Temporal and vertical variations of lipid biomarkers during a bottom ice diatom bloom in the Canadian Beaufort Sea: further evidence for the use of the IP25 biomarker as a proxy for spring Arctic sea ice

Variations in the concentrations of the sea ice diatom biomarker, IP25 (Ice Proxy with 25 carbon atoms), were measured in the bottom 10 cm of sea ice collected from the eastern Beaufort Sea and Amundsen Gulf from January to June 2008, as part of the International Polar Year-Circumpolar Flaw Lead system study. Temporal and vertical changes in IP25 concentrations were compared against other biomarkers and indicators of ice algal production. IP25 was not detected in sea ice samples collected from mid-winter to early spring, likely as a result of light-limiting conditions for algal growth and accumulation. From early March to mid-June, IP25 concentrations correlated well with those of fatty acids (r = 0.79; P < 0.001), less so with total sterols (r = 0.63; P < 0.001) and qualitatively with chlorophyll a concentrations and diatom cell abundances from adjacent sea ice cores. Approximately 90% of the total sea ice IP25 accumulation occurred from mid-March to late-May, coincident with the ice algal bloom period. The majority (ca. 87-93%) of IP25 was biosynthesised within the lower 5 cm of the sea ice where brine volume fractions were > 5% which is consistent with the hypothesis that brine channel connectivity limits the internal colonisation of sea ice by diatoms. Maximum IP25 concentrations occurred at 1-3 cm from the ice-water interface providing further evidence for a selective sea ice diatom origin for this biomarker. In contrast, vertical concentration profiles for fatty acids and sterols indicated mixed sources for these biomarkers.