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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 ( Lgross) ranged from −27 to 54 mg·m−2·day−1 ( n = 461), while long-term net rates in whole ecosystems ( Lnet) 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. [ABSTRACT FROM AUTHOR]