Energetics

In contrast with streams of temperate and tropical biomes and because of their flood-shaped channel morphology, desert streams generally are not shaded by adjacent riparian vegetation. As a consequence, incidence of photosynthetically active radiation (PAR) reaching desert streams is high, and rates of instream primary production, the process by which energy is captured and organic matter is produced in ecosystems, are among the highest documented for benthic algae. The accrual of algal biomass in turn represents the energetic basis for stream food webs, and is central to the overall ecosystem dynamics of arid streams. For example, the abundance of high-quality benthic algae, together with warm temperatures and selection for rapid growth, result in among the highest rates of secondary production reported for benthic invertebrates. Moreover, owing to high standing stocks and growth rates, invertebrates play an important role in organic matter dynamics and nutrient cycling in desert streams. Indeed, the quantity of organic matter ingested by stream invertebrates can be 2-6 times greater than primary production. Finally, the emergence of desert stream insects represents an important resource for predators in adjacent terrestrial habitats.

- Bare sand

---Diatoms

Cladophora Bluegreens Mat

10 20 30 40 Days after flooding

Figure 4 Recolonization of Sycamore Creek, AZ by primary producers. Redrawn from Fisher SG, Gray LJ, Grimm NB, and Busch DE (1982) Temporal succession in a desert stream ecosystem following a flash flood. Ecological Monographs 52: 93-110.

At the ecosystem level, high rates of algal productivity in desert streams set them apart from streams of forested regions with respect to the relative rates of production (P) and respiration (R). Specifically, desert streams are often autotrophic (P > R). This is in striking contrast to streams of other biomes that receive the bulk of organic matter from outside the stream ecosystem and are often highly heterotrophic (P << R). Productivity of desert streams is also influenced by the disturbance regime. Flash floods scour stream channels, decimate existing organisms, and initiate a suite of algal and macroinvertebrate successional processes that correspond to temporal changes in photosynthesis and respiration (Figure 4). Post-flood recovery of heterotrophs is enhanced by availability of organic matter that was stranded or deposited on the stream margins and in the riparian zone during dry periods.

In addition to metabolic changes associated with flash floods, spatial patterns of photosynthesis and respiration and post-flood successional dynamics are further influenced by hydrologic exchange between hyporheic and parafluvial subsystems and the surface stream. Specifically, rates of photosynthesis and the speed of post-flood recovery are greatest where nutrient-rich water from hyporheic and parafluvial sediments enters the surface stream. Conversely, rates of respiration are greatest where oxygen and organic matter from the surface stream enters subsurface and lateral sediments (Figure 5). When both surface and hyporheic processes are taken into account, desert streams may more closely approximate a balanced metabolism (P = R), which highlights the connection between the two subsystems.

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