TRAVERSE CITY, Mich. (AP) — Food supplies for fish andother organisms are declining in some areas
of the Great Lakes,particularly Lakes Huron and Michigan, according to a newly releasedscientific report.The
study, based on years of data compiled bygovernment agencies and university researchers, found evidence
ofdrop-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 likelythat
invasive quagga and zebra mussels have played a significant role bygobbling plankton, according to the
paper, which was published onlinethis month in the journal BioScience. The mussels arrived in the GreatLakes
in the 1980s after being scooped into cargo ships’ ballast tanksin foreign ports and hauled across the
Atlantic.Another crucialfactor is government policies that have reduced the flow of phosphorus —a key food
source for plankton — as a means of preventing runaway algaeblooms."As we shrink the base of the food
web, it ultimatelywill 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 salmonpopulation already has crashed in Lake Huron because of steep declinesin the forage
fish they eat.The study was designed to documenttrends in Great Lakes food webs and determine whether the
webs wereinfluenced more by the feeding habits of top predator fish or bydevelopments at the lower end of
the chains.For the most part,"bottom-up" factors were found to have a greater effect. The
phosphorusshortfall in deeper waters and the mussel infestation closer to shorewere most evident in Lakes
Huron and Michigan. Invertebrates that feedprey fish were noticeably absent in both, along with Lake
Ontario.Declinesin prey fish numbers were documented in Lakes Michigan, Huron andSuperior. But only in Lake
Michigan was it evident that the heavyappetites of predator fish at least partly caused the
drops."Foodisn’t available for those prey fish, and it’s not because we’veoverstocked" bigger
fish, said Tom Nalepa, a scientist with theUniversity of Michigan Water Center. "It’s because of
changes in thelower food web."While each lake has unique characteristics, thedata revealed increased
water clarity everywhere except in Lake Erie,which has been plagued by excessive algae. Clear water can be a
telltalesign of invasive mussels.Despite the massive amounts ofinformation used to compile the report, the
scientists said there weresignificant data shortages that show the need for more intensivemonitoring 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.___Follow John Flesher on Twitter at http://twitter.com/JohnFlesherCopyright 2013
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