Is Bythotrephes not invasive at home due to prey adaption?

CONTENT OF RESEARCH PROJECT Bythotrephes longimanus is a large predatory water flea home to European clear water lakes. After invading North American lakes several decades ago, Bythotrephes caused substantial changes in food webs of the invaded lakes. In comparison to the intense research carried out in North America, little is known about the importance of Bythotrephes in its native area, which includes lakes in and around the Alps. Supposedly lower impacts of Bythotrephes on its prey in the native range than in the invaded regions has been credited to long time co-evolution, but this assumption has never been tested. Instead, we suggest that Bythotrephes may have a more significant role than previously considered also in the native lakes. We propose to investigate the performance of Bythotrephes and its impacts on zooplankton populations in Austrian montane lakes, with focus on mechanisms of prey defense adaptations in lakes where the predator is present. HYPOTHESES Does Bythotrephes alter the zooplankton prey community in Austrian montane lakes? Are there so many Bythotrephes in montane lakes that they can have strong impacts on its prey similar as what has been shown in North America? Does long time coexistence of Bythotrephes and zooplankton in native range lead to development of prey features to avoid or reduce predation? METHODS We will use a mix of several field and laboratory approaches, both standard and novel. Lake sampling will combine classical zooplankton net-sampling with new high frequency sonar/ADCP and IR-video profiles for overall abundances and vertical migration behavior. We will employ cutting-edge methods based on molecular gut analysis for prey preference and diet composition. The field studies will be accompanied with laboratory experiments on prey adaptations to Bythotrephes presence. WHAT IS NEW AND/OR SPECIAL ABOUT THE PROJECT We have to date a very limited knowledge about the impact of Bythotrephes on the pelagic food web in highly sensitive ecosystems of montane lakes of its native region. We argue that role of Bythotrephes might have been underestimated and that it might increase in time in connection with global climate change.

Background Bythotrephes longimanus is a large predatory water flea (body of up to ~ 3 mm + tail-spine of up to 6 mm length) occurring naturally in clear lakes around the Alps and in northern Europe. Several decades ago, this species invaded due to human activities numerous North American lakes, causing substantial and lasting changes in food webs of these lakes. However, knowledge of its impacts from the native region, which might be useful for both short and long term predictions, have turned out to be rather limited. In this project, we aimed to mitigate this gap and studied role of Bythotrephes in several Austrian montane lakes together with mechanisms of prey defense adaptations in lakes where the predator is present. Specifically, we asked: 1) whether and how Bythotrephes alters the zooplankton prey community in Austrian montane lakes?; 2) does long time coexistence of Bythotrephes and zooplankton in native range lead to development of prey features to avoid or reduce predation? Methods To tackle our questions, we used a combination of field, experimental and laboratory approaches. In the year 2018, we conducted an intense field sampling, utilizing both standard and novel methods, in two pairs of lakes. In each pair of otherwise similar lakes, one did host Bythotrephes, the other one not, so that we could follow and compare development of planktonic communities. The field studies were accompanied with laboratory experiments on prey adaptations to Bythotrephes presence. Additionally, we employed molecular gut analysis for prey preference and diet composition of Bythotrephes. Main results First, we concluded that, due to suboptimal sampling techniques used earlier, densities and therefore impacts of Bythotrephes on the plankton food webs were so far in Austrian lakes underestimated, up to 3-4 times. Second, Bythotrephes seem to be able to significantly affect zooplankton in the studied lakes. This has been observed as both: 1) direct lethal effects - changes in abundances and population size structures of the prey, in comparison with zooplankton development in a similar lake lacking the predator, 2) indirect non-consumptive effects - such as enforcing vertical migration of zooplankton prey during day to deeper and colder layers (influencing thus negatively the reproduction), together with shifting the size spectrum of the prey populations. Third, the in-situ diet study of Bythotrephes revealed, besides supporting earlier observations that this predator is a generalist capable to capture broad range of prey species, that in the studied lakes were consumed predominantly species occurring mostly in epilimnion and not migrating diurnally, curiously species known for fast escape responses (Eudiaptomus gracilis and Diaphanosoma brachyurum). In the frame of the project, we collected enough evidence to support the hypothesis that Bythotrephes can significantly impact prey zooplankton communities even in the native lakes.