Reviews and Syntheses: Carbon and Nitrogen Stable Isotope Analysis of Antarctic Macroalgae: a 35-Year Record for Environmental Change

Abstract. Stable isotope analysis for Antarctic macroalgae has received minimal research interest outside of trophic ecology. However, carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analysis provides insights into nutrient uptake, productivity and response to environmental change. This review collates all available CN, δ¹⁵N and δ¹³C data available for macroalgae from the Antarctic continent. Fieldwork was also carried out in late 2023 to the northern West Antarctic Peninsula to collect specimens for stable isotope analysis. Prior to the 2023 survey, only 29 publications were found spanning a 35-year period from 1987−2022 with a strong bias towards data collection in the South Shetland Islands. A bias was found towards the Rhodophyta and Phaeophyta phyla, with H. grandifolius and Desmarestia sp. being the most studied species. This review highlights Antarctic seaweeds had a lower average CN compared to the global average, likely driven by the non-nitrogen limiting environment. Both δ¹⁵N and δ¹³C varied between species as well as spatially, δ¹³C recorded a wider range (~25 ‰) than for δ¹⁵N (~16 ‰). Spatial variation is assumed to be driven by site specific differences as well as latitudinal changes with light availability. δ¹³C was more negative for the Rhodophyta phylum, with some species plotting below –30 ‰, however no significant difference was found between Rhodophyta and Phaeophyta species for δ¹³C and all recorded average δ¹³C values indicative of both passive CO₂ uptake and carbon concentration mechanism use. When plotted temporally, δ¹³C showed variation with sea ice extent across the 35-year record. Within the last decade, both sea ice extent and macroalgae δ¹³C became more variable and suggesting an unexplored link between primary producer response and sea ice instability. Significantly more research is required for stable isotope analysis of Antarctic seaweeds. Variability in both δ¹⁵N and δ¹³C will impact trophic food web studies, both spatially and temporally. There is a lack of data for sites outside of the Antarctic Peninsula as well as ~100 species currently having no data available. This review also highlights the potential for macroalgae to be an additional proxy for monitoring primary producer response to ongoing climatological changes for the Southern Ocean.