Biological Controls and Biogeochemical Outcomes of Marine Elemental Stoichiometry

Redfield proportions (106C:16N:1P:138-O2) have been used to describe ocean biogeochemical patterns since the 1950s. However, recent research demonstrates variation in this ratio over latitudinal gradients, time, and seasons. As a result, determining the controls on, and predictability of, variable stoichiometry will improve our understanding of global biogeochemical cycle dynamics. To address these missing components, I examined the environmental and physiological conditions associated with changes in stoichiometry and performed predictive modeling to demonstrate the impact of variable stoichiometry on marine biogeochemical cycles. I examined the direct environmental and physiological effects on fluctuations in stoichiometry in two ways. First, I synthesized scientific literature and revealed that specific physiological mechanisms have a strong impact on stoichiometry in nutrient-rich environments, whereas biogeochemical interactions are important in the oligotrophic gyres. Second, I utilized the 2015 El Niño as an example of extreme environmental conditions, where high temperatures and low nutrient availability were expected to affect stoichiometric ratios. These conditions resulted in a reduction in POM concentrations and an increase in C:N, C:P, and N:P ratios. El Niño conditions are representative of future scenarios, as such, this study provides evidence that stoichiometry is affected by extreme environmental shifts. To estimate the impact of variable stoichiometry on global biogeochemical cycles, I modelled and predicted future carbon and oxygen concentrations. First, I created a simple ocean box model to estimate the atmospheric CO2. I found that variation in allocation of cellular resources led to higher carbon export and declines in atmospheric CO2 compared to Redfield proportions. Second, I quantified the variability in the respiration quotient, or the oxygen required to oxidize one unit of carbon, r_(-O2:C) in marine organic matter. The r_(-O2:C) ratio was found to vary regionally due to temperature controls on plankton community composition and their physiological state. Deoxygenation, which had previously been estimated to occur during high temperature conditions, increased in regions where r_(-O2:C) was low. In summary, my work has demonstrated the importance of understanding, utilizing, and quantifying variable stoichiometry in order to better characterize global biogeochemical cycles