Chlorophyll- a in Antarctic Landfast Sea Ice: A First Synthesis of Historical Ice Core Data

Abstract Historical sea‐ice core chlorophyll‐a (Chla) data are used to describe the seasonal, regional and vertical distribution of ice algal biomass in Antarctic landfast sea ice. The analyses are based on the Antarctic Fast Ice Algae Chlorophyll‐a dataset, a compilation of currently available sea‐ice Chla data from landfast sea‐ice cores collected at circum‐Antarctic nearshore locations between 1970 and 2015. Ice cores were typically sampled from thermodynamically grown first‐year ice and have thin snow depths (mean = 0.052 ± 0.097 m). The dataset comprises 888 ice cores, including 404 full vertical profile cores. Integrated ice algal Chla biomass (range: ‐2 – 219.9 mg m‐2, median = 4.4 mg m‐2, interquartile range = 9.9 mg m‐2) peaks in late spring and shows elevated levels in autumn. The seasonal Chla development is consistent with the current understanding of physical drivers of ice algal biomass, including the seasonal cycle of irradiance and surface temperatures driving landfast sea‐ice growth and melt. Landfast ice regions with reported platelet‐ice formation show maximum ice algal biomass. Ice algal communities in the lower‐most third of the ice cores dominate integrated Chla concentrations during most of the year, but internal and surface communities are important, particularly in winter. Through comparison of biomass estimates based on different sea‐ice sampling strategies, i.e., analysis of full cores versus bottom‐ice section sampling, we identify biases in common sampling approaches and provide recommendations for future survey programs: e.g., the need to sample fast ice over its entire thickness and to measure auxiliary physico‐chemical parameters.