Submarine Groundwater Discharge (SGD) is described as any and all flow of water on continental margins from the seabed to the coastal ocean. As SGD flows through the aquifer it may undergo diffusion, chemical reactions, dissolution, circulation and mixing. Therefore the chemical signature of the water is greatly modified from that which entered the aquifer as rainfall, runoff or surface water. Groundwater has a great capacity to become contaminated as it flows underground. One such contaminant is carbon (C).
Image 1: Example of SGD at Kinvara Bay. SGD can be seen swirling from the rocks to the ocean.
Carbon can be transported in SGD in both organic and inorganic forms. Dissolved organic matter (DOM) is a complex, heterogeneous mixture of organic molecules and represents one of the largest reservoirs of organic carbon on Earth. It is mainly derived from decaying organisms, such as plants and algae. DOM can be classified according to two main types of compounds: humic-like components and protein-like components. Humic-like components consist of humic and fulvic acids. These are produced from the biodegradation of dead organic matter and are principle components in soil. These can be easily incorporated in SGD as it filters through the earth (allochthonous DOM) and from biodegradation of organic material in the water (autochthonous DOM). Protein-like components are derived from organisms and include amino acids, which may be free or bound in proteins.
It is ubiquitous in aquatic ecosystems and has many important influences. It has been shown to be a driver in microbial loop dynamics and hence, is regenerated in the food web as it is incorporated into microbial biomass and returned to higher trophic levels. It is the main source of energy for heterotrophic bacteria; this produces energy for the ecosystem as a whole. It can have a strong effect on light penetration; therefore, with higher DOM content light may become less available to aquatic organisms influencing biological production. DOM also serves as a media for transporting trace metals and elements. A high concentration of metals increases toxicity and affects pH. The pH in turn affects the alkalinity of waters, which has an impact on the buffering capacity of the water.
However, characterising and quantifying DOM is no easy task. So how is it done?
Image 2: EEMF, equipment used to analyse DOM
Coloured DOM is the optically active portion of DOM which is measured by using fluorescence, which is then inferred to trace and characterise the wider DOM pool. Emission-excitation matrix fluorescence is a technique, which collects emission scans over a range of excitation wavelengths to obtain a large amount of spectroscopic data. This data is interpreted using modelling software in order to determine the relative concentrations of humic-like and protein-like components. This is carried out by using the local maxima and minima of the excitation and emission wavelengths. The actual determination of individual compounds is exceptionally difficult due to the complex nature of the mixture. Relative concentrations of broad fluorescing phenomena are characterised instead. The main peaks, which are determined, are tyrosine-like, tryptophan-like (both amino acids), marine humic-like and humic like. From these peaks, information about the sources (marine or terrestrial) and types (protein-like or humic-like) of DOM present can be inferred.
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