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 Significant size changes in mussel shells suggest that the river
environment has been altered.
Scientists at the Illinois Natural History Survey (INHS) of the
PRI selected specimens of the Threeridge mussel and Mapleleaf
mussel from the INHS Mollusk Collection that had been collected
from the Illinois River in the years 1897, 1912, 1966, and 2013.
They also examined Illinois State Museum shell collections from
archaeological excavations along the Illinois River that were
dated to 1,000–1,200 years before present (~850 AD).
The researchers studied mussel growth, maximum size, and diet.
Annual rings are produced in shells as they grow and age, much
like tree rings. Mussels are filter feeders of algae and
bacteria, which are modified by human-induced environmental
changes such as river impoundments and nutrient-rich pollution,
so changes in food quality and availability are reflected in
their growth.
The team was surprised to discover that mussel shell size and
growth rate remained approximately the same from the year 850 to
1897, and then increased by more than 50 percent over the course
of the 20th century, according to Andrea Fritts, former
postdoctoral research associate at INHS.
“There was a clear separation in size between the time before
the late 1800s and in the 20th century,” Fritts said. “The
changes that we observed were likely driven by the input of
excess nutrients.”
The Chicago Ship and Sanitary Canal, built in 1900, directed
untreated sewage into the Illinois River and, in the early
1900s, toxic river conditions killed a significant number of
fish and mussels, eliminating most aquatic life from the upper
Illinois River. At a downriver site near Havana, IL, 45 mussel
species were reported historically, but only 18 species remained
by the 1960s.
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Water quality improved in the 1970s and 1980s with the introduction
of more effective sewage treatment, a reduced amount of water
diverted from Chicago, and legislation protecting the river and its
inhabitants. However, the growth rate and size of mussels found in
2013 are still elevated compared with prehistoric times.
“My co-authors and I want to caution those who think that increased
growth in mussels is a good thing,” Fritts said. “Mussels that grow
faster often have a shorter life span.”
Larger sizes have also coincided with reduced numbers and species of
mussels in the Illinois River. That mussel size remains larger today
may indicate that the Illinois River is still impaired, Fritts said.
This PRI-funded study was recently published in the Journal of
Science of the Total Environment.
[Lisa A. Sheppard] About
the Prairie Research Institute: The Prairie Research Institute (PRI)
at the University of Illinois at Urbana-Champaign comprises the
Illinois Natural History Survey, Illinois State Archaeological
Survey, Illinois State Geological Survey, Illinois State Water
Survey, and Illinois Sustainable Technology Center. PRI provides
objective natural and cultural resource expertise, data, research,
service, and solutions for decision making, the stewardship of
Illinois’ resources, and the public good.
www.prairie.illinois.edu
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