LAUSR

Abstract Knowledge of the effects of environmental variables on estuaries with intermittent upstream rivers is scarce. Thus, understanding how environmental filters affect fish assemblages in these estuaries is important when an atypical drought occurs. Three zones in the Mamanguape River estuary designated according to the salinity gradient were sampled during an atypical climatic event in 2015. A total of 18,084 fishes of 125 species were recorded. Density and richness showed significant differences between seasons, while density, biomass, diversity and evenness showed significant differences in spatial pattern. By ecological guild, marine estuarine-dependent, solely estuarine, marine estuarine-opportunist and estuarine and marine were the most representative groups in the estuary during the sampling period. Chlorophyll a and salinity were major filters that explained only the spatial distribution of fish assemblages, relating mainly to richness increasing from the upper to the lower parts of the estuary. Our study supports the need to understand the filters for species richness and spatial distribution patterns. Changes in those filters may shift fish assemblages, hence altering ecosystem function.