The water quality is very important for the health of ecosystems and to people own, therefore it is important to know it and to direct efforts to preserve it. In this sense, the evaluation of the eco-logical state through using bioindicators is very useful and simple.
The objective of our study was the analysis of the ecological status of seawater using mid-littoral commu-nities from rock substrate as a bioindicator. It has applied the CARLIT methodology (Ballesteros et al., 2007) which is based on mapping of communities living along the coast through the use of geographic in-formation systems (GIS). Specifically, the mapping of communities consists on displaying and identifying communities through the dominant species observed for each coastal stretch of about 50 meters. From here, it indicates a value of environmental sensitivity for each community it depends on its tolerance to pollution, and it is determined the ecological status of the area by applying a rate of Environmental Qua-lity .
The methodology has been applied in 2015 and 2016 in a area of study that includes all the coastal border of Platja d'Aro. Once developed sampling, the results have indicated us that the most abundant communi-ty in the study area has been Cystoseira mediterranea ahead of other as Corallina elongata or Cystoseira com-pressa, especially in far areas from the major population centers, with high hydrodynamics and well lit. Moreover it has been observed that communities of Corallina elongata and green algae are mainly located in restricted areas very near to urban centers or areas of artificialised coast.
In summary, it has been diagnosed that the ecological status of the sampled area is excellent and is close to that observed in unaltered places by human action
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