Wahleach Project Water Use Plan Wahleach Reservoir ... - BC Hydro

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Executive Summary Fertilization of Wahleach Reservoir was completed in 2004-2006 to restore productivity to the reservoir where large water level fluctuations and altered hydraulic regime have reduced littoral and pelagic productivity to unnaturally low levels. Agricultural grade fertilizers were added in twenty weekly applications between June and October of each year. A suite of physical, chemical and biological parameters, used to examine the ecosystem response, were examined on a monthly basis from June through October. This report summarizes results of the limnological program and the fish stock assessments. Nutrient loading to the reservoir was variable during each study year. Between 4.7- 5.5 MT of 10-34-0 liquid ammonium polyphosphate and 13.2-17.0 of 28-0-0 ureaammonium-nitrate was added to the reservoir in 2004-2006. The total phosphorus load increase from ~186 mg·P·m -2 to ~204 mg·P·m -2 in 2006. Nitrogen loading also increased during the study period from ~1070 mg·N·m -2 to ~1520 mg·N·m -2 in 2006. During each year of the experiment the molar N:P ratio increase from 13.1 to 13.6 and finally to 14.9 in 2006. Water chemistry indicated that the fertilizer was rapidly taken up by the phytoplankton assemblage. Dissolved inorganic phosphorous (SRP) ranged from 1.4-2.7 µg·L -1 , which is below the concentration known to limit phytoplankton growth. Dissolved inorganic nitrate+nitrite concentrations were below 10 µg·L -1 in the summer growing season, again below the concentration known to limit phytoplankton growth. The phytoplankton community responded to increased nutrient loading in 2006 by nearly a 2 fold increase in cell densities and nearly a 3 fold increase in phytoplankton biomass. The phytoplankton community was composed of a mixed assemblage of Chryosphytes/Chryptophytes (heterotrophic nanoflagellates) and by Bacillariophytes (diatoms). The nanoflagellates (2.0-20.0 µm) are the preferred size class of phytoplankton that are consumed by herbivorous zooplankton and a healthy contribution of flagellates are considered necessary for optimal growth of key zooplankton species. The zooplankton community was composed of cladocerans, copepods, a mixed assemblage of rotifers and the occasional ostracod, a benthic invertebrate. The zooplankton assemblage was numerically dominated by cladocerans, principally by large Daphnia sp. which accounted for up to 90% of the community. Densities of Daphnia sp. increased in 2006 and are now over 14 fold higher than densities in 1999. Seasonal average total zooplankton biomass more than doubled over the study period, from 20.2 μg·L -1 in 2004 to 54.5 μg·L -1 in 2006. The biomass was largely dominated by other cladocerans and Daphnia sp which accounted for between 95 and 99% of the total zooplankton biomass in 2004, 2005 and 2006. The continued “top down” pressure on the stickleback population by sterile cutthroat throat has reduced the stickleback population and has decreased the forage pressure on the zooplankton community. In turn, the kokanee have responded to favorable food availability as spawner numbers are now approaching 9000 iii
spawners, from 0 spawners at the start of the experiment in 1995. Spawners were found at all three index creeks with the highest number of spawners observed in Boulder Creek likely due to imprinting of stocked kokanee. 2004 was the last year of kokanee stocking and in 2005 and 2006 stocking was ceased which will allow for determination of natural recruitment in Wahleach Reservoir. The large numbers of spawners are a result of stocking and successful rearing to adult stage through fertilization. A hydroacoustic survey was completed in 2006 and average aerial transect fish densities was approximately 99 fish·ha -1 and ranged from 0 to 268 fish·ha -1 . These densities were far lower than densities computed from surveys conducted in the 1990’s and in 2000. It is highly possible that the 2006 survey may have underestimated fish in the shallower portions of the reservoir which accounted for 38% of the total reservoir area. The abundance estimate for all fish from the average aerial density is approximately 19,000. This is considered very low in respect of the previous population estimate of 424,263 in 2000. There is a strong likelihood that the near shore habitat which was not sampled may have contained a large component of stickleback which would significantly increase the 2006 abundance estimate. From changes in seasonal G trap catch rate data there is supporting evidence that stickleback are more numerous in near shore habitats in spring and early summer than in the fall. This would be consistent with the previous survey data which surveyed into these shallow habitats. A combination of optimal forage and rearing conditions by fertilization and reduced competition for herbivorous zooplankton by sticklebacks, suggests that pelagic conditions are favorable for kokanee and rainbow trout restoration. Currently the iv
Page 1 and 2: Wahleach Project Water Use Plan Wah
Page 3: Citation: Harris, S.L., D. Sebastia
Page 7 and 8: Table of Contents EXECUTIVE SUMMARY
Page 9 and 10: List of Figures Figure 1 Map of stu
Page 11 and 12: List of Tables Table 1. Annual fert
Page 13 and 14: 1 Introduction The Wahleach Reservo
Page 15 and 16: 3. Methods 3.1 Fertilizer Additions
Page 17 and 18: Photo 1. Old tank farm, consisting
Page 19 and 20: Nitrogen Load (mg·N·m -2 ) 140 12
Page 21 and 22: Vertical profiles of dissolved oxyg
Page 23 and 24: y gravid females and the lengths of
Page 25 and 26: hydroacoustic surveys was used in o
Page 27 and 28: 4. Results 4.1 Reservoir elevation
Page 29 and 30: 4.3 Water Chemistry The mean pH was
Page 31 and 32: Silicic acid is used by diatoms to
Page 33 and 34: Density (cells mL -1 ) 4000 3000 20
Page 35 and 36: Chrysoflagellates and Chryptoflagel
Page 37 and 38: 4.5 Zooplankton The zooplankton com
Page 39 and 40: Density (individuals·L -1 ) Biomas
Page 41 and 42: 4.6 Spawner Surveys The number of k
Page 43 and 44: providing some spawning opportuniti
Page 45 and 46: Kokanee spawners 10000 8000 6000 40
Page 47 and 48: Catch Per Unit Effort (# Fish·Net
Page 49 and 50: Weight (mm) Fork Length (mm) 60 55
Page 51 and 52: 4.8 Hydroacoustics Good weather pro
Page 53 and 54: strengths suggested these fish were
Page 55 and 56:
Table 5 lists the range and the mea
Page 57 and 58:
Wahleach Reservoir had lower total
Page 59 and 60:
6. Recommendations 1) Fertilization
Page 61 and 62:
7. References APHA. 1985. Standard
Page 63 and 64:
Pick, F.R. and D.R.S. Lean. 1987. T
Page 65 and 66:
Appendix 1. Phytoplankton species a
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Wahleach Project Water Use Plan Wahleach Reservoir ... - BC Hydro

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