Statistical analysis of nitrogen and phosphorus in six streams entering Lake Tahoe

Lake Tahoe is a popular national resource for recreational and scenic attractions, but is impaired due to excess fine sediment and nutrients (NDEP, 2011). Nutrients can stimulate algal growth, which also reduces clarity. The nutrients of greatest impact to water quality are nitrogen and phosphorus. The approach of this study is to use a trend analysis of long-term water quality and discharge data for six streams that enter Lake Tahoe during the late 1980’s through 2008. The results of the model confirm that nutrient loading is influenced strongly by season, especially spring runoff from snowmelt. The highest concentrations or loads occurred in the late 1980’s and early 1990’s, typically during high flows, followed by significant decreases in concentration in most streams for total nitrogen. The results for total phosphorus, however, have a less noticeable reduction, and in some cases appear stable, and therefore, uninfluenced by BMP implementations. The six streams are located north, south, east, and west of the lake, indicating a regional improvement in stream management compared to the concentrations found the early 1990’s. However, most of the streams in this study indicated an increase in concentrations after 2000. A possibility for this is the BMPs are either requiring maintenance, or an increase in anthropogenic activities has surpassed the ability of the existing BMPs to prevent nutrients from entering the streams during high flux. The increase in concentration during base flow may be due to increased concentrations of nutrients in the groundwater that enters the streams.