Interactive effects of fish predation and sublethal insecticide concentrations on freshwater zooplankton communities.

Stresses imposed by insecticides and predators are possibly the most rigorous filters to which aquatic organisms are exposed in rivers and lakes associated with agricultural lands. However, their interactive effects on zooplankton communities are still unclear. This study elucidated the zooplankton community response to fish predation, the insecticide chlorpyrifos (CLP), and a combination of both factors, using a 30-day mesocosm experiment. The zooplankton assemblage was influenced by fish presence prior to CLP toxicity. Fish predation reduced microcrustacean density leading to a community dominated by microzooplankton (i.e.: rotifers and copepod nauplii). CLP decreased the species richness in treatments with and without fish, yielding an increase in the abundance of bdelloid rotifers, in the genera Lepadella and Trichocerca. The zooplankton:phytoplankton (<20 µm) ratio decreased substantially when the two stressors, fish predation and insecticide toxicity, were combined. Although CLP dissipated relatively rapidly in the aqueous phase and accumulated in sediment and fish tissue, zooplankton richness was unable to recover. A possible explanation for this could be the inhibitory effect of CLP on resting stage hatchings in the sediment. Therefore, the combined effects of fish predation and CLP might influence zooplankton richness, leading to an assemblage dominated by rotifers that appeared to be resistant to both factors, with a limited capability to control phytoplankton growth. Thus, the effects of natural and anthropogenic stressors should be considered together when assessing community dynamics in aquatic ecosystems.

Disruption of the hatching dynamics of zooplankton egg banks due to glyphosate application.

Hatching rhythms of eggs banks are important processes because they favor species co-existence and promote resilience of ecosystems after natural disturbances. Anthropogenic stressors can disrupt such natural hatching dynamics. This work examines the effects of concentrations ranging from 1 to 8 mg l-1 of a commercial glyphosate-based formulation (Sulfosato Touchdown®) on the hatching dynamics of zooplankton dormant stages, present in the sediment of a natural lake. Sediment samples were collected from the surface sediment (<10 cm deep) of an isolated shallow lake free from pesticide pollution. An ex situ emergence assessment method was carried out and four treatments plus one control (without pesticide) were performed with three replicate each. Zooplankton hatching from the resting stages was monitored during 30 days. In total, 30 zooplankton taxa were recognized. The species diversity decreased significantly at concentration above 2.7 mg l-1 glyphosate. The proportion of cladocerans within hatchling organisms decreased, while that of rotifers Bdelloidea increased in all treatments with glyphosate. Time of the first hatching (TFH), time of maximum hatching (TMH) and the frequency of hatchings (FH) of most zooplankton species were also altered. In conclusion, the application of a glyphosate-based pesticide selectively affected the hatching dynamic of zooplankton egg banks, which suggest that these resting structures are highly sensitive to the toxicity of the pesticide.