Spatial and Interannual Variability of Antarctic Sea Ice Bottom Algal Habitat, 2004-2019

In the Antarctic, sea ice algae use the highly dynamic sea ice as a platform for growth. Antarctic sea ice extent has recently been highly variable, first showing a slight increase and then a record decrease starting in 2016. We investigated the response of Antarctic ice algal habitat to variations in sea ice and other environmental forcings during 2004-2019. Combining an ice growth model, remote sensing and reanalysis data, and a radiative transfer model, we assessed whether light penetration to the bottom ice was sufficient for ice algal growth. Trends in the inputs over the 16 years were relatively small: there were no changes in ice thickness or bottom ice melt date, a 6.4% decrease in snow depth, a 1.2% decrease in incident light, and a 0.8 & DEG;C decrease in air temperatures. Eighty-one percent of the sea ice cover was habitable by ice algae for & GE;14 days each year. The Antarctic has a larger extent and duration of potential ice algal habitat than the Arctic. Over time, the spatially averaged seasonal duration of habitat increased because a higher proportion of each pixel became habitable on average, compensating for the 2016-2019 reduction in sea ice extent. The spatial variability in potential habitat was strikingly high, even within geographic sectors. Bottom ice melt date (bloom termination) far surpassed other environmental factors in explaining variation (45%) in ice algal habitat on the 25 km scale. Because melt date depends on the ice-atmosphere heat balance, Antarctic ice algal habitat may be highly sensitive to future climate changes. Sea ice algae are single-celled photosynthetic organisms that grow within or at the bottom of sea ice. They tend to bloom early in the spring each year before other carbon fixers and are therefore an important food source for small marine animals. We examined how ice algae responded to spatial and temporal changes in the Antarctic environment during 2004-2019, a period that featured large swings in the amount of area covered by sea ice. We paired satellite data, which can measure where sea ice is and the depth of the snow on top of the ice, with other atmospheric data (e.g., incoming sunlight and air temperature) from models and observations to estimate where and when sea ice was potential algal habitat. There were some increases in habitable sea ice, but more importantly, we found that the spatial distribution of habitat was very patchy. This spatial unevenness suggests that whether or not sea ice is habitable is highly affected by relatively small changes in the environment and that ice algae may be quite sensitive to future climate changes. About 81% of Antarctic sea ice cover transmitted sufficient light to the bottom ice to support & GE;2 weeks of net ice algal growth each yearAn increase in the proportion of habitable area prevented a decline in potential habitat during the 2016-2019 reduction in sea ice areaPotential habitat was highly spatially variable and best explained by bottom ice melt date. It is likely sensitive to future climate change

Automated summary

This study investigated the response of Antarctic ice algal habitat to variations in sea ice and other environmental forcings. The research found that the Antarctic has a larger extent and duration of potential ice algal habitat than the Arctic. The bottom ice melt date was identified as the most important factor in explaining the variation in ice algal habitat. The study suggests that Antarctic ice algal habitat may be highly sensitive to future climate changes.