Application and reassessment of the reduced species list index for macroalgae to assess the ecological status under the Water Framework Directive in the Atlantic coast of Southern Spain

According to the Water Framework Directive (WFD) macroalgae are one of the Biological Quality Elements proposed to assess the ecological status of coastal water bodies. In the case of the North East Atlantic coastal shores, two methodologies have been implemented (RSL – reduced species list – in the U.K.; CFR – quality of rocky bottoms – in the Spanish Cantabric Sea). However, the ecological differences between these shores and the Atlantic coasts of Southern Spain imply a reassessment of these indices when applied to this water body. In this study, the RSL index has been reassessed for the rocky shores of the Atlantic coast of Andalusia (south-western Spain). In addition, an ecological and a morphological approximation to this index have been compared. After successive field sampling in the period 2006–2010, a reduced species list was developed for this shore. Based on anthropogenic pressures (water turbidity, nutrients, metal concentration and the distance to sources of stress), 19 sites along the coast were classified in five quality status (high, good, moderate, poor and bad) as proposed in the WFD. According to this classification the RSL index was calibrated. Finally, the results of the reassessed RSL-index were compared with the water quality. Overall, most of the elements yielded a significant relationship with the water quality and showed significant differences among the ecological quality classification. The less significant boundary among ecological status is the one lying between good and high. The results showed that both approximations of the RSL index were suitable to assess the ecological status, being the ecological approximation more suitable. Furthermore, the data analysis pointed out the existence of two coastal fringes with a different intertidal composition of algal species: Atlantic Cádiz and the Gibraltar Strait.

► The reduced species list index (RSL) uses macroalgae to assess the ecological status.
► The RSL was adapted for the WFD implementation in the Atlantic coast of Andalusia.
► Two approximations to this index has been compared in a gradient of pressures.
► Both approximations were suitable to assess the ecological status.
► The ecological approximation was more accurate than the morphological approach.