More than 70,000 large dams have been built worldwide. With growing water stress and demand for energy, this number will continue to increase in the foreseeable future. Damming greatly modifies the ecological functioning of river systems. In particular, dam reservoirs sequester nutrient elements and, hence, reduce downstream transfer of nutrients to floodplains, lakes, wetlands, and coastal marine environments. Here, we quantify the global impact of dams on the riverine fluxes and speciation of the limiting nutrient phosphorus (P), using a mechanistic modeling approach that accounts for the in-reservoir biogeochemical transformations of P. According to the model calculations, the mass of total P (TP) trapped in reservoirs nearly doubled between 1970 and 2000, reaching 42 Gmol y −1 , or 12% of the global river TP load in 2000. Because of the current surge in dam building, we project that by 2030, about 17% of the global river TP load will be sequestered in reservoir sediments. The largest projected increases in TP and reactive P (RP) retention by damming will take place in Asia and South America, especially in the Yangtze, Mekong, and Amazon drainage basins. Despite the large P retention capacity of reservoirs, the export of RP from watersheds will continue to grow unless additional measures are taken to curb anthropogenic P emissions.phosphorus | river damming | biogeochemical cycles | nutrient retention | eutrophication
Many physical, chemical, and biological processes in freshwater ecosystems mobilize the nutrient phosphorus (P) from sediments, which in turn may contribute to the formation of harmful algal blooms. Here, we critically reviewed internal P loading in Canadian fresh waters to understand the geographic patterns and environmental drivers of this important process. From 43 publications, we consolidated 618 estimates of internal P loading from Canadian freshwater ponds, lakes, reservoirs, and coastal wetlands (n = 70). Expressed in terms of total P, short-term gross rates in sediment samples (L gross ) ranged from −27 to 54 mg·m −2 ·day −1 (n = 461), while long-term net rates in whole ecosystems (L net ) ranged from −1694 to 10 640 mg·m −2 ·year −1 (n = 157). The main environmental drivers of this variation were oxygen, pH, geology, and trophic state. Internal P loading tended to be higher during the open-water season and most prominent in small prairie lakes. Priorities for future research on internal P loading should include resolving methodological problems, assessing the relative importance of different mechanisms, examining the influence of anthropogenic activities, and quantifying rates in understudied ecosystems. Résumé :De nombreux processus physiques, chimiques et biologiques dans les écosystèmes d'eau douce mobilisent le phosphore (P), un élément nutritif, des sédiments, ce qui peut favoriser la formation de fleurs d'eau néfastes. Nous avons effectué un examen critique de l'apport interne de P dans les plans d'eau douce canadiens afin de comprendre les motifs de répartition géographique et les facteurs environnementaux qui influent sur cet important processus. À la lumière de 43 publications, nous avons colligé 618 estimations de l'apport interne de P d'étangs, de lacs, de réservoirs et de milieux humides littoraux d'eau douce canadiens (n = 70). Exprimés en termes de P total, les taux d'apport de P à court terme pour les échantillons de sédiments (L gross ) vont de −27 à 54 mg·m -2 ·jour -1 (n = 461), alors que les taux nets à long terme à l'échelle de l'écosystème (L net ) vont de −1694 à 10 640 mg·m -2 ·an -1 (n = 157). Les principaux facteurs environnementaux influant sur ces variations sont l'oxygène, le pH, la géologie et l'état trophique. L'apport interne de P tend à être plus élevé durant la période d'eau libre et le plus marqué dans les petits lacs de prairie. La priorité des travaux futurs sur l'apport interne de P devrait porter sur la résolution de problèmes méthodologiques, l'évaluation de l'importance relative de différents mécanismes, l'examen de l'influence des activités humaines et la quantification des taux dans les écosystèmes sous-étudiés. [Traduit par la Rédaction]
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