Fish assemblage stability in an intermittently flowing stream from the Brazilian semiarid region

Flooding and drought are important hydrological disturbances occurring in intermittent streams located in semiarid regions. The present study aims to analyse the effects of flood and drought on the diversity and stability of an intermittent stream fish community from the Brazilian semiarid region over the 1996 hydrological cycle. Fourteen fish collections were conducted during the rewetting, wet and drying phases, and 808 individuals were collected. Diversity was measured by using Simpson's Index (S) and community stability was analysed by using the variation in the abundance, then tested by applying Kendall's (W) test of concordance over the relative abundance rankings. This test supplies information about the stability of communities by means of the concordance of rankings. A community that is stable or near equilibrium has more constant rankings of its constituent species compared with a fluctuating one. The diversity increased progressively during the rewetting phase (S = 0.480-0.693; coefficient of variation = 0.131), reached its highest values during the wet phase (S = 0.745-0.861; coefficient of variation = 0.072), and remained high during the drying phase, with a lower oscillation than in the other phases (S = 0.699-0.801; coefficient of variation = 0.050). The abundance per collection varied significantly (Kruskal- Wallis 6.94, P = 0.031) throughout the hydrological phases. The stability of the community was higher during the rewetting (W = 0.58, P < 0.001) and drying (W = 0.86, P < 0.001) phases than during the wet phase (W = 0.45, P = 0.1818). In the Brazilian semiarid region, flooding disrupted the stability of the fish community studied and increased the diversity, whereas the absence of floods (expressed by the drying phase) caused a higher stability of the fish community and led to less variation in diversity.