Temporal and spatial trends in marine carbon isotopes in the Arctic Ocean and implications for food web studies

Abstract

The Arctic is undergoing unprecedented environmental change. Rapid warming, decline in sea ice extent, increase in riverine input, ocean acidification and changes in primary productivity are creating a crucible for multiple concurrent environmental stressors, with unknown consequences for the entire arctic ecosystem. Here, we synthesized 30 years of data on the stable carbon isotope (δ13C) signatures in dissolved inorganic carbon (δ13C‐DIC; 1977–2014), marine and riverine particulate organic carbon (δ13C‐POC; 1986–2013) and tissues of marine mammals in the Arctic. δ13C values in consumers can change as a result of environmentally driven variation in the δ13C values at the base of the food web or alteration in the trophic structure, thus providing a method to assess the sensitivity of food webs to environmental change. Our synthesis reveals a spatially heterogeneous and temporally evolving δ13C baseline, with spatial gradients in the δ13C‐POC values between arctic shelves and arctic basins likely driven by differences in productivity and riverine and coastal influence. We report a decline in δ13C‐DIC values (−0.011‰ per year) in the Arctic, reflecting increasing anthropogenic carbon dioxide (CO2) in the Arctic Ocean (i.e. Suess effect), which is larger than predicted. The larger decline in δ13C‐POC values and δ13C in arctic marine mammals reflects the anthropogenic CO2 signal as well as the influence of a changing arctic environment. Combining the influence of changing sea ice conditions and isotopic fractionation by phytoplankton, we explain the decadal decline in δ13C‐POC values in the Arctic Ocean and partially explain the δ13C values in marine mammals with consideration of time‐varying integration of δ13C values. The response of the arctic ecosystem to ongoing environmental change is stronger than we would predict theoretically, which has tremendous implications for the study of food webs in the rapidly changing Arctic Ocean.

Spatial and Temporal Variability of Ice Algal Trophic Markers—With Recommendations about Their Application

“…This ratio is widely applied in trophic food web analyses, but the basis for its specific use to contrast sea ice algae vs. phytoplankton is the relative shortage of inorganic carbon for photosynthesis in spatially limited brine channels where high biomass densities might accumulate, as opposed to a usually well replenished surface layer of the ocean. As a result, δ 13 C values in sympagic algae become enriched over time by 13 C. A strong impact of both nutrient availability as well as taxonomic composition on sympagic particulate organic carbon (POC) δ 13 C was described previously [28], and the implications for tracing the fate of ice algal production in Arctic food webs is discussed extensively both in this work, as well as more recently [29]. One caveat of this approach is that Arctic phytoplankton can also exhibit considerable base-line variation in its isotope values which can result in overlaps with sea ice algae, complicating foodweb analyses [30].…”

“…This is in agreement with other studies, which identified phytoplankton growth rate, as well as the availability of carbon, light, and nutrients to affect isotopic fractionation and the δ 13 C-POM values. In addition to these biological factors, there are also temporal and regional differences in the δ 13 C baseline in inorganic carbon [29] which also affect δ 13 C in POM. Thus, even the baseline signal in sea ice might undergo a pronounced temporal variation, and thereby also contribute to the high variability in δ 13 C-POC ice .…”

Atlantic walrus signal latitudinal differences in the long-term decline of sea ice-derived carbon to benthic fauna in the Canadian Arctic

“…The δ 13 C Phe of Atlantic walrus in Foxe Basin declined at a rate of 0.6‰ per decade from 1982 to 2016 in association with a decreasing sea ice concentration and earlier sea ice breakup, and was 0.8‰ lower in 2006 than in 1996 in Jones Sound, suggesting less sea ice-derived carbon in the benthos over time in both areas which aligns with our H-Print estimates. The decadal declines of δ 13 C Phe in Atlantic walrus from both study areas is six to eight times higher than the decrease of δ 13 C of dissolved inorganic carbon in Arctic marine waters (−0.11‰ per decade) attributed to increasing concentrations of anthropogenic CO 2 in the atmosphere known as the Suess effect [ 47 ]. Additionally, our decadal declines in δ 13 C Phe parallel estimates from [ 47 ] where the entire pool of δ 13 C of particulate organic carbon in the Arctic Ocean decreased by 0.6‰ per decade with declining sea ice in combination with increased phytoplankton primary productivity [ 5 ].…”

Latitudinal consistency of biomass size spectra - benthic resilience despite environmental, taxonomic and functional trait variability

“…The relationship between δ 13 C and benthic standing stock is less obvious, especially in this study, where we reported higher standing stock in fjords characterized by lower δ 13 C values. Depleted δ 13 C is usually interpreted as indicating a higher contribution of terrestrial or partially decomposed organic matter 54,55 . In the studied locations δ 13 C was higher in northern Norway fjords (low precipitation and terrestrial inflows) than in southern Norway (high precipitation and dense land vegetation) and Svalbard (glacial erosion) 56 .…”