Freshwater ecosystems are directly affected by higher temperatures and the impacts of changing thermal structure and lake chemistry. At high latitudes and/or altitudes, ice cover is reduced and productivity increases, leading to increased algal abundance and in some cases fish production. In tropical lakes, in contrast, reduced productivity has been observed due to more stable stratification and reduced nutrient upwelling. As a consequence, freshwater ecosystems have changed in composition, which sometimes has led to detrimental consequences for human health caused by increased cyanobacteria populations. In river systems, numerous diverse changes have been observed in response to warming, including displacement of thermophilic species to the north (Rhone) and shifts in the timing of fish migration (North American rivers).
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