CHAMPAIGN, Ill. – Man-made levees and water pollution have made an impact on the fish and other fauna of the Illinois River throughout the 20th century, but researchers at the Prairie Research Institute (PRI), University of Illinois, have taken an even longer view of human-induced changes in freshwater mussels, dating back to pre-Columbian times.
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.
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.
Media contact: Andrea Fritts, firstname.lastname@example.org; 608-781-6284.
Photo: Andrea Fritts collecting and sorting mussels from the Illinois River.