| Several lines of evidence suggest small alpine lakes and ponds are sensitive to nitrogen deposition. Paleolimnological studies, nutrient bioassays, and mesocosm experiments show the positive effects of nitrogen on aquatic alpine primary producers. In particular, alpine pond ecosystems have been inferred to be nitrogen-limited based on low availability of dissolved inorganic nitrogen relative to total phosphorus. However, nitrogen-limitation of alpine ponds has never been tested at the whole-ecosystem level. I performed a replicated in situ whole-pond experiment, consisting of two crossed treatments (2 nitrogen x 2 phosphorus levels) applied across 16 natural alpine ponds (n = 4) immediately following ice-out in 2008. Surprisingly, neither nutrient amendment stimulated phytoplankton or zooplankton abundance although subtle shifts in community composition were detected over a two-month period. Intensive grazing pressure exerted by high densities (>100 individuals/m 2) of herbivorous Branchinecta paludosa (fairy shrimp) may have suppressed planktonic responses to nutrient additions. Another ecological explanation for the lack of a positive effect of nutrients on phytoplankton abundance was competition from periphyton, which are comparatively more abundant in most shallow ponds on an areal basis. Therefore, density-dependent ecological interactions (competition and predation) may mediate the responses of phytoplankton to nitrogen deposition over ponds situated in extreme environments. |
