Cultured periphyton in a subapline environment: Results of a Tahoe Basin pilot system for water quality improvement

Target pollutants of the Total Maximum Daily Load (TMDL) under development in the Lake Tahoe Basin include phosphorus and very fine (<20 µm) particles. Research in the Basin has implicated these pollutants as the key causes in the dramatic decline in the clarity of the lake over the last three decades. Structural best management practices commonly used to reduce nonpoint-source pollution may not be sufficient to achieve the water quality characteristics required to reverse the decline in clarity of this subalpine, oligotrophic lake. Periphyton has several characteristics potentially useful for water quality improvement including the creation of a mucilaginous matrix that physically traps fine particles and the ability to remove dissolved nutrients directly from the water column. A pilot periphyton-based cultured ecology system designed to remove soluble nutrients and fine particles from stormwater runoff was tested at the Tahoe Environmental Research Center, Tahoe City, CA in 2006. Cylindrical translucent fiberglass tanks containing cylindrical screens to serve as substrate were seeded with locally-occurring periphyton assemblages. Biomass was harvested every 30 days and total nitrogen (N) and phosphorus (P) concentrations analyzed. Influent and effluent concentrations of P and N were sampled every 14 days. Synthetic stormwater was created using road sweepings and introduced on two occasions. Estimated annual P removal was 10g/m²/yr. Nitrate influent concentrations of 10 to 81µg/l were consistently reduced to 3µg/l or less. Influent soluble reactive phosphorus (SRP) concentrations of 42 to 64µg/l were reduced to 26 to 46µg/l. SRP removal appears to have been nitrogen limited. Higher removal rates were correlated with higher nitrate input. Turbidity levels during the synthetic stormwater tests were reduced from 135 NTU to 13. Almost 50% of particles removed were 11µm in diameter or smaller.