In surface area, Williston Reservoir is British Columbia's largest lacustrine ecosystem. The reservoir shows remarkably little spatiotemporal variance among its three major reaches (Finlay, Parsnip and Peace), despite the immensity and biogeoclimatic diversity of its drainage basins. Williston is ultra-oligotrophic with average concentrations of total dissolved phosphorus (TDP) ranging from 3 to 5 µg/L and nitrate-nitrogen (NO3-N) from 60 to 65 µg/L. The pelagic plankton community of Williston Reservoir is dominated by bacteria, pico-cyanobacteria, nano-flagellates and small diatoms, and the zooplankton community is comprised largely of copepods in spring and small cladocerans in summer and fall. Primary production rates are very low and most carbon is produced by the pico- and nano-sized plankters that appear to be primarily light limited due to high turbidity and frequent wind-mixing episodes, i.e., shallow light compensation depth and deep epilimnetic depth. Microbial food webs are predominant in Williston through much of the growing season, with free-living bacteria and pico-cyanobacteria as the primary carbon template, and nano-flagellates, ciliates and micro-zooplankton as the principal grazers. Loading models indicate that after impoundment in 1968, Williston was initially a moderately productive ecosystem, but the system has progressively lost nutrients, namely P, through sedimentation and outflow, but also by the scarcity of littoral carbon production due to severe water level fluctuations (drawdown) and winter ice-scouring. Hence, within the past 2 to 3 decades the ecosystem has gradually lost biogenic productive capacity and now lies within the ultra-oligotrophic trophic status and supports a low level of fish production. Daily rates of carbon production are presently at levels more typical of British Columbia fast-flushing, ultra-oligotrophic coastal ecosystems than large, interior-type oligo- and mesotrophic British Columbia lakes. Based on average values of most limnological variables we demonstrate that the Finlay Reach is currently the most productive region of Williston, and estimates of pelagic fish from hydroacoustic/trawl and gillnet surveys support this contention.