TRAVERSE CITY, Mich. (AP) — Food supplies for fish and other organisms are declining in some areas of the Great Lakes, particularly Lakes Huron and Michigan, according to a newly released scientific report.
The study, based on years of data compiled by government agencies and university researchers, found evidence of drop-offs in phytoplankton — tiny plants essential to many food chains — since the late 1990s. A decline in tiny invertebrates and prey fish, such as alewives and round gobies, also was detected.
It's likely that invasive quagga and zebra mussels have played a significant role by gobbling plankton, according to the paper, which was published online this month in the journal BioScience. The mussels arrived in the Great Lakes in the 1980s after being scooped into cargo ships' ballast tanks in foreign ports and hauled across the Atlantic.
Another crucial factor is government policies that have reduced the flow of phosphorus — a key food source for plankton — as a means of preventing runaway cyanobacteria blooms (sometimes referred to as blue-green algae blooms).
"As we shrink the base of the food web, it ultimately will constrain the amount of fish we have," said David "Bo" Bunnell, lead author of the report and a fishery biologist with the U.S. Geological Survey's Great Lakes Science Center in Ann Arbor. The salmon population already has crashed in Lake Huron because of steep declines in the forage fish they eat.
The study was designed to document trends in Great Lakes food webs and determine whether the webs were influenced more by the feeding habits of top predator fish or by developments at the lower end of the chains.
For the most part, "bottom-up" factors were found to have a greater effect. The phosphorus shortfall in deeper waters and the mussel infestation closer to shore were most evident in Lakes Huron and Michigan. Invertebrates that feed prey fish were noticeably absent in both, along with Lake Ontario.
Declines in prey fish numbers were documented in Lakes Michigan, Huron and Superior. But only in Lake Michigan was it evident that the heavy appetites of predator fish at least partly caused the drops.
"Food isn't available for those prey fish, and it's not because we've overstocked" bigger fish, said Tom Nalepa, a scientist with the University of Michigan Water Center. "It's because of changes in the lower food web."
While each lake has unique characteristics, the data revealed increased water clarity everywhere except in Lake Erie, which has been plagued by excessive algae. Clear water can be a telltale sign of invasive mussels.
Despite the massive amounts of information used to compile the report, the scientists said there were significant data shortages that show the need for more intensive monitoring of Great Lakes ecosystems.
"The biggest gap in the study is that we need to do a better job of estimating mussel populations," Nalepa said.
Clarification: An earlier version of this story referred to "algae blooms" in Lake Erie. These are really bacterial blooms (cyanobacteria) that look like algae. The copy has been clarified above.