Using a Coupled 1-D Physical-Biological Model of the Chukchi Sea to Understand Recent Changes in Arctic Ocean Biogeochemistry

In 2010 and 2011, phytoplankton were observed blooming in the water column underneath first-year sea ice in the Chukchi Sea, an environment that has changed in recent years as ice has thinned and melt ponds have proliferated. To better understand how these recent changes have affected the seasonal cycle of primary productivity (PP) in this region, we coupled the 1-D General Ocean Turbulence Model (GOTM) physical modeling framework to a previously published 1-D biogeochemical model parameterized for the Chukchi Sea [Palmer et al., 2014]. Using data from the 2010-2011 ICESCAPE cruises, we validate this new model and perform sensitivity testing of model parameters for both years. We then utilize the model to explore how the timing of ice retreat, under-ice (UI) light availability, and initial nitrate concentrations impact key biogeochemical variables, including the timing and magnitude of peak and annual PP, and the dynamics of zooplankton, bacteria, and particulate carbon. Results show that although interannual variability is high, UI blooms are a common feature of this region, formed when shade-adapted phytoplankton take advantage of abundant winter nutrients and UI light to grow and multiply. However, there is a threshold in light availability necessary for these blooms to develop, and even small changes in the timing of UI light and ice retreat can have a large impact on both peak and annual PP. The early timing of UI blooms effectively extends the length of the growing season, such that annual PP is enhanced in years where phytoplankton bloom first under the ice. Results from nutrient experiments imply that a future increase in nitrate to the region may sustain larger overall populations of zooplankton as well as enhanced annual PP. However, zooplankton dynamics are closely linked to peak PP, an important implication for the regional ecosystem as modeled peak PP is highest in open water blooms.