Abstract
Sumithion, an organophosphate,is widely used as insecticide for crop safety and eradication
of tiger bug from larval rearing aquaculture pond. The aim of the present study was to
assess the effects of sumithion on certain structural (phytoplankton, zooplankton, macroinvertebrates and periphyton) and functional (organic matter decomposition) endpoints of
freshwater ecosystems, and early development of zebrafish. In addition, the effects of high
temperature on acute toxicity of sumithion in adult zebrafish were evaluated. In the first
attempt, sumithion 50 EC was applied in each of the 12 microcosms (PVC tanks having 400
L of dechlorinated tap water) at nominal concentrations of 0, 25, 50 and 100 µgL-1 at 4-day
interval over a period of 4 weeks. Hence, zooplankton, macro-invertebrates and some water
quality parameters were affected by the exposure of sumithion in microcosm levels. Second
attempt was to evaluate the effect of sumithion on water, sediment quality, plankton and
benthic invertebrates in aquaculture ponds for 30 weeks. Three treatments were tried in
duplicate: no sumithion (control), weekly application of 0.5 mgL-1 sumithion and 1.0 mgL-1
sumithion. The results showed that water quality parameters, such as temperature,
transparency, pH, dissolved O2, total alkalinity, NO3-N and PO4-P remained similar among
the treatments throughout the study period. The pH, organic matter, available P and total N
contents of bottom-sediment also did not vary significantly among the treatments. No
distinct changes were observed in phytoplankton population densities. On the other hand,
the zooplankton population densities were significantly decreased with toxicity of
sumithion after 30 days to end of experimental period in both doses compared to control.
Seven genera of benthic invertebrates belonging to Chironomidae, Oligochaeta and
Mullusca were identified and their abundances were significantly decreased in both doses of
sumithion, compared to control. In the third attempts, 100 each of fertilized eggs and larvaе
of zebrafish were exposed to six concentrations of sumithion (0, 0.1, 0.2, 0.4, 0.8 and 1.6 mgL
1) in three replicates. Sumithion exerted developmental toxicity to embryos and larvae of
zebrafish. Finally, a two day renewal bioassay was used to determine the 96h LC50 value of
sumithion at three temperature regimes, such as 25, 30 and 35°C. Sumithion significantly
reduced dissolved O2 but increased free CO2 though pH and alkalinity did not change at the
highest temperature. The 96h LC50 values were estimated to be 4.59, 3.58 and 3.10 mgL-1 at
25, 30 and 35°C, respectively. Sumithion at the highest temperature showed cytotoxic
damage in the zebrafish. Increased water temperature enhanced the toxicity of sumithion.
Blood glucose level, frequencies of micronucleus, erythrocytic nuclear and cellular
abnormalities were increased with the increase of temperature regime. The overall findingssuggest that sumithion had adverse effect on abundance of plankton, benthic invertebrates,
embryos, larvae and adult fish that might have negative impact on culture animals in
aquaculture pond.