This chapter reviews the diversity and composition of algal, invertebrate and fish communities in the Llobregat basin and analyses the factors affecting their distribution. Phytoplankton develops only during short periods and in particular areas where the hydraulic residence time is high. Diatoms are by far the most diverse and abundant in autotrophic benthic biofilms. The invertebrate community is highly rich taxonomically, whereas the native fish richness is relatively low, with about eight freshwater species and some others of marine origin. Some of the fish native species are threatened and two have been extirpated. Many exotic fish species have been introduced into the basin and are suspected to have contributed to the decline of native fish. Salt inputs, organic pollution and hydrological conditions lead to further alterations in water quality in the middle part of the river and downstream, thus reducing ecological quality. Multiple pressures on fluvial biodiversity severely compromise the integrity of this river’s ecosystem. To recover the structure and functioning of the biological communities, it is important to maintain the integrity of headwater ecosystems to ensure a species pool for colonisation downstream and reduce water abstraction, regulation and nutrient and pollutant inputs
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