Viability of zooplankton resting eggs in rice field sediment after pesticide applications.

Abstract

Many herbicide products are commonly used in agricultural areas to prevent and eliminate weeds. Contamination from these toxicants in water might affect aquatic organisms not only in the active stage, but also in the diapause stage. To test the effect of herbicide on the resting eggs of zooplankton, we prepared two rice fields: one field without the application of pesticides (RF-NPA) and one with the application of pesticides (RF-PA) in a sampling year. We conducted a hatching experiment for 30 days. Twenty-four taxa of zooplankton were found. Sixteen species of these were rotifers, seven species were cladocerans and one taxon was an unidentified nauplius copepod. The species richness of zooplankton between RF-NPA (17 taxa) and RF-PA (16 taxa) was close, but species compositions between RF-NPA and RF-PA were different, indicated by the similarity index of 0.545. Lecanidae was the most diverse family of rotifers in both rice fields with nine species, while Chydoridae was the most diverse family of cladocerans (four species). The total abundance of zooplankton of RF-NPA was higher than RF-PA with 1,897 and 1,286 individuals, respectively. The Shannon-Wiener diversity index (H´) and Pielou's evenness (J) in RF-NPA were higher than in RF-PA. The high species richness of zooplankton in both rice fields occurred on days 18 to 30. On the other hand, the highest abundance was recorded on day 18 for RF-NPA and on day 24 for RF-PA. The non-metric multidimensional scaling (NMDS) demonstrated significant differences in zooplankton community composition between RF-NPA and RF-PA (p < 0.05; ANOSIM test). According to the diversity indices, the RF-NPA has more diversity than the RF-PA, which might be a result of herbicide application in the sampling year. This study suggests that the toxicity of glyphosate should be a concern in terms of the biodiversity of rice field ecosystems.