Multiple effects of anthropogenic climate change on the growth and calcification of marine phytoplankton
PhD: University of Colorado Boulder, 2018.
Phytoplankton living in the surface ocean are an influential force on the global carbon cycle and the base of the marine food web. Over the next century the surface ocean is expected to increase in inorganic carbon content, acidify, warm, and stratify as a result of anthropogenic climate change. Calcifying phytoplankton called coccolithophores are thought to be especially susceptible to ocean acidification. These tiny, photosynthetic calcifiers have an important role in the global carbon cycle, substantially contributing to global ocean calcification, ballasting organic matter to the deep sea, and influencing ocean-atmosphere CO2 exchange. Despite their potential vulnerability to ocean acidification, our in situ study showed that coccolithophores in the North Atlantic have been increasing in abundance over the past two decades in response to rising levels of dissolved inorganic carbon, but model testing is needed to rule out the influence of other factors, such as increasing temperature. We investigated the role of temperature on marine phytoplankton net primary production in the Community Earth System Model (CESM), showing that anthropogenic warming causes declines in phytoplankton productivity on a global scale with variable regional responses. In order to better understand coccolithophore responses to ocean change, we compiled field and laboratory studies to synthesize overarching, across-species relationships between environmental conditions and coccolithophore growth rates and relative calcification. These relationships were used to parameterize coccolithophores as a phytoplankton functional type in CESM, the first coccolithophore parameterization in an Earth system model in which growth rate and calcification are sensitive to carbonate chemistry. We performed CESM integrations under preindustrial (285 μatm), modern (400 μatm), and end-of-the-century (900 μatm) CO2 concentrations. From preindustrial to modern CO2 levels, coccolithophores show widespread decreases in calcification, but these decreases are offset by a carbon fertilization effect, allowing coccolithophores to flourish in areas where they were previously limited in growth by CO2. However, under end-of-the-century CO2 conditions, decreases in calcification from ocean acidification are the predominant force, resulting in a negative net change in calcification by coccolithophores relative to preindustrial CO2 levels. These results highlight how multiple simultaneous changes in the marine environment modulate biological responses on a global scale.