Time-series of direct primary production and phytoplankton biomass in the southeastern Bering Sea: responses to cold and warm stanzas

Abstract

Sub-Arctic and Arctic regions are warming faster than nearly all other areas of the global ocean, leading to significant changes in ice quality and the duration of ice-covered periods. The impacts of this warming and sea ice variability on higher trophic levels in the Bering Sea is well documented, but the effects on lower trophic levels are less well understood. Phytoplankton biomass (as chlorophyll a [chl a]) and primary and nitrogen production measurements in the Bering Sea are presented from 2006-2016, a period that covers relatively colder (2007-2012) and warmer (2014-2016) temperature regimes. In warm spring periods, relative to cold spring periods, the frequency of subsurface chl a maxima increased, but with no significant differences in integrated chl a inventories. In contrast, cold fall periods were characterized by greater integrated chl a inventories than warm fall periods. Integrated net primary production (NPP) increased from the cold period (2007-2011) to the warm period (2014-2016). The difference in patterns in chl a and NPP resulted in higher phytoplankton growth rates during warm periods. Nitrate uptake rates increased from spring to fall during cold periods, while rates decreased from spring to fall during warm periods, suggesting changes in the balance of new versus regenerated production. While changes in phenological timing cannot be ruled out, changes in phytoplankton growth rate appear more important than changes in chl a biomass underlying increasing daily NPP. This distinction directly impacts our understanding of the linkages between warming temperatures and phytoplankton production and its implications in evaluating and understanding energy flow to higher trophic levels.