Chicago - Great Lakes shorelines are becoming clogged by algae blooms fed by agricultural run-off, while invasive mussels decimate the food chain in deeper waters, an environmental group said on Tuesday.
The five lakes, which contain one-fifth of the world's fresh water and supply tens of millions of people, may be “veering close to ecosystem collapse,” the report by the National Wildlife Federation said.
“Too much food is causing massive algal blooms in Lake Erie and other coastal systems, while too little food is making fish starve in Lake Huron's offshore waters,” said the group's Great Lakes director, Andy Buchsbaum.
There are many problems afflicting the Great Lakes, which in other ways have grown healthier after years of pollution.
This past summer, Lake Erie was choked by toxic algae blooms up to 2 feet (0.7 metres) thick and 10 miles (16km) wide, and algae coated some Lake Michigan coastlines. Water treatment removes the toxin, at a cost, but often creates an unpleasant odour, one of the report's authors, Julie Mida Hinderer, said in an interview.
In deeper water, prolific quagga mussels have beaten out zebra mussels and colonised vast stretches of the lake bottoms, filtering out vital plankton that is the base of the lakes' food web. This has starved small fish such as alewives, bloater fish, and rainbow smelt that in turn has hurt populations of top lake predators such as whitefish and salmon.
Both mussel species are not native and arrived in the ballast water of ocean-going ships, but quagga mussels are more of a hazard because they colonise more of the lake bottom than zebra mussels.
There has been a 95 percent decline in fish biomass in Lake Huron, one of three larger lakes, in the past 15 years, according to the report. Freshwater shrimp that are key to Lake Michigan's fishery have declined by 94 percent in 10 years.
Scientists blame too much phosphorous from farm fertiliser runoff for the algae blooms, which decompose and create an oxygen-depleted “dead zone” in the Lake Erie.
Among the report's recommendations are for farmers to leave buffer zones between farmland and waterways, and better enforcement of clean water laws. Scientists are also experimenting with open water applications of a microbial agent that kills mussels, and with introducing mussel-eating fish.
The problems predate the threat to the lakes' $7 billion fishery posed by Asian carp, which authorities are desperately trying to block from migrating from the Mississippi River watershed - though the invading carp could prove to be a benefit in that they eat an algae carpeting lake beaches. - Reuters