Dynamics of a Boreal Lake Ecosystem during a Long-Term Manipulation of Top Predators |
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Authors: | David L Findlay Michael J Vanni Michael Paterson Kenneth H Mills Susan EM Kasian Willie J Findlay Alex G Salki |
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Institution: | (1) Fisheries and Oceans, Freshwater Institute, 501 University Crescent, Winnipeg, Manitoba, R3T 2N6, Canada;(2) Department of Zoology, Miami University, Oxford, Ohio 45056, USA |
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Abstract: | We assessed the long-term (16 years) effects of introducing piscivores (northern pike) into a small, boreal lake (Lake 221,
Experimental Lakes Area) containing abundant populations of two planktivorous fish species. After the introduction, pearl
dace were extirpated and yellow perch abundance was greatly reduced. Daphnia species shifted from D. galeata mendota to larger bodied Daphnia catawba, but the total zooplankton biomass did not increase, nor did the biomass of large grazers such as Daphnia. Phytoplankton biomass decreased after the northern pike introduction, but increased when northern pike were partially removed
from the lake. Phosphorus (P) excretion by fish was ∼0.18 mg P m−2 d−1 before pike addition, declined rapidly to approximately 0.03–0.10 as planktivorous perch and dace populations were reduced
by pike, and increased back to premanipulation levels after the pike were partially removed and the perch population recovered.
When perch were abundant, P excretion by fish supported about 30% of the P demand by primary producers, decreasing to 6–14%
when pike were abundant. Changes in phytoplankton abundance in Lake 221 appear to be driven by changes in P cycling by yellow
perch, whose abundance was controlled by the addition and removal of pike. These results confirm the role of nutrient cycling
in mediating trophic cascades and are consistent with previous enclosure experiments conducted in the same lake. |
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Keywords: | food web trophic cascade bottom-up top-down Experimental Lakes Area nutrients fish phytoplankton zooplankton piscivores |
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