Adaptive biogeochemical responses in the Sargasso Sea in response to reductions in winter mixing

For three decades, the Bermuda Atlantic Time-series Study (BATS) program in the northwestern Sargasso Sea has been documenting changes in the physical, chemical and biological environments to quantify and understand the fluxes of carbon and associated biogenic elements through the marine ecosystem. The wintertime North Atlantic Oscillation (NAO) index is an highly influential climate-related driver that impacts seasonal and annual processes. Here, we compare and contrast two periods of similar winter NAO index values but different biogeochemical responses. There have been two periods where the winter NAO index was substantially positive (>0.5) for multiple years, 1990-1995 and 2014-2018, however, the response of the system was dramatically different. During the winters of 1990-1995, mixed layer depths were deeper (200-300m) compared to 2014-2018 (≤200 m), leading to the accumulation of heat in the upper 100 m. During winters in the 2014-2018 period, there were more dramatic reductions in vertical phosphate fluxes and subsequent decrease in phytoplankton biomass stocks and net primary production. The reductions in biomass and productivity were partially mitigated by an ecosystem response to retain phosphorus, resulting in substantial increases in suspended particulate carbon to phosphorus ratios in 2014-2018 compared to 1990-1995. While data for the carbon to phosphorus ratio in exported particles is only available for the 2014-2018 period, there was a marked three-fold increase in the ratio beginning in 2014 relative to data from 2004-2013; suggesting further retention of phosphorus in this system. These adaptive biogeochemical responses are important but currently are poorly captured in most earth system models, leading to potential underestimates of carbon export in a future warmer ocean.