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.