Now you see it, now you don't: Nutrient co-limitation of alpine pond ecosystems

Background/Question/Methods

Anthropogenic nitrogen (N) deposition is considered a major stressor of high-elevation ecosystems, especially in cases where N is the limiting nutrient.  Further, recent evidence of the ubiquity of nutrient co-limitation across aquatic and terrestrial ecosystems highlights how anthropogenic phosphorus (P) inputs can synergistically amplify the the fertilizing effect of elevated N deposition.  However, the nature of nutrient limitation likely varies among and within ecosystems because of differences local environmental heterogeneity and the scale of scientific inquiry.  Therefore, we first conducted a survey of 35 alpine waterbodies to chemically infer changes in nutrient limitation along a size-gradient ranging from lakes to ponds located along the Eastern Front Ranges of the Canadian Rockies.  Nutrient bioassays using water collected from each sampled waterbody were also performed to quantify the single and combined effects of N and P on primary production.  The findings provided us with the basis for formulating the hypothesis that while alpine lake ecosystems are P-limited, ponds are co-limited by N and P.  The co-limitation hypothesis was then tested by conducting a crossed two-factor (N x P) fertilization experiment using 12 whole natural alpine ponds.

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Results/Conclusions

Although experimental nutrient additions increased by three-fold the ambient N and P concentrations in the amended ponds, these did not significantly stimulate phytoplankton growth while only N marginally enhanced the abundance of sediment-dwelling primary producers (epipelon).  However, P amendments significantly increased the abundance of naturally-occurring high densities of herbivorous fairy shrimp (Branchinecta paludosa).  Nutrient-herbivory bioassays revealed that intensive grazing pressure by the fairy shrimp had suppressed the otherwise NP-limited phytoplankton and N-limited epipelon from responding positively to the nutrient amendments.  Our findings demonstrate how environmental heterogeneity involving predator-prey interactions and habitat heterogeneity can mediate the nature of nutrient limitation in ecosystems, thereby precluding simple prediction of their responses to anthropogenic nutrient deposition based solely on the known direct effects of N and P on primary producers.