Biology Reference
In-Depth Information
TABLE 4
. Methyl Mercury Biomagnification (ng/g dry weight)
Mukooda Lake,
Ryan Lake,
Almaden
MN
MN
Reservoir, CA
Mercury source
Atmospheric
Atmospheric
Mining
Water (wet weight)
0.000018
0.00019
0.00037
Seston (mostly algae)
3.5
8.3
4.1
Zooplankton
17
197
640
Young fish (age 1)
181
943
4,830
Northern Pike (55 cm)
1,130
9,725
Note:
Methyl mercury biomagnifies in aquatic food webs. In Voyageurs National Park, Minne-
sota, concentrations in young Yellow Perch were more than one million times greater than those
in water, and in adult predatory pike more than ten million times greater. In the South Bay's Al-
maden Reservoir, near a historic mercury mine, concentrations were higher in the water than in
the Minnesota lakes, which is reflected in the elevated transfer up to zooplankton.
Source:
MN from Wiener et al., University of Wisconsin-La Crosse; CA from Kuwabara et al., USGS.
fill only five ponds. Scientists call such holes in the bay floor “sediment
sinks.” The project design took his calculations into account, focusing tidal
restoration plans on higher elevation ponds.
The bay's sediment budget will also determine whether the new
marshes will survive or be obliterated in coming decades. Though sea level
has been rising slowly for decades, USGS experts project a much faster
rise of between four feet and five feet by 2100. Wetlands must rise too or
they will become inundated. Scientists estimate that to keep up with the
projected rise over the next century, bay marshes would need more than
half a billion cubic yards of material (sediments, decomposing plants, and
organic matter) but that only about 30 percent to 60 percent of this may be
available from natural erosion and runoff processes.
Though sediment is a key ingredient in bayland restoration, it may har-
bor another ingredient that isn't so welcome: mercury. Numerous restora-
tion projects include monitoring of how reintroduced wetland processes
are interacting with the region's mercury legacy, and the San Francisco
Estuary Institute has been trying to identify mercury hot spots in the bay.
The chemical interactions and bacterial and decomposition processes
occurring naturally at the mud and water interface of wetlands can con-
vert innocuous inorganic mercury into toxic methyl mercury. In general,
methyl mercury tends to be higher in sediments closer in to shore. At low
tide, damp mud baking in the sun fuels bacterial activity in the oozes that
may boost methylation. Unlike many other contaminants, methyl mer-
cury gets hundreds and thousands of times more concentrated each time
it moves up the food chain (see Table 4). Though mercury may not hurt a
small fish, it can harm the offspring of the larger animals that eat them—
hence the warning labels on cans of tuna.
