Recent trends in sea ice-associated phytoplankton blooms in the northern Bering and Chukchi Seas

The northern Bering and Chukchi Seas are among the most productive marine ecosystems in the world and act as important carbon sinks, particularly during May and June when seasonal sea ice-associated phytoplankton blooms occur throughout the region. Sea ice melt and breakup during spring strongly drive this production by enhancing light availability in the water column, enabling stratification and stabilization of the water column, and introducing a new source of nutrients to surface waters. Recent variability in sea ice extent in the northern Bering and Chukchi Seas may therefore have profound impacts on spring phytoplankton production. In particular, passive microwave (SMMR and SSM/I) satellite data show strong interannual variability in sea ice area during winter and early spring months over the 1978-2008 time series, but no statistically significant trends. However, for months adjacent to the ice-free season (June-November), significant declines occurred for June (-3222 km2 or -8.5%/decade), July (-902 km2 or - 24.5%/decade), September (-1105 km2 or -23.8%/decade), October (-3879 km2 or - 20.0%/decade), and November (-10521 km2 or -13.0%/decade), suggesting that sea ice is breaking up earlier and reforming later each year. In this study, we focus on utilizing multi-sensor satellite data to investigate key temporal and spatial linkages between sea ice variability and chlorophyll biomass throughout the northern Bering and Chukchi Sea regions. We incorporate AMSR-E passive microwave measurements of sea ice extent and Aqua-MODIS derived concentrations of chlorophyll-a over the years 2002-2008. Remotely sensed measurements are validated with in situ data collected onboard the US Coast Guard Cutter Healy during the spring of 2006, 2007 and 2008. Surface phytoplankton blooms and peaks in chlorophyll-a concentrations are nearly coincident with the onset of sea ice degradation and begin to decline within approximately two weeks of the start of the blooms. Recent trends of earlier sea ice breakup in the northern Bering and Chukchi Seas therefore directly affect the timing and also likely the intensity of phytoplankton blooms. With earlier sea ice breakup, although sea ice melt still enables stratification and provides nutrients to surface waters, sunlight conditions may not yet be ideal for phytoplankton blooms and the intensity of chlorophyll-a peaks may thus be dampened. The potential implications of these shifts in the timing of sea ice breakup might reasonably include lowered overall productivity if seasonal sea ice retreat continues as expected.