The role of fog in the ecohydrology and biogeochemistry of the Namib Desert Keir Stephen Soderberg Germantown, MD MSc, University of Cape Town, 2003 BSE, Princeton University, 2000 A Dissertation presented to the Graduate Faculty of the University of Virginia in Candidacy for the Degree of Doctor of Philosophy Department of Environmental Sciences University of Virginia May, 2010 Dr. Stephen A. Macko Dr. Robert J. Swap Dr. Paolo D’Odorico Dr. P. Paxton Marshall ii Abstract The Namib Desert on the west coast of southern Africa is hyper-arid in terms of rainfall, but receives up to 100 days of fog each year. This project adds to the understanding of the occurrence of fog in the Namib, the importance of fog as a water source for this ecosystem, and the role of fog in carbon, nitrogen and sulfur cycles. Stable isotopes (δ2H, δ18O, δ34S) of fog, groundwater, plant water and soil water (collected from 2007 to 2009) indicate that plants growing in or near the ephemeral Kuiseb River have a distinctly depleted water isotope signal, similar to groundwater in the alluvial aquifer. In contrast, shrubs and grasses growing on the dunes and gravel plains tend to have an enriched signal, representing a minimum of 5 to 40% fog water usage in these plants. Notably, the very long-lived gymnosperm Welwitschia mirabilis, has a depleted water isotope signal, statistically similar to the trees growing in the Kuiseb River. This finding is consistent with long-term observations associating growth with annual rainfall. Together these studies can dispel the popular perception that W. mirabilis survives in the desert via fog water uptake, which is significant because this plant is important as a national symbol and potentially threatened by proposed uranium and copper mines. The δ34S of plant material tends to be more enriched in plants that use more fog water, providing a first-order corroboration of the water isotope results. W. mirabilis has significantly more enriched δ13C values in lower rainfall areas, but also exhibits high variability in δ13C (range of 4 ‰) within a single community, suggesting the influence of microhabitat on photosynthesis. The δ15N of C3 plants becomes more depleted closer to the coast in an apparent reversal of the trend found across the Kalahari. iii Table of Contents Abstract ............................................................................................................................. ii Table of Contents ............................................................................................................. iii Figures .............................................................................................................................. vii Tables ................................................................................................................................ ix Acknowledgements ........................................................................................................... x Chapter 1 : Introduction ................................................................................................ 11 1.1 Water in the Namib Desert ..................................................................................... 11 1.2 Water demands in the Namib .................................................................................. 16 1.3 Quantifying fog input .............................................................................................. 17 1.4 Microenvironmental controls on photosynthesis .................................................... 26 1.5 Fog and nutrient cycling ......................................................................................... 26 1.5 Hypotheses .............................................................................................................. 28 1.6 Objectives ............................................................................................................... 29 References ..................................................................................................................... 31 Chapter 2 : Methods ....................................................................................................... 36 2.1 Ground-based measurements .................................................................................. 36 2.2 Climate model predictions ...................................................................................... 41 2.3 Sampling – water .................................................................................................... 41 2.4 Sampling – plants and soils..................................................................................... 42 2.5 Sampling – aerosols ................................................................................................ 43 2.6 Analysis – plants and soils ...................................................................................... 44 2.7 Analysis – water samples ........................................................................................ 47 2.8 Analysis – aerosols ................................................................................................. 48 References ..................................................................................................................... 51 Chapter 3 : Precipitation, soil moisture and climate change in the Namib Desert ... 53 Abstract ......................................................................................................................... 53 iv 3.1 Introduction ............................................................................................................. 54 Fog and dew in ecosystems....................................................................................... 55 Fog and climate change ............................................................................................ 57 3.2 Methods................................................................................................................... 58 Fog observation and monitoring ............................................................................... 58 Climate model data ................................................................................................... 61 3.3 Results ..................................................................................................................... 62 Rain, fog and dew deposition.................................................................................... 62 Soil moisture ............................................................................................................. 66 Fog and relative humidity ......................................................................................... 70 Climate model data ................................................................................................... 72 3.4 Discussion ............................................................................................................... 73 Rain, fog, dew and hours of wetness ........................................................................ 73 Soil moisture ............................................................................................................. 75 Fog, relative humidity and climate change ............................................................... 75 3.5 Conclusions ............................................................................................................. 77 References ..................................................................................................................... 78 Chapter 4 : Fog as a supplemental water source for plants in the Central Namib Desert ............................................................................................................................... 80 Abstract ......................................................................................................................... 80 4.1 Introduction ............................................................................................................. 81 Water in the Namib ................................................................................................... 81 Stable isotopes in fog studies .................................................................................... 83 4.2 Methods................................................................................................................... 85 Study site and plant species ...................................................................................... 85 Sampling and analysis ............................................................................................... 86 4.3 Results ..................................................................................................................... 88 Water and soil samples - δ2H and δ18O ..................................................................... 88 Plant samples - δ2H and δ18O .................................................................................... 89 v Sulfur isotopes .......................................................................................................... 93 4.4 Discussion ............................................................................................................... 95 4.5 Conclusions ........................................................................................................... 103 References ................................................................................................................... 103