Aquatic organisms may have high mercury concentrations even in low source areas of the US far from cities because conditions are right for efficiently methylating the mercury raining down from the atmosphere. Most studies of mercury bioaccumulation have been done in lakes, reservoirs, and wetlands, whereas much less is known about mercury bioaccumulation in rivers and streams.
Benthic invertebrates, such as caddisflies and mayflies, are important food items for fish and so may transfer mercury to fish and higher organisms.
Together with concurrent sampling of other media:
- In 2003, sampling was done at three stream sites in the WMIC representing one urban and two rural/non-cultivated watersheds (low and high wetland percentages). Sites were sampled twice per year (June and August) for benthic invertebrates.
We collected three composite samples each of two species of invertebrates that were thought to be important prey items for the forage fish collected (or the predator fish collected), including at least one species of algae-eating invertebrate to compare with mercury concentrations in benthic algae. Samples were analyzed for total mercury and methyl mercury. Stable isotopes (carbon and nitrogen) also were analyzed in invertebrates to provide information on food web relations. Invertebrates were collected by hand picking with gloved hands and plastic forceps.
Detailed Sampling Sites - 2003
Oak Creek in South Milwaukee, WI
Evergreen River near Langlade, WI
Pike River near Amberg, WI
Data for additional chemical concentrations in other media are still being compiled and reviewed. Results are being included in reports scheduled for completion in 2006-7.