Water level fluctuations in a tropical reservoir: the impact of sediment drying, aquatic macrophyte dieback, and oxygen availability on phosphorus mobilization
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Reservoirs in semi-arid areas are subject to water level fluctuations (WLF) that alter biogeochemical processes in the sediment. We hypothesized that wet–dry cycles may cause internal eutrophication in such systems when they affect densely vegetated shallow areas. To assess the impact of WLF on phosphorus (P) mobilization and benthic P cycling of iron-rich sediments, we tested the effects of (i) sediment drying and rewetting, (ii) the impact of organic matter availability in the form of dried Brazilian Waterweed (Egeria densa), and (iii) alternating redox conditions in the surface water. In principle, drying led to increased P release after rewetting both in plant-free and in plant-amended sediments. Highest P mobilization was recorded in plant amendments under oxygen-free conditions. After re-establishment of aerobic conditions, P concentrations in surface water decreased substantially owing to P retention by sediments. In desiccated and re-inundated sediments, P retention decreased by up to 30 % compared to constantly inundated sediments. We showed that WLF may trigger biochemical interactions conducive to anaerobic P release. Thereby, E. densa showed high P release and even P uptake that was redox-controlled and superimposed sedimentary P cycling. Macrophytes play an important role in the uptake of P from the water but may be also a significant source of P in wet–dry cycles. We estimated a potential for the abrupt release of soluble reactive phosphorus (SRP) by E. densa of 0.09–0.13 g SRP per m2 after each wet–dry cycle. Released SRP may exceed critical P limits for eutrophication, provoking usage restrictions. Our results have implications for management of reservoirs in semi-arid regions affected by WLF.
KeywordsWater level change Egeria densa Nutrient cycling Eutrophication Water management Semi-arid Itaparica reservoir
The authors would like to thank S. Calado for her support during several sampling campaigns in Brazil. We are grateful to F. Selge, D. Lima, and M. Rodriguez for their help with field work. Our colleagues H.-J. Exner, C. Herzog, A. Lüder, S. Jordan, and M. Uber helped with technical support. We also thank H. Nottebrock, G. Gunkel, and two anonymous reviewers for critical comments and for improving the manuscript. This study was funded by the Federal Ministry of Education and Research (BMBF) within the project “INNOVATE” (01LL0904C).
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Conflict of interest
The authors declare that they have no competing interests.
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