| Citation |
E.U. Schindler, K.I. Ashley, R. Rae, L. Vidmanic, H.Andrusak, D. Sebastian, G. Scholten, P. Woodruff, F. Pick, L.M. Ley and P.B. Hamilton (2008) Kootenay Lake Fertilization Experiment, Years 11 and 12 (2002 and 2003) Ministry of Environment, Sumac Writing and Editing, Limno-Lab Ltd., Redfish Consulting Ltd., BC Conservation Foundation, University of Ottawa, University of British Columbia, Canadian Museum of Nature. |
| Abstract/Description or Keywords |
This report examines the results from the eleventh and twelfth years (2002 and 2003) of the Kootenay Lake fertilization experiment. Experimental fertilization has occurred with an adaptive management approach since 1992 in order to restore productivity lost as a result of upstream dams. One of the main objectives of the experiment is to restore kokanee (Oncorhynchus nerka) populations, which are a main food source for Gerrard rainbow trout (Oncorhynchus mykiss). Kootenay Lake is located between the Selkirk and Purcell mountains in southeastern British Columbia. It 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 quantity of agricultural grade liquid fertilizer (10-34-0, ammonium polyphosphate and 28-0-0, urea ammonium nitrate) added to Kootenay Lake in 2002 and 2003 was similar to that added from 1992 to 1996. After four years of decreased fertilizer loading (1997 to 2000), results indicated that kokanee populations had declined, and the decision was made to increase the loads again in 2001. The total load of fertilizer in 2002 was 47.1 tonnes of phosphorus and 206.7 tonnes of nitrogen. The total fertilizer load in 2003 was 47.1 tonnes of phosphorus and 240.8 tonnes of nitrogen. Additional nitrogen was added in 2003 to compensate for nitrogen depletion in the epilimnion. The fertilizer was applied to a 10 km stretch in the North Arm from 3 km south of Lardeau to 3 km south of Schroeder Creek. The maximum surface water temperature in 2002, measured on July 22, was 22 ľC in the North Arm and 21.3 ľC in the South Arm. In 2003, the maxima were recorded on August 5 at 20.6 ľC in the North Arm and on September 2 at 19.7 ľC in the South Arm. The maximum water temperature in the West Arm was 18.7 ľC on September 2, 2003. Kootenay Lake had oxygen-saturated water throughout the sampling season with values ranging from about 11–16 mg/L in 2002 and 2003. In both years, Secchi depth followed the expected pattern for an oligo-mesotrophic lake of decreasing in May, June, and early July, concurrent with the spring phytoplankton bloom, and clearing again as the summer progressed. Total phosphorus (TP) ranged from 2–11 _g/L in 2002 and 2–21 _g/L in 2003. With average TP values generally in the range of 3–10 _g/L, Kootenay Lake is considered to be an oligotrophic to oligo-mesotrophic lake. Total dissolved phosphorus (TDP) followed the same seasonal trends as TP in 2002 and 2003 and ranged from 2–7 _g/L in 2002 and from 2–10 _g/L in 2003. Total nitrogen (TN) ranged from 90–380 _g/L in 2002 and 100–210 _g/L in 2003. During both the 2002 and 2003 sampling seasons, TN showed an overall decline in concentration with mid-summer and fall increases at some stations, which is consistent with previous years’ results. Dissolved inorganic nitrogen (DIN) concentrations showed a more pronounced declining trend over the sampling season compared with TN, corresponding to nitrate (the dominant component of DIN) being used by phytoplankton during summer stratification. DIN ranged from 7–176 _g/L in 2002 and from 8–147 _g/L in 2003. During 2003, discrete depth sampling occurred, and a more detailed look at the nitrate concentrations in the epilimnion was undertaken. There was a seasonal decline in nitrate concentrations, which supports the principle of increasing the nitrogen loading and the nitrogen to phosphorus (N:P) ratio during the fertilizer application period. Chlorophyll a (Chl a) concentrations in Kootenay Lake were in the range of 1.4–5.1 _g/L in 2002 and 0.5–4.9 _g/L in 2003. Over the sampling season, Chl a at North Arm stations generally increased in spring corresponding with the phytoplankton bloom, decreased during the summer, and increased again in the fall with mixing of the water column. The trend was similar, but less pronounced, at South Arm stations in these years, and spring Chl a concentrations were lower. During 2002, total algal biomass averaged during June, July and August was lower in the North Arm than the South Arm. This was the first time this occurred since the commencement of the North Arm fertilization experiment. Results in 2002 indicated Kootenay lake continues to be a diatom dominated lake (80 to 89% of the total average biomass). The overall trend observed throughout the 2003 sampling season was one of a slight decline in algal biomass from the North Arm stations towards those in the South Arm. Kootenay Lake continued to be a diatom-dominated lake (76–83% of total average biomass). Synedra spp. and some Asterionella, as in the previous three years, dominated the early biomass increase in 2003, but the peak biomass in July was largely due to Tabellaria. Depth profiles of biomass in 2003 showed that the distribution of algae was not uniform with depth in the top 20 m of the water column. This was particularly evident in the fertilization zone in August when exceedingly high biomass was reached in the upper surface waters and it declined rapidly with depth. In particular, station KLF 1 reached a total algal biomass of 3.1 g/m3 at 2 m; this was largely due to Tabellaria (contributing to 40% of the total), Fragilaria, Cyclotella, and Asterionella. Tabellaria tended to decline with depth at North Arm stations. The same surface “bloom” was not as pronounced from stations KLF 2 through KLF 4, although a peak was observed at the latter station at 5 m due to high abundance of both Fragilaria and Tabellaria. In contrast, depth profiles from stations in the South Arm tended to exhibit higher biomass at greater depths (below 10 m) in mid-summer. The composition of the samples at depth indicated a greater proportion of diatoms contributing to the biomass; the diatoms were most likely derived from earlier epilimnetic growth. The zooplankton populations in Kootenay Lake were a diverse species assemblage, with a relatively consistent population density in 2002 and 2003. The Kootenay Lake zooplankton density is numerically dominated by copepods, which averaged 91% and 85% of the population in 2002 and 2003 respectively. Daphnia spp. comprised 3% and 5% respectively, and cladocerans other than Daphnia spp. comprised 6% and 10% respectively. The decline in the proportion of cladocerans in 2002 may have been due to a decrease in the biomass of grazeable phytoplankton (nanoplankton, 2–22 _m). As a result, zooplankton biomass may have declined and may not have been high enough to keep pace with the grazing rate imposed by the higher number of kokanee in the lake. Zooplankton biomass had similar trends in both the North and South arms of Kootenay Lake. In 2002, total biomass decreased in both arms, as did the biomass of other cladocerans and of Daphnia. Copepod biomass decreased in the South Arm but increased in the North Arm of the lake. However, in 2003 biomass in all categories increased in both the North and South arms. There was a distinct increase (more than three fold) in the biomass of other cladocerans in the North Arm, as well as an increase (more than two fold) in Daphnia biomass in both the North and South arms. The significant increase of total zooplankton biomass in 2003 was due to increases in the density of Diaphanosoma brachiurum and Daphnia spp., which was reflected in increased biomass. During 2002 and 2003, a sharp decrease in mysid abundance was recorded. During the study period from 1993 onward, mysid densities at deep stations fluctuated along the length of the lake. Average mysid density was higher in the South Arm in 1993, 1994, 2001, and 2002. However, in the period from 1995–2000 and again in 2003, density was higher in the North Arm. During the season, densities increased through summer and declined in winter. Mysid density and biomass tended to be higher at the deep sites than at near-shore sites. Near-shore samples predominantly contained juveniles and immature males and females, while mature and breeding males and females were rare. In 2002 and 2003, mysids in Kootenay Lake were most actively breeding from January to April. During the breeding season, deep samples contained a higher proportion of mature and breeding individuals than near-shore samples. The number of brooding females was low in the fall-winter seasons of 2001–2002 and 2002–2003, which was reflected in the lower number of juveniles during the summer of 2002 and 2003 and in decreased mysid density. Estimated kokanee escapement to Meadow Creek was 0.35 million in 2002, representing the third consecutive low escapement since 1992 when lake fertilization commenced and the lowest escapement since 1991. The three years of low numbers contrast with most escapements in the latter part of the 1990s, which ranged from 0.5–1.1 million. The explanation for this major decrease in 2000–2002 is believed to be linked to the reduced fertilizer loadings from 1997–2000. Despite the small escapement in 2002, the spawning channel was filled to capacity (~300,000 kokanee). In sharp contrast, the 2003 Meadow Creek numbers were close to 0.9 million spawners, nearly triple the 2002 numbers. The 2003 estimate represents the first sizeable increase in numbers in the last four years, but there were still fewer fish than in the parent year (1999) which had about 1.2 million. Mean size of female kokanee returning to Meadow Creek in 2002 was slightly higher (23.3 cm) than the 37-year average of 22.2 cm. Mean size of 2003 kokanee was slightly lower than the 37-year average (males 21.5 cm, females 21.4 cm). Mean fecundity in 2003 was 208 eggs, much lower than the 37-year average, but similar to the levels recorded in the mid-1980s and late 1990s. Decreased mean size and fecundity in 2002 and again in 2003 likely signals a density-growth response as the whole lake population rebuilds following increased fertilization that began in 2001. As the kokanee population rebuilds towards lake carrying capacity, it is predicted that fecundity and fish length will decline and stabilize close to the long-term average. Kokanee fry production from Meadow Creek in the spring of 2002 was about 23 million with 94% produced in the spawning channel. This estimate was the second highest in 27 years of records and nearly twice the average of about 11.9 million. The highest fry production on record occurred in 1994, as a result of high fecundity. The 2003 fry production estimate was slightly lower than the 2002 estimate with approximately 17.9 million produced from the channel and a total of 18.3 million fry from the whole system. Kokanee fry-to-adult survival rates for the 1996–1998 year classes were low and the Meadow Creek recruit:spawner ratios were also very low with replacement not achieved for these cycles. The impact of nutrient reduction commencing in 1997 and continuing through 2000 should be most evident with the 1996–1999 cohorts. This appears to have been the case based on the adult survival estimates four years later (2000–2002). If lake fertilization positively influences kokanee survival as contended in the above analysis, then fertilizer loading increased to the rates applied from 1992–1996 should result in improved in-lake survival for the 2000–2003 cohorts. The increases in 2003 escapements and in-lake abundance estimates lend support to this hypothesis. The trend data suggest that 2004 escapements will be high, possibly one million fish at Meadow Creek. It is believed that the status of the Gerrard rainbow trout population is closely tied to the abundance of kokanee. The increased numbers of kokanee observed throughout most of the 1990s appear to have resulted in very good rainbow trout fishing conditions and escapements in the latter part of the 1990s. The 2001 and 2002 rainbow trout sport fisheries were very poor but some improvement was evident in 2003. There was an increase in the success rates, not only for the smallest trout but also for those in the 2–5 kg size category, and in 2003 there was a slight increase in the 5–7 kg category. There appears to be a time lag of about three years between increased kokanee abundance and increased rainbow trout abundance. The extent to which increasing predation pressure affects kokanee recovery has not been quantified, although it is possible that greater numbers of Gerrard rainbow trout in the late 1990s contributed to the rapid decline of kokanee during the period of reduced fertilization in 1997–2000. |
