CHANGES IN BENTHIC COMMUNITY COMPOSITION IN RESPONSE TO RESERVOIR AGING

The effects of reservoir aging on the benthic macroinvertebrate commun ity in Pawnee Reservoir were documented by comparing species compositi on and biomass of samples collected from October 1991 through Septembe r 1992, to a similar survey conducted in 1968-70 by Hergenrader and Le ssig (1980). Filling of the basin with sediment and associated materia l and the subsequent change in the benthic environment, has resulted i n a relatively homogenous bottom substrate at each of the three sampli ng transects (dam, middle, and inflow). Sediment enrichment has limite d the benthic fauna to species tolerant of brief periods of bottom ano xia and increased levels of organic matter, which has resulted in the disappearance of many taxa and a decrease in the abundance of remainin g invertebrates. Significant differences in total biomass were found a t each transect, as well as for the whole lake, between study periods. The dam, middle, inflow and total biomasses for the 1968-70 study per iod 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 dis appearance of eight invertebrate taxa, in particular two sphaerid clam species, and significant declines in other dominant taxa such as Chao borus punctipennis and Chironomus sp. accounted for these major differ ences in biomass between study periods. Reductions in the number of ta xa present has resulted in an increase in benthic faunal similarity at each transect, with tubificid oligochaetes, Coelotanypus sp., C. punc tipennis, and Chironomus sp., comprising 90% of both the total density and biomass of benthic invertebrates in Pawnee Reservoir.