| Citation |
E.U. Schindler, D. Sebastian, H. Andrusak, L. Vidmanic, S. Harris, G.F. Andrusak, F. Pick, L.M. Ley, P.B. Hamilton, D. Johner, P. Woodruff, M. Bassett, K.I. Ashley (2010) Kootenay Lake Nutrient Restoration Program, Year 16 (North Arm) and Year 4 (South Arm) 2007, Ministry of Environment, Redfish Consulting Ltd., Limno-Lab Ltd., University of Ottawa, Canadian Museum of Nature, British Columbia Conservation Foundation, and BC Institute of Technology. |
| Abstract/Description or Keywords |
This report summarizes results from the sixteenth year (2007) of nutrient additions to the North Arm of Kootenay Lake and four years of nutrient additions to the South Arm. These additions were conducted using an adaptive management approach in an effort to restore lake productivity lost as a result of nutrient retention and uptake in upstream reservoirs. The primary objective of this experiment is to restore kokanee (Oncorhynchus nerka) populations, which are the main food source for Gerrard rainbow trout (Oncorhynchus mykiss) and bull trout (Salvelinus confluentus). Nutrients added were in the form of agricultural grade liquid fertilizer (10-34-0, ammonium polyphosphate (phosphorus, P) and 28-0-0, urea ammonium nitrate (nitrogen, N). The total amount added to the North Arm in 2007 was 46.2 tonnes of P and 246.9 tonnes of N while in the South Arm 245 tonnes of nitrogen were added but zero P. Kootenay Lake has an area of 395 km2, a maximum depth of 150 m, a mean depth of 94 m, and a water renewal time of approximately two years. The lake is warm monomictic generally mixing from late fall to early spring and stratifying during the summer. Surface water temperatures were warmest in August at 20o C and 22o C in the North Arm and South Arm, respectively. The concentration of oxygen in vertical profiles were similar to previous years with the lake being well oxygenated from the surface to the bottom depths at all stations. Similar to past years, Secchi disc measurements at all stations in 2007 indicated a typical seasonal pattern of decreasing depths associated with increased phytoplankton biomass, followed by increasing depths as the algae biomass gradually diminished in the fall. The concentrations of total phosphorus (TP) ranged from 2 - 10 _g/L and tended to decrease as summer advanced in the North Arm, whereas it was uniform in the South Arm with a peak in September at one station. Over the sampling season dissolved inorganic nitrogen (DIN) concentrations decreased, with the decline corresponding to nitrate (the dominant component of DIN) being utilized by phytoplankton during summer stratification. Owing to the importance of epilimnetic nitrate that is required for optimal N:P ratios and to ensure growth of edible phytoplankton, discrete epilimnetic water sampling was undertaken in 2007 to more accurately monitor changes in euphotic zone nitrate concentrations. As expected, there was a seasonal decline in nitrate concentrations, thus supporting the strategy of increasing the nitrogen loading in both arms. These in-season adjustments emphasize the need for an adaptive management approach to ensure the nitrogen to phosphorus (N:P) (dissolved fraction) ratio does not decrease below 15:1 (weight:weight) during the fertilizer application period. Phytoplankton composition determined from integrated samples (0-20m) was dominated by diatoms, followed by cryptophytes and chrysophytes. In 2007, the contribution of Kootenay Lake Nutrient Restoration Program, Year 16 (North Arm) and Year 4 (South Arm) (2007) cryptophytes to total biomass was greater in the North Arm than in the South Arm (31% vs. 18%); a higher contribution than in 2006. Phytoplankton in the discrete depth samples (2, 5, 10, 15 and 20 m) was dominated by chryso-cryptophytes in June and then shifted to bacillariophytes being dominant from July through September, a trend similar to other years. There were no large blue-green (cyanobacteria) populations in 2007. The trend of chyrso-cryptophytes being dominant in the spring and decreasing in the summer and fall months coincides with the increase in Daphnia spp. biomass, indicating that grazing on the phytoplankton is likely occurring. Depth integrated 14C primary production rates ranged from oligotrophic to mesotrophic conditions (50 to 430 mg C/m2/d) with seasonal and interannual variability. The contribution of nanoplankton (i.e., 2 to 20 u cell diameter) to total productivity ranged from 25% in 2006 to 47% in 2007. Results in 2004 and 2005 ranged between 39% and 46%. Nanoplankton production is considered the size class preferred by Daphnia spp. which in turn is the main food source for kokanee. In 2007, seasonal average zooplankton abundance and biomass in the main body of the lake slightly decreased compared to 2006. Total zooplankton density and densities of copepods and Daphnia in the West Arm decreased while Cladocera other than Daphnia spp. increased compared to 2006. Zooplankton density was numerically dominated by copepods and biomass was dominated by Daphnia spp. The annual average mysid biomass data at deep stations indicated that the North Arm of Kootenay Lake supported slightly less biomass than the South Arm in 2007. Mysid densities increased through the summer and decreased into fall. The mean whole lake values remain within pre-nutrient addition densities. Kokanee escapement to the Meadow Creek spawning channel slightly increased in 2007 to approximately 386,000 spawners compared to 371,000 in 2006. The Lardeau River escapement increased from 2006 with 100,000 spawners to approximately 147,000 spawners. Similar to the last decade, kokanee spawner numbers in the South Arm tributaries remained virtually at zero. The mean size of female and male kokanee from Meadow Creek was the largest on record (28.2 cm and 27.7 cm, respectively); the long term average has been 22.4 cm. Fecundity increased in 2007 with an average of 411 eggs per female. The increased kokanee size in 2007 is believed to reflect a density dependent growth response owing to good growing conditions. Spring hydroacoustic survey estimates indicated very low densities of kokanee in the South Arm with highest densities at the northerly stations where most of the fry are produced. The relationship between the number of fry produced from Meadow Creek and the number of fry estimated in the fall hydroacoustic survey were similar, a trend that has occurred during most study years. By the fall, the distribution of kokanee fry was fairly uniform throughout the lake, as observed in previous years. Fall hydroacoustic estimates Kootenay Lake Nutrient Restoration Program, Year 16 (North Arm) and Year 4 (South Arm) (2007) for all age groups increased to approximately 23 million kokanee in 2007 thus foreshadowing increased spawner numbers in the near future. The results of the 2007 nutrient additions indicate that the trophic level response has been positive. Nutrient additions to the North and South arms have resulted in sufficient phytoplankton composition and biomass suitable for Daphnia spp. growth. Pelagic kokanee numbers size and biomass increased; all indicative of successful trophic level transfer to planktivores, and indicative of a positive response to our closely monitored seasonal applications of limiting macronutrients. |
