Applicability of the trophic index TRIX in two transitional ecosystems: the Mar Menor lagoon (Spain) and the Mondego estuary (Portugal)

Ecological indicators are commonly used to provide synoptic information about the state of ecosystems. Their main attribute is that they combine a range of environmental factors in a single value, which is thought useful for management and for making ecological-quality concepts easily understandable by the general public. In some European countries, the trophic state of marine and coastal waters is characterized using the trophic index TRIX. We used TRIX in the studies of the Mar Menor coastal lagoon ( southeast Spain) and the Mondego estuary ( northwest Portugal). These areas are under environmental stress, and eutrophication processes have been monitored in recent years. The results demonstrate the inefficacy of TRIX as a trustworthy tool to classify eutrophication status in estuarine waters. The causes of the malfunction of the index are discussed, and we suggest that, if the index is to be applied over wide areas, the classification criteria will have to be adapted to specific environments.