A bloom of blue-green algae of nuisance proportions occurred at Cochiti Reservoir in the summer of 1976. The bloom was characterized as predominately Aphanizomenon with a smaller biomass of Anabaena. Bioassays of extracts prepared from this Aphanizomenon bloom indicated that the bloom was toxic to vertebrate organisms. In the summer of 1977 the surface bloom of Aphanizomenon was reduced by an order of magnitude and Anabaena was approximately equal in abundance to Aphanizomenon. Because of increased light penetration and enhanced photosynthetic activity at greater depths, phytoplankton primary productivity decreased only to about half the 1976 values. The decrease in blue-green algae in 1977 was associated with lower levels of total nitrogen and phosphorus in reservoir water. Cochiti Reservoir was characterized as eutrophic in the summer of 1976 and mesotrophic in 1977.

At Abiquiu Reservoir the blue-green algal population and primary productivity was considerably lower than at Cochiti Reservoir. In the summer of 1976 Aphanizomenon was abundant in algal samples while Anabaena was rarely observed. In 1977 smaller populations of Aphanizomenon occurred with an increased diversity of diatoms and green algae. Primary productivity and nutrient concentrations in Abiquiu Reservoir were characteristic of mesotrophic conditions in 1976-77.

Project No. 3109-069, A-058-NMEX