Nutrient ratios in summer snowpack are in Redfield proportions

All life depends on the availability of the critical elements nitrogen (N) and phosphorus (P). The ratio of N and P present in ecosystems often resembles the so-called Redfield ratio of 16N:1P (molar), a value first derived in classic studies of the open ocean. An enduring conundrum in ecology is how such regularity in N:P is achieved. To gain new insights into this puzzle, we assessed N and P concentrations and ratios in mountain summer snowpacks, physiochemically varied, dynamic, and understudied ecosystems that contribute to the headwaters of global water cycling. Similar to oceans, in mountain snowpacks unicellular algae (snow algae) dominate the base of the food web but, unlike marine ecosystems, important nutrient cycling feedbacks such as nitrogen fixation and denitrification are absent. We expected that N:P of snow would be widely divergent from classical values given wide variation in geochemical and physical processes that affect the nutrient chemistry of snowpack. Alternatively, snow N:P would reflect biological, Redfield-like values if dominant biota growing in snow (e.g., snow algae and associated microbes) act to retain N and P in balanced proportions during their proliferation. To test these ideas, we sampled ~100 snowfields in two mountain regions of the western U.S.A. Molar N:P ratios in summer snow were remarkably close to Redfield with a value of 16.8 (±1SE=1.02) for total N:P (TN:TP) while particulate N:P (via two methods) was 13.8 (±1.0) and 23.0 (±1.02). In contrast, the N:P of winter snow (lacking biota) had a mean TN:TP ratio of 46.5 (±4.03). Since Redfield N:P proportions in algae are indicative of nutrient-sufficient growth, to test for N and P sufficiency of snow algae we conducted a series of field nutrient enrichment experiments and indeed observed no response to fertilization. These findings provide strong evidence that the Redfield ratio at the ecosystem level in mountain summer snow is regulated by a biological attractor set by the characteristic composition of nutrient-sufficient snow algae. Our findings have implications for our broader understanding of elemental coupling in ecosystems as well as of the impacts of snow algae proliferation on ecological and hydrological processes in the global cryosphere.