Nutrient dynamics and fertilization effects in two oligotrophic coastal mountain B.C. lakes

Selected nutrient dynamics and the effects of nitrogen and phosphorus fertilization on Twin Lakes, British Columbia, were examined for this research. Twin East Lake and Twin West Lake are small oligotrophic lakes with adjoining watersheds in the Insular and Coast Mountain region of British Columbia. They are currently being studied by the Fisheries Research Branch of the B.C. Ministry of Environment as part of a lake fertilization program and thus were chosen as a site for this research. The effects of nitrogen and phosphorus enrichment on primary production, chlorophyll a, and algal species composition were studied using closed microcosm containers placed in Twin West Lake. Experiments were conducted using three phosphorus loading rates - 30, 60 and 90 µg/L, and three nitrogen to phosphorus (N:P) ratios of 3:1,10:1 and 20:1 at a fixed P loading rate of 60 µg/L. Experiments were conducted in May, July, and August, 1989. The optimum N:P ratio in terms of highest primary production, chlorophyll a, and desirable algal species was found to vary. A low N:P ratio or the addition of phosphorus alone produced the best results in the spring (May), while the highest N:P ratio (20:1) was optimum for the summer experiments (July and August). Both an input and an output to the nutrient pools of Twin Lakes were studied as part of this research, to gain a better understanding of nutrient dynamics. Littoral zone deciduous vegetation was studied as a source of nitrogen and phosphorus to the lake system Laboratory microcosms containing water, lake sediment and hardhack leaves were used to study nutrient leaching from vegetation that has been carried into the lake. Leaf litter bags placed in the shoreline littoral zone of Twin West Lake were used to examine leaching during periodic flooding and rainfall events. Both types of experiments found vegetation nutrient loss to be occurring, probably through both leaching and microorganism decomposition. Infrared air photo analysis was used to determine the extent of deciduous vegetation adjacent to the shoreline of Twin East and Twin West Lakes, and thus the possible nutrient supply available. Twin East Lake, the more productive of the two lakes, was found to have 12 ha of deciduous vegetation surrounding the lake whereas Twin West Lake had an area of 3 ha. The process of sedimentation as a nutrient removal mechanism was studied in Twin Lakes. Triplicate sets of sediment traps were placed at 2 locations at depths of 5 and 10 m in Twin East Lake. In Twin West Lake, sets of traps were placed at 5 and 10 m depths at four locations. In addition, sets of traps were placed at a 25 m depth at two locations in Twin West Lake. Sediment trap samples were collected four times in 1989 and trap contents were analyzed for nitrogen and phosphorus. Sedimentation rates were found to vary by depth and season. Sedimentation rates in Twin East Lake were 2 to 4 times higher than those in Twin West Lake. In terms of the average nutrient mass in Twin West Lake, sedimentation losses represented a phosphorus turnover rate of 2.3 years and a nitrogen turnover rate of 1.7 years. For Twin East Lake, turnover rates were 0.4 years for nitrogen and 0.2 years for phosphorus.Applied Science, Faculty ofCivil Engineering, Department ofGraduat