Demand of carbon prevails over nutrients in proglacial streams subjected to glacier retreat: a nutrient manipulation bioassay

Glacier-fed streams across the world’s major mountain ranges are consistently energy limited, contributing low concentrations of bio-reactive organic carbon (C) to downstream recipients. Yet, climate-driven glacial retreat is expected to alter both C and nutrient supply in glacial-fed streams with consequences for downstream elemental fluxes and ecosystem functioning. As sources shift from glacier melt to groundwater and snowmelt, reductions in stream power and turbidity promote primary production, giving rise to a “greening effect” that favours autochthonous supply of organic C. In conjunction, lower flow turbulence may also reduce phosphorus (P) inputs from erosion-driven rock weathering. Yet, the impacts of altered energy and nutrient stoichiometry on microbial energetics and C cycling remain unknown across high-mountain catchments. In this study, we established chamber bioassays to measure metabolic rates and changes in dissolved organic carbon (DOC), nitrate, and phosphate concentrations over 24 h in, using sediments and stream water from clean ice glacier, rock glacier, and groundwater-fed headwaters, as well as from downstream recipients. Bioassays included three nutrient treatments (C+N, P+N, and C+N+P) together with an ambient stream water control, incubated at 8 °C under dark (12 h) and light (12 h) conditions. Gross primary production and ecosystem respiration metabolism rates were quantified with high resolution optical oxygen monitoring. We found that both microbial degradation and production of DOC increased in headwaters without clean ice glacier inputs, with the highest metabolic rates and greatest reductions in DOC concentrations observed in sediments receiving rock glacier inputs. The sediments from rock glacier and groundwater-fed headwaters were also C limited, whereas the clean ice glacier showed no response to C additions. Interestingly, we found no evidence for microbial P limitation in any site, despite low ambient P concentrations. These results demonstrate that microbial C cycling and energy demand in proglacial headwaters can be expected to increase with glacial retreat, imposed by a switch in the microbial communities from chemolithotrophic to heterotrophic and photoautotrophic dominance. Although microbial biomass growth increased and stream water stoichiometry predicted C and P co-limitation, the unexpected absence of P limitation in bioassays suggests flexibility in stoichiometric strategies, allowing for a wide range in C:P ratios of microbial biomass present in proglacial streams. To resolve the impacts of glacial retreat on stream ecosystem functioning, we thus stress the need for complementing indirectly inferred nutrient limitation with direct nutrient manipulation experiments.