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Historically important sport and commercial fish in the lake - such as smallmouth bass - have increased with the decrease of phosphorus coming into the lake. But other fish species, such as walleye, have begun to decline.
"There are trade-offs to Lake Erie becoming cleaner," said Roy Stein, a study co-author and director of Ohio State University's Aquatic Ecology Lab. According to 1997 figures, Lake Erie's sport fishing industry averaged an annual $243 million.
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"We need to decide whether to manage for continued ecosystem rehabilitation or for the success of only a few economically important species," said Stein, who is also a professor in the department of evolution, ecology and organismal biology at Ohio State.
The research appears in a recent issue of the journal Ecological Applications.
The scientists looked at 28 years' worth of water quality and fish community data gathered from two distinct areas of Lake Erie - the warm, shallow west basin and the cooler, deeper central basin. The Ohio Department of Natural Resources-Division of Wildlife collected the data from 1969 to 1996.
Reductions in phosphorus pollution led to the loss of one-quarter of the total number of fish species in the west basin (from nearly 20 to about 15), while diversity increased by one-quarter in the central basin (from less than 12 species to nearly 15.) Data from the east basin, the coolest and deepest part of the lake, wasn't available.
Phosphorus is a byproduct of municipal and industrial wastewater plants and from fertilizer use on agricultural lands. Too much phosphorus in a body of water can lead to eutrophication - a condition in which algae thrive and use much of the available oxygen near the bottom of the lake. This can, in turn, harm fish intolerant of low oxygen levels, such as smallmouth bass and lake whitefish.
In the last 30 years, clean water initiatives to reduce the amount of phosphorus flowing into Lake Erie have been successful, Stein said, creating cleaner, oxygen-rich water.
"We know the effects of increased phosphorus in a body of water," he said. "But we're just starting to learn what happens to fish in a lake when phosphorus levels are reduced."
The researchers looked at 18 key fish they called "signal species" - species captured at least four years out of the 28-year data set. None of the signal species served as walleye prey, since predators can drive shifts in prey abundance. Walleye is the lake's top predator.
Signal species varied in their ability to tolerate eutrophic conditions: Tolerant species included black crappie, channel catfish and common carp; intolerant species included smallmouth bass, lake whitefish and minnows.
The researchers used the abundance of signal species throughout the years to better understand how fish communities in Lake Erie responded to changes in phosphorus levels.
The number of species intolerant to eutrophication increased in both basins (from one to more than four in the central basin, and two to four in the west basin.) The levels of tolerant species declined in the west basin (from 11 to less than six) and remained the same in the central basin (about six).
"Species diversity typically increases with a decline in lake pollution," Stein said. "But in the west basin, that diversity reached a peak with moderate levels of phosphorus, and declined as phosphorus decreased. Even so, these changes are positive signs of ecosystem recovery."
Sport and commercial fishermen are most concerned about the livelihood of intolerant species. Walleye and smallmouth bass are two of the most economically important species in Lake Erie, but each prefers slightly different conditions, Stein said.
"Whether a cleaner lake is good or bad depends on your perspective," Stein said. "We're pushing the system to be as clean as possible, and walleye like moderate levels of phosphorus in their habitat. So we've seen a decline in walleye in the west basin. On the other hand, smallmouth and rock bass are increasing in the lake.
"As these systems continue to improve in water quality and clarity, we will likely see corresponding changes in the fish community," Stein said.
The U.S. Fish and Wildlife Service and the Ohio Department of Natural Resources-Division of Wildlife supported this work.
Stein co-authored the study with Stuart Ludsin of the University of Windsor; Mark Kershner of Kent State University; Karen Blocksom of the U.S. EPA in Cincinnati; and Roger Knight of ODNR-DOW.
Contact: Roy Stein, 614-292-1613; Stein.4@osu.edu
Written by Holly Wagner, 614-292-8310; Wagner.235@osu.edu
Journal
Ecological Applications