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Environmental Controls on Marine Particulate C:N:P Ratios


Elemental ratios of particulate organic matter (POM) are key to linking biogeochemical cycles. Microbial uptake and allocation of essential biogenic elements (carbon (C), nitrogen (N), and phosphorus (P)) influence the distribution of nutrients throughout the ocean. This dissertation evaluates the role of environmental stress and underlying phytoplankton diversity in driving regional variation in the ratio of particulate C:N:P. Competing hypotheses predict C:N:P equally well due to regional co-variance in environmental conditions and biodiversity. The Indian Ocean offers a unique positive temperature and nutrient supply relationship to test these hypotheses. We collected 248 POC:N:P observations in the eastern Indian Ocean along this environmental gradient. As phytoplankton community composition was constant, biodiversity changes could not explain the elemental variation. Instead, our data supports the nutrient supply hypothesis over the influence of temperature.

Nutrients concentrations are often below detection limits in subtropical ocean regions. We develop two methods to predict nutrient stress to further evaluate its role in particulate C:P regulation. In the first method, we develop a global remote sensing estimate of surface phosphate. Using a mechanistic framework, we develop an artificial neural network to provide a robust basis for developing a remote sensing estimation of surface phosphate. However, C:P predictions using only phosphate did not match observations in either the South Indian or Pacific subtropical gyres. To address this challenge, we develop a second method by applying genomic shifts among microbial communities as ‘biosensors’ for the in situ nutritional environment. We find that our genome-based trait-model significantly improves our prediction of particulate C:P across ocean regions. Furthermore, we detect previously unrecognized ocean areas of iron, nitrogen, and phosphorus stress. Ultimately, we find a combination of nutrient stress accounts for global variation in particulate C:P.

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