Missing effects of anthropogenic nutrient deposition on sentinel alpine ecosystems

Abstract

Anthropogenic nitrogen (N) deposition affects unproductive remote alpine and circumpolar ecosystems, which are often considered sentinels of global change. Human activities and forest fires can also elevate phosphorus (P) deposition, possibly compounding the ecological effects of increased N deposition given the ubiquity of nutrient co-limitation of primary producers. Low N : P ratios coupled with evidence of NP-limitation from bioassays led us to hypothesize that P indirectly stimulates phytoplankton by amplifying the direct positive effect of N (i.e. serial N-limitation) in alpine ponds. We tested the hypothesis using the first replicated N × P enrichment experiment conducted at the whole-ecosystem level, which involved 12 alpine ponds located in the low N deposition backcountry of the eastern Front Range of the Canadian Rockies. Although applications of N and P elevated ambient N and P concentrations by 2-5×, seston and plankton remained relatively unaffected in the amended ponds. However, additions of ammonium nitrate elevated the δ(15) N signals of both primary producers and herbivores (fairy shrimp; Anostraca), attesting to trophic transfer of N deposition to consumers. Further, in situ bioassays revealed that grazing by high ambient densities of fairy shrimp together with potential competition from algae lining the pond bottoms suppressed the otherwise serially N-limited response by phytoplankton. Our findings highlight how indirect effects of biotic interactions rather the often implicit direct effects of chemical changes can regulate the sensitivities of extreme ecosystems to nutrient deposition.

Seasonal methane dynamics in high-elevation lakes in the Sierra Nevada California: the role of elevation, temperature, organic matter, and inorganic nutrients

“…Higher elevation lakes exhibit lower nutrient and DOC concentrations than mid-elevations lakes (Moser et al 2019;Urmy and Warren 2019); in this area, microbial activities may be constrained by nutrient concentrations. Previous studies have found that alpine ecosystems are highly sensitive to modest N deposition (Baron et al 2000;Wolfe et al 2003;Vinebrooke et al 2014). Consistently, lakes in this study were likely N-depleted for the majority of the season, which may explain why CH4 concentrations were predicted to be higher when N was more available either in the NO2form or as a higher DIN:DIP ratio.…”

Interactive effects of precipitation and nitrogen enrichment on multi-trophic dynamics in plant-arthropod communities

“…For instance, atmospheric warming decreases soil moisture [ 8 ] and may thereby influence the fertilization effects of N deposition. In addition, N and phosphorus co-limit plant production in ecosystems around the world [ 71 , 89 , 90 ], and human activities are simultaneously increasing the availability of these nutrients, particularly in aquatic systems [ 91 , 92 ]. Also, fossil fuel combustion is increasing CO 2 availability for primary production [ 93 ], which in combination with nutrient enrichment can cause responses of primary producers indicative of co-limitation [ 94 – 96 ].…”

Alpine limnology of the Rocky Mountains of Canada and the USA in the context of environmental change

“…As water levels decline, numerous buffered lakes could shift towards shallower ponds with important consequences for biodiversity and ecosystem function (Parker et al 2008). Shallow alpine ponds are considered especially sensitive to environmental change due to their small volume and high solar energy absorption (Vinebrooke et al 2014). Pond ecosystems are polymictic so they lack the stratification needed to buffer against rapid changes in temperature (Lewis 1983).…”

“…. However, top-down effects by grazers (i.e., crustacean zooplankton) and competition between algal species may limit algal growth in some systems (Van Geest et al 2007;Vinebrooke et al 2014). Grazing pressure as well as algal dynamics could reduce the negative impacts of N inputs but understanding these confounding influences is difficult.…”