Patterns of community assembly in a fourth order stream

Spatial patterns in community structure were studied for one year along a f ourth order stream in NW Spain, the Tea Stream. Differences were observed f or most biological variables examined between upper-middle lotic reaches an d downstream reaches in autumn, a season characterised by occurrence of hig h and frequent spates. Diversity and richness values were high at the upper stream reaches throughout the year, even when discharge was high. Downstre am sites were inhabited by species-poor communities during autumn and winte r. In spring and summer, however, richness and diversity values increased, and invertebrate abundance was higher at these sites than at the upper reac hes. Observed spatial patterns in community structure were mainly correlate d with physical variables, such as a combination of morphological channel f eatures, dissolved salts and oxygen concentration, and surface water veloci ties. The spatio-temporal differences in community structure observed in th is study are greater than expected for a fourth-order stream. The greatest discontinuity in community composition occurred between high and low gradie nt sites during periods of high flow. The flow conditions during autumn-win ter seemed to exert a strong abiotic influence on communities inhabiting do wnstream reaches, when species richness and abundances remained low. The up per-middle reaches of the Tea supported high richness throughout the year, with more resilience to high flows than downstream communities.