Potential causes of enhanced transfer of mercury to St. Lawrence River Biota: implications for sediment management strategies at Cornwall, Ontario, Canada

Abstract

We examined factors and pathways involved in the transfer of mercury (Hg) to the food web in St. Lawrence River embayments near Cornwall, Ontario, where natural remediation of contaminated sediments (eventual burial by settling of cleaner sediments) has been adopted as a management strategy. Yellow perch (Perca flavescens) from one of the study zones (Zone 1) along the river by Cornwall contained significantly higher total mercury (THg) concentrations than perch from other equally contaminated zones. While THg concentrations in benthic invertebrates did not vary among contaminated zones, THg concentrations in yellow perch and invertebrate prey recovered from the perch stomachs were 1.5-2.5 times higher in Zone 1 than those from other zones, suggesting that prey selection affects THg accumulation more than habitat location. No significant differences were found in THg concentrations among different prey species within Zone 1, although there were significant differences in THg concentrations in the same prey species within Zone 1. In contrast, THg concentrations among different prey species increased significantly with trophic level in other contaminated and reference zones. The lack of correspondence between trophic position and THg accumulation in Zone 1 suggests two possibilities: (1) yellow perch in Zone 1 are highly mobile and are assimilating THg from a wide range of prey across Zone 1 with variable THg concentrations and (2) there may be an important non-dietary source of THg to the Zone 1 food web. Potential waterborne Hg sources to Zone 1 were investigated. Whereas THg and MeHg values in discharges from a disused canal were similar to Zone 1 surface water values (0.97 and 0.04 ng l -1 , respectively), concentrations in storm sewer and combined sewer overflows discharging in the vicinity of Zone 1 were 19-45-fold (THg) and 2-4-fold (MeHg) higher than upstream river water. Contributions of Hg to the water column from sediment-water diffusion, estimated using a simple, well-mixed reactor model, ranged 0.05-0.1% of the surface water THg concentration and 1-2% of the MeHg concentration measured in summer months in Zone 1. Although not investigated in the other zones, a strong correlation (r 2 = 0.82) was found between Guest editors: M. Power, J.

Bioconcentration of methylmercury in microzooplankton in a temperate river

“…For example, zooplankton MeHg ranged from 7.4 to 11 ng/g in the St. Lawrence River, from 6 to 161 ng/g in northern Wisconsin lakes, and from 4 to 240 ng/g in Canadian lakes [12,36,37]. In the present study, the mean concentration of MeHg in microzooplankton was approximately four times the mean amount of MeHg in microseston, which is similar to the literature values (4.6 times [12]).…”

Influence of a chlor‐alkali superfund site on mercury bioaccumulation in periphyton and low‐trophic level fauna

“…Ridal et al [64] attributed significant differences in THg concentrations observed for perch but not for benthic invertebrates within contaminated zones of the Cornwall Area of Concern to prey selection, zone-specific Hg sources, and/or heterogeneity of habitat influencing Hg dynamics. For most taxa, differences between sites in biota MeHg concentrations were non-significant, with the exception of shiners (p<0.001) and juvenile smallmouth bass (p=0.0193) (both highest at CrossPower and Gorham on an among-site basis (Figure 4, and Table S4)).…”

“…Additionally, the presence of dams within this system likely impose restrictions on transport of Hg to downstream reaches, affecting bioaccumulation, and the implications of dam and/or sediment removal on transport of legacy pollutants within river systems (e.g. [54, 64]) is an important avenue of continuing research.…”

“…Although the mayflies appear more closely linked to sediment and water column Hg concentrations (particularly with regard to THg), this relationship is not transferred consistently through trophic levels, partially because of feeding preferences (omnivores such as crayfish that feed at different trophic levels often do not exhibit clear patterns of bioaccumulation) and other confounding factors discussed. Ridal et al [64] attributed significant differences in THg concentrations observed for perch but not for benthic invertebrates within contaminated zones of the Cornwall Area of Concern to prey selection, zone-specific Hg sources, and/or heterogeneity of habitat influencing Hg dynamics. For most taxa, differences between sites in biota MeHg concentrations were not significant, with the exception of shiners (p < 0.001) and juvenile smallmouth bass (p ¼ 0.0193; both highest at Cross Power and Gorham on an among-site basis [ Figure 4, and Supplemental Data, Table S4]).…”

Simplified sample preparation procedure for measuring isotope‐enriched methylmercury by gas chromatography and inductively coupled plasma mass spectrometry

“…The sediment samples (Zone 1, St. Lawrence River, Cornwall, ON, Canada) used in the present study were highly heterogeneous, being a composite of brown-black clayed mud with wood fibers and oil patches resulting from the historical industrial contamination of the site [23][24][25] and bacterial decay. The sediment heterogeneity therefore influenced the high variability of ambient mercury concentrations (MeHg: 2.6 AE 0.53 ng/g, n ¼ 24; total Hg: 1,212 AE 489.4 ng/g, n ¼ 35) in the samples.…”