Effects of river regulation on Rio Grande fish communities revealed by stable isotope analysis of historical and contemporary samples

Background/Question/Methods

We studied spatial and temporal dynamics of energy sources and trophic structure of the fish community in the middle Rio Grande, New Mexico by evaluating stable isotopes of carbon and nitrogen at four locations and over a time span of 70 years.  At present, the Rio Grande is regulated for irrigation and flood control, and consequently the frequency and duration of flooding is diminished compared to the past.  Prior to regulation, the Rio Grande had extensive overbank flooding and increased connectance to adjacent floodplain (bosque) habitats.  We hypothesized that river modification should have caused a switch from terrestrial production to instream (algal) production as the dominant source of carbon for the entire fish food web.  Additionally, we predicted that trophic structure would be diminished as river habitats were homogenized through channelization.  We tested these hypotheses using a new analytical framework that characterizes and tests for differences in dispersion, central tendencies, and trajectories of δ¹³C and δ¹⁵N values across space and time.

Results/Conclusions

Our analyses revealed that instream carbon sources predominately fueled both historical and contemporary fish food webs, but energy sources to the food web were less heterogeneous over time (as evidenced by diminished dispersion in the δ¹³C dimension), indicating that primary production has become increasingly autochthonous.  There was concomitant reduction in dispersion in δ¹⁵N, suggesting homogenization of trophic structure in the fish community.  The Rio Grande appears to have suffered losses in ecological diversity that coincide with habitat homogenization and loss of biodiversity.