How does plankton distribution and activity influence the variability of carbon dioxide uptake in the North Atlantic?
This study combines two invaluable datasets that have been collected on-board volunteer observing ships to analyse the variability of the carbon dioxide (CO2) sink in the North Atlantic at a range of spatial and temporal scales. Phytoplankton indices collected from the continuous plankton recorder (CPR) and the concentration of CO2 within the surface waters show that at seasonal time-scales phytoplankton play an important role in maintaining the carbon drawdown within the northeast Atlantic, while sea surface temperature (SST) drives the seasonal signal in CO2 flux in the subtropics. The North Atlantic remained a significant sink of CO2 between 2002 and 2013, despite strong inter-annual variability in CO2 flux that was correlated to changes in the North Atlantic Oscillation and the influence that this had on SST. Discrete dissolved inorganic carbon, total alkalinity and dissolved oxygen samples were collected during 4 voyages between April 2012 and February 2013. Using these measurements this study successfully developed and implemented a simple and inexpensive technique to estimate net community production in the surface ocean, with the potential to extend coverage of such measurements over wider regions at low cost. Two key observations were made in the northeast Atlantic. Firstly, the increase in SST was significantly correlated with the increase in phytoplankton colour index measured by the CPR between 1960 and 2012, despite other micro and nanophytoplankton counts decreasing over this time frame. This suggests that as the surface ocean warms and stratification is enhanced, pico-phytoplankton (which contribute to the colour index but not the phytoplankton counts) may be better equipped to dominate the system, compared to larger species that are more nutrient dependent. Secondly, the CO2 uptake capacity has decreased compared to the 1990s. Combined, these two results will likely have a significant impact on carbon flux, export efficiency and ecosystem dynamics.
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