Environmental controls on nitrogen cycling in coastal sediments

Human activity is currently altering global elemental cycles to an unprecedented degree. For example, the invention of the Haber Bosch process, and with it, the ability to make fertilizer from atmospheric nitrogen, has greatly increased dissolved nitrogen (nitrate, nitrite, and ammonium) inputs to coastal waters. Because nitrogen is often a limiting nutrient in aquatic systems, the resulting increase in primary production and subsequent organic matter deposition can lead to O2 depletion at depth. This phenomenon, termed eutrophication, can induce harmful algal blooms (HABs) or the development of anoxic “dead zones”. If we are to effectively manage our costal resources, we must be able to understand and predict the impact of this nutrient loading. Such an understanding requires numerical models that can accurately simulate physical, chemical, and biological processes. Unfortunately, a major source of uncertainty in these biogeochemical models is a lack of knowledge as to how the microbial communities that catalyze many biogeochemical processes will respond to environmental perturbations. Two reasons for this are: 1) the lack of an explicit representation of the microbial communities carrying out the relevant processes, and 2) a lack of understanding of the fundamental principles controlling the organization of microbial communities.

My research attempts to address this weakness by seeking to improve the representation of microbial processes in biogeochemical models. This will be investigated by studying nitrogen cycling in coastal sediments in the context of three interrelated themes. Theme I will examine nitrogen cycling dynamics in response to the changes in nutrient and carbon loading in two different coastal systems; 1) Bedford Basin, Halifax NS, and 2) sediments underlying salmonid aquaculture farms along the southern coast of Nova Scotia. Aquaculture operations alter carbon and nutrient inputs to sediments and so provide an ideal “natural laboratory”, to study these effects. Theme II will examine the theory of Maximum Entropy Production (MEP) as a principle of ecosystem organization. This theory from non-equilibrium thermodynamics states that ecosystems will organize to dissipate gradients in free energy by the fastest allowable pathway. Theme III will work on improving the representation of (meta)genomic data in biogeochemical models. Over the last 10 years, the application of genomic sequencing to environmental systems has provided an unprecedented view into the diversity and functional potential of microbial communities, however biogeochemical models are not yet formulated in a way to take advantage of these data.

Through these three themes, this research will strengthen our ability to predict the impacts of coastal nutrient loading, therefore improving our ability to manage our coastal resources.