Relevance

This indicator measures the average concentration of total nitrogen for each water year in the seven routinely monitored Tahoe streams. Nitrogen is a nutrient important to the growth and reproduction of plants, and it is considered a pollutant of concern in the Lake Tahoe Basin. Nitrogen and phosphorus together support the growth of algae in Lake Tahoe. Free-floating algae (phytoplankton) occur throughout Lake Tahoe and contribute to the decline in water transparency by absorbing light for photosynthesis. Attached algae (periphyton) coat rocks in the nearshore, adversely affecting nearshore aesthetics. From an ecological perspective, algae are a dominant component of the aquatic food web, providing an important source of energy and nutrients that support other organisms in the food web (e.g., zooplankton and herbivorous fish). However, persistently high levels of algae in Lake Tahoe would be considered undesirable. Nitrate and nitrite are inorganic forms of nitrogen that are directly available for use by plants, whereas total nitrogen includes all organic and inorganic forms of nitrogen that are directly and indirectly available to plants. Sources of nitrate in stream water include atmospheric deposition, urban runoff, and decomposition of organic matter, especially from alder, which is an important nitrogen-fixer.

Human and Environmental Drivers

All the tributaries within the Tahoe Basin deliver sediment and nutrients to a single downstream waterbody: Lake Tahoe. The Tahoe Basin has 63 individual tributaries and associated watersheds, each with its own drainage area, slope, geology, and land-use characteristics resulting in high variability throughout the Region. Furthermore, variability in the amount, timing, and type of precipitation strongly influences runoff patterns. A substantial rain shadow exists across the basin from west to east; precipitation can be twice as high on the west shore relative to the east shore of Lake Tahoe. Both new and legacy disturbances to the landscape can affect the volume of runoff, erosion rates, and the ability of the watershed to retain sediment and nutrients. Landscape disturbances including, but not limited to, impervious road and parking lot surfaces, residential and commercial development, wildfire, and the degradation of stream environment zones, can contribute to sediment and nutrient inputs to the Lake or its tributaries. Weather variations and their effects on stream hydrology (particularly the extremes of droughts and floods), and long-term climate change are considered among the most important environmental drivers of tributary runoff.

Monitoring Programs

• Lake Tahoe Interagency Monitoring Program Stream Monitoring