Particulate organic carbon dynamics along the land–ocean aquatic continuum
Date: 18 November 2019
University of Exeter
PhD in Physical Geography
The anthropogenic and natural input and transport of particulate organic carbon (POC) into and along stream systems from land to ocean is not yet well described (Regnier et al, 2013). Accelerated soil erosion due to changes in land use (e.g. expansion and intensification of agriculture) contribute to POC transport from land to ocean, ...
The anthropogenic and natural input and transport of particulate organic carbon (POC) into and along stream systems from land to ocean is not yet well described (Regnier et al, 2013). Accelerated soil erosion due to changes in land use (e.g. expansion and intensification of agriculture) contribute to POC transport from land to ocean, while artificial damming of river systems, such as hydropower, trap the suspended particulate fractions and contribute to limnic storage in reservoirs. These perturbations on the landscape impact the volumes, transport and fate of suspended organic material. While mainly geomorphic processes control erosion and deposition of inorganic sediments, the mobility and transformation of POC is further subject to within-catchment stabilisation processes, sequestration by burial into sediments, and microbial fermentation which contribute to the vertical emission of potent greenhouse gases like CO2 and CH4. These sources, sinks and transformations of POC along the land-ocean aquatic continuum (LOAC) are dynamic and depend on input, transport, and deposition of suspended organic materials. Particulate organic carbon dynamics has been studied broadly in the past, both in freshwater systems, estuaries and oceans. However, its role in the global carbon cycle has been highlighted especially in recent years due to the lack of quantifiable pathways of POC sequestration, transport and mineralisation in freshwater systems. This project studied POC in two contexts: 1) POC transport and fate in a tropical reservoir was modelled and measured, and 2) the effects of flocculation on riverine DOM was measured by experiments. In the first study, erosion rates were calculated from fallout radionuclides (FRN) in a highly perturbed and largely deforested tropical catchment in Brazil. The results from these field observations were used to quantify soil erosion and sedimentation in the catchment. The results also validated modelled output with Revised Universal Soil Loss Equation (RUSLE) which was used to model soil erosion for the whole catchment. The contribution of allochthonous organic carbon from catchment soils into the reservoir was then quantified. The second study utilized the same field site, and addressed the relative inputs of allochthonous and autochthonous POC into the reservoir. Physical and chemical protection of organo-mineral aggregates and flocs determine the fate of catchment-derived POC. This second part of the study investigated whether allochthonous POC derived from terrestrial soils was more important than autochthonous POC (e.g. from macrophytes, biofilms and algae) for limnic storage in post-flooding sediments of a tropical reservoir. POC was calculated from C inventories of reservoir sediment cores collected along the delta–reservoir gradient and linked to organic matter content by loss on ignition (LOI). POC sources were determined by analysis of carbon-nitrogen ratio (C/N). Finally a POC budget was calculated for the post-flooding reservoir sediments. The third part of this project investigated flocculation dynamics of riverine organic matter in the LOAC. Flocculation experiments were undertaken to determine the effects of three treatments with coagulants on water DOM quality from eight streams draining a rural landscape with headwaters in Exmoor, UK. Through flocculation experiments stream samples were reacted with added clay and salt standards which simulated three flocculation boundaries along the LOAC. The three treatments were T1: increased input of minerogenic coagulants, such as clay, by soil erosion into streams, T2: saline mixing at the estuarine boundary, and T3: a mixture of salt and clay representing soil erosion at the estuarine boundary. Residual DOM quality by each treatment was analysed by mass spectrometry, UV-Vis absorbance and synchronous fluorescence. The results showed preferential removal of humic-like components in residual DOM and the efficiency of the chosen coagulant standards, which demonstrated the dynamics of chemical change in riverine organic matter along the LOAC.
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