Changes in macroinvertebrate tissue chemistry in response to winter carbon-limitation in Alaskan spring-streams

Perennially flowing spring-fed streams in arctic Alaska provide winter habitat for benthic macroinvertebrates whereas other headwater-stream habitats freeze solid. The water temperatures of these streams stay relatively constant though light cycles from 24-hours of light in summer to little to no light during winter. Such uncoupling of temperature and light regimes in arctic spring-streams provides a unique opportunity to assess the effects of annual light-cycles independent of temperature. Previous investigations at one of our study sites, Ivishak Spring, found consumer winter carbon-limitation as metabolic demand exceeded net primary production. Here we ask whether higher water temperatures exacerbate the severity of carbon-limitation driving corresponding changes in consumer tissue stoichiometry. Annual cycles in the stoichiometry of macroinvertebrates from five spring streams (mean annual temperature range: 1-12℃) were assessed with semi-monthly samples from June 2022 – June 2023. We measured whole-body carbon (C):nitrogen (N)phosphorus (P), δ¹³C and δ¹⁵N relative abundances, and lipids were quantified. We predict that evidence of C-limitation during winter will be manifested as increases in whole-body δ¹³C and δ¹⁵N due to preferential respiration of ¹²C and excretion of ¹⁴N. Additionally, C:P, and lipid content will decrease as C stores are mobilized and lipids oxidized during winter months when primary productivity is negligible. Finally, we predict relative winter C-limitation will increase with temperature due to higher metabolic demands. We anticipate that findings from our ongoing study will contribute to the understanding of light-temperature interactions in controlling stoichiometric compensatory mechanisms to reduce the scope of winter C-limitation in these unique arctic ecosystems.