The aquatic macrofauna of the Guadalquivir estuary were sampled (1 mm mesh persiana net) at 5 sampling
sites located along the entire (except the tidal freshwater region) estuarine gradient of salinity (outer 50 km). A
total of 134 fish and macroinvertebrate species was collected but only 62 were considered common or regularly present
in the estuary. Univariate measures of the community structure showed statistically significant differences among sampling
sites: species richness, abundance, and biomass decreased in the upstream direction, being positively correlated
with the salinity. Temporal differences of these three variables were also statistically significant. While a clear seasonal
pattern (minimum densities in winter and maximum in spring-summer) was observed for abundance and biomass, no
such pattern existed for the number of species. Mysids was the most dominant group throughout the estuary (96% to
99% of abundance; 49% to 85% of biomass), although fish biomass was also important at the outer estuary (36% to
38%). Multivariate analyses indicated highly significant spatial variation in the macrofaunal communities observed along
the salinity gradient. These analyses suggest that the underlying structure was a continuum with more or less overlapping
distributions of the species dependent on their ability to tolerate different physicochemical conditions. There were also
significant temporal (intermonthly 1 interannual) variation of the estuarine community; the relative multivariate dispersion
indicated that monthly variation was more considerable (relative multivariate dispersion . 1) at the outer part of
the estuary during the wet year (last 20 km) and was higher in the inner stations during the dry year (32 to 50 km from
the river mouth). Since a clear negative exponential relationship was observed between the freshwater input (from a
dam located 110 km upstream) and water salinity at all sampling stations, it is concluded that the human freshwater
management is probably affecting the studied estuarine communities. While the higher seasonal (long-term) stability of
the salinity gradient, due to the human control of the freshwater input, may facilitate the recruitment of marine species
juveniles during the meteorologically unstable early-spring, the additional (short-term) salinity fluctuations during the
warm period may negatively affect species that complete their lifecycle within the estuary.