| 3.2
Results |
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The oxygen concentrations were measured
at "non-detectable" for the nitrogen incubations and 10%
air saturation (=16Torr partial pressure) for the "trimix".
The pH value of the water bubbled with trimix reached
5.5 after the initial 10 min of vigorous bubbling. The
aerobic and nitrogen bubbled seawater maintained their
pH at 8. The incubations showed clearly that "trimix"
kills organisms considerably faster than incubations in
pure nitrogen Table 1. The shrimp and crabs incubated
in "trimix" were dead after 15 min and 75 min, respectively.
Even a transfer into aerated water did not result in any
movement. The brittle stars incubated under nitrogen started
to move again after transferred into aerated water. All
the mussels incubated in nitrogen and "trimix" were open
after 95 min but only the ones in nitrogen still responded
to tactile stimuli by closing their shells. The barnacles
were judged dead after incubation in "trimix" when they
did not withdraw their feet when disturbed, the ones incubated
in nitrogen still behaved normally. The plankton sample
mainly contained copepods. They stopped moving after 15
min and could not be revived in nitrogen and "trimix"
incubations. The results are summarized in Table 1. |
| 3.3
Discussion |
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Low oxygen concentrations
in water are a common natural phenomenon and their effects
on live organisms have been widely discussed in the past.
Oxygen may not be available to an organism because no
water for respiratory purposes is present, e.g., during
low tide in the intertidal zone. Oxygen may also be removed
in stagnant waters due to bacterial or other "life based"
actions, e.g., in ocean basins, fjords, tide pools, or
in waters with high organic content and consequently high
bacterial counts, e.g., in sewage, mangrove swamps, paper
mill effluent. In addition, oxygen can also be removed
by chemical reactions, e.g., in hot springs, industrial
effluents. The manuscript by Tamburri et al. (2000) summarizes
survival of a variety of larvae and adults of organisms
including some which may be significant as "nuisance species"
under hypoxic conditions. The publication supports extensively
that most organisms only survive strongly hypoxic conditions
for a few hours and only a few adults for several days.
The authors suggest that 72 h of hypoxia will be sufficient
to kill most eucaryotic organisms, adults or larvae in
ballast water.
The effects of high CO2 on organisms
in natural waters have become a research focus because
of proposals to dispose atmospheric CO2 in the deep ocean
(Haugan 1997, Omori et al. 1998, Seibel and Walsh 2001).
Two effects have to be distinguished when looking at "trimix"
incubations in seawater: a) the lowering of the pH from
pH 8 to about 5.5 and b) the raised CO2 concentrations
in the water. While the pH change caused by the incubations
in "trimix" are in the range of published experiments,
the CO2 concentration in "trimix" (about 14%) is much
higher than those investigated in the published literature
(0.1% to 1%). Therefore, the effects of "trimix" incubations
should be much stronger than those published previously.
Several publications have shown the
detrimental effect of lower pH values and high CO2 levels
on aquatic life. In a recent publication, Yamada and Ikeda
(1999) tested ten oceanic zooplankton species for their
pH tolerance. They found that the LC50 (=pH causing 50%
mortality) after incubations of 96 hours was between pH
5.8 and 6.6 and after 48h it was between pH 5.0 and 6.4.
Therefore, the pH value caused by incubations with "trimix"
is well within the lethal range for this zooplankton.
Huesemann et al. (2002) demonstrate that marine nitrification
is completely inhibited at a pH of 6. Larger organisms
were also investigated, a drop in seawater pH by only
0.5 diminishes the effectiveness of oxygen uptake in the
midwater shrimp Gnathophausia ingens (Mickel and Childress
1978) and. Deep sea fish hemoglobin may even be more sensitive
to pH changes (Noble et al. 1986). It appears that a common
metabolic response to raised CO2 levels and concomitant
lowered pH is a metabolic suppression (Barnhart and McMahon
1988, Rees and Hand 1990). Most recently, first papers
were published investigating the effects of environmental
hypercapnia in detail (Poertner et al. 1998, Langenbuch
and Poertner 2002).
The trimix combines both of these effects
on organisms - hypoxia and hypercapnia. Preliminary results
demonstrate the effectiveness of this combination in quickly
killing a variety of sample organisms. Contrary to methods
using additions of biocides or any chemicals in general,
nothing is added to the ballast water and, therefore,
nothing will be released into the environment when it
is released again. Methods using radiation, heating, or
filtering ballast water before or during a ship's trip,
are much more expensive. The equipment needed to establish
a rapid gassing of ballast water is available off the
shelf and has been used in the marine environment. The
plumbing and gas release equipment has been optimized
and has been used in application such as aquaculture,
sewage treatment and industrial uses. Extensive supporting
literature and research about the design and optimization
of equipment for the aeration of water is available from
public resources. Inert gas generators are available for
fire prevention purposes on ships and other structures
and are already installed on many ships, mainly tankers.
They can use a variety of fuels including marine diesel
to generate the inert gas.
Several topics have to be further investigated
before a conclusive recommendation about the treatment
of ballast water with "inert gas" can be made: a) how
are larvae, eggs, and plankton effected and b) what is
the affect of trimix type inert gas in fresh water? If
ballast water is taken up through a screen, larger animals
will not be included. The initial tests were made with
adults because of easy access to them. However, if adults
of a species are effected by "inert gas" it is most likely
that their larvae will also be effected probably even
more so.
Future tests will be conducted with
specimens from plankton and larval cultures and with incubations
of mixed plankton collected from the ocean. Determinations
of viability will be made by microscopic observations
(e.g. movement of mouthparts, swimming behavior), ATP
measurements (the ATP levels rapidly decreases after death
of an organism), and the ability to bioluminescence (many
planktonic organisms emit light, an ability which ceases
after death). Fresh water organisms will be of interest
because the pH change is not as much as in seawater. Freshwater
in its natural environment can have pH values around 5.5.
It has to be proven that raised CO2 concentrations in
combination with hypoxia will also affect these species.
Only then can the method be used for both, fresh and salt
water ballast. |
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