Floodplain habitats have been inferred to
provide a variety of functions for aquatic organisms,
yet few studies have documented movement between
channel and aquatic floodplain habitats. We exploited
spatial variation in stable isotope ratios of gizzard
shad (Dorosoma cepedianum) to document movement
between floodplain lakes and the main river channel
of the Brazos River, Texas, during a period of
frequent hydrologic connectivity. Additionally, we
examined stomach contents of shad to determine if
ontogenetic diet shifts or faunal exchange best
explained variation in isotopic ratios. Regression
analysis indicated significant relationships between
gizzard shad size and isotopic ratios in oxbow lakes,
whereas these relationships were not significant for
the main channel. Plots of individual fish in each
habitat suggested that adult shad migrated into oxbow
lakes during floods whereas juveniles assimilated
material produced in oxbows. Some adults in oxbows
had signatures similar to juveniles, and these individuals
were probably long-term oxbow residents. The
proportion of adults with a “river” signature was
greater in the oxbow with the shortest flood recurrence
interval where opportunities for faunal exchange
were more frequent. Analysis of stomach
contents indicated almost total overlap between adult
and juvenile diets indicating that movement between
habitats having different isotopic ratios of basal
resources rather than ontogenetic dietary shifts best
explained patterns of isotopic variation in Brazos
River gizzard shad.