Changes in benthic community composition in response to reservoir aging

Abstract

The effects of reservoir aging on the benthic macroinvertebrate community in Pawnee Reservoir were documented by comparing species composition and biomass of samples collected from October 1991 through September 1992, to a similar survey conducted in 1968–70 by Hergenrader & Lessig (1980). Filling of the basin with sediment and associated material and the subsequent change in the benthic environment, has resulted in a relatively homogenous bottom substrate at each of the three sampling transects (dam, middle, and inflow). Sediment enrichment has limited the benthic fauna to species tolerant of brief periods of bottom anoxia and increased levels of organic matter, which has resulted in the disappearance of many taxa and a decrease in the abundance of remaining invertebrates. Significant differences in total biomass were found at each transect, as well as for the whole lake, between study periods. The dam, middle, inflow and total biomasses for the 1968–70 study period were 2.4, 1.5, 2.3 and 2.0 g m−2, respectively, compared to 0.2, 0.2, 0.3, and 0.2 g m−2 for the 1991–92 study period. The total disappearance of eight invertebrate taxa, in particular two sphaerid clam species, and significant declines in other dominant taxa such as Chaoborus punctipennis and Chironomus sp. accounted for these major differences in biomass between study periods. Reductions in the number of taxa present has resulted in an increase in benthic faunal similarity at each transect, with tubificid oligochaetes, Coelotanypus sp., C. punctipennis, and Chironomus sp., comprising 90% of both the total density and biomass of benthic invertebrates in Pawnee Reservoir.