Experiment and natural variation in N and P inputs elucidate complexity in freshwater ecosystems

Extending ecological inference from local experiments to broader spatial extents remains a challenge. Here, we leveraged the natural, self-contained aquatic ecosystem of the pitcher plant (Sarracenia purpurea) to test whether locally observed effects of nutrient enrichment are consistent with patterns across a regional gradient of nutrient deposition. We conducted a field experiment at a single bog using realistic nitrogen (N) and phosphorus (P) additions and evaluated 10 ecosystem responses, representing both nutrient pools (stocks) and fluxes. We then compared these to a survey spanning 28 sites from Florida to Maine. The spatial grain of the individual pitcher leaf was held constant while the spatial extent varied by more than 2000 km. Enrichment increased fluid P by an amount equivalent to 92% and 20% of the observed range in the experiment and the survey, respectively, and increased tissue P by amounts equivalent to 276% and 21% of the observed range in the experiment and the survey, respectively. Enrichment also increased tissue N by 23% of its observed range in the survey. Enrichment also elevated Chlorophyta biovolume, whereas microbial responses were primarily mediated by indirect effects via detrital nutrient pools and grazing pressure. More than 80% of the coefficients representing nutrient fluxes in the local experiment matched those from the regional survey, with only slightly stronger nutrient effects in nutrient addition. These results indicate that well-designed site-based nutrient addition manipulations can robustly predict ecosystem responses to nutrient deposition across broad spatial gradients.