Investigation of Under‐Ice Phytoplankton Growth in the Fully‐Coupled, High‐Resolution Regional Arctic System Model

In July 2011, observations of a massive phytoplankton bloom in the ice‐covered waters of the western Chukchi Sea raised questions about the extent and frequency of under‐ice phytoplankton growth and its contribution to the carbon budget in the Arctic Ocean. To address some of these questions, we use the fully‐coupled, high‐resolution Regional Arctic System Model to simulate Arctic marine biogeochemistry over a 30‐year period. Our results demonstrate the presence of extensive under‐ice phytoplankton growth in the western Arctic (WA) in summer. In addition, similar growth, yet of lower magnitude, occurs annually in the eastern Arctic (EA). We investigate the critical levels of nitrate concentration and photosynthetically available radiation (PAR) that are necessary for under‐ice phytoplankton growth to occur. Our results show that while the majority of ice‐covered Arctic waters have sufficient surface nitrate levels to sustain growth, PAR reaching the ocean surface through the sea ice in early summer only exceeds critical levels in the WA. We therefore conclude that the EA high chlorophyll‐a concentrations shown in our simulations did not develop under sea ice, but were instead, at least in part, formed in open waters upstream and subsequently advected by ocean currents beneath the sea ice. Plain Language Summary In July 2011, scientists conducting research in the western Arctic (WA) Ocean observed a large phytoplankton bloom under the sea ice. Traditionally, such blooms were believed to be rare. Using our state‐of‐the‐art Arctic system model, we were able to demonstrate that in fact, areas with high concentrations of under‐ice phytoplankton have been occurring annually for the past several decades. In the WA, under‐ice phytoplankton growth begins when sufficient sunlight penetrates through the sea ice to the ocean, and ends when nutrient concentrations become too low to sustain the phytoplankton. In the eastern Arctic (EA), our model shows that phytoplankton can still grow beneath the sea ice even when there is little sunlight penetrating the ice and entering the water column. We conclude that the EA high chlorophyll‐a concentrations shown in our simulations did not develop under sea ice, but were instead, at least in part, formed in open waters upstream and subsequently advected by ocean currents beneath the sea ice. Key Points Model simulations show strong under‐ice phytoplankton growth similar to observations in summer 2011 Under‐ice phytoplankton growth