systems entered an acute period after the addition of Fomesafen. The abundance of
Lecane sp. decreased at addition concentrations of more than 20 mg/L of fomesafen,
the number of Aeolosoma hemprichi decreased significantly at concentrations of
more than 0.6 mg/L, Cyclidium glaucoma perished, and the population of Philodina
erythrophthalma increased and then decreased from herbicide addition in the subacute period. The abundance of Lecane sp. decreased at addition concentrations of
5 mg/L or more during the chronic period, Aeolosoma hemprichi significantly
decreased at concentrations of more than 0.6 mg/L, Cyclidium glaucoma perished
at concentrations of more than 2 mg/L, and Philodina erythrophthalma increased
and decreased in abundance through the addition of fomesafen. As for the phytoplankton, Chlorella sp. decreased to about half of its original population size in all
microcosms, and there were few changes. The population of Scenedesmus
quadricauda tended to increase incrementally, but it exhibited chronicity even
when there was a tendency toward increasing and then decreasing at concentrations
of more than 30 mg/L. Tolypothrix sp. increased in abundance but decreased with the
addition of more than 20 mg/L of Fomesafen.
From the viewpoint of B 16–30 (days 16–30 of the experiment), there was no
significant decrease in the abundance of any organisms at an addition concentration
of 0.5 mg/L, but the population of zooplankton decreased by more than half with
concentrations of greater than 0.6 mg/L. From this, it was estimated that the
m-NOEC was 0.5 mg/L for the structural parameter. Additionally, a strong influence
on zooplankton was observed with the addition of Fomesafen, and a decrease in the
abundance of Chlorella sp. was observed, while an increase was recognized for the
populations of both Scenedesmus quadricauda and Tolypothrix sp. This depended
on the difference in the resistance of the microbial species in the microcosm, and it is
thought that the presence or absence of the influence differed and might be produced
by different choices of herbicide, resulting in different effects on a given plant.
There was no observed difference in the functional parameter (DO concentration)
at addition concentrations of 0.6 mg/L or 10 mg/L, but activity decreased at 30 mg/L,
and the DO concentration decreased with the addition of 50 mg/L of fomesafen (Fig.
7.9). The P/R ratio was stable at ~1 for addition concentrations of 0.6 mg/L and
10 mg/L, but some variation was observed at a concentration of 30 mg/L, and the
value greatly declined with the addition of 50 mg/L of fomesafen (Fig. 7.9). From
these results, it can be estimated that the m-NOEC was approximately 30 mg/L for
the functional parameter.
7.3.3 Simazine
Simazine only affected the growth of Tolypothrix sp., which was measured for
15 days after the addition of the herbicide, and it decreased to 1/40th of its original
value with the addition of 0.08 mg/L or more of Simazine. The influence of this
chemical was mitigated by the diversity of the constituent species in the microcosm.
Simazine remained very stable in the microcosm. It was found that the toxicity—the
7 Example Assessments of the Microcosm N-System
85
Lecane sp. decreased at addition concentrations of more than 20 mg/L of fomesafen,
the number of Aeolosoma hemprichi decreased significantly at concentrations of
more than 0.6 mg/L, Cyclidium glaucoma perished, and the population of Philodina
erythrophthalma increased and then decreased from herbicide addition in the subacute period. The abundance of Lecane sp. decreased at addition concentrations of
5 mg/L or more during the chronic period, Aeolosoma hemprichi significantly
decreased at concentrations of more than 0.6 mg/L, Cyclidium glaucoma perished
at concentrations of more than 2 mg/L, and Philodina erythrophthalma increased
and decreased in abundance through the addition of fomesafen. As for the phytoplankton, Chlorella sp. decreased to about half of its original population size in all
microcosms, and there were few changes. The population of Scenedesmus
quadricauda tended to increase incrementally, but it exhibited chronicity even
when there was a tendency toward increasing and then decreasing at concentrations
of more than 30 mg/L. Tolypothrix sp. increased in abundance but decreased with the
addition of more than 20 mg/L of Fomesafen.
From the viewpoint of B 16–30 (days 16–30 of the experiment), there was no
significant decrease in the abundance of any organisms at an addition concentration
of 0.5 mg/L, but the population of zooplankton decreased by more than half with
concentrations of greater than 0.6 mg/L. From this, it was estimated that the
m-NOEC was 0.5 mg/L for the structural parameter. Additionally, a strong influence
on zooplankton was observed with the addition of Fomesafen, and a decrease in the
abundance of Chlorella sp. was observed, while an increase was recognized for the
populations of both Scenedesmus quadricauda and Tolypothrix sp. This depended
on the difference in the resistance of the microbial species in the microcosm, and it is
thought that the presence or absence of the influence differed and might be produced
by different choices of herbicide, resulting in different effects on a given plant.
There was no observed difference in the functional parameter (DO concentration)
at addition concentrations of 0.6 mg/L or 10 mg/L, but activity decreased at 30 mg/L,
and the DO concentration decreased with the addition of 50 mg/L of fomesafen (Fig.
7.9). The P/R ratio was stable at ~1 for addition concentrations of 0.6 mg/L and
10 mg/L, but some variation was observed at a concentration of 30 mg/L, and the
value greatly declined with the addition of 50 mg/L of fomesafen (Fig. 7.9). From
these results, it can be estimated that the m-NOEC was approximately 30 mg/L for
the functional parameter.
7.3.3 Simazine
Simazine only affected the growth of Tolypothrix sp., which was measured for
15 days after the addition of the herbicide, and it decreased to 1/40th of its original
value with the addition of 0.08 mg/L or more of Simazine. The influence of this
chemical was mitigated by the diversity of the constituent species in the microcosm.
Simazine remained very stable in the microcosm. It was found that the toxicity—the
7 Example Assessments of the Microcosm N-System
85