a1 National Institute of Biology, Večna pot 111, 1000 Ljubljana, Slovenia
a2 Faculty of Environmental Sciences, University of Nova Gorica, Vipavska 13, 5000 Nova Gorica, Slovenia
a3 Anton Melik Geographical Institute, ZRC SAZU, Gosposka ulica 13, 1000 Ljubljana, Slovenia
a4 Department of Biology, State University of Ghent, K.L. Ledeganckstraat 35, 9000 Ghent, Belgium
Two consecutive earthquakes temporary changed a zooplankton community in a high-mountain Lake Krn (altitude 1383 m a.s.l.). It was dominated by the eurytherm copepod, Cyclops vicinus, until 1998, when the first earthquake hit the lake (EMS=5.6). After the earthquake, the population of C. vicinus collapsed and the thermophilic cladoceran, Ceriodaphnia quadrangula, took over. After the second earthquake in 2004 (EMS=4.0), C. vicinus became untraceable. In 2008, few copepods reappeared and by 2010 they became the sole dominant again. Only Secchi-disc depth showed a statistically significant increase over time, while Ntot, Ptot and temperature showed an increasing trend, yet the relationship was insignificant. To compare multi-parameter properties of the water column, the studied period was divided into Period 1 (before the first earthquake), Period 2 (between earthquakes) and Period 3 (after the second earthquake). A Hotteling T2 test confirmed a statistically significant difference between Periods 1 and 2 & 3 (P<0.01), but not between Periods 2 and 3 (P>0.1). During simple laboratory experiment, specimens of C. vicinus were covered with a thin layer of sediment, to mimic the earthquake's effect on their survival. A hypothesis was that the timing of both earthquakes had been crucial for decimation of C. vicinus population as they re-suspended sediment with hibernating copepodites. As these became subsequently buried they were deprived of a re-activation signal and exposed prolonged anoxic conditions there. C. quadrangula temporary filled the void left by the copepod, which needed 6 years to regain its dominance.
(Received May 10 2011)
(Accepted October 18 2011)
(Online publication March 20 2012)