| Summary: | Experimental fertilization was conducted on Kootenay Lake, B.C. from 1992 to 1997 to
compensate for nutrients lost behind hydroelectric dams upstream of the lake. Declining nutrient
loads were correlated with lower in-lake nutrient concentrations, chlorophyll a concentrations,
and macrozooplankton densities, and a dramatic decline in kokanee salmon {Oncorhynchus
nerka) stocks. A simulation model of the lake suggested that increased zooplankton production
resulting from fertilization might be shunted into increased abundance of Mysis relicta, an exotic
crustacean that competes with kokanee, and that nutrient additions might actually hasten the
kokanee decline. In an attempt to test this prediction, nutrients were applied at the north end of
the lake, and the response of the food web was monitored along the expected longitudinal
productivity gradient.
The food web structure along the lake suggests that a trophic gradient of grazeable
phytoplankton abundance was established, but that M. relicta may have grazed down any
increase in zooplankton production in the fertilized end of the lake. Kokanee distribution and
size-at-age along the lake did not correlate with the nutrient gradient. Surprisingly, M. relicta
abundance decreased during the experiment, while kokanee abundance increased four-fold, and
Gerrard rainbow trout {Oncorhynchus mykiss), which prey mainly on kokanee, also increased in
abundance. M. relicta is vulnerable to mortality due to export out of the lake during high flow
years, whereas zooplankton replace flow-related mortality through rapid reproduction and
kokanee can actively avoid export. High surface water turnover rates, due to large winter snow
accumulation during the experiment, likely contributed to increased M. relicta mortality. This
physical factor may have shifted the competitive equilibrium between kokanee and M. relicta, by
suppressing an increase in M. relicta abundance, and allowed kokanee to take advantage of
increased zooplankton availability.
Caution should be exercised in extrapolating the results of fertilization in Kootenay
Lake to other large lakes where fish populations have been affected by hydroelectric dams or
competition from exotic species introductions. Nutrient additions may not reach the desired
target species unless the responses of exotic competitors are suppressed by physical factors
operating independently of the dynamics of the food web. === Science, Faculty of === Zoology, Department of === Graduate
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