Calcification in the planktonic foraminifera <i>Globigerina bulloides</i> linked to phosphate concentrations in surface waters of the North Atlantic Ocean

Marine calcifiers, such as planktonic foraminifera, form a major component of the global carbon cycle, acting as both a source and sink of CO2. Understanding factors that affect calcification in these organisms is therefore critical in predicting how the oceans will respond to increased CO2 concentrations in the atmosphere. Here, size-normalised weights (SNWs) of the planktonic foraminifera Globigerina bulloides, collected from the surface waters of the North Atlantic Ocean, are compared with in situ carbonate ion concentrations ([CO32–]), sea-surface temperature, optimum growth conditions and nutrient concentrations. Changes in phosphate concentrations ([PO43–], range: 0.04–0.39 μM) explained the majority of G. bulloides SNW variation, with reduced test masses at higher concentrations. Two factors already known to influence calcification in foraminifers, [CO32–] and temperature, were also positively correlated over the range of values examined (148–181 μM kg−1 and 10.3–12.7 °C respectively). No evidence was found for increased SNWs under apparent optimum growth conditions, indicated by G. bulloides abundances. However, "growth potentials" (μ), derived from modelled growth rates (d–1), were positively correlated with SNWs, suggesting that this may be a better proxy for optimum growth conditions. These findings point to the potential importance of [PO43–] in determining calcification intensities in foraminifera, a factor which has been overlooked by previous studies on these organisms. The confirmation of this via carefully controlled culture studies is recommended in the future.