No Project associated with this Finding

What is Periphyton?

The accumulation of periphyton (attached algae) on natural rock surfaces, piers, boats and other hard-bottomed substrates is perhaps the most striking indicator of Lake Tahoe’s declining water quality for the largely shore-bound population. An increase in periphyton growth coincided with the period of rapid growth and development within the basin during the 1960s and could be attributed to an increased nutrient loading from the surrounding watershed via urban and stream runoff as well as groundwater discharge (Goldman 1974, 1981; Loeb and Goldman 1979). Periphyton grows in the littoral (shore) zone of Lake Tahoe. Periphyton biomass is characterized by consistent seasonal growth patterns each year. As nutrient concentrations diminish and shallow, nearshore water temperatures warm with the onset of summer, this community rapidly dies back. The algae can slough from the substrate and wash onshore, creating an unsightly mess with a rather foul odor, in those areas where biomass is high.

Historical Monitoring Efforts

Studies of nearshore attached algae at Lake Tahoe began in the late 1960s and early 1970s as scientists appreciated the link between periphyton abundance and the early onset of eutrophication (e.g., Goldman 1967, 1974; DWR 1971). In the early-to-mid-1980s attention was turned to detailed studies of measuring primary productivity and nutrient cycling in periphyton and its relationship to nutrient input (e.g., Goldman et al., 1982; Loeb and Reuter, 1984; Loeb 1986; Reuter et al., 1986b). It was at this time that the monitoring program for eulittoral periphyton was initiated. Routine monitoring was re-initiated in 2000 and has continued through the present (e.g., Reuter et al., 2001; Hackley et al., 2004, 2005, 2006, 2007, 2008, 2010, 2011).

Periphyton Trends

A seasonal pattern was persistent year-after-year with a seasonal peak in late winter/spring (occasionally in the fall) and minimum biomass in the summer. Areas close to urban zones and/or nutrient input had higher overall Chlorophyll-a (Chl-a) concentrations (refer to Figure 15-4). This is most notable at Tahoe City, Pineland Dollar Point where maximum annual concentrations were in the range of approximately 100-200 Chl-a/m², 75-125 Chl-a/m² and 75-100 Chl-a/m², respectively. On the much less urbanized east shore (Deadman Point, Sand Point, Zephyr Point, Incline West) values were close to 20 Chl-a/m² and almost always <50 Chl-a/m². At many of these east shore locations it is noteworthy that biomass levels began to increase somewhat around 2007.

Recommended Monitoring Plan

The current monitoring program offers higher spatial and temporal resolution; including synoptic sampling, once a year, at approximately 40 sites (see Figure 5) monitored for biomass accumulation along with the nine routine sites discussed above. This synoptic monitoring is timed as much as possible to correspond to peak periphyton growth in each region of the lake, and which typically occurs in the spring. It is important to note that the peak annual biomass does not occur simultaneously around the entire lake, with certain areas reaching peak levels sooner or later than others. To make the whole-lake data comparable, the specific timing for this type of sampling is coordinated with conditions in the field.

For additional information, view the Nearshore Evaluation below.