The susceptibility of cladocerans in North Andean Patagonian lakes to volcanic ashes

1. 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 degrees 35S, 72 degrees 06W, 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. 2. We combined field data and laboratory experiment data to determine the effect of the volcanic ashes suspended in the water column on cladoceran populations. 3. During the summer of 2011-2012 (6months 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), 18months after the eruption, a recovery of cladoceran populations was observed, with a concomitant decrease in total suspended solids. 4. 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 8mgL(-1) negatively affected survival and fecundity. Populations exposed to 8mgL(-1) of ash cannot persist; these organisms fail to produce offspring because they die before reaching reproductive age. 5. 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. 6. 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.