Characterizing the long-term taxonomic and functional variability of a stream fish assemblage

While much research has been devoted to characterizing congruency in the spatial distribution of taxonomic and functional structure, the temporal aspect of this relationship is poorly known. We examined taxonomic and trophic function based variability of a stream fish assemblage using a 12-year data set of spring and summer surveys. We found the occurrence of the most abundant species was persistent, while their relative abundance was variable. Rare, satellite species formed less than 3 % of the assemblage and contributed little to variations in relative abundance. Assemblage level trophic function proved to be relatively stable across years, but showed clear seasonality. The pattern of long-term taxonomic composition indicated that seasonal differences in trophic function were primarily due to a switch to alternate food resources by some species between seasons and not due to periodic immigration/emigration of fish with different functions. Overall, our study showed significant directional changes in taxonomic variability in time, but relative stability in trophic function, and suggested that local processes (i. e. population dynamics of resident species and seasonality of food resources) influenced assemblage dynamics, while the relative role of regional scale processes (periodic immigration/emigration) was low. The study also suggests considering more direct measures for evaluating the assemblage level function of fish than using rigid guild-based categorizations.