Organic carbon and nitrogen export from a tropical dam-impacted floodplain system

[摘要] Tropical floodplains play an important role in organic matter transport,storage, and transformation between headwaters and oceans. However, thefluxes and quality of organic carbon (OC) and organic nitrogen (ON) intropical river-floodplain systems are not well constrained. We explored thequantity and characteristics of dissolved and particulate organic matter(DOM and POM, respectively) in the Kafue River flowing through the KafueFlats (Zambia), a tropical river-floodplain system in the Zambezi Riverbasin. During the flooding season, > 80% of the Kafue Riverwater passed through the floodplain, mobilizing large quantities of OC andON, which resulted in a net export of 69–119 kg OC km⁻² d⁻¹ and3.8–4.7 kg ON km⁻² d⁻¹, 80% of which was in the dissolved form.The elemental C : N ratio of ~ 20, the comparatively highδ¹³C values of −25‰ to−21‰, and its spectroscopic properties(excitation-emission matrices) showed that DOM in the river was mainly ofterrestrial origin. Despite a threefold increase in OC loads due to inputsfrom the floodplain, the characteristics of the riverine DOM remainedrelatively constant along the sampled 410-km river reach. This suggests thatfloodplain DOM displayed properties similar to those of DOM leaving theupstream reservoir and implied that the DOM produced in the reservoir wasrelatively short-lived. In contrast, the particulate fraction was¹³C-depleted (−29‰) and had a C : N ratio of~ 8, which indicated that POM originated from phytoplanktonproduction in the reservoir and in the floodplain, rather than from plantdebris or resuspended sediments. While the upstream dam had little effect onthe DOM pool, terrestrial particles were retained, and POM from algal andmicrobial sources was released to the river. A nitrogen mass balance overthe 2200 km² flooded area revealed an annual deficit of 15 500–22 100 t Nin the Kafue Flats. The N isotope budget suggests that these N losses arebalanced by intense N-fixation. Our study shows that the Kafue Flats are asignificant local source of OC and ON to downstream ecosystems andillustrates how the composition of riverine OM can be altered by dams andfloodplains in tropical catchments.

[学科分类] 地球化学与岩石