Doctoral Dissertation Research: Determining the functional relationship between simultaneous co-limitating light and nutrient conditions on phytoplankton growth

The growth rate of phytoplankton, single-celled marine algae that take up carbon dioxide and produce oxygen, is controlled in the ocean by the availability of both light and nutrients. However, no laboratory study has generated the data required to quantify how nutrient and light interact to control phytoplankton growth. Some models assume that light and nutrients act independently to control phytoplankton growth, while others assume that light and nutrients act in concert.

These two different assumptions about how light and nutrients control phytoplankton growth can produce vastly different predictions about the speed of this process and the magnitude of phytoplankton blooms. The researchers will use a series of laboratory experiments to determine if the effects of nutrients and light on phytoplankton growth is best modeled assuming that the two limiting resources act independently or in concert.

This experiment will help to improve marine ecosystem models implemented throughout the world’s oceans, but will be particularly relevant in the Arctic, where light and nutrient availability are both rapidly changing as a result of anthropogenic climate change.

The researchers will grow Arctic diatoms across a range of growth-limiting nitrate (NO3-) and light conditions. Individual cultures will be maintained under constant temperature, light, and nutrient conditions in chemostats. There will be 25 unique interacting NO3- and light-limited treatment combinations conducted using five light and five NO3- conditions, as well as five additional treatments at light-saturated but NO3- deplete conditions.

Following acclimation of cultures to the treatments, the researchers will measure phytoplankton biomass and photosynthesis vs. irradiance (P-E) relationships, and phytoplankton growth rates will be determined from the dilution rate of the media. Light and nutrient limitation terms will then be calculated for each treatment. Finally, the best functional relationship that best describes the relationship between co-limiting light and nutrient conditions will be determined by comparing the two established (minimum and multiplicative) relationships to a variety of non-linear relationships between the limitation terms.

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