This project has been recently funded for the next three years by the highly competitive National Research Programme of the Spanish Ministry of Science and Technology (Ministerio de Ciencia y Tecnología). This study aims to evaluate the impact of several phitocenosis at coastal habitats on the natural variability of sea water pH, assessing the implications of these pH regimes and the ones foreseeable due to climate change for the larval development, settlement, survival and growth of key calcifying invertebrates (sea urchin species: Diadema aff. antillarum and Paracentrotus lividus). We intend to incorporate the natural patterns of pH levels, recorded at different algal communities, into a series of laboratory experiments and proceed with in situ transplants of cultured organisms to test how experimental pH regimes affect their survival and growth in the sea.