Increasing coccolithophore abundance in the subtropical North Atlantic from 1990 to 2014

Kristen M. Krumhardt, National Center for Atmospheric Research, Boulder, CO, United States, Nicole S Lovenduski, University of Colorado, Boulder, CO, United States, Natalie M Freeman, University of Colorado at Boulder, Boulder, CO, United States and Nicholas Robert Bates, Bermuda Institute of Ocean Sciences, BATS, St. George's, Bermuda
As environmental conditions evolve with rapidly increasing atmospheric CO2, biological communities will change as species reorient their distributions, adapt, or alter their abundance. In the surface ocean, dissolved inorganic carbon (DIC) has been increasing over the past several decades as anthropogenic CO2 dissolves into seawater, causing acidification (decreases in pH and carbonate ion concentration). Calcifying phytoplankton, such as coccolithophores, are thought to be especially vulnerable to ocean acidification. How coccolithophores will and are responding to increasing carbon input has been a subject of much speculation and inspired numerous laboratory and microcosm experiments, but how they are currently responding in situ is less well documented. In this study, we use coccolithophore pigment data collected at the Bermuda Atlantic Time-series Study (BATS) site together with satellite estimates of surface chlorophyll and particulate inorganic carbon (PIC), to show that coccolithophore populations in the North Atlantic Subtropical Gyre have been increasing significantly over the past two decades. We further demonstrate that coccolithophore abundance is positively correlated with DIC (and especially the bicarbonate ion) in the upper 30m of the water column. Previous studies have suggested that coccolithophore photosynthesis may benefit from increasing CO2, but calcification may eventually be hindered by low pH (<7.7). We speculate that coccolithophore photosynthesis and perhaps calcification may be increasing in response to anthropogenic CO2 input. Over this time period, decreasing carbonate ion concentration and pH do not appear to be negatively affecting coccolithophore growth in the North Atlantic.