Resistance and resilience of Najas marina to disturbance by flash floods were studied in a permanent fluvial pool of a Brazilian semiarid intermittent stream. A total of 21 macrophyte samples was collected in the high-rainfall season during two annual cycles (1996–1997). Decreases in macrophyte biomass were positively correlated with flood magnitude (Pearson, P = 0.047), varying from 25 to 53 percent when discharges were lower than or equal to 0.5 m3/sec and between 70 and 100 percent when discharges were higher than 1.0 m3/sec. Macrophyte resilience was greater after floods of low magnitude. After floods of 0.5 m3/sec, three weeks were necessary to re-establish 88 percent of biomass lost, and after a flood of 1.4 m3/sec, six months were needed to initiate N. marina regrowth. This population of N. marina in Avelós stream has higher stability in response to small disturbances, but as expected, its resistance and resilience decreases with larger disturbances. In general, the high resistance and resilience of N. marina in response to small disturbances have been observed in intermittent tropical streams. The absence of large floods during the study period and the low variability of water temperature in this tropical region may have influenced these results.
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Vol. 33 • No. 4