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Abstract-Videotapes of dye dispersion experiments in the epilimnion of a small lake were successfully analyzed with image-processing techniques. The rate of horizontal dispersion from dye releases in the center of the lake was consistent with previous observations in oceans and large lakes. We derive a power-law relationship that relates the apparent diffusivity to the characteristic length scale of the dye patch. This relationship applies to all three data sets and is valid over length scales ranging from 10 m to more than 100 km. Dye releases from a point on the shore of the same lake dispersed slightly faster than the central releases. This observation, while clearly not universal, has prompted the successful shore-based fertilization of several small lakes.

Kootenay Lake, B.C. (395 km2), has experienced a series of major perturbations in the past 50 years, resulting in the 1980s’ collapse of South Arm kokanee salmon stocks and a dramatic decrease in the abundance of North Arm stocks. Historical data indicate the collapse was due to reservoir construction together with subsequent impoundment and nutrient retention on both main inflow tributaries (Kootenai and Duncan rivers) to Kootenay Lake, combined with the introduction of an exotic mysid shrimp which is an efficient competitor with kokanee for zooplankton. Nutrients (47.1 t of P and 206.7 t of N) are now being added annually to the 174 km2 North Arm of Kootenay Lake (271.3 mg-m-2 P and 1,190.5 mg-m-2 N) in a 5-year (1992-96) experiment with the goal of restoring historical kokanee salmon biomass and productivity. The concentration of total phosphorus in the North Arm currently ranges from 4–10 µg-L-1 P, which indicates oligotrophic to mesotrdphic conditions, whereas the concentration of dissolved inorganic nitrogen remains in the oligotrophic range (i.e., <200 mg L-1 N) throughout the year. The seasonal mean zooplankton density observed in 1994 was higher than in 1992 and 1993, and similar to the highest densities observed from 1972 to 1984. Cladocerans comprised about 7.5% of the zooplankton in the North Arm in 1994, 12% in 1993 and 7.5% in 1992 as compared with less than 5% between 1949 and 1991. Combined North Arm kokanee escapement (Lardeau River and Meadow Creek) was 1.25 million fish in 1994 and was the highest observed since 1986, but lower than some escapements observed in the 1970s. Kokanee spawner size and fecundity have also increased. No significant trends have been observed in the Gerrard rainbow trout population. To date, the fertilization experiment has been successful at increasing the abundance of clado-ceran zooplankton and restoring kokanee populations in the North Arm of Kootenay Lake.

To examine the effect of hypolimnetic aeration on key components (circulation, decomposition, major nutrients, major ions, and pH interactions) of a lake ecosystem, I used a small naturally eutrophic lake. I divided the lake in half with a plastic curtain, and installed and operated a hypolimnetic aerator in the experimental half for a year. Hypolimnetic aeration had no effect on thermal stratification during the ice-free season but circulated the entire experimental half under ice cover. Aeration increased hypolimnetic turbidity via reduced detrital sedimentation but did not affect epilimnetic transparency. Hypolimnetic oxygen concentrations were increased as was oxygen consumption as both sediment and water column oxidation of organic material was enhanced. Hypolimnetic ammonia concentrations were reduced and sufficient oxygen was added for nitrification to occur. Internal phosphorous loading and hypolimnetic orthophosphate concentrations were reduced but aerobic P regeneration increased. Aeration vented accumulated CO2 from the hypolimnion and decreased its calcium, magnesium, bicarbonate, and orthophosphate content via calcium carbonate phosphate coprecipitation. Management applications of hypolimnetic aeration include reduction of internal nutrient loading for eutrophication control, improvement of water quality for domestic use, and prevention of fish winterkill.

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We used two montane lakes (4.5 and 7.8 ha) in a before-after control-impact pair experimental design to measure the responses of rainbow trout (Oncorhynchus mykiss) and their prey to 5 years of inorganic phosphorus and nitrogen (liquid ammonium polyphosphate and urea ammonium nitrate) additions. Epilimnetic total phosphorus (TP) concentration, euphotic zone chlorophyll a concentration, limnetic macrozooplankton biomass, and annual trout yield increased significantly in the fertilized lake. Chlorophyll a concentration and zooplankton biomass varied directly with epilimnetic TP. Trout yield varied as TP0.5. Fertilization increased trout reproductive output, growth, and yield but did not alter yearling survival or the mean age of mature fish. Prey utilization by trout did not change. DeltaC data indicated that trout in the fertilized lake obtained 65% of their body carbon from benthic insects and 35% from zooplankton. Increased biomass of Hesperodiaptomus accounted for most of the zooplankton response. Benthic insect emergence decreased with increasing water depth but was greater at a given depth in the fertilized lake. Resources accounted for smaller proportions of the variance of consumer biomass with increasing trophic level.