Many of the world’s clear lakes are affected by a new phenomenon: At the shore, where people play or swim, the lake bottom is covered with green carpets of algae. These are mass accumulations of filamentous algae that have recently appeared even in remote mountain lakes as well as in some large lakes such as Lake Tahoe (USA) and Lake Baikal (Russia). An international group of lake researchers from different institutions, including the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), has drawn attention to the problem and has compiled possible reasons. Because filamentous algae blooms change the ecosystem profoundly and can cause problems for water use.