2016 Periphyton Trend Analysis

No Project associated with this Finding

Finding Details

Periphyton, or the algae attached to substrate, has become an increasing concern for the nearshore. There are three dominate algae types within the nearshore that are referred to as periphyton: (1) stalked diatoms, (2) filamentous green algae (FGA), and (3) cyanophytes. During the mid-to-late-winter/spring stalked diatoms and FGA can be seen rapidly colonizing in the nearshore environment, only to die and slough from the substrate in late spring/early summer. Cyanophytes, found further offshore, are responsible for the slimy, crusty, or furry coating over substrate which can be found throughout the year.

Periphyton, which includes all three alga types, was sampled for Chlorophyll-a (Chl a), and Ash Free Dry Weight (AFDW) to determine algal biomass from: 1982-1985, 1989-1993, 2000-2003, and 2005-2015. From 1982-2015, there were no lake-scale nor regional-scale significant temporal trends of algal biomass. On an individual-scale only one site exhibited any statistically significant trends, Sugar Pine Point exhibited a negative trend.

At a finer temporal resolution from 2000-2015, no significant lake-wide trends were detected. Regionally, a statistically significant upward trend was detected in the North (Tahoe City, Dollar Point and Incline West). On an induvial scale, Pineland and Incline Village, exhibited small statistically significant upward trends.

In addition to the Chl a and AFDW, a measurement known as Periphyton Biomass Index (PBI) was calculated. PBI is calculated by visually examining the site to determine the percent cover of algae, then multiplying by the thickness (cm) of the algae. Therefore, PBI can rapidly assess the abundance of algae in a region. Regionally, PBI scores are listed from highest to lowest: West, North, South, and East Shore.

PBI and Chl a were examined in relation to the level of development (i.e. high, medium, low) and land cover (i.e. residential structures, commercial structures, roadways) onshore adjacent to the sites, referred to as the intervening zone (IVZ). For all years measured, lake-wide PBI was greatest in stream mouth/tributary IVZs. General trends for Chl a identified higher algal biomass values associated with IVZs containing high development, although there was variability amongst the data.

Regardless of temporal trends, periphyton growth continues to be an issue of great concern for Lake Tahoe. The future of periphyton monitoring should continue to focus on the data from 2000-2015, as it is nearly continuous. Additionally, the development of the Total Maximum Daily Load (TMDL) has established the importance on the reduction of fine sediment particles, nitrogen, and phosphorus pollution to the Lake. For additional information, view the UC Davis Evaluation of Trends in Nearshore Attached Algae: 2015 TRPA Threshold Evaluation Report below.