traffic may have been responsible for the higher
mids. Omnivores were the most common func-
turbidity in February, but only one site was moni-
tional group at offshore sites. Within the naviga-
tored during February, and all March and April
tion channel, only 37 taxa were collected. None
measurements (except one) were collected at three
were unique or dominant. It was stated that ex-
other sites. It is questionable whether a compari-
tended season navigation could impact benthic
son of samples collected at different sites on dif-
invertebrates through loss of food and cover if
ferent dates can be used to infer navigation-in-
aquatic macrophytes are damaged, through in-
duced turbidity. Further, the single March "con-
creased turbidity or sedimentation, or by caus-
trol" measurement taken at the same site as the
ing direct contact with ice. This in turn would
February measurements was less than the maxi-
affect the abundance of food available to fish.
mum light penetration recorded during February.
However, no evidence of damage to aquatic mac-
They also suggested that the greater light pen-
rophytes was cited, and no basis for hypothesis
etration at their low-impact sites supported the
was provided.
claim of ship-induced turbidity. However, their
During the winter of 1974-75, the Great Lakes
data from their March control period show this
Fisheries Laboratory conducted a field study of
same relation between sites, suggesting natural
the macrobenthos in the Lake Nicolet portion of
variations may have contributed. Further, since
the St. Marys River to assess what, if any, effects
Hodek et al. (1986) found turbidity levels to vary
could be attributed to navigation in ice (Hiltunen
significantly with location (even for essentially
1979). Results from the sampling sites were also
simultaneous samples at a single site), drawing
compared against control sites in the West Neebish
conclusions on ship effects by direct comparison
channel, which was closed to winter navigation.
of turbidity levels at sites more than 3000 ft apart
Although the winter of 1974-75 had the largest
is tenuous. Interestingly penetration was greater
number of vessel transits and greatest tonnage
in April than in March despite heavier vessel traffic.
shipped beyond the traditional navigation sea-
They felt that this may be due to the fragmenta-
son, Hiltunen found no significant decline in the
tion of the solid ice cover in April.
population densities of any macrozoobenthos or
macrophytes in either test or control site areas.
Poe et al. (1980) sampled fish, benthic macro-
Benthic macroinvertebrates
Liston and McNabb (1986) sampled benthic
invertebrate populations and drift at four sites
macroinvertebrates on the St. Marys River dur-
on the St. Marys River as part of the 1978-79 En-
ing two years without winter navigation (1982
vironmental Evaluation Work Group effort. Too
and 1983). A total of 670 samples were analyzed
few fish and eggs were sampled to examine the
to estimate the populations occurring at selected
effects of vessel passage on fish distribution or
sites along the river. Total benthic invertebrate
abundance. Their results indicated no decrease
abundance was generally high throughout the
in the density of benthic macroinvertebrates due
river, ranging from 21 to 64,278 organisms per
to vessel-related disturbances, but they mentioned
square meter, with a median between 7,000 and
that their data were subject to some statistical un-
8,000. Abundance was almost always less in the
certainty. Examination of their drift net records
navigation channel (the median was less than 1,000
led them to conclude that only in February could
per square meter) than at other locations. Fur-
an "unequivocal demonstration" be made of the
ther, abundance was about three times greater on
effects of vessel passage. At other times vessel
the western shore than on the eastern shore. This
passage was too frequent to gather background-
was considered to be a reflection of a lower en-
level data for comparison. Although they noted
ergy environment there due to prevailing winds
that the available data were not sufficient to as-
from the west.
sess the significance of vessel-induced drift, they
A total of 162 taxa were identified, with 118
nonetheless postulated that if an increase in trans-
taxa within the vegetated littoral zone and 41 taxa
port were to occur it could indicate a net loss of
unique to that zone. Organisms characterizing the
energy (biomass) in the system.
littoral zone include odonates, lepidopterans, co-
Poe and Edsall (1982) conducted a follow-up
leopterans and nonchironomid dipterans. Both
study of vessel-induced drift on the St. Marys River
herbivores and predators were well represented.
during the period from January through May 1980.
At offshore sites, 120 taxa were identified, of which
Their objective was to determine how the com-
42 were unique. Organisms characterizing that
position and amount of drift varied between the
zone include mollusks, trichopterans and chirono-
following conditions: ice covered with vessel traf-
27
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