Desert streams

Desert streams are found in arid and semiarid regions, defined by low annual precipitation. They are characterized by a variable hydrologic regime with both flash floods and periods of drying that vary in recurrence and severity from diel to century scales. Biota inhabiting desert streams show adaptation to both high discharge and drying in their behavior (such as burrowing or emigration) and in physiology (such as osmoregulatory solutes), or morphology (such as buttressed fins). Stream biota rapidly recolonize following floods and are highly productive due to high light availability and, in hot deserts, high temperatures. Under these conditions, primary productivity often exceeds respiration, resulting in autotrophic ecosystems that may be net exporters of organic material. Nutrient dynamics of desert streams are strongly influenced by surface water-groundwater interactions both within the stream channel and between the stream and riparian zone. Transformations of nutrients occur rapidly in stream sediments which are colonized by microbiota, and variable rates of processing result in spatial patchiness in nutrient availability in the water column. Nutrients and organic matter may accumulate along stream margins or in riparian zones during dry periods and become available following high flows, resulting in large temporal variability in nutrient dynamics with consequences for recolonization of stream biota following high-flow events. Desert streams are particularly threatened by human appropriation of water to support agricultural expansion and burgeoning cities in arid regions worldwide.