| Citation |
E.U. Schindler, L. Vidmanic, D. Sebastian, H. Andrusak, G. Scholten, P. Woodruff, J. Stockner, K.I. Ashley and G.F. Andrusak (2008) Arrow Lakes Reservoir Fertilization Experiment, Years 6 and 7 (2004 and 2005), Ministry of Environment, Limno-Lab Ltd., Redfish Consulting Ltd., BC Conservation Foundation, Eco-Logic Ltd., University of British Columbia. |
| Abstract/Description or Keywords |
This report presents the sixth and seventh year (2004 and 2005) of monitoring results of an ongoing fertilization experiment on the Arrow Lakes Reservoir. The Arrow Lakes food web has been influeneced by several anthropogenic stressors which include the introduction of mysid shrimp (Mysis relicta) in 1968 and 1974 and the construction of large hydroelectric impoundments in 1969, 1973 and 1983. Construction of Hugh Keenleyside Dam (1969) at the lake outlet resulted in flooding that eliminated an estimated 30% of the available kokanee spawning habitat in Lower Arrow tributaries and at least 20% of the Upper Arrow tributaries. The Mica Dam (1973), built on the Upper Columbia River, blocked upstream migration of all fish species including kokanee and influences Arrow Lakes Reservoir water level fluctuations. The Revelstoke Dam (1983) flooded 150 km of the mainstem Columbia River and 80 km of tributary streams which were used by Arrow Lakes Reservoir kokanee, bull trout, rainbow trout and other species. Formation of upstream reservoirs also resulted in nutrient retention which ultimately redcued Arrow Lakes Reservoir productivity to an ultra-oligotrophic state. Kokanee are typically the first species to respond to oligotrophication. To address the ultra-oligotrophic status, an experimental bottom-up approach was undertaken with additions of nutrients (nitrogen and phosphorus in the form of liquid fertilizer) that commenced in 1999. Two objectives of the experiment were to replace lost nutrients as a result of upstream impoundments and restore productivity in the Upper Arrow basin that would in turn restore kokanee and other sport fish. During 1999 to 2003, the seasonally adjusted blend of fertilizer was modeled on the Kootenay Lake fertilization experiment. Results from 2003 onwards suggested that a closer examination of monthly phytoplankton biomass, species composition and water chemistry parameters was required to adaptively manage the weekly nutrient loading schedule . In 2004 and 2005, less phosphorus and more nitrogen was added than the previous five years in an attempt to ensure an adequate nitrogen to phosphorus (N:P) ratio for optimal phytoplankton growth. In 2004, 39 tonnes of phosphorus and 277 tonnes of nitrogen were added to the reservoir while in 2005, 45 tonnes of nitrogen and 279 tonnes of nitrogen were added.. During 2005, a change in the area of lake fertilized occurred; nutrients were added over a 15 km north-south transect in the Upper Arrow basin versus a much smaller east-west transect area with more frequent additions via the Galena Bay ferry.Monthly monitoring indicates Arrow Lakes Reservoir is oligotrophic according to nutrient and chlorophyll a concentrations from the 0-20 m integrated samples. Additional discrete-depth sampling in 2004 and 2005 indicated that the reservoir was not nitrogen limited as it was in 2003. Adaptively managing the weekly nutrient loading rates in 2004 and 2005 resulted in an improved N:P ratio compared to 2003. The phosphorus additions was less than in previous years and this likely assisted with keeping the N:P ratio favourable for phytoplankton growing conditions. Monitoring of nutrient loads from the main tributary streams in 2004 was similar to the 2001 and 2003 levels. Total dissolved phosphorus input to the reservoir was lowest on record since 1997 while dissolved inorganic nitrogen input was the third lowest on record since 1997. Phytoplankton biomass from July-November 2004 was dominated by the diatoms Fragilaria spp., Diatoma elongatum and Asterionella Formosa.There was a decline in flagellates, dinoflagellates, greens and cyanobacteria. In 2005, diatoms were abundant but more evenly dispersed than previous years and did not form large populations in Upper Arrow or Lower Arrow basins. Although large diatoms dominated, there was a shift to the diatom Cyclotella spp and an increase in flagellates, which are considered edible to zooplankton. Change to the nutrient dispersal method in 2005 resulted in more dispersed phytoplankton populations in Upper Arrow and the narrows area. The largest populations had shifted southward to the narrows area in early June, contrasted to late June or early July in previous years. During September and October, phytoplankton densities in Upper Arrow were lower than densities in Lower Arrow, the first time this has been observed since nutrient additions commenced in 1999. There was also more station-to-station variability in abundance and biomass and the absence of a clear north-south gradient on most dates compared to previous years. The seasonal sampling of phytoplankton at discrete vertical depths in Upper and Lower Arrow in 2004 and 2005 resulted in small and temporary differences in abundance and biomass between depths and among stations and dates. Zooplankton biomass in Upper Arrow continued with the trend of being less productive than Lower Arrow. In 2005, density in all zooplankton categories in both basins decreased compared to 2004, except for Daphnia density in Lower Arrow. Copepods had distinct peaks in July in both basins, while other Cladocera and Daphnia were not present in higher numbers, especially in Upper Arrow. These changes were also mirrored in significant biomass fluctuations. In Lower Arrow, Daphnia composed only 11 % of the total biomass in 2004 but was 40% in 2005. In 2005, an increase of Daphnia occurred in Lower Arrow, and Daphnia made up 40% of the total zooplankton biomass. The shift in copepods comprising a significant portion of the zooplankton population compared to previous years could be explained by an increased grazing pressure on Daphnia from mysids and kokanee. It could also be attributed to a shift in the phytoplankton populations in 2004 to large colonial diatoms, which are considered inedible by zooplankton. The annual average mysid density and biomass data in 2004 and 2005 (deep stations) indicated Upper Arrow is more productive than Lower Arrow. Densities were not uniform along the length of the reservoir, but averaged over the course of the year, they resulted in a greater density in Upper Arrow. Mysid densities tend to fluctuate annually but average biomass in 2004 and 2005 was higher than the pre-fertilization years (1997 and 1998). Increased biomass of Mysis relicta means continued predation pressure on larger zooplankton such as Daphnia. Mean size-at-maturity of Hill Creek spawners was the smallest on record in 2004 and weas only slightly larger in 2005 most likely reflecting a density growth response. In addition to a decline in size, Hill Creek kokanee fecundity also decreased in 2004 and 2005. Fry-to-adult survival rates and the spawner recruit ratios both indicate substantial changes have taken place with Arrow Lakes Reservoir kokanee since lake fertilization began in 1999. The kokanee hydroacoustic survey abundance estimates in 2004 and 2005 decreased compared to 2003 but remain higher than the estimates for the pre-fertilization years. The decline in 2004 and 2005 was attributed to poor fry production at the Hill Creek spawning channel when virtually no fry were produced in either year. The mean kokanee biomass density prior to fertilization (1993-1998) was estimated at ~3 kg/ha, while the post fertilization average was ~10 kg/ha; a threefold increase. Spawner biomass also increased post-fertilization from 1.4 kg/ha pre-fertilization and 3.4 kg/ha post-fertilization. Results of the sixth and seventh year of the Arrow Lakes Reservoir fertilization experiment indicate that the bottom up approach to restoring kokanee has been successful by replacing nutrients lost as a result of upstream impoundments. The adaptive management approach applied to the Arrow Lakes Reservoir nutrient addition program needs to be continued to ensure continued favourable growing conditions for kokanee and other sport fish, however the long-term objective of replacing 500,000 kokanee lost to upstream impacts in Upper Arrow is yet to be achieved. |
