Abstract
Reservoir creation often leads to a “trophic upsurge” of nutrients that may affect communities living near impoundments. We determined the duration of the nutrient upsurge and associated changes in plankton in a study of a new reservoir (Lake 979; L979) in northwestern Ontario that included 2 years pre-impoundment, 16 years of impoundment, and 2 years postimpoundment. Secondarily, we determined allochthonous versus autochthonous carbon (C) use by zooplankton with reservoir development. For the first 6–14 years of impoundment, mean concentrations of total phosphorus (TP), total nitrogen (TN), and dissolved organic carbon increased 3.6, 2.4, and 1.7 times above pre-impoundment levels, respectively, decreasing thereafter. L979 shifted from a net sink to a source of TP and TN for the first 6 years of impoundment. Mean annual biomasses of bacteria, phytoplankton, and zooplankton increased 4.6, 17, and 32.6 times above pre-impoundment levels, respectively, with associated changes in community composition. Bacteria and phytoplankton returned to pre-impoundment levels within 4 and 10 years, respectively, but zooplankton densities remained elevated even after 18 years. Changes in δ13C suggested that postimpoundment zooplankton biomass was not supported predominantly by allochthonous C.
Highlights
Reservoir creation often leads to a “trophic upsurge” of nutrients that may affect communities living near impoundments
We address the following questions: 1) what were the longer-term effects of impoundment on nutrient dynamics and how long did initial increases in nutrient concentrations persist?; 2) what were the consequences of changes in nutrients and physical conditions such as WRT on planktonic food web structure?; and 3) were increases in zooplankton biomass following impoundment supported by allochthonous C, as hypothesized by Paterson et al (1997)?
total phosphorus (TP) concentrations increased in L979 following impoundment relative to preimpoundment levels and concentrations in the inflow (Figure 1a; Table 1)
Summary
Reservoir creation often leads to a “trophic upsurge” of nutrients that may affect communities living near impoundments. Concentrations of nutrients and biomasses of bacteria and zooplankton increased following impoundment. The effect of impoundment on phytoplankton and zooplankton abundance, biomass, and species composition was determined from collections made during the ice-free season in the same years and frequencies as water chemistry, except 1991.
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