Biogeochemical dynamics of a tropical river-floodplain system
Inland waters are increasingly recognized for their potential to alter the luxes of organic carbon from terrestrial ecosystems to the oceans. Rivers are considered net heterotrophic systems, with large quantities of carbon dioxide emitted to the atmosphere. In the global budgets, tropical rivers are of interest because of the large quantities of terrestrial organic matter they transport and the amounts of greenhouse gases that are being emitted to the atmosphere. Riverine biogeochemistry, especially in tropical rivers with seasonal rainfall patterns, is strongly impacted by interactions of the river with its loodplains. Once a lood pulse spills onto low-lying riparian areas, the looded loodplain develops distinctly ecological functions that are distinctly different from the bordering river and the terrestrial ecosystems.
In this thesis the biogeochemical functioning of tropical river-loodplain systems was examined, with the largely pristine Barotse Plains along the Zambezi River as the main case study. The research focused on understanding the seasonal patterns of organic matter cycling, linking hydrology and biogeochemical processes, and tracing the origin of riverine organic matter. Considering the increasing anthropogenic pressure on inland waters, it is imperative to understand the impact of dam construction on tropical luvial biogeochemistry, as dam construction is expected to boom in Africa, South America and Asia. To this end, the biogeochemistry of the Barotse Plains was compared with that of the dam-impacted Kafue Flats. To capture the dynamics of the system, large scale ield campaigns were complemented with long-term sensor deployment. Riverine organic matter composition was characterized with regard to carbon and nitrogen content, isotopic compositions, radiocarbon content, and biomarkers.
Particulate and dissolved organic matter in the Barotse Plains exhibited different seasonal trends: maximum loads of particulate organic matter occurred during the initial phases of looding, while dissolved organic matter loads peaked at the same time as peak low. During an annual cycle, particles and associated phosphorus were eroded from the loodplain, while organic matter was retained, with varying dynamics between seasons. The seasonal trends are strongly linked 2 to the balance of primary production and degradation on the loodplain and in the river, with degradation dominating during initial looding, primary production dominating during continued and maximum looding, and more degradation during receding water levels.
The interaction between the river and the loodplain is a driving force for riverine biogeochemistry. A mass balance approach showed that during peak low, roughly half of the downstream discharge had spent time on the loodplain. Furthermore, distinct hysteresis patterns were observed between biogeochemical parameters and both discharge and travel time of the lood pulse over the loodplain. At longer travel times, more degradation of organic matter occurred on the loodplain, making the loodplain a stronger hotspot for greenhouse gas emissions.
Particulate organic matter in the river carried a distinct soil-derived signature year-round, but the input from vegetation showed distinct seasonal variations, as shown using a variety of isotopic and biomarker analyses. During low-water conditions, organic matter was mostly derived from terrestrial vegetation and mineral soils. In contrast, when the loodplain was inundated the riverine organic matter showed large contributions from topsoil and loodplaine gvetation.
The dams in the Zambezi catchment trap particles and organic matter and release easily degradable aquatic biomass, changing the quantity and signature of the downstream organic matter. Nutrients are retained in the reservoir, and the timing of and ratio in which nitrogen and phosphorus become available to downstream ecosystems change. The altered hydrograph also changes looding dynamics of downstream ecosystems, which can lead to encroachment of woody plants onto the loodplain and altered biogeochemical functioning of the loodplain.
The results in this thesis highlight the strong seasonal dynamics of tropical riverloodplain systems, and how the biogeochemistry is heavily linked to the hydrology of the system. Understanding these interactions between river, loodplain, and vegetation is crucial in light of changing climate and altered water balances.