Freshwater mussels in an impacted watershed: Influences of pollution from point and non-point sources

The Grand River watershed in a densely populated region of Ontario supports one of the richest assemblages of freshwater mussels in Canada. However, water quality in this watershed is influenced by urban development, agriculture, and industry. Mussel populations and water chemistry in the lower Grand River and the Boston Creek tributary were evaluated to determine whether point sources of pollution such as discharges of domestic wastewater and industrial effluent, and non-point sources of pollution are affecting mussel distribution and population structure. Semi-quantitative population surveys conducted at 9 study sites identified 20 mussel species, including 3 Species at Risk. Mussel abundance (34-160 mussels/search hour) and species richness indicated that mussel populations in the lower Grand River watershed are continuing to recover from historical lows reported in the 1970s. However, changes in populations at some sites were consistent with altered water chemistry. Most notable was that the three most abundant mussel species in the Boston Creek tributary down-stream of a gypsum plant discharge were significantly smaller in length than those upstream of this site. The water chemistry in this habitat was characterized by elevated conductivity (similar to 2000 mu S/cm) and calcium (similar to 500 mg/L), as well as concentrations of sulfate (similar to 1000 mg/L) that can be toxic to freshwater mussels. In the Grand River downstream of the confluence with Boston Creek, there tended to be (p > 0.05) fewer mussels (mean 34 +/- 20/search h) compared to upstream (mean 67 +/- 15/search h) and this corresponded to altered water chemistry, including elevated sulfate (239 mg/L) downstream of the confluence relative to upstream (58 mg/L). These data indicate that chronic exposures to high levels of major ions is likely driving changes to mussel population structure. In addition, the discharges of wash water from a gypsum plant may be impacting sensitive biota in the main stem Grand River well beyond the immediate tributary receiving environment.

Automated summary

The Grand River watershed in Ontario supports a diverse population of freshwater mussels, with ongoing recovery from historical lows reported in the 1970s. However, changes in mussel populations at specific sites appear to be linked to altered water chemistry, particularly in areas with point source pollution. Chronic exposure to high levels of major ions and discharges from industrial plants may be driving these changes and impacting sensitive biota beyond immediate receiving environments.