Stability of Periphyton and Macroinvertebrates to Disturbance by Flash Floods in a Desert Stream

Resistance, resilience, and patterns of succession were evaluated for periphyton and macroinvertebrates of Sycamore Creek, Arizona, between 1984 and 1987. During this period, 35 flash-flood disturbances occurred, ranging in magnitude (peak discharge) from 0.2 m3/s to 58 m3/s; peak discharge of the largest flash floods exceeded base flow by 3-4 orders of magnitude. Macroinvertebrates and algal assemblages dominated by diatoms were more resistant, i.e., showed less change in response to spates, than macroalgae (filamentous Chlorophyta) and cyanobacterial mats, but resistance of all groups declined with increasing disturbance magnitude. Biota showed little resistance to events large enough to move substrata. Twenty sequences of postflood succession were analyzed to characterize resilience and patterns of recovery. Resilience was very high compared with other streams and other ecosystems, because of high rates of biotic production in this desert stream. Resilience of periphyton (as indicated by recovery of total chlorophyll a) was highest in summer and autumn, while macroinvertebrate resilience (in terms of density or biomass) did not differ among seasons. In many sequences, recovery curves of macroinvertebrate and algal standing crops were linear or asymptotic; however, macroinvertebrates declined precipitously during later stages of five successional sequences, primarily because of declines in the dominant collector-gatherers. These organisms reproduce continuously and have short life cycles, so declines cannot be explained by synchronous emergence. We propose instead that declines are caused by food quality limitation. Sustained high rates of macroinvertebrate consumption and use of autochthonous detritus are possible only if bacterial conditioning (and immobilization of nitrogen) can increase the N content of this material, which is halved with each gut passage. During many sequences, bacterial conditioning is probably limited by nitrogen, which is known to limit autotrophic processes in this stream. This suggests that productivity at higher trophic levels may be influenced by availability of a limiting nutrient not only to primary producers but to microconsumers.