Lake Tahoe Nearshore Periphyton Study

Completed

By Nevada Water Science Center December 18, 2017

Periphyton, a type of algae, is growing on bottom sediment and rocks along nearshore areas of Lake Tahoe. Periphyton is seen as a nuisance and negatively impacts the recreational value of the lake. Periphyton biomass (PB) data collected along the nearshore of Lake Tahoe exhibit increasing trends over the last decade. However, the mechanisms that have caused these changes are not well understood.

Locations of sampling transects for the Lake Tahoe nearshore periphyton study.

We worked with the University of Nevada-Reno, in cooperation with the Lahontan Regional Control Board, to investigate the cause and effect relationships between periphyton biomass and chemical and physical characteristics of the lake and shallow groundwater surrounding Ward Creek, near Tahoe City, California.

Our research provided important information on what are considered the most influential variables controlling the growth of periphyton including

Is Ward Creek an important source of nutrients to periphyton assemblages at distal locations along the Lake Tahoe shoreline?
What chemical and physical variables are important to periphyton growth?
Specifically, are there correlations among these variables to patterns in productivity, nutrient limitation, and taxonomic composition?

Five monitoring transects were installed to help to distinguish whether nutrients are coming from Ward Creek, other on-shore locations, groundwater, or from lake upwelling to the Pineland Periphyton Monitoring Site (PPMS). Ultimately, our results may assist in providing the context for managing water quality in the Tahoe basin.

Chlorophyll levels at 4 sites along the shore of the Ward Creek Watershed at Lake Tahoe — Conceptual diagram of how nutrients may move within the Ward Creek watershed and chlorophyll levels at 4 sites along the shore of the Ward Creek Watershed at Lake Tahoe.

Monitoring Strategy

We sampled five transects on the west shore of Lake Tahoe, north of Ward Creek.

Temperature, electrical conductance, dissolved oxygen and chlorophyll-a were measured at 20 locations (4 locations per transect) using hand-held sensors shown in yellow on map.
Sediment and lake water quality samples were obtained from approximate locations (green triangles on map).
Temperature probes were installed at two locations per transect (three at PPMS) shown in red on map.
Lake stage and head was monitored by a sensor, temperature rod (TROD), and pressure transducer inside a piezometer driven into lake bed sediments.
Turbidity was measured at 1 location in transect 4. Four light loggers were installed in transect 4 and 1 light logger was installed at transects 1 and 3.
Temperature was monitored with iButtons in the Lake column and with the TROD for lake bed sediments at 6 depths.
Groundwater influence was measured by a 1-inch diameter PVC piezometer installed at the PPMS to a depth of 1 m.

Pineland Periphyton Monitoring Site

Automated (hourly) measurements of temperature, electrical conductance, and dissolved oxygen were collected. Samples of nitrogen and phosphorus were collected in the lake water columns and shallow pore water at 3 points (green triangles).

Diagram of sampling sites along a transect at the Lake Tahoe nearshore periphyton study site — Diagram of sampling sites along a transect at the Pineland Periphyton monitoring site, Lake Tahoe.

Water-Quality, Physical, and Nutrient Data

Underwater view of periphyton on rocks and the Lake Tahoe shoreline.

We sampled transects at a high frequency over a 10-month period on the west shore of Lake Tahoe, north of Ward Creek. All data and sample analysis results underwent quality assurance/quality control evaluation according to USGS protocols. Data has been uploaded into the USGS National Water Information System (NWIS). Results from Ward Creek surface water samples has been compared to concurrent Lake Tahoe Interagency Monitoring Program (LTMP) samples for inter-laboratory comparison.

Green periphyton (algae) on rocks on the Lake Tahoe shoreline.

Data Collected

Temperature
Solar Radiation
Dissolved Oxygen
Specific Conductance

Turbidity
Chlorophyll-a
Nitrogen
Phosphorus

Additional Data

Tracked periphyton assemblage dynamics by estimating standing stocks of biomass.
Identified shifts in taxonomic structure.
Quantified rates of net primary productivity.
Assessed potential nutrient limitation.

A trend analysis provided information about how measured parameters varied at seasonal and sub-seasonal time scales. Multivariate statistics (e.g., regression analysis, principal components analysis) were used to determine which physical and chemical conditions are most correlated to periphyton metabolic rates and biomass.

Correlation and Trend Analysis

Comparison between measured periphyton biomass at the Pineland monitoring site (green line) and statistical model using multivariate regression (yellow line). All values were normalized (value-mean)/(standard deviation). Data provided by Hackley and Rueter.

We graphed time series of all measured parameters in order to evaluate qualitative patterns and statistical moments. Multivariate analysis included analysis of variance and principal component analysis. When necessary, structural equation modeling was used to explore direct and indirect relations between possible explanatory variables and seasonal changes in periphyton biomass and periphyton metabolic rates. These analyses are intended to reveal correlation between the seasonal and sub-seasonal variability in physical and chemical properties of lake and pore-water as explanatory variables for the variability and growth of periphyton.

Structural equation modeling and/or mechanistic algal hydrodynamic modeling were used to explore direct and indirect relations between possible explanatory variables and seasonal changes in periphyton biomass and periphyton metabolic rates.