Using optical tools to improve understanding of dissolved organic carbon fluxes along the land-ocean aquatic continuum

Abstract

The transport of terrigenous dissolved organic matter (tDOM) from land to sea is a significant and changing component of the global carbon cycle. This thesis uses optical tools (absorbance and fluorescence spectrometry) to investigate the composition and fate of tDOM across a range of aquatic settings. Results show that agricultural expansion within the Belize River Watershed has increased tDOM overlying an adjacent portion of the world’s second largest barrier reef system, with the potential for deleterious effects. A substantial decoupling of ‘coloured’ DOM (cDOM) and dissolved organic carbon (DOC) is identified, likley the result of a sizeable optically ‘invisible’ DOC fraction (iDOC). In the same watershed, iDOC was found to represent 52 % of the total DOC pool. The molecular properties which render iDOC optically invisible should, in theory, render it more bioavailable than cDOC, but cDOC was positively correlated with bioavailable DOC (BDOC) and biodegradation rate (k) whereas iDOC was positively correlated with refractory DOC (RDOC). This suggests that it is cDOC rather than iDOC which drives biodegradation rates in this system. A new method is proposed with which to further partition the cDOC fraction according to its fluorescent properties. This method is demonstrated within the Belize River Watershed, where a traditional approach whereby changes in cDOM fluorescence were extrapolated to the DOC pool was deemed appropriate for the qualitative assessment of relative concentration shifts, but yielded unrealistic results in terms of the contribution of each fraction to the DOC pool. In Great Britain (GB), iDOC accounted for 21 % of the annual riverine flux (0.23 Tg C yr-1) and was primarily explained by rainfall, base flow index, and dairy cattle density. These results were placed in a wider context using a collated data set of ~ 3,000 samples obtained from a wide range of aquatic, geographic, and climatic settings across five continents where, on average, iDOC represented 26 % of the DOC pool (range = 0 – 97%). cDOC and iDOC were estimated across 12 GB estuaries at quarterly intervals (n = 60 transects) to investigate the influence of each fraction on estuarine DOC export, and cDOC transport was almost universally conservative whereas iDOC transport was almost universally non-conservative. Shifts in iDOC concentration therefore appear to be the dominant control over bulk DOC behaviour within these estuaries. The overarching conclusion of this thesis is that iDOC is a globally significant carbon pool, the broad-scale importance of which has yet to be studied in earnest. Large uncertainties exist with regards the source, behaviour, and fate of iDOC. It is critical that these uncertainties are addressed in order to better constrain the land-ocean carbon flux and, more broadly, the global carbon cycle.

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Identifiers

ORCID for Stacey Louise Felgate: orcid.org/0000-0002-9955-4948

ORCID for Jasmin Godbold: orcid.org/0000-0001-5558-8188

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