| Citation |
E. U. Schindler, D. Sebastian, L. Vidmanic, H. Andrusak, J. Stockner, M. Bassett, K.I. Ashley (2009) Arrow Lakes Reservoir fertilization experiment, Year 8 (2006) report, Ministry of Environment, Limno-Lab Ltd., Redfish Consulting Ltd., Eco-Logic Ltd., University of British Columbia. |
| Abstract/Description or Keywords |
This report presents the eight year (2006) of monitoring results of an ongoing fertilization experiment on the Arrow Lakes Reservoir, in southeastern BC. The reservoir’s food web has been negatively influenced by several anthropogenic stressors which include the introduction of mysid shrimp (Mysis relicta) in 1968 and 1974 and the construction of large upstream and downstream 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, at least 20% elimination of the Upper Arrow tributaries as well as extensive littoral productivity from the former Arrow Lakes. The Mica Dam (1973), built on the Upper Columbia River, blocked upstream migration of all fish species and influenced Arrow Lakes Reservoir water level fluctuations. The Revelstoke Dam (1983) flooded 150 km of the mainstem Columbia River, blocked 80 km of tributary streams which were used by Arrow Lakes Reservoir kokanee, bull trout, rainbow trout and other species. Formation of the two upstream reservoirs also resulted in nutrient retention which ultimately reduced 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 strategy was adopted with seasonal addition of nutrients (nitrogen and phosphorus in the form of liquid fertilizer) that commenced in 1999. The objectives of the experiment were to (1) replace lost nutrients as a result of upstream impoundments and (2) to restore productivity in the Upper Arrow basin that would in turn rebuild kokanee and other sport fish populations. During 1999 to 2003, the seasonally adjusted blend of fertilizer was modeled on the successful Kootenay Lake fertilization experiment. Results from 2003 onward 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 to optimize food web transfer of ‘new’ carbon. From 2004 to 2006 less phosphorus and more nitrogen was added than the 1999 to 2003 period to ensure an adequate nitrogen to phosphorus (N:P) ratio for optimal phytoplankton growth. In 2006, 41.6 tonnes of phosphorus and 244.9 tonnes of nitrogen were added to the reservoir. Adaptively managing the 2006 weekly nutrient loading rates resulted in sufficient nitrogen to phosphorus ratios suitable for phytoplankton growth. Monthly monitoring indicated Arrow Lakes Reservoir remained oligotrophic according to nutrient and chlorophyll a concentrations taken from 0-20 m integrated epilimnetic samples. Additional discrete-depth photic zone sampling at two stations in 2006 indicated that the reservoir was not nitrogen limited, an occurrence observed during 2001 to 2003. Overall phytoplankton abundance and biomass was lower in 2006 at all stations than in previous years. The trend in large diatom populations as in previous years was minimized, indicating carbon was being transferred through the pelagic food web instead of sinking into the reservoir hypolimnion. Zooplankton density and biomass was higher in 2006 compared to the previous two years. Lower Arrow had higher zooplankton biomass than Upper Arrow, a trend that followed previous years. Compared to 2005, density of all zooplankton in 2006 increased in Upper Arrow while in Lower Arrow only copepods and Daphnia increased. In 2006, Daphnia in Upper Arrow and Lower Arrow comprised of 70% of the zooplankton biomass, the highest contribution to total zooplankton since 2000. This increase in Daphnia biomass was likely responsible for the observed decrease in phytoplankton biomass as a result of herbivorous grazing. The overall trend of mysid density and biomass in 2006 was similar to the 2004 and 2005 results. Mysid density and biomass was higher in the fertilization period relative to the two year period prior to nutrient additions. The number of kokanee spawners in tributaries to Arrow Lakes Reservoir declined in 2006 relative to the 2004 and 2005 escapements. Escapements have been increasing since the commencement of fertilization in 1999, with the peak occurring in 2004. Mean size-at-maturity and fecundity of Hill Creek kokanee spawners increased in 2006 from previous years, indicating a density-dependent growth response since reservoir abundance declined in 2004 and 2005. The age and size of the kokanee have been variable during the fertilization period (1999 to 2006) with a temporary shift in age of maturity to age 2+ during 2000 to 2002. Since 2002, kokanee are maturing at age 3+. A kokanee hydroacoustic survey estimate in the fall of 2006 indicated a higher pelgic kokanee density relative to 2005. The total of all age classes in the reservoir was 9 million kokanee, compared to only 5 million in 2005. Transect densities in 2006 ranged from 218-638 fishŠha-1 compared to 160-693 fishŠha-1 in 2004 and 77-497 fishŠha-1in 2005. The mean kokanee biomass in 2006 was 11.5 kg/ha. During the pre-fertilization ‘control’ period (1993-1998), biomass was estimated at ~3 kg/ha. The average kokanee biomass throughout the entire fertilization period (1999 to 2006) has been 10.7 kg/ha. An overall assessment of the eight years of nutrient additions to Arrow Lakes Reservoir indicates that the bottom up approach to restoring kokanee was successful in 2006. The adaptive management approach of applying nutrients to Arrow needs to be continuously applied to ensure favourable water quality and pelagic food resources for kokanee and higher trophic level insectivorous and piscivorous fish. |
