In just four years after the initial discovery in 1986 zebra mussels spread across the Great Lakes (except Lake Superior), quickly colonizing all suitable areas. Although lake-wide Dreissena surveys greatly underestimated the size of D. polymorpha populations both in space and time, Dreissena initiated all the drastic changes in water quality seen in the Great Lakes early in the Dreissena spp. invasion. Quaggas colonized Great Lakes later than zebras due to their slower spread, but by the early 2000s they outcompeted zebras and became the dominant species. Quaggas eventually occupied even the deepest bottom areas of the lakes. In shallow Lake Erie quaggas reached a population maximum by the end of 1990s-early 2000s and are now declining. In contrast, in deeper lakes quaggas have currently reached their carrying capacity only in shallow to mid depth zones, while densities are still climbing deeper than 100 m.  Therefore the lag time between the initial colonization and maximum population density depends on Dreissena species and lake morphometry, and is longer in deep lakes