Spatial variability of ocean fertilizing nutrients in the dust‐emitting ephemeral river catchments of Namibia

• Published in: Earth surface processes and landforms 2018-03, Vol.43 (3), p.563-578
• Main Authors: Dansie, A.P., Thomas, D.S.G., Wiggs, G.F.S., Munkittrick, K.R.
• Format: Article
• Language: English
• Subjects: Arid zones; Atmospheric particulates; Basins; Bioavailability; Biological fertilization; Catchment area; Catchments; Climate and vegetation; Dust; Dust emission; dust sources; Dust storms; Earth; Earth surface; Eolian dust; Fertility; Fertilization; Fluvial sedimentation; Fluvial sediments; Geology; Geomorphology; Hot spots (geology); Hydrology; Iron; Lake basins; Lakes; Land use; Landforms; Mineral nutrients; Namibian ephemeral rivers; Nitrogen; Nutrient enrichment; Nutrients; ocean fertilization; Oceans; Phosphorus; Precipitation; River catchments; River systems; River valleys; Rivers; Sediment; sediment biogeochemistry; Sediments; Spatial distribution; Spatial variability; Spatial variations; Valleys; Vegetation
• ISSN: 0197-9337; 1096-9837
• DOI: 10.1002/esp.4207

Research into global hot spots of dust emission has focused on exposed fine‐grained sediments in palaeo‐ or ephemeral dryland lake basins including Etosha (Namibia) and Makgadikgadi (Botswana) in southern Africa. Namibia's western ephemeral river valleys are also known to produce dust but have remained largely overlooked as a regionally significant source. Nutrient enrichment of valley sediments and proximity to the South Atlantic suggests aeolian dust could play an important role in ocean fertilization. The fertility of valley dust is dependent on fluvial sediments originating in the upper catchments on the Southern African Central Plateau. In this study we investigate climate, geology, vegetation and land use variability and how these may influence the nitrogen, phosphorus and iron availability in the catchments. We intensely sampled the Huab, Kuiseb and Tsauchab river systems to map the spatial distribution of nutrients from upper catchments to river termini. Samples were analysed for the bioavailable fractions of iron, nitrogen and phosphorus as well as total nitrogen and phosphorus. Results show that the lower valley reaches are sources of aeolian dust enriched in nutrients. Nitrogen levels correlate with precipitation and vegetation levels and phosphorus levels with geology. However, differences in upper catchment sediment nutrient levels were not representative of downstream nutrient differences between valleys. Rather, it is the hydrological and geomorphological processes of the ephemeral river systems that are key for producing the enriched sediments in the lower reaches. We demonstrate that the ephemeral river valleys of western Namibia are an extensive and enriched source of mineral dust that could play a critical role in marine productivity of the southern Atlantic. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Surface sediments in the dust‐emitting regions of Namibia's ephemeral river valleys, within the hyper‐arid Namib Desert, are shown to have the highest nutrient concentrations found anywhere in the catchments. These nutrient‐enriched lower‐valley sediments occur irrespective of upper catchment climate gradient, geology, vegetation, land use and cattle stocking densities. These riverine dust sources are considered to play a significant, and overlooked, contribution to ocean productivity as may geomorphologically‐similar systems worldwide.