Stage of bloom succession drives trophic marker variability in sea ice algal communities in Hudson Bay

Sea ice algae are a vital food source in Arctic marine ecosystems, yet the variability of lipid- and isotope-based trophic markers within natural sea ice algal communities remains poorly understood and challenges the use of trophic marker approaches. This study tracked a bottom sea ice algal bloom in northwestern Hudson Bay, examining how environmental factors influence polyunsaturated fatty acids (PUFAs), highly branched isoprenoids (HBIs), and δ¹³C ratios of fatty acids. Trophic markers showed high spatiotemporal variability that was significantly correlated with sea ice nutrients and dissolved inorganic carbon (DIC) concentrations. Early bloom stages were characterized by high nutrient levels, peak bloom samples by highest percentages of PUFAs, while late stages showed increased 16:1(n-7) levels, suggesting storage lipid formation. Gradually increasing DIC depletion corresponded with enriched δ¹³C signals. The highest IP25 values occurred at nearshore sites where bloom progression was least advanced, suggesting possible benthic-sympagic coupling. Bloom succession stage differed between sampling sites and was the dominant factor influencing trophic marker variability leading to spatial variability along the transect.