Comparison of bottom sea-ice algal characteristics from coastal and offshore regions in the Arctic Ocean

Ice-core observations were conducted at three arctic sites in early April 2003: one clean sediment-free site (Chukchi), one sediment-rich site (Beaufort) on landfast sea ice offshore Barrow, and one oceanic Ice Exercise (ICEX) site. Concentrations of inorganic nutrients at coastal sites were similar to but higher than those at the oceanic site. Chlorophyll-a (Chl-a) concentration was much higher in the Chukchi bottom ice than at the other two sites. However, large size Chl-a (>20 μm) dominated (>70%) the bottom-ice algal Chl-a at both the Chukchi and Beaufort sites, whereas the oceanic ICEX site was evenly occupied by large (>20 μm), medium (5–20 μm), and small Chl-a (0.7–5 μm). These in situ data were incorporated into an ocean-ecosystem model. The model results revealed that: (1) strong light attenuation by trapped sediments controlled ice-algal production at the Beaufort site; (2) the peak in ice algae occurred later at the oceanic site than at the Chukchi site because of thicker ice and the consequently reduced amount of light reaching the ice algae at the ice bottom; and (3) maximum production at the oceanic site reached only 10% the level at the Chukchi site because of nutrient limitation.