of any type of pollution.
Bioindication began in Europe in the mid-19th century. Kolenati (1848) and Cohn (1853) found relationships between certain species and the degree of water quality. In the 20th century, Kolkowitz and Marsson (1908, 1909) proposed the saprobic system for Germany, as a measure of the degree of contamination by organic matter. Lorenz, van Dijk, van Hattum, and Cofino (1997) developed a system of bioindicators for the Rhine River (Germany) based on theoretical concepts describing natural rivers which included morphology of the systems, stream order and the River Continuum Concept (RCC).
Biotic indices were created to improve and shorten the time required to assess the quality of freshwater environments, based on the sensitivity and ranges of tolerance of organisms. One of these indices is the Biological Monitoring Working Party (BMWP) score system, which basis is the presence/absence of aquatic macroinvertebrates and their ranges of tolerance to organic pollution. It ranks macroinvertebrates families in a range which goes from 1 to 10, where the highest scores are given to those highly sensitive families to organic pollution, while the lowest scores indicate those taxa with a wider spectrum of tolerance to polluted conditions.
The BWMP score system has been broadly used worldwide to typify the quality of running waters due to its ease of use and cheap methodology. Over the past 40 years, it has been used in Europe. In the Rhine River basin, bioindication started in the early 1970s when typical physicochemical characterization was complemented with biological monitoring. Initially, the fish community was the only component. Later, macroinvertebrates, algae, and zooplankton were added for continuous monitoring.
Nowadays, biomonitoring involves a holistic approach where ecologists, taxonomists, environmental scientists, water sector industries and policymakers take part. Integrated initiatives such as Youth for the Rhine represent ideal scenarios for the establishment and development of diverse strategies to evaluate and have a closer view of the ecological status of the rivers.