Award: PLR-1303617

The long-standing paradigm for phytoplankton blooms in the Arctic Ocean is that they form in the open water near the edge of the pack ice as it retreats northwards during spring. The idea is that not enough light can penetrate fully consolidated ice to trigger significant bloom activity, and it is not until the ice melts that the plant cells can utilize the nutrients in the surface water in the presence of the enhanced light levels. However, in summer 2011, a science team aboard the US Coast Guard cutter Healy serendipitously discovered a massive phytoplankton bloom in the northern Chukchi Sea, north of Bering Strait. The discovery was extraordinary for two reasons: (1) the bloom was occurring underneath fully consolidated pack ice that was 1 meter thick; and (2) it was one of the largest blooms ever recorded in the World Ocean. Not surprisingly, this raised a host of questions about how such blooms can form, and how their occurrence might be related to the warming climate. Such was the backdrop for our program entitled "The Study of Under Ice Blooms in the Chukchi Ecosystem," or SUBICE. The purpose of SUBICE was to document the conditions before, during, and after under-ice blooms by obtaining physical and biological measurements of both the pack ice and the water column. The plan was to get into the field as early in the season as possible, before any bloom activity had begun. This would allow us to document the "initial conditions" and then observe the entire sequence from under-ice blooms to subsequent open water blooms. Unfortunately, late-spring 2014 was colder than normal, and consequently we did not measure any significant under-ice bloom activity. However, our unique early-season cruise to the Chukchi Sea revealed a vast amount of information about the pre-bloom state of the shelf. Surprisingly, we found that greater than 95% of the water in the northeast Chukchi Sea was very cold, high-nutrient winter water. This was true even outside of the well-known pathways of Pacific-origin water that extend northward from Bering Strait. Furthermore, the winter water was nearly homogenous. We used a polynya model driven by realistic atmospheric forcing, together with a one-dimensional mixing model, to demonstrate that when leads in the ice open up, it would take on average less than a day for convective overturning to mix the water all the way to the bottom. In particular, the re-freezing leads add salt to the surface water, making it denser and hence causing it to sink. Notably, even though our study domain was nearly 100% ice covered, it was filled with small leads within the ice – many of them re-freezing. This suggests that winter water is formed throughout the Chukchi shelf via convection within small openings in the ice. Furthermore, the local convection would be expected to stir nutrients into the water column from the sediments, which explains the high nutrient concentrations observed throughout the shelf. Lastly, based on our biological measurements and a statistical model, we showed that blooms are unable to form in the leads despite the fact that they allow large amounts of light into the water column. This is because the convection quickly transports any newly growing phytoplankton to the bottom where the light levels are very low. Our results have enhanced the community?s understanding of water mass modification and bloom dynamics in the late-spring – a time of year that is critical for the seasonal functioning of the ecosystem. Last Modified: 12/22/2017 Submitted by: Robert S Pickart