Seasonal trends in Subantarctic plankton δ¹³C and δ¹⁵N are driven by phytoplankton dynamics and nutrient preference

Abstract The carbon and nitrogen isotope ratios (δ 13 C and δ 15 N) of marine plankton record biogeochemical processes at the base of the food web. In the Southern Ocean, such data, predominantly from summer, have been used to infer surface CO 2 concentrations and the potential for biological carbon export. However, variability in plankton δ 13 C and δ 15 N remains poorly understood, with the lack of seasonal measurements from the Southern Ocean emerging as a particular limitation. Here, we investigate the δ 13 C and δ 15 N of suspended particulate matter (SPM) and zooplankton collected from the Subantarctic Ocean in winter, summer, and autumn. The low summertime δ 13 C SPM and δ 15 N SPM can be explained by strong diatom reliance on nitrate supplied during winter mixing. A subsequent increase in δ 13 C SPM and decrease in δ 15 N SPM by autumn is consistent with iron limitation in mid‐ to late‐summer favoring diatom succession by nano‐phytoplankton that consume mainly recycled ammonium. By winter, bacterial decomposition of biomass outpaces new biomass production, decreasing δ 13 C SPM and raising δ 15 N SPM . The δ 13 C and δ 15 N of contemporaneously sampled zooplankton generally reflect the variability in δ 13 C SPM and δ 15 N SPM , with one autumn mismatch suggesting that in situ SPM is not always the dominant zooplankton food source. Our study shows how nutrient dynamics and phytoplankton community composition shape the seasonality of the Subantarctic's isotopic baselines, emphasizing a key role for iron availability. This work has implications for isotope‐based food web studies, highlighting the need to consider seasonal variability in δ 13 C SPM and δ 15 N SPM , as well as the different turnover times of phytoplankton (i.e., SPM) vs. zooplankton biomass.