surface but that it was not a continuous relation.
systems of the upper Great Lakes. The major spe-
Rather, large events caused dislocation onto the
cies of phytoplankton were diatoms, but their bio-
mass was low (on the order of 1 mg/m3 based on
ice, while events below some threshold did not.
They felt that vessels transiting within existing
chlorophyll a). Submersed macrophyte commu-
speed limits should cause little damage to benthic
nities were simple in species composition, with
populations. Loss of benthos to the ice surface
three species dominating: Chara globularis (charo-
was considered insignificant in comparison with
phyte), Isoetes riparia (quillwort) and Nitella flexilis
total annual mortality due to all causes.
(charophyte). The maximum annual biomass of
submersed stands ranged from 10 to 70 g/m2 (ash-
Hudson et al. (1986) examined the distribu-
tion and abundance of macrozoobenthos, aquatic
free dry weight). Extensive emergent wetlands
macrophytes and juvenile fishes during the 1983
were well developed in shore zones that were
and 1984 open-water seasons in the SCDRS. A
protected from waves, currents and shifting sand.
total of 756 benthic samples were collected along
Emergent wetlands were dominated by Scirpus
21 transects. The diversity of macrozoobenthos
acutus (hardstem bullrush) at 64% of the areas
was highest in the upper Detroit River, with 101
mapped, followed by secondary dominants
taxa, and lowest in Lake St. Clair, where 65 were
Sparganium eurycarpum (bur reed) at 16% and
recorded. Elsewhere, 98 taxa were identified in
Eleocharis smallii (spikerush) at 13%.
the upper St. Clair River, 95 in the lower St. Clair
Submersed and emergent plant communities
River, and 80 in the lower Detroit River.
varied little in species composition, location, size
Hudson et al. (1986) concluded that the benthic
and annual maximum biomass from year to year
communities observed in their study did not ex-
during the period of study. Submersed plant com-
hibit obvious ill effects from existing levels of win-
munities were 510 times more productive and
ter navigation. However, winter vessel traffic had
emergent wetlands 300 times more productive than
already occurred for many years, and there was
phytoplankton, indicating their importance in the
no truly unaffected baseline from which to judge
food chains of the St. Marys River.
prior effects. A major ice jam on the St. Clair River
Aerial photographs revealed evidence of emer-
in 1984 did, however, afford an opportunity to
gent wetland erosion at some locations in Lake
examine conditions representing perhaps a worst-
Nicolet, but its cause was not readily apparent.
case scenario of winter ecosystem disruption by
Apparently dead rootstocks of emergent plants
were evident in shallow water offshore of exist-
ice. While the St. Clair and Detroit Rivers normally
ing emergent stands. Ground surveys found these
remain nearly ice free, this jam persisted for three
to be relics of previous bulrush stands in some
weeks, from late April into early May, and included
locations. Liston and McNabb (1986) felt that
significant ship traffic and icebreaking operations.
patterns of vegetation and channels in the adja-
Macrozoobenthos populations appeared to be
cent emergent stands supported the conclusion
the most adversely affected of the three groups
that erosion was occurring. The rate of erosion
examined in their study. Densities of ten taxa and
was not determined, however, since it was too
total biomass were lower in 1984 than in 1983,
slow to be measured from the photographs. At
and most declines occurred in the lower St. Clair
other sites along the Lake Nicolet shoreline, the
River. It is not known whether these declines were
outer fringe of the wetland had apparently un-
due to ice scour, lower temperatures or some other
dergone little change for over 30 years. Further,
factor or combination of factors. If the low densi-
scuba observations and collections and Ponar
ties had not recovered by the fall of 1984, it might
dredge sampling have shown that the location
have seemed reasonable to postulate that the ice
and species of submerged macrophytes in the St.
jam had caused long-term damage. However, most
Marys River tended to be stable from year to year.
of the affected taxa had recovered by the fall of
Further studies of the wetlands by personnel
1984 to levels equaling or exceeding those in the
of the Detroit District, COE, using photointer-
fall of 1983, and the remaining taxa were within
pretation of historical aerial photographs has in-
30% of the fall 1983 values.
dicated a correlation between the outer bound-
ary of wetlands and water levels.* It is question-
Aquatic plants
Liston and McNabb (1986) examined aquatic
plants and primary productivity on the St. Marys
River. Composition, distribution, biomass and pro-
* Personal communication, Don Williams, Detroit Dis-
ductivity were found to be typical of oligotrophic
trict, COE.
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