Do Organics Matter?

Our previous post ‘What puts the C in SGD?’ described the process by which dissolved organic matter (DOM) transported to the coastal ocean via submarine groundwater discharge (SGD) is characterised using excitation-emission matrix fluorescence (EEMF). EEMF collects emission scans over a wide range of excitation wavelengths (measured in nanometers, nm). This information is then used to group compounds according to their characteristic fluorescence patterns and thus, infer the composition of DOM in the sample.

PARAFAC is a mathematical modelling tool used to visualise the excitation-emission matrices (EEMs) and determine the main groups of compounds found in the sample set. This technique identifies the most common components and models these. The main groups of components are divided into humic-like (humic and fulvic acids, common in soils) and protein-like (amino acids, either free or bound in proteins) and can be marine or terrestrial in nature.

kinvara
Submarine groundwater discharge point in Kinvara Bay

The below model was produced for samples taken in Kinvara, Co. Galway (The Hidden Depths of Kinvara: Underground Rivers). This karstic aquifer represents an ideal study site for analysing SGD as discharge is focused in Kinvara Bay, and can actually be seen discharging from the land at low tide. Due to limited surface drainage, SGD studies are particularly important as it transports large concentrations of nutrients and carbon to the coastal ocean.

160 samples from a range of sources, including marine, the bay and the catchment area were collected, processed and then analysed. The below contour plots are produced which highlight the excitation and emission maxima, which in turn are used to determine the type of components present. The colours represent the intensity of fluorescence, which in turn can be used to determine relative concentrations of components to one another. For quick interpretations, the left-hand side of the plot represents marine sources, and the right-hand side represents terrestrial sources. Furthermore, components near the front of the plot represent labile material (easily broken down), and components near the back represent recalcitrant material (resistant to breakdown).

kelly fig2
An EEM and surface plot of component 1 of the PARAFAC model. The position of the main peak (left) is characteristic of a marine source. The smaller peak represents a mixture of marine and terrestrial sources (due to mixing of groundwater and seawater in the estuary at the discharge point).
kelly fig4
Component 2 of the PARAFAC model. This peak location (right) is characteristic of terrestrial humic-like composition, again, containing a smaller peak representing a mixture of terrestrial and marine sources.
kelly fig6
Component 3 of the PARAFAC model is characteristic of a marine source. The peak to the front is not represented as a component but is characteristic of labile material and will need to be studied further.

EEMF-PARAFAC is a useful tool in characterising SGD-derived DOM as it allows us to determine sources (marine vs terrestrial), types (humic-like vs protein-like, labile vs non-labile) and relative concentrations of DOM components. This helps us to understand important biogeochemical processes occurring in the groundwater throughout the region.

Authored by Tara KellyBiogeochemistry Research Group, Geography, TCD

 

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