able whether the sediments were actually erod-
disruption of the community by ice. The distri-
ing, and the outer boundaries of the wetlands were
bution and occurrence of aquatic macrophyte taxa
probably responding to increases in water levels.
changed little over this period. Plant bed devel-
Both submersed plant communities and emer-
opment was delayed in the St. Clair River and at
gent wetlands occurred on clay sediments, lead-
Belle Isle in 1984, but by September the beds were
ing Liston and McNabb to suggest that their de-
little different than in 1983. There were signifi-
struction could lead to increased turbidity in the
cant differences in biomass between the two years,
river. It might also be argued that the clays are
but there were no consistent differences between
more stable than sands, allowing colonization by
locations or months. No impacts on submersed
plants. Periods of winter ice were identified as a
macrophytes could be attributed to the jam, ex-
time of potential damage to the rootstocks of
cept perhaps for delayed development due to
emergent vegetation. Possible mechanisms include
lower water temperatures.
vertical movement of an ice cover frozen to the
bed during water level fluctuations and scour-
Fish
ing by moving ice floes during ice cover breakup.
Gleason et al. (n.d.) documented conditions
Jude et al. (1986) found no strong seasonal dif-
at potential coregonine (lake whitefish and lake
ferences in the occurrence of macrophytes in their
herring) spawning grounds as part of the Envi-
St. Marys River drift samples (48% of all samples
ronmental Evaluation Working Group studies
in winter, 57% in summer). However, only six mac-
during the winter of 1978-79. There was concern
rophytic taxa were collected in winter compared
that winter navigation could adversely affect in-
with 10 in the summer. At one site, Frechette Point,
cubating eggs due to excessive sedimentation,
macrophytes were both more frequent and more
localized current alterations and dislocation. Their
diverse in summer (89%, nine taxa) than in win-
objectives were to identify spawning areas, de-
ter (77%, five taxa). In addition, the dominant
termine species composition by sampling eggs,
plants were found to change seasonally at Fre-
quantify the rate of sedimentation, and determine
chette Point and Point Aux Frenes.
the composition of sediments in those areas. Their
Hudson et al. (1986) sampled aquatic macro-
literature review generally indicated that sedi-
phytes in the Detroit and St. Clair Rivers during
mentation can adversely affect spawning grounds,
the open-water seasons of 1983 and 1984. Using
but opinions in the literature often conflicted,
a Ponar grab sampler they collected a total of 18
perhaps due to a lack of quantification of mortal-
taxa of submersed macrophytes on the St. Clair
ity vs. sedimentation rates. It was also unclear
River and 19 on the Detroit River. The average
whether coregonines have a "home" spawning
depths at which submersed plants were retrieved
ground, which could make individual sites im-
varied only between 6 and 8 ft for all sites, de-
portant.
spite wide ranges in light transmission and wa-
Nine potential spawning areas were identified,
ter velocity. Mean light transmission varied from
but attempts to sample eggs were unsuccessful.
a low of 2% measured on the Detroit River to 86%
Sampling included visual searches by divers and
at a site on the St. Clair River. Near-bottom water
dredging of areas thought to have a high poten-
velocities ranged from essentially still water to
tial as spawning beds. A single egg was recov-
2.5 ft/s. Light transmission was typically two to
ered at one site, and "several" at a second site.
three times greater and water velocities two times
None of the areas were conclusively shown to be
greater in the St. Clair River than in the Detroit
spawning areas. However, the authors felt that
River.
the lack of success may have been due to low ini-
Hudson et al. (1986) concluded that the aquatic
tial egg populations, high predation rates, dislo-
plant communities observed in their study did
cation of eggs by water currents, or inability to
not exhibit obvious ill effects from existing lev-
locate discrete spawning areas. It was recom-
els of winter navigation. However, winter vessel
mended that future attempts take place during
traffic had occurred for many years, and there
or immediately following spawning, along with
was no truly unaffected baseline from which to
sampling of fish at the sites.
judge prior effects.
Liston and McNabb (1986) sampled fish popu-
As discussed in the section on benthos, how-
lations in the St. Marys River during years with-
ever, a major ice jam on the St. Clair River in 1984
out winter navigation past December (1982-83)
afforded an opportunity to examine conditions
with the objective of providing baseline informa-
representing perhaps a worst-case scenario from
tion for analyzing the effects of winter naviga-
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