Assessing the response of coccolithophores and foraminifera to enhanced ocean alkalinity as a CO2 sequestration technique

Abstract:

The alkalinity of seawater sets the overall capacity of the ocean to hold carbon dioxide in dissolved forms. Variations in past alkalinity, related to changing weathering or carbonate compensation, may have played an important role in moderating or controlling past variations of atmospheric pCO₂. Future manipulation of ocean alkalinity by direct addition of suitable chemicals to seawater, or through enhanced weathering on land, has also been suggested as one possible route to intentionally draw CO₂ from the modern atmosphere and mitigate the impacts of future climate change [1]. Although we know an increasing amount about how species and ecosystems respond to changes in pH, we know much less about their response to changes in alkalinity. Calcifying plankton play a crucial role in modulating the surface ocean carbonate system and its buffering of alkalinity perturbations [2]. Here we investigate the growth and calcification response of both coccolithophores and foraminifera to elevated ocean alkalinity through a series of carefully designed batch culture laboratory experiments. Alkalinity is raised by two different methods during the experiments: by (i) addition of NaHCO₃ and (ii) addition of Na₂CO₃ and CaCl₂. These differing elevated alkalinity treatments allow us to constrain whether factors other than raised alkalinity affect plankton growth and/or calcification; NaHCO₃ addition raises alkalinity and DIC in solution without altering the Ca²⁺, whereas addition of NaCO₃ + CaCl₂ raises alkalinity, DIC and Ca²⁺, with possible impact on calcification (Ca has also been suggested to be toxic in high levels to several species of plankton [3]). This will allow us to constrain how physiology and calcification respond to these two different modes of alkalinity manipulation. I will present results from experiments with three species of coccolithophores: Emiliania huxleyi, Gephyrocapsa oceanica and Coccolithus braarudii, as well as two species of foraminifera: Gloigerinoides ruber and Globigerinella siphonifera.

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