Desert streams are highly variable over time, at a range of temporal scales. In addition to the seasonality that typifies many streams, temporal dynamics of desert streams are strongly influenced by disturbances at two extremes (flash flooding and drying) of a hydrologic spectrum. Researchers have largely focused on temporal dynamics at decadal and lower scales; however, decade- to century-scale channel change establishes a geomorphic template upon which these higher-frequency dynamics play out. Our discussion will consider temporal change from low-to high-frequency events (Figure 2).
The concept ofdisturbance has various meanings, but in stream ecology disturbance is usually associated with hydrologic extremes that change ecosystem structure and processes. Using terms from disturbance ecology, we can a o <n o a. E œ
Climate change Climate variability
Succession life cycle
Particle Subreach Section
Assemblage Reach Catchment Spatial scale (log km)
Particle Subreach Section
Assemblage Reach Catchment Spatial scale (log km)
Figure 2 Temporal scales considered in this section (ordinate) are correlated with spatial scales (abscissa), and each phenomenon discussed is associated with a characteristic range of time and space scales.
characterize a disturbance regime, which has features such as interannual (or interdecadal) variability, seasonality, and timing, frequency, and magnitude of individual events. Disturbance is intimately connected to succession, which is most simply defined as the change in ecosystem properties on a site following disturbance. Ecosystem components that are affected by a disturbance are those that undergo succession after the disturbance; for example, a flash flood that removes algae and invertebrates but does not affect streamside vegetation initiates succession in the stream but not in the riparian zone.
Successional patterns depict the temporal changes in stream and riparian communities and processes that are superimposed upon a larger temporal scale of variability. For longer-lived riparian vegetation, successional patterns and time frames may be similar to those of terrestrial communities but for stream biota, succession often plays out against a seasonal backdrop. Thus, successional patterns differ between seasons, with faster increases in biomass during warmer months. Successional patterns also vary depending upon the size and nature of the initiating disturbance as well as antecedent conditions (which are themselves influenced by the disturbance regime -timing and clustering of individual events). Whereas disturbances that occur over short timescales may produce predictable recovery sequences, biota recover from longer-term, infrequent disturbances with less regularity. Effects on biota of pronounced interannual variation that characterizes deserts include shifts in community composition of invertebrates and differences in the relative importance of nitrogen-fixing cyanobacteria versus nonfixer algae in the primary producer assemblages.
Flash flooding and drying are the primary disturbances that characterize desert streams. Flood magnitude is usually described by the peak discharge, but other aspects of a flash flood hydrograph - the steepness of its rise and the length of its tail - also determine flood effects. Floods are important geomorphic agents, shaping channel form, as well as disturbances that initiate biotic succession. In deserts, floods connect elements of the landscape, from ridgetops to large rivers to groundwater, which are otherwise isolated and disconnected. Drying is at the opposite hydrologic extreme but is more difficult to treat as a discrete disturbance because it represents a protracted reduction and ultimately loss of stream flow. As drying progresses, there is first a concentration of mobile biota that precedes a concentration of dissolved materials (through evaporation); there may be isolation of sections of a stream and distinctive patterns of surface-water loss; direction of surface-subsurface water exchange may flip; organisms may move into sediments; and, ultimately, surface flow is lost entirely. Drying ends when surface flow resumes, either during a flood or as a gradual increase in discharge.
At the scale of centuries, events that occur only every 50-100 years or so can shape channel form and initiate riparian succession. For example, in the southwestern USA, a period of erosion occurred forming arroyos or gullies and draining the riverine wetlands that were once characteristic of these desert environments. This period left a geomorphic structure that persists today in many southwestern river-riparian ecosystems. Dramatic changes such as these can affect groundwater-surface water interactions and change species composition of the riparian vegetation. Indeed, such large-scale changes have repercussions for many stream characteristics, underscoring the importance of the hydrogeomorphic template in establishing structure and function of stream-riparian ecosystems.
Decadal variability resulting in relatively wet and dry periods in the southwestern USA is related to quasi-cycles of the Pacific Decadal Oscillation and El Niño Southern Oscillation (ENSO). For the southwestern USA, a strong ENSO signal is seen in decadal patterns of winter runoff from the Puerco and Grande Rivers in New Mexico and Sycamore Creek in Arizona. During wetter periods, frequent high-discharge events remove active-channel vegetation, leaving open gravel bars (parafluvial zone). Although these particular characteristics may be unique to desert streams of the southwestern USA, the important point is that larger-scale forcing from global climatic patterns can result in decadal shifts in near-stream riparian vegetation that have profound consequences for stream ecosystem function.
Given the high degree of interannual variability of desert environments, annual averages often carry little information and long-term trends are masked. Years vary not only in the total amount of runoff but also in the temporal distribution and timing of individual events or clusters of events. During the five wettest years of a 30-year period for Sycamore Creek, frequent floods meant the ecosystem was in an early successional state most of the year, whereas stream organisms experienced severe drying conditions over much of the year during the five driest years. Furthermore, years with the same total annual runoff may differ substantially in seasonal distribution of that runoff with consequences for the seasonal patterns of drying. A single, large late-summer flood in 1970 in Sycamore Creek carried the same total volume of water as nine total floods distributed more evenly across the spring season in 1988, with the result that much of the stream was dry during the hottest months in 1970 but was undergoing succession during summer 1988 (Figure 3).
Although seasonality in desert streams may be strongly dependent upon the distribution of disturbance events, other variables in addition to discharge, such as temperature and day length, can influence the biota of desert streams. Deserts are defined only by low precipitation; thus, there is a broad range across the world's deserts in both flow seasonality and annual temperature distributions. Flow seasonality may vary from highly unpredictable and
1970 (solid) 1988 (dashed)
Figure 3 Contrasting seasonal discharge patterns in 2 years with nearly identical total annual discharge. Bars at bottom show time periods likely to be influenced by postflood succession (hatched), drying (open), or neither (solid). Redrawn from Grimm NB (1993) Implications of climate change for stream communities, pp. 293-314. In: Kareiva P, Kingsolver J, and Huey R (eds.) Biotic Interactions and Global Change. Sunderland, MA: Sinauer Associates.
Apr episodic events to a relatively predictable and sustained increased in discharge associated with a distinct wet season, with consequences for successional patterns. Temperatures of stream water in cool deserts may fluctuate seasonally from near-freezing to 20 °C; in hot deserts, daytime stream temperature can reach >30 °C but may be ameliorated by extensive evaporative cooling.
Temperature variation over the course of a single 24-h period may be nearly as great as seasonal variation. High albedo and low heat capacity of desert land surfaces cause extensive diel fluctuations in air temperatures, leading to wide (though comparatively muted) ranges in stream temperature. Particularly in summer when evapotranspiration rates are very high, streamflow varies measurably over 24 h, causing stranding at stream margins. At points where drying streams sink into the sediments, the end of the stream can migrate up and down the channel by several meters! Desert streams of Antarctica show extreme diel variation in discharge, but by a very different mechanism. Streamflow is generated by solar melting of the vertical walls (ice cliffs) of glaciers. During summer, when the Sun circles around the horizon at a low angle, melting (and streamflow) stops when the cliffs are in shadow.
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