Background/Question/Methods Abiotic factors are regarded typically as more important than biotic interactions (e.g., competition, predation) in structuring plant communities in extreme environments. For example, increased nitrogen deposition is expected to stimulate primary producers in alpine ecosystems in part due to the assumed absence of high grazing pressure. We addressed this hypothesis by first performing a replicated in situ whole-ecosystem experiment, consisting of two crossed treatments (2 nitrogen x 2 phosphorus levels) applied across 12 naturally-occurring alpine ponds (N = 3).

Results/Conclusions Surprisingly, neither nitrogen or phosphorus amendments stimulated phytoplankton abundance over a two-month period following ice-out in 2008. Unexpected high densities of herbivorous Branchinecta paludosa (fairy shrimp) were observed in the experimental ponds during the experiment and we deduce that intensive grazing suppressed the response of primary producers to nutrient additions. However, an in vitro grazing experiment involving the use of late-summer adult fairy shrimp demonstrated a weak effect of grazing. An ontogenetic dietary shift from planktonic juvenile grazers towards omnivorous, benthic-feeding adults is a potential explanation for the apparent lack of planktonic herbivory. Interestingly, high abundances of fairy shrimp have also been discovered in other extreme environments, such as arctic ponds. Therefore, we suspect that intensive grazing may mediate the responses of nitrogen-limited primary producers to elevated nitrogen deposition in extreme environments.