Bio-optical properties and radiant heating rates in the borderlands region of the Chukchi Sea: The roles of phytoplankton biomass and sea ice cover

Abstract

Hydrographic, irradiance, nutrient and chlorophyll-a concentration data were collected in the Chukchi Borderlands region of the northern Chukchi Sea 2011 and 2015 during Korean Arctic program research cruises. Goals included investigation of the spatial and temporal variability of optical and radiant heating properties in the water column, with a focus on the roles of phytoplankton and sea ice. The distribution of phytoplankton in the Chukchi Borderlands exhibits significant east-west variations, derived from two distinct water body types: Type-C and Type-M. Type-C water is influenced by the relatively low-nutrient Pacific Summer Water and upper layers of the central Beaufort Gyre, while Type-M water is associated with relatively higher-nutrient waters found in upwelling and shelf regions. In addition, the differences between these two water types are also reflected in their thermohaline properties, sea ice concentration, optical properties, and radiant heating rates. Under conditions of high sea ice concentration, we observed that phytoplankton are concentrated in the upper layers where more light is available, which in turn concentrates radiative energy at the surface. This mechanism partly reduces the impact of sea ice cover and increases radiant heating in early melt season, aiding further sea ice melt. The near-surface temperature maximum (NSTM) $T_{\max }$ is strongly correlated with the radiant heating rate (R² = 0.79; p < 0.001), and the presence of sea ice decreases the heating rate. On balance, as Arctic sea ice continues to decline, we estimate that the heat stored in the NSTM would contribute to melting 35.3 cm thickness of existing sea ice. These findings highlight the crucial roles of phytoplankton and sea ice in the dynamics of Arctic Ocean radiant heating.