Bear Lake is a hardwater lake located in a limestone basin on the border of Utah and Idaho with a surface area of 282 km2, maximum depth of 63 m, and a mean depth of 10 m. The lake was formed by tectonic activity approximately 28,000 years B.P. with no natural outfall. Inflow mainly from small tributaries probably did not equal evaporation. These conditions resulted in a concentration of carbonate salts and a unique macrochemistry, with magnesium as the predominant divalent cation. The isolation from any major drainages also led to the development of endemic fish species. In 1912, Utah Power and Light Company completed a series of canals diverting water from the Bear River into the lake during the spring and later released for downstream irrigation and power needs. Diversion of the Bear River into the lake increased water flow and presumably loadings by as much as 70 percent above historic conditions. A 2-year study recently completed found nutrient loadings into Bear Lake at meso-eutrophic levels, but the lake limnologically oligo-mesotrophic. Phosphorus was the principal limiting nutrient. Because of this apparent anomaly in trophic status and the known relationship between calcium carbonate and phosphorus in marl lakes, this study was undertaken to quantify the reduction of potential algal biomass through coprecipitation of phosphorus. Initially, three different phosphorus levels were added to synthetic Bear Lake medium without algae to determine if coprecipitation would occur under ideal conditions. The pH of the medium was raised artificially with NaOH to 8.5, a value not uncommon in Bear Lake. After 4 days 100 percent of the phosphorus had precipitated in the 10 μg P/l treatment. Bioassays were then conducted in softwater and Bear Lake media withSelenastrum capricornutumat 10 different phosphorus levels. At similar nutrient levels the maximum biomass reached twice that of the biomass in the Bear Lake medium. These may explain the low primary production experienced in many hardwater lakes and Bear Lake in particular. It may also be inferred that a potential self-cleansing mechanism exists within Bear Lake that would allow a rapid reversion to historic water quality if nutrient loadings were reduced.