Phytoplankton community response to reservoir aging, 1968-92

The effects of reservoir aging on the phytoplankton community of amidwestern U.S. reservoir constructed in 1965 (Pawnee Reservoir) werestudied by comparing algal biovolume and species composition from April 1992through November 1992 to surveys conducted in 1968-73 and 1990. Meansummer total phosphorus, nitrate-nitrogen, Secchi disk depth, totalsuspended solids, chlorophyll a, and phytoplankton species composition datacharacterized Pawnee Reservoir during 1968-69 as a high nutrient,relatively clear water environment. Phytoplankton biomass was relativelylow, consisting mainly of cyanophytes and non-flagellated chlorophytes.During 1970-73, water clarity was poor, total suspended solids werehigh, and total phosphorus was lower, but was still greater than 100 µgl^sup -1^. The 1970-73 phytoplankton biomass was high and wasdominated by cyanophytes. Mean summer total phosphorus remained >100µg l^sup -1^, water clarity remained poor, but phytoplanktonbiomass decreased significantly during 1990-92. The dramatic drop inchlorophyll a and low mean volatile suspended solids indicated thatinorganic suspended sediments, rather than phytoplankton, accounted for themajority of the turbidity in 1990-92. In addition to lower phytoplanktonbiomass, community composition shifted away from buoyancy-regulatingcyanophytes toward flagellated chlorophytes. These data suggest that asreservoirs located in agricultural watersheds age, (1) inorganic suspendedsediments have a significant effect on the light environment as well asphytoplankton biomass and species composition, (2) the control ofphytoplankton biomass and species composition shifts away from nutrients tolight and suspended sediments, and (3) there is a 1-2 year lag in theresponse of phytoplankton biomass to maximum nutrient loading during thetrophic upsurge period. Thus, sedimentation has been shown to be a primarydeterminant of plankton and benthic macroinvertebrate community compositionas Pawnee Reservoir aged.[PUBLICATION ABSTRACT]