The Use of Algal Assays to Evaluate Algal Biomass in Regulated Streams
Burtwistle, Jennifer Mae Carr
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The problem. This study was undertaken to evaluate the algalgrowth potential of the Des Moines River as it passes through Saylorville and Red Rock Reservoirs. Procedure. The test organism, "Selenastrum capricornutum" Printz, was grown in an algal assay which used various additions of nutrient spikes to waters collected above and below each reservoir. Seasonal river samples were collected from December 1978 to October 1979. Growth response was measured gravimetrically. Findings. Addition of trace metals in combination with iron and manganese most frequently increased algal growth at all sampling stations, followed by the addition of trace metals in combination with phosphorus and nitrogen. Actual algal yield correlated with predicted algal yields using phosphorus and nitrogen content of the waters at all stations during most sampling periods. Using N:P ratios, phosphorus is most often the limiting factor in the river. Above- and below reservoir differences in algal productivity were not distinct. Samples taken from the station below Saylorville Reservoir responded more frequently to nutrient additions than did samples taken from the corresponding above-reservoir station. Differences above and below Red Rock Reservoir (below the City of Des Moines) were less distinct. Conclusions. Nutrient spike additions to Des Moines River water dId not consistently increase algal yield, although actual yield correlated with predicted algal yield. Above- and below-reservoir differences were not distinct. Nutrient additions more frequently increased algal yield at the stations above the City of Des Moines than below it. Recommendations. Further study should include a year-round survey using more sampling sites and frequent collections to determine the algal growth potential and serve as a basis for predicting future trends. Use of individual nutrient spikes, rather than combinations of nutrients, would be useful. Adding a few sampling sites along the Raccoon River would be helpful to determine where algal growth potential changes occur in the downst ream river. Use of EDTA to chelate heavy metals prior to assay could be recommended.
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