Publication Date

2019-07-03

Availability

Open access

Embargo Period

2019-07-03

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Marine and Atmospheric Chemistry (Marine)

Date of Defense

2019-06-07

First Committee Member

Dennis A. Hansell

Second Committee Member

Kimberly J. Popendorf

Third Committee Member

Jingfeng Wu

Fourth Committee Member

Larry Brand

Abstract

The cycling of bioactive elements on Earth and their allocation into major reservoirs is driven through their biogeochemical transformations. Carbon, for example, is biologically transformed by primary producers that convert CO2 to organic carbon during photosynthesis, which is then respired back to the gaseous form by heterotrophs. The cycling of other elements such as nitrogen, phosphorus and iron, considered essential nutrients for living organisms, are directly linked to carbon through the formation of organic matter resulting from primary production. In the ocean, a portion of organic carbon resists immediate degradation in the euphotic layer and can be exported to the deep layers to be respired far away from surface waters, thus contributing to the development of an oceanic sink for atmospheric CO2. However, controls on the production of resistant fractions with potential for export are both unresolved and the subject of this dissertation. The first chapter contains a general introduction to organic carbon dynamics in the ocean, its state-of-the-art, and the itemized objectives of this dissertation. In the second chapter, I use incubations of natural microbial populations to demonstrate that the production of a resistant dissolved organic carbon (DOC) fraction is controlled by the initial availability of nutrients to the microbial community. In the third chapter, I apply this concept to the upper northeast Pacific Ocean, where seasonal measurements of organic and inorganic parameters were taken from multiple cruises carried out between 2017 and 2018. I show that organic carbon production and accumulation in the region is seasonally variable: in the wintertime, vertical mixing brings inorganic nutrients to the euphotic layer that fuel organic carbon production in spring and summer, with a variable fraction accumulating as DOC. For the same region, in the fourth chapter, I present a novel application of Bio-Argo float (i.e., BGC float) data to estimate net organic carbon production with high temporal resolution, with ability to differentiate total and dissolved organic carbon fractions. The method was applied to Bio-Argo data collected from 2009 to 2018, and improved temporal resolution of net organic carbon production estimations without the need of ship-based sampling. The highest and lowest production happened during the two consecutive summers of 2014 and 2015, respectively, and controls on the variability were investigated. Using Bio-Argo float and historical data from the region, I show that warm events driven by ocean-atmospheric oscillations, occurring between 2013 and 2016, restricted winter mixing during two consecutive years. The enhanced stratification decreased the nutrient stocks in the upper ocean, which greatly reduced further organic carbon production and, thus, its prospects for export to the deep ocean. The last section of this dissertation is an Appendix that contains preliminary results from an incubation experiment. The study aimed to evaluate the preferential consumption of particulate versus dissolved organic carbon (POC versus DOC, respectively) by surface ocean microbes. The incubation outcome indicates that chemical compounds from the POC fraction are remineralized faster than those comprising the DOC fraction. Although the result is preliminary, it provides insights on the preferential consumption of each organic carbon pool once they are freshly released to the environment.

Keywords

dissolved organic matter; marine biogeochemistry; net community production; warm events; el niño; dissolved organic carbon; dissolved organic nitrogen; carbon cycle

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