Summary Explosive volcanic eruptions are one of the few natural phenomena that can produce global catastrophic effects. On 4 June 2011, an eruption in the Puyehue volcanic complex (40°35′S, 72°06′W, Chile) discharged massive amounts of ash and pumice into the surrounding landscape in Argentina (North Andean Patagonia). The ejected material affected nearby aquatic environments, producing an increase in total suspended solids. We combined field data and laboratory experiment data to determine the effect of the volcanic ashes suspended in the water column on cladoceran populations. During the summer of 2011–2012 (6 months after the eruption), zooplankton populations in these lakes decreased, mainly due to the disappearance of cladocerans. This decrease in zooplankton was not caused by food shortage; chlorophyll a concentrations in the lakes were higher than in previous years or in the following season, and no change in phytoplankton size was observed. By the following summer (2013), 18 months after the eruption, a recovery of cladoceran populations was observed, with a concomitant decrease in total suspended solids. We performed a life‐table experiment, examining Daphnia commutata survival and fecundity at a series of ash concentrations. Ash concentrations of 2, 3, 5 and 8 mg L−1 negatively affected survival and fecundity. Populations exposed to 8 mg L−1 of ash cannot persist; these organisms fail to produce offspring because they die before reaching reproductive age. We conclude that the disappearance of cladocerans was due to the presence of ash. As the sedimentation process occurs, ash concentrations decrease, favouring population recovery, as observed the following summer. When the lakes recovered their original transparency with low total suspended solids values, the cladoceran populations also returned to their historical abundances. Previous data on other explosive volcanic eruptions and the present data indicate that immediately after ash fall, the zooplankton suffer reduced abundance, particularly of non‐selective filter feeders such as daphnids. However, recovery of zooplankton population growth can be expected within a few years.
Diel vertical migration (DVM) is a common behavior in zooplankton to avoid visual predation as well as potentially hazardous light wavelengths. In deep transparent lakes of Patagonia, the dark pigmented mixotrophic ciliate Stentor araucanus inhabits the upper layers of the epilimnion and is resistant to ultraviolet radiation (UVR). Here, we investigated if the ciliate pigment called stentorin increases oxidative stress in its predators. We studied the DVM behavior of Mesocyclops araucanus and the presence of stentorin in field‐collected copepods and evaluated in the laboratory the rate at which the copepod releases stentorin. S. araucanus has a C : P ratio ∼ 170 (atomic), which is one half of that of the bulk seston of the lake resulting in a very good food source in a system with very low food quality. Compared to an alternative prey without stentorin, when feeding on Stentor, the copepod suffered high oxidative stress (increased glutathione S‐transferase activity) and the reduced glutathion levels increased from dark to visible and ultraviolet radiation. However, we also determined that exposure to only visible light was sufficient to cause oxidative stress. In the field, we observed that M. araucanus displays a larger amplitude DVM than other crustaceans, while the ciliate Stentor remained in the upper levels of the epilimnion. The DVM protects the copepods from stentorin‐induced oxidative stress during daytime. Our findings are the first to show that a compound of a zooplankton prey item can influence the vertical behavior of predators in order to minimize the negative effect.
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