Biomarkers and diatoms as tracers of phytoplankton communities and past sea ice conditions in the southwestern Ross Sea, Antarctica: drivers and variability over the last 200 years

The Ross Sea, Antarctica, is among the most seasonally productive regions of the global ocean, where different classes of phytoplankton, such as diatoms, dinoflagellates, and haptophytes ( Phaeocystis antarctica) , play key roles in marine ecosystems and the carbon cycle. Sea ice dynamics strongly influence Ross Sea phytoplankton blooms, yet the effects of recent sea ice changes on bloom composition and productivity remain poorly constrained, and longer baseline records are required to understand changes in sea ice and resulting impacts on phytoplankton and climate. The last 200-year period provides a natural baseline before the onset of the warming and rising atmospheric carbon dioxide (CO 2 ) associated with industrialisation, making it critical for contextualising recent and projected change. We used sedimentary biomarkers and diatom assemblages to examine the relationship between physical changes in sea ice and phytoplankton community composition, which in turn influences the chemical signatures preserved in marine sediments. We investigated phytoplankton-derived lipid biomarkers (fatty acids, highly branched isoprenoids; HBIs, sterols) and diatom assemblages in six marine sediment core tops and three short sediment cores collected along a north-south transect extending from McMurdo Sound coastal polynya to south of Terra Nova Bay. Diatoms and biomarkers in core tops reveal increased proportions of open-ocean diatom species and bacterial fatty acids towards the southern end of the transect near McMurdo Sound driven by lower summer sea ice concentration and yearly duration, along with a phytoplankton community dominated by diatoms and higher summer biomass. In contrast, the northern end of the transect, near Terra Nova Bay, is characterised by higher proportions of diatoms associated with sea ice, as well as increased concentrations of sea ice diatom-derived fatty acids, and HBIs, driven by greater sea ice concentrations. Similarly, Phaeocystis antarctica -derived fatty acid biomarkers increase towards the northern end of the transect, likely driven by differences in the phytoplankton community. A Principal Component Analysis (PCA) of the biomarker and diatom dataset confirms this spatial structure, resolving three main variance groupings: diatoms and fatty acids associated with heavier sea ice ( Fragilaropsis curta group, C 24:0 ), which are inversely related to open-water and fast ice diatom assemblages ( Chaetoceros resting spores, open ocean, cold water, and sea ice diatoms), while fatty acids and HBIs form a third, independent grouping consistent with shared post-depositional degradation. Within the short cores, sea ice associated diatoms such as Fragilariopsis curta and the sea ice proxy, PIPSO 25 , indicate an increase in sea ice extent over the last 200 years, accompanied by declining open ocean diatom species, a trend not captured by phytoplankton-derived fatty acids alone, which show little change in community composition over the same period. Overall, our records reveal a 200-year increasing sea ice trend in the southwestern Ross Sea, consistent with existing regional sea ice extent reconstructions from ice cores. Biomarkers in the southwestern Ross Sea sediment independently distinguish between pelagic diatoms, P. antarctica , and sea ice-associated diatoms, offering a valuable tool for developing longer decadal resolution records of sea ice and phytoplankton community changes.