hemprichi was strongly affected by thiurum. However, at the 1.0 mg/L addition
concentration, the population of Aeolosoma hemprichi decreased before then
increasing. The populations of Philodina erythrophthalma and Lecane
sp. increased 2 days after the addition of thiurum. At the thiurum addition concentrations of 4.0 mg/L and 8.0 md/L, the populations of Philodina erythrophthalma
and Lecane sp. first decreased and then increased remarkably. This was because of
the monopoly of Lecane sp. and Philodina erythrophthalma on the food source that
occurred due to the extinction of Cyclidium glaucoma and Aeolosoma hemprichi.
Additionally, the increase in the bacterial population after the addition of thiurum
was related to the decrease in predation pressure by microanimals, which decreased
due to the addition of thiurum. Furthermore, this was because the survival of
microanimals recovered in the thiurum-added microcosm. Photolysis and biological
degradation were both possible with the addition of thiurum, and it was therefore
assumed that microanimals were able to multiply remarkably again.
After analyzing the residual properties of thiurum through high-performance
liquid chromatography (HPLC), the thiurum concentration exhibited little change
in the control system where there were no microorganisms, and it was shown that
photolysis did not occur under the microcosm culture conditions. In the 1.0 mg/L
thiurum addition system, the thiurum concentration in supernatant was reduced to
half 1 day after addition and decreased to below the detection limit by 2 days later.
From this, it was thought that thiurum was biodegraded by the microorganisms in the
microcosm within a short period of time after its addition. After analyzing thiurum
concentrations in the microorganisms sampled by filtration, the characteristics of
thiurum accumulation were not accepted.
From the evaluation of thiurum toxicity in the single-species culture test, the EC 50
of Philodina erythrophthalma and Aeolosoma hemprichi were estimated as 6.0 mg/L
and 1.5 mg/L, respectively. However, the concentration at which an increased
influence was observed in comparison with the control system was determined as
the influence concentration in the microcosm test. The concentration at which the
influence of thiurum increased in the single-species culture test was also determined
to compare both test methods. Philodina erythrophthalma was affected at a concentration of 4.0 mg/L of added thiurum in the single-species culture test, the same as in
the microcosm test. Aeolosoma hemprichi was affected at a concentration of 1.0 mg/
L of added thiurum in the single-species culture test, which was also the same as in
the microcosm test. Generally, the influence of thiurum was more difficult to observe
in the single-species culture test, but it was readily observed in the microcosm test.
These findings are thought to have resulted from the differences in the basal media of
the microorganism populations between the two methods.
7.3 Herbicides
Selective herbicides control specific weed species while leaving the desired crop
relatively unharmed. Meanwhile, nonselective herbicides (sometimes called total
weed killers in commercial products) can be used to clear plants from the ground,
82
K. Murakami et al.
concentration, the population of Aeolosoma hemprichi decreased before then
increasing. The populations of Philodina erythrophthalma and Lecane
sp. increased 2 days after the addition of thiurum. At the thiurum addition concentrations of 4.0 mg/L and 8.0 md/L, the populations of Philodina erythrophthalma
and Lecane sp. first decreased and then increased remarkably. This was because of
the monopoly of Lecane sp. and Philodina erythrophthalma on the food source that
occurred due to the extinction of Cyclidium glaucoma and Aeolosoma hemprichi.
Additionally, the increase in the bacterial population after the addition of thiurum
was related to the decrease in predation pressure by microanimals, which decreased
due to the addition of thiurum. Furthermore, this was because the survival of
microanimals recovered in the thiurum-added microcosm. Photolysis and biological
degradation were both possible with the addition of thiurum, and it was therefore
assumed that microanimals were able to multiply remarkably again.
After analyzing the residual properties of thiurum through high-performance
liquid chromatography (HPLC), the thiurum concentration exhibited little change
in the control system where there were no microorganisms, and it was shown that
photolysis did not occur under the microcosm culture conditions. In the 1.0 mg/L
thiurum addition system, the thiurum concentration in supernatant was reduced to
half 1 day after addition and decreased to below the detection limit by 2 days later.
From this, it was thought that thiurum was biodegraded by the microorganisms in the
microcosm within a short period of time after its addition. After analyzing thiurum
concentrations in the microorganisms sampled by filtration, the characteristics of
thiurum accumulation were not accepted.
From the evaluation of thiurum toxicity in the single-species culture test, the EC 50
of Philodina erythrophthalma and Aeolosoma hemprichi were estimated as 6.0 mg/L
and 1.5 mg/L, respectively. However, the concentration at which an increased
influence was observed in comparison with the control system was determined as
the influence concentration in the microcosm test. The concentration at which the
influence of thiurum increased in the single-species culture test was also determined
to compare both test methods. Philodina erythrophthalma was affected at a concentration of 4.0 mg/L of added thiurum in the single-species culture test, the same as in
the microcosm test. Aeolosoma hemprichi was affected at a concentration of 1.0 mg/
L of added thiurum in the single-species culture test, which was also the same as in
the microcosm test. Generally, the influence of thiurum was more difficult to observe
in the single-species culture test, but it was readily observed in the microcosm test.
These findings are thought to have resulted from the differences in the basal media of
the microorganism populations between the two methods.
7.3 Herbicides
Selective herbicides control specific weed species while leaving the desired crop
relatively unharmed. Meanwhile, nonselective herbicides (sometimes called total
weed killers in commercial products) can be used to clear plants from the ground,
82
K. Murakami et al.