Temporal trends in nutrient ratios: chemical evidence of Mediterranean ecosystem changes driven by human activity

Over the last few decades, the Mediterranean ecosystem has experienced changes in biodiversity due to climatic and environmental change or to accidental inputs of exotic species. But the plankton community, which is the base of the food chain and remains only partly described, is also probably experiencing a drastic change. Observed changes in nutrient concentrations and ratios in the deep waters of the western Mediterranean, as well as differences between the eastern and western Mediterranean, suggest that shifts have occurred in the relative distribution of nutrients and therefore probably phytoplankton species over the whole sea. A shift from a diatom-dominated ecosystem to a nonsiliceous one (as already observed in some coastal areas, with increasing algal blooms and eutrophication events) may involve the whole Mediterranean Sea and have consequences for fishery and tourism activities. (C) 2002 Elsevier Science Ltd. All rights reserved.