Impact of terrestrial runoff on organic matter, trophic state, and phytoplankton in a tropical, upland reservoir
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The impact of organic matter inputs from agricultural, forest and domestic sources on aquatic processes has been considerably less studied in tropical reservoirs relative to temperate systems despite the high number of these small aquatic systems in the tropics. Here we present the results of an in situ mesocosm study that examined the impact of allochthonous organic matter on a headwater reservoir in Northern Vietnam. We examined the impact of wastewater and soils from floodplain paddies, Acacia mangium plantations and from upland slopes on the metabolic status of the reservoir. The addition of floodplain paddy soils to the reservoir water led to a rapid switch in metabolic status from net autotrophic to net heterotrophic. In contrast, the addition of wastewater in low concentrations had less impact on the metabolic status of the reservoir, reflecting the low population density in the area. The addition of floodplain paddy soils also increased phytoplankton diversity and evenness relative to the control. In summary, soils from floodplain paddies and from A. mangium plantations had the highest impact on the reservoir, with upland soils and wastewater having less of an impact. We also found that primary production in this reservoir was nitrogen limited. In order to avoid accelerating the impact of runoff on the reservoir, future management options should perhaps focus on minimizing water and sediment runoff from upstream paddy fields and from A. mangium plantations. These results also underline the importance of studying these upland tropical water bodies that can contribute an important but, on the whole, ignored part of the global carbon balance.
KeywordsLimiting factor Aquatic mesocosm Incubation Vietnam
This research was funded by the LOTUS N° 44/2012/HĐ-NĐT and the NAFOSTED No. 104.99-2014.41, Ministry of Science and Technology, Vietnam (to Trinh Anh Duc); PHC Hoa Sen Lotus N° 23970QM (to Emma Rochelle-Newall), JEAI BioGEAQ (to Trinh Anh Duc, Didier Orange and IRD), the Institute of Chemistry, Vietnam Academy of Science and Technology, Vietnam, the French Institut de Recherche pour le Développement (IRD) and the UMR iEES-Paris. The MSEC (Multi-Scale Environmental Changes; http://web2.obs-mip.fr/msec/) network is thanked for providing meteorological data. The lead author is also grateful to the PANACEA Erasmus Mundus Program for support in writing this paper.
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