et al. (1980) for ship passage in ice, Jude et al.
presence of reasonable benthic densities and pro-
(1986) disagreed with the conclusion that there
duction rates, Jude et al. concluded that the ef-
would be a considerable increase in drift density
fects of winter shipping have been minimal. How-
for ship passages in ice. While they agreed that
ever, they felt that the available ship passage data
the data collected during February 1979 for peri-
and the observation that previous shipping did
ods with and without vessel passages showed
not appear to detrimentally affect the benthos is
significant ship-induced drift, they noted that it
not a sufficient basis on which to forecast the fu-
is the only documented occurrence known to exist.
ture.
Due to increased levels of ship traffic during field
Gleason et al. (1979) studied the loss of benthos
periods subsequent to the one in January, no data
through nearshore, shore-parallel cracks along the
could be collected to represent the without-ship-
St. Marys River. A system of one or more of these
ping case for those later data sets.
cracks can form due to water level fluctuations
In arriving at their conclusion that ship pas-
large enough to fail the ice cover in flexure. The
sage in ice would increase drift rates in general,
first crack generally forms at the offshore limit of
Poe et al. (1980) and Poe and Edson (1982) pointed
shore-fast grounded ice, and other offshore cracks
out that drift catch per unit time was roughly equal
have sometimes occurred at sharp changes in river
for field periods in March and April 1979, even
bathymetry. With frequent water level fluctua-
though there were only four vessel passages while
tions these cracks may not have time to refreeze,
drift nets were deployed in March and 22 while
leading to open or active cracks persisting through-
nets were deployed in April. Since water flow rates
out the winter. For large, fast-moving ships it has
and temperatures were comparable for the two
been observed that the associated drawdown and
periods and limnological conditions that affect
surge can be large enough to cause water, sedi-
catch were considered little changed, they con-
ment, vegetation and even small fish to be sprayed
cluded in the 1980 report that greater drift rates
through these nearshore cracks (Wuebben 1978).
occur with ship passage in solid ice than for ship
At three sites Gleason et al. (1979) monitored
passage under floe ice (broken ice) conditions. In
fluctuations in the elevation of the ice surface dur-
their 1982 report, Poe et al. expanded this con-
ing ship passage, sampled the quantity and com-
clusion to state that drift rates were considerably
position of materials washed through a crack, and
higher for vessel passages with an ice cover than
collected dredged samples of bed material to de-
for ice-covered conditions without navigation or
fine site characteristics. During the study period,
ice-free conditions with navigation.
24 recorded ship-induced ice level fluctuations
Jude et al. (1986), however, were not convinced
ranged from 2.8 to 72.5 cm. Twenty samples were
that the apparent ship-induced drift pulse con-
retrieved from events causing material to pass
tinue to be evident with additional ship passages.
through the active cracks. Of these, there were
They instead interpreted the data to indicate that
only five that contained benthic organisms, and
ship passage through ice may cause a pulsed in-
they were collected during the three vessel pas-
crease in drift density during some events (Janu-
sages causing the largest recorded vertical ice dis-
ary data), while at other times (March data) it may
placements.
cause little apparent effect. Further, they felt that
The most abundant organisms found in the bed
short, ship-induced pulses of benthic drift in win-
samples were snails at 45%, followed by dipter-
ter may not be subject to predation as severe as
ans at 17%, annelids at 17% and pelecypods at
at other times due to generally lower metabolic
12.4%. The most abundant in the crack samples
rates. However, they were concerned about any
were dipterans (75%), followed by annelids (15%)
additional loss of benthic populations at a time
and ostracods (10%). From the bottom sample data
when numbers and productivity are minimal, and
the average benthic density was 9593 organisms
about disruption or loss of preferred habitat. They
per square meter, while the 20 crack samples
postulated that ship-generated drawdown and
yielded a total of 21 organisms. This yield of about
surge during spring breakup may result in ice
one organism per vessel passage was extrapolated
floes disrupting the bed through direct contact.
to 10 organisms per meter of crack per vessel pas-
However, no documentation or observations were
sage, or about 0.1% of the existing benthos popu-
cited to support this proposed mechanism for
lation along the vessel track.
increased drift rates.
They concluded that there was a correlation
In view of several years of year-round navi-
between the magnitude of vessel-induced fluc-
gation under the demonstration program and the
tuation in ice elevation and benthic loss to the ice
29
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