Role of Nitrogen, Phosphorus and Iron in Occurrence of Algal Blooms at Abiquiu and Cochiti Reservoirs

The alkaline reservoirs of Abiquiu and Cochiti in northern New Mexico were examined over the period of 1977-1979 to determine which nutrients influence the cyanobacterial blooms and to characterize phytoplanktonic activities. Nutrient bioassays were performed using Algal Assay Procedure (AAP) bottle test with Selenastrum capricornutum and Anabaena flos-aquae as indicator organisms. Additionally, an in situ method for evaluating nutrient and chemical additions was developed. Nutrients found to limit growth, singularly or when combined, included nitrogen, phosphorus and iron. Aquazine, copper sulfate and paraquat were evaluated as chemical control agents for phytoplankton. Decreases and changes in the population of phytoplankton were observed after copper sulfate and Aquazine additions; however, paraquat eliminated all phytoplankton.

Seasonal variations in phytoplankton populations, primary productivity, and nitrogen fixation were characterized. The diurnal F changes in the depth distributions of cyanobacteria and nitrogen fixation were assessed during an Aphanizomenon bloom.

The dynamics of iron movement through the reservoir water were followed in laboratory experiments using radio-labeled iron. Iron associations are quite complex due to the appreciable salt level and alkaline ph of the water.

Nitrogen, phosphorus and iron concentrations were measured in reservoir water and at the inlets and outlets throughout this study. Nutrient balance analyses indicated that 46% of the nitrogen and 488 of the phosphorus entering Abiquiu Reservoir were retained. For Cochiti Reservoir during a low runoff year nitrogen retention was 78% and phosphorus retention was 43%. In a high runoff year retention was 55% for nitrogen and 55% for phosphorus. Surface loading of nitrogen and phosphorus occurs at extremely high levels; however, the rapid movement of water through these reservoirs minimizes the impact of excessive nutrient loading. The abundance of phosphorus relative to nitrogen in influent waters strongly favors nitrogen-fixing cyanobacteria resulting in occasional Aphanizomenon blooms during the summer months.

Project No. 1345632