News & Events

PhD defense - Kristen Krumhardt

Monday, April 09, 2018, 8:00AM - 10:00AM


Kristen M. Krumhardt



SEEC room S228 (Sievers Conference Room)


Multiple effects of anthropogenic climate change on the growth and calcification of marine phytoplankton


Phytoplankton living in the surface ocean perform half the photosynthesis on Earth each year, making them an influential force on the global carbon cycle, as well as the base of the marine food web. As anthropogenic CO2 in the atmosphere continues to rise, the surface ocean environment changes in several ways, altering the habitat of marine phytoplankton. 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. In response, biological communities will change as species reorient their distributions, adapt, or alter their abundance. 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 coordination with 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 over the 21st century on a global scale with variable regional responses. Despite the important impacts of coccolithophores on the global carbon cycle, these calcifying phytoplankton are not explicitly simulated in most Earth system models, including CESM. In order to more explicitly 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 an explicit 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 three atmospheric CO2 concentrations: preindustrial (285 μatm), modern (400 μatm), and end-of-the-century (900 μatm). 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.


Open to the public.