Résumé
In a natural world increasingly transformed by human activities, the contemporary acceleration of biodiversity loss is now widely accepted. Coastal and estuarine environments are some of the most productive ecological systems on Earth and are able to provide many essential ecosystem goods and services, such as protein from fish or nutrient exports to adjacent marine ecosystems. Unfortunately, coastal lagoons are under increasing human pressure, and constraints on fish biodiversity are of major concern. Using fish samples from the Terminos coastal lagoon (southern Gulf of Mexico), we found a significant decrease in taxonomic diversity over 18 yr while variations in richness and evenness were mainly associated with seasonal and spatial variations. In other words, 2 randomly chosen individuals or 2 randomly chosen species within a fish assemblage were more likely to be closely related in the 1998 to 1999 period than in the 1980 to 1981 period. This result suggests a loss of functional diversity and a biotic homogenization in the lagoon fish community. At the same time, we observed a significant salinity increase between the 1980 to 1981 period and the 1998 to 1999 period, and the water temperature and transparency increased between the 2 annual periods. These results suggest a loss of estuarine conditions in the Terminos lagoon between the 1980 to 1981 and the 1998 to 1999 periods and a shift in hydrological conditions from hypohaline to euhaline/hyperhaline status. In parallel, some fish families such as Mugilidae, Serranidae, Lobotidae, Achiridae, Belonidae, Elopidae and Stromatide are better represented within the Terminos lagoon now than they were in the past. As a feedback, this loss of taxonomic diversity can lead to a loss of ecological responsiveness to environmental fluctuations and a loss of ecological functioning.