An ephemeral perspective of fluvial ecosystems: viewing ephemeral rivers in the context of current lotic ecology.
Jacobson, Peter James
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Hydrologic and material dynamics of ephemeral rivers were investigated in the Namib Desert to assess how hydrologic regimes shape the physical habitat template of these river ecosystems. An analysis of long-term hydrologic records revealed that the variation in mean annual runoff and peak discharge were nearly four times higher than the global average, rendering the rivers among the most variable fluvial systems yet described. Further, a pronounced downstream hydrologic decay characterized all of the rivers. The high spatio-temporal variability in flow was reflected in patterns of material transport. Retention of woody debris increased downstream, in contrast to patterns typically reported from more mesic systems, largely attributable to hydrologic decay. Woody debris piles were the principal retentive obstacles and played an important role in channel dynamics. They were also key microhabitats for various organisms, forming "hotspots" of heterotrophic activity analogous to patterns reported from perennial streams. Large amounts of fine particulate and dissolved organic matter (FPOM and DOM) deposited in the lower reaches of the rivers serve to fuel this heterotrophic biota. As a result of the hydrologic decay, sediment concentration (both organic and inorganic) increased downstream and the lower reaches of these rivers acted as sinks for material exported from their catchments. FPOM and DOM concentrations were among the highest reported for any aquatic system, and, contrary to patterns reported from more mesic systems, FPOM dominated the total organic load transported in these rivers. Inorganic solute concentration also increased downstream, resulting in a downstream increase in soluble salt content in floodplain soils. Soils within the river's lower reaches served as effective long-term integrators of hydrologic variability. The mean extent of floods entering the lower river was defined by an alluviation zone, evident from the convexity exhibited in the lower section of the rivers' longitudinal profiles. A downstream increase in the proportion of silt within floodplain soils is associated with increased sediment deposition. Silt deposition had a positive influence on moisture availability, plant rooting, and habitat suitability for various organisms, including fungi and invertebrates. In addition, a strong positive correlation was observed between silt, organic matter, and macronutrients. Thus, the hydrologic control of transport and deposition patterns has important implications for the structure and function of ephemeral river ecosystems. Finally, an examination of the influence of elephants upon riverine vegetation highlighted the importance of these systems as isolated resource patches interspersed in an arid and hostile landscape. Further, it illustrated that flooding was a key ecological process and that hydrologic alterations would affect the fluvial ecosystem as well as the regional landscape they drain.
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