| Citation |
E.U. Schindler, R. Pieters, L. Vidmanic, H. Andrusak, D. Sebastian, G. Scholten, P. Woodruff, J. Stockner, B. Londsay, and K.I. Ashley (2008) Arrow Lakes Reservoir Fertilization Experiment, Years 4 and 5 (2002 and 2003), Ministry of Environment, University of British Columbia, Limno-Lab Ltd., Redfish Consulting Ltd., BC Conservation Foundation, Eco-Logic Ltd. |
| Abstract/Description or Keywords |
This report presents the fourth and fifth year (2002 and 2003, respectively) of a five-year fertilization experiment on the Arrow Lakes Reservoir. The goal of the experiment was to increase kokanee populations impacted from hydroelectric development on the Arrow Lakes Reservoir. The impacts resulted in declining stocks of kokanee, a native landlocked sockeye salmon (Oncorhynchus nerka), a key species of the ecosystem. Arrow Lakes Reservoir, located in southeastern British Columbia, has undergone experimental fertilization since 1999. It is modelled after the successful Kootenay Lake fertilization experiment. The amount of fertilizer added in 2002 and 2003 was similar to the previous three years. Phosphorus loading from fertilizer was 52.8 metric tons and nitrogen loading from fertilizer was 268 metric tons. As in previous years, fertilizer additions occurred between the end of April and the beginning of September. Surface temperatures were generally warmer in 2003 than in 2002 in the Arrow Lakes Reservoir from May to September. Local tributary flows flows to Arrow Lakes Reservoir in 2002 and 2003 were generally less than average, however not as low as had occurred in 2001. Water chemistry parameters in select rivers and streams were similar to previous years results, except for dissolved inorganic nitrogen (DIN) concentrations which were significantly less in 2001, 2002 and 2003. The reduced snow pack in 2001 and 2003 would explain the lower concentrations of DIN. The natural load of DIN to the Arrow system ranged from 7200 tonnes in 1997 to 4500 tonnes in 2003; these results coincide with the decrease in DIN measurements from water samples taken in the reservoir during this period. Water chemistry parameters in the reservoir were similar to previous years of study except for a few exceptions. Seasonal averages of total phosphorus ranged from 2.11 to 7.42 _g/L from 1997 through 2003 in the entire reservoir which were indicative of oligomesotrophic conditions. Dissolved inorganic nitrogen concentrations have decreased in 2002 and 2003 compared to previous years. These results indicate that the surface waters in Arrow Lakes Reservoir were approaching nitrogen limitation. Results from the 2003 discrete profile series indicate nitrate concentrations decreased significantly below 25 _g/L (which is the concentration where nitrate is considered limiting to phytoplankton) between June and July at stations in Upper Arrow and Lower Arrow. Nitrogen to phosphorus ratios (weight:weight) were also low during these months indicating that the surface waters were nitrogen deficient. These results indicated that the nitrogen to phosphorus blends of fertilizer added to the reservoir need to be fine tuned and closely monitored on a weekly basis in future years of nutrient addition. Phytoplankton results shifted during 2002 and 2003 compared to previous years. During 2002, there was a co-dominance of potentially ‘inedible’ diatoms (Fragilaria spp. and Diatoma) and ‘greens’ (Ulothrix). Large diatom populations occurred in 2003 and these results indicate it may be necessary to alter the frequency and amounts of weekly loads of nitrogen and phosphorus in future years to prevent the growth of inedible diatoms. Zooplankton density in 2002 and 2003, as in previous years, indicated higher densities in Lower Arrow than in Upper Arrow. Copepods and other Cladocera (mainly tiny specimens such as Bosmina sp.) had distinct peaks, higher than in previous years, while Daphnia was not present in higher numbers particularly in Upper Arrow. This density shift in favour to smaller cladocerans was mirrored in a weak biomass increase. In Upper Arrow, total zooplankton biomass decreased from 1999 to 2002, and in 2003 increased slightly, while in Lower Arrow the biomass decreased from 2000-2002. In Lower Arrow the majority of biomass was comprised of Daphnia throughout the study period except in 2002, while in Upper Arrow the total biomass was comprised of copepods from 2000-2003. The decrease in zooplankton density in 2002 could be explained by the heavy grazing pressure caused by the sharp increase in kokanee numbers. Furthermore, the abundance of M. relicta, another predator of zooplankton, has increased since fertilization began in 1999. The increase of M. relicta increased predation pressure on larger zooplankton such as Daphnia and had an impact on its density and biomass. Since Daphnia is the preferred prey of both kokanee and mysids, predation may be suppressing the standing stock biomass of Daphnia. The shift in phytoplankton species to a higher biomass of inedible diatoms also could have affected the zooplankton density and biomass. Since 1999, mysid densities have increased, reaching a number similar to that of more productive lakes. The annual average mysid density and biomass data in 2002 and 2003 (deep stations) suggests that Upper Arrow was more productive than Lower Arrow. In previous years, productivity was higher in Lower Arrow from 1997 to 2001. The trend of higher mysid productivity in Upper Arrow began in 2002 and continued in 2003. Kokanee densities in 2002 ranged from 300 – 1450 fishŠha-1 in October in Upper Arrow compared with 300–1000 fishŠha-1 in 2001. Densities in Lower Arrow ranged from 500 to 3750 fishŠha-1 in 2002 compared with 500 – 2621 fishŠha-1 in 2001. In Upper Arrow both age 0+ and age 1-3+ fish densities were consistently higher in the lower half of the basin as in 2001. The most notable change since fertilization began was the low number of age 1-3+ fish in the middle of the basin from Halfway River south to Nakusp at only 100 fishŠha-1. These densities were more typical of pre-fertilization densities in the same area. Compared to Upper Arrow, kokanee densities were again higher and more variable in Lower Arrow. Similar to 2001 the small basins between the Narrows and Lower Arrow had very high fry densities at 1300–3100 fishŠha-1. In the main part of Lower Arrow, kokanee fry densities were comparable to Upper Arrow but age 1-3 densities were notably higher. This was supported by trawl sampling which indicated that age 2 fish were more abundant in Lower Arrow. Mean kokanee densities in October 2002 were 651 and 856 fishŠha-1 for Upper and Lower Arrow, respectively. Kokanee densities in 2003 ranged from 184-629 fishŠha-1 in Upper Arrow, and from 173-938 fishŠha-1 in Lower Arrow. In Upper Arrow both age 0+ and age 1-3+ fish densities were consistently higher in the lower half of the basin as in 2001. This was in contrast to 2002 when the highest densities of all age groups were found near the fertilization zone. In Lower Arrow age 0+ fish densities, as in previous years, were highest in the narrows and lowest mid-basin. The distribution of age 1-3+ fish in Lower Arrow has changed the last three years from highest numbers at the upper end in 2001, evenly distributed in 2002 and congregated in the lower half of the basin in 2003. The 2002 Upper Arrow kokanee escapements (263,000) of index tributaries were higher than in 2000 and 2001, (177,000 and 198,000) as well as higher than recorded escapements in the 1990s. The 2002 Lower Arrow escapements (340,500) of index tributaries were higher than 2000 but lower than 2001 (248,500 and 405,000). The 2003 escapement estimates were incomplete due to the serious fire hazard that prevented aerial estimates to be conducted, however the escapement at the Hill Creek spawning channel was higher than 2002 and 2001 but lower than 2002. Size-at-maturity of Hill Creek spawners decreased slightly in 2001 and more so in 2002, most likely reflecting a density growth response since numbers in the lake have been increased dramatically. The size of 2003 spawners was comparable to the sizes observed in the 1980s and early 1990s. The 2001 data was the first indication of a density dependent growth response and the 2002 data indicated size of fish should decrease as numbers increase. The growth curves for Hill Creek spawners illustrated changes from rapid growth during initial years of fertilization followed by a decline as time advanced. A major shift in age at maturity of Hill Creek spawners was initially detected in 2000 and length frequency analysis and age data indicated that ages 2+ and 3+ were present in the 2001 spawning population. Prior to 2000, age-at-maturity had been confirmed as predominately age 3+. Age data for 2000 and 2001 Hill Creek kokanee comprised of equal numbers of age 2+ and 3+. No age data was available for 2002, however the size and age frequency analysis of trawl caught fish suggested a mix of ages 2+ and 3+. The 2003 data was determined as age 3+ to be the primary age at spawning based on analysis of size and age of the trawl caught fish. Kokanee data collected through 2003 indicated a change in the kokanee population has occurred. Higher escapements, increased growth, higher fecundity, higher fry-to-adult survival rates, and spawner-recruit ratios > 1 all support improved growing conditions now exist within the reservoir. Results of the fourth and fifth year of the Arrow Lakes Reservoir fertilization experiment indicate that kokanee benefited from nutrient addition since 1999. Phytoplankton biomass and nitrogen to phosphorus results need to be closely monitored in order to fine-tune nutrient loading to the reservoir in future years. A continued adaptive management approach needs to be applied to the Arrow Lakes Reservoir to ensure that the preferred food of kokanee is available. |
