Zebra mussels arrived in the Great Lakes in the late 1980s, almost certainly in ballast water from Eurasian shipping, and spread through connected waterways within a few years. Unlike native freshwater mussels they attach to hard surfaces with byssal threads and encrust in layers, reaching very high densities on rock, pipes, hulls and each other.
Each mussel filters roughly a litre of water a day, and a colonised lake bed can process a large fraction of the overlying water column. The visible result was increased water clarity in Lake Erie and elsewhere. The invisible result was a redistribution of energy: plankton that formerly fed larval fish in open water was intercepted at the bottom and returned there as faeces and pseudofaeces, favouring benthic algae and bottom-dwellers.
Secondary effects followed, including expanded nuisance Cladophora growth on newly sunlit substrate, and links have been drawn to botulism outbreaks in waterbirds. The costs are also mundane and large: mussels clog intake pipes for municipal water and power plants, and control is a permanent operating expense rather than a one-off cleanup. Clearer water, in this case, is a symptom.