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The effects of one year of fertilization on the primary productivity of the Arrow reservoir Roushorne, Meghan Elizabeth

Abstract

The Arrow Reservoir, which consists of an upper and lower basin separated by a narrows region, was created with the impoundment of the Upper and Lower Arrow Lakes in 1969. This, followed by the construction of the Mica (1974) and Revelstoke (1983) dams upstream on the Columbia River, the major tributary to the reservoir, has had a significant impact on the aquatic ecosystem. Preliminary studies conducted in 1997 and 1998 revealed that the nutrient load to the reservoir was likely insufficient to support the ecosystem. A fertilization and monitoring project was initiated in 1999 with the intent of reversing the oligotrophication of the reservoir. The Arrow Reservoir was studied in 1998, the pre-treatment year, and in 1999, the first year of fertilization, to examine the effects of one year of fertilization on the activity and abundance of bacteria and phytoplankton. Primary productivity and phytoplanktonic abundance were estimated from radiolabeled bicarbonate incorporation studies and chlorophyll-a analyses, respectively. Bacterial activity was examined using I4C-glucose incorporation studies along with CTC staining and counting, while DAPI staining and counting was employed for bacterial abundance estimates. Samples were collected on a monthly basis from May to September in 1998 and from April to September in 1999. Six depths (the surface, 1, 2, 5, 10 and 15 m) were sampled at a single, central location in each basin. In addition to these vertical transects, horizontal transects of the upper basin were performed in June, August and September of 1999. Horizontal transect samples were collected at a depth of 2 m at 6 approximately equidistant locations in a north-south transect of the basin. The primary productivity and bacterial activity and abundance in the upper basin, where nutrient addition occurred, appear to have been enhanced by fertilization, as these parameters were each significantly greater in this basin in 1999 than in 1998. The lower basin did not appear to benefit from the nutrient enrichment during the first year of fertilization as these parameters did not differ significantly in this basin from 1998 to 1999. A decrease in algal biomass in the lower basin from 1998 to 1999, which was likely a function of the colder, wetter year in 1999, was not observed in the fertilized upper basin. Algal bioassays using Selenastrum capricornutum were used to assess the bioavailability of nutrients in the tributaries of the Arrow Reservoir. The nitrogen and phosphorus concentrations of all of the tributaries assayed were severely growth limiting. The majority of tributaries were nitrogen and phosphorus co-limiting, while samples from the agriculturally-influenced Innonoaklin Creek were limited to a greater extent by nitrogen.

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