Diatoms from gut contents of museum specimens of an endangered minnow suggest long-term ecological changes in the Rio Grande (USA)

Diatoms consumed by Rio Grande silvery minnows (Hybognathus amarus) collected 104 years apart were used to deduce ecological requirements of this endangered species and to infer a possible cause of its decline based on environmental conditions in the Rio Grande. In 1874, foraged diatoms were largely motile, silt tolerant generalist and epipelic species (e.g. Navicula capitatoradiata, Navicula cryptotenella, Nitzschia palea, Sellaphora pupula) somewhat tolerant to pollution and indicative of eutrophic conditions, low dissolved oxygen, and high biological oxygen demand (BOD). In contrast, diatoms foraged in 1978 were mainly nonmotile epipsammic species (Fragilariaceae) sensitive to pollution and characteristic of low nutrient, oligotrophic conditions with high dissolved oxygen and low BOD. The large-scale difference in composition of the foraged diatom assemblages is consistent with a decline in nutrients and a shift in trophic state commonly associated with river regulation by dams. The results suggest that facilitating allochthonous input of detritus and nutrients into the Rio Grande ecosystem to meet foraging requirements for algivorous and detritivorous fish species such as H. amarus would be a good river restoration strategy.