Biogeochemical characterisation of particulate organic matter at sequential stages of transport in suspended, sinking, and benthic fractions

Abstract

The export of particulate organic matter (POM) from the surface ocean to depth forms the basis of the biological carbon pump (BCP). It is important for modulating atmospheric carbon dioxide concentrations, amongst a range of other significant processes. Coastal zones play a significant role in organic matter cycling and burial, and have the capacity to affect a range of important biogeochemical cycles at a global scale. Determining the source and biogeochemical composition of POM, is essential in order to determine its fate; whether POM is recycled in the water column, or exported to depth. POM is heterogeneous in nature with inputs from terrestrial, estuarine, and marine sources. Diverse sources of POM result in a wide spectrum of POM present in the water column, which can be loosely categorised into suspended particulate material (SPM) and sinking fractions. These fractions can be compositionally distinct, and contribute to carbon export to different extents. This thesis addresses key questions surrounding characterisation of POM of these different fractions, determining its origin, and reactivity. The multifaceted approach taken includes detailed micro-phytoplankton community dynamics, molecular-level biogeochemical analysis of POM, and reactivity (using oxygen consumption as a proxy for reactivity) measures across a seasonal cycle and is unprecedented. An intensive sampling campaign was carried out in highly dynamic coastal waters in the Firth of Lorne, western Scotland. Microphytoplankton community composition, biochemical composition, and environmental drivers (wind speed and pycnocline depth) were found to be related to community sedimentation rates. The origin of POM was mixed as indicated by C:N ratio, d13C values, and fatty acid biomarkers. SPM had a larger terrigenous input compared to sediment trap material and sediments. A seasonal shift in SPM source from marine dominated POM in spring, to increasing terrestrial inputs into winter, which corresponded to periods of high rainfall, was observed. SPM was more labile relative to sinking and benthic fractions, and generally concentrations of organic carbon and nitrogen, amino acids, fatty acids, and carbohydrates decreased with depth. The decreasing trend in reactivity observed in SPM and sediment trap material from summer to winter, coincided with the shift in source material, with lowest reactivity occurring when terrigenous inputs were highest. Relationships were found between SPM and sediment trap reactivity, and lability parameters such as amino acids, fatty acids, and carbohydrate concentration. The BCP is complex, and a good understanding of POM characteristics and composition is essential in order to better understand POM cycling and export efficiency. This is especially important given the predicted changes to the BCP as a result of a changing climate.