| 2.4
Theory |
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Except for ballast
water exchange, essentially all treatment concepts involve
the chemical change of the water to cause an environment
lethal for ANS. The chemical changes described in Tamburri
et al. (2002) and McMahon (1995) offer promising results,
i.e., reduce the dissolved O2 in the one case, and carbonate
and reduce the pH in the other case. In both cases the
process involves the exchange of gases, the extraction
of the dissolved O2 and the introduction of CO2. Surface
contact area and partial pressure differentials permit
the gas exchanges to occur. The deoxygenation of the ballast
water is based on Henry's Law of gas solubility: The relative
proportion of any dissolved gas including oxygen in the
ballast water is a function of the concentration, equivalent
to partial pressure of the gas (e.g. oxygen), within the
mixed gases over the ballast water. The depletion of oxygen
in the ballast water is primarily a function of the shared
surfaces and concentrations at the interfaces of the inert
gases and water.
The pH of the ballast water is lowered
by the chemical reaction: |
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| All
systems described thus far in the literature, including
ballast transfer, has left untreated the sediment buildup
in the bottom of the tanks. If the orifices in the lattice
work of piping pointed down, then the sediment could be
stirred up facilitating the kill of the embedded ANS.
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| 3.
LABORATORY TEST AT THE SCRIPPS INSTITUTION OF OCEANOGRAPHY |
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| 3.1
Methods |
The purpose of
the preliminary experiments described here was to obtain
initial data on the effects of "inert gas" on marine organisms.
"Inert gas", hereinafter called trimix, a commercially
available gas mixture of 2% oxygen, 12% CO2 and 84% nitrogen
resembles the gas generated by commercially used marine
"inert gas generators". Adult or young adult animals were
chosen for two reasons a) to make the size of specimens
amenable for the experimental setup and b) to raise the
significance of possible effects since adults of a species
are typically more tolerant of environmental changes than
juveniles or larvae. All animals were collected fresh
from the coastal waters off La Jolla, CA and used immediately.
The plankton sample was collected with a plankton net
from a small boat.
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The schematic of the experimental setup
is shown in Figure 1. Three parallel incubations were
done for each experiment. Several organisms were incubated
in 1.5L of seawater at 22°C in large Erlenmeyer flasks.
Each incubation was equilibrated with the respective gas
using aquarium stones before any organisms were introduced.
The aerobic control was bubbled from an aquarium pump
for approximately 15 min and left open to the atmosphere
after addition of specimens. An anaerobic incubation was
bubbled with 99.998% nitrogen for 15 min. After introduction
of the organisms, the bubbling was continued for another
10 min and then the container was closed with a rubber
stopper or the bubbling was continued. The incubation
in trimix was treated similarly except that the gas mix
was used instead of nitrogen. The oxygen concentrations
were measured after the initial bubbling period using
a Strathkelvin oxygen electrode with a Cameron instruments
OM-200 oxygen analyzer. Ph values were determined using
a combination electrode and a Radiometer pH meter.
Survival of the specimens was determined visually by checking
for motile responses to tactile stimulus (e.g. mussels
do not close their shells, barnacles to not withdraw their
feet, shrimp do not move their mouthparts, worms appear
limp and motionless). After each testing of the animals,
the incubation flasks were bubbled for 10 min to reestablish
original conditions. To verify survival of the specimens,
they were relocated to aerobic conditions and checked
again after 30 min. If they still did not respond, they
were considered dead.
This setup allowed us to compare responses to nitrogen
and "trimix" while making sure that test specimens were
not gravely affected by other experimental parameters.
Incubation in pure nitrogen allow for a comparison with
published results by others. |
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