Nutrient supplies for algae in Elephant Butte Reservoir were investigated by chemical analysis of water samples, laboratory bioassays with unialgal cultures, and field experiments in which the effect of nutrient additions on primary productivity of the natural phytoplankton population was determined. Laboratory experiments were conducted with unialgal cultures to quantify algal growth at various concentrations of nitrogen and phosphorus in a Rodhe’s nutrient solution. Growth and nitrogen content of algae colonizing artificial substrates suspended in the Reservoir were determined throughout a one year period.

The average nitrate concentration was essentially the same in the lower (near the outlet) and upper Reservoir over a one year sampling period with a mean value of 0.59 mg/liter as nitrogens. The average total phosphate and orthophosphate concentrations were 0.395 and 0.090 mg/liter as phosphorus in the lower Reservoir while average values for total and orthophosphate in the upper Reservoir were 0.642 and 0.176 mg/liter as phosphorus, Condensed phosphates were not detected in water samples analyzed for this form of phosphorus at several times during the year.

Nitrogen was found to be a limiting nutrient for algal growth in all laboratory bioassays of Reservoir water conducted with green algae . Phosphorus was also found to be limiting in most bioassays of Reservoir water conducted in the laboratory. In field experiments conducted in August, 1972 and July, 1973 nitrogen was found to severely limit primary productivity. In a July, 1973 field experiment phosphorus was also found to be severely limiting. Nutrients did not clearly limit productivity in a field experiment conducted in January, 1973, although this interpretation may have been complicated by possible toxic effects due to the addition of Na2Si03 and iron and trace elements.

Growth of algae colonizing artificial substrates suspended in the Reservoir was correlated with the nitrate concentration of Reservoir water. The nitrogen content of the algal material varied from 0.95% to 4.89% on an ash free dry weight basis and was not correlated with the nitrate concentration of Reservoir water. These observations suggest that nitrogen was limiting accumulation of algal biomass on the artificial substrates.

Cultures of two predominate species of green algae occurring in the Reservoir, Chlamydomonas sp. and Platymonas sp., were found to produce acid and alkaline phosphatases when grown with growth limiting orthophosphate concentrations. These enzymes would enable algae to convert monoesters of phosphoric acid to orthophosphate which then could be utilized by the algae. This is significant because much of the soluble phosphorus in the Reservoir is not in the ortho form.

Growth of algae in nutrient solutions with varying nitrate concentrations in the laboratory indicated that the nitrate levels in Reservoir water would severely limit growth if other nutrients were present at the levels supplied in the bioassays. Thus the nitrate concentrations measured in the Reservoir substantiated the observation that nitrogen was a limiting nutrient in field experiments and laboratory bioassays. The total phosphate concentration measured in Reservoir water was sufficient to support considerable algal growth based upon laboratory experiments, however, phosphorus was found to be limiting in most laboratory bioassays and in one field experiment. Apparently the total phosphate measured at some sampling times did not correspond to the phosphorus available for algal growth, Thus algal growth in Elephant Butte Reservoir seems to generally be limited by nitrogen while at times phosphorus is also limiting.

Project No. A-040-NMEX