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Project: San Gabriel River Watershed Pilot Study: Bioassessment Using Algae

Background and Objective

Nutrient over-enrichment is a common cause of stream impairment in the San Gabriel River watershed. Excessive nutrient levels are often associated with macroalgal blooms that can further impair beneficial uses of streams by reducing their ability to support stream biota or causing unpleasant aesthetics or odors. Existing Indices of Biotic Integrity (IBIs) for streams, based on benthic macroinvertebrate assemblages (BMIs), do not necessarily capture impacts to algal-based food web changes from nutrient over-enrichment.

Algal bioassessments represent a valuable environmental monitoring tool for several reasons. As primary producers, algae belong to the biotic community most directly responsive to nutrients and therefore are well poised to reflect the net effect of nutrients on the ecological health of streams. Another benefit of observing algal communities is their ability to colonize virtually any stream substratum, which means that this indicator could be monitored across the diverse range of stream types found in California. Algal taxa also tend to have higher dispersal and growth rates and shorter generation times than benthic macrofauna, allowing relatively rapid responses to change in the environment. Adding algal assessment as a complement to other bioindicators would facilitate a weight-of-evidence approach, which is recommended by the US Environmental Protection Agency for assessing stream health.

The objective was to determine the feasibility of developing and implementing an algal IBI for southern California streams by examining the relationship between algal assemblages and water quality data in streams of the San Gabriel River watershed.

Left: Photo of macroalgal bloom in the upper San Gabriel watershed; Right: Microscopic views of some soft-bodied algal specimens


This project was completed in 2006.


Benthic algae (diatoms and soft-bodied algae) and water chemistry samples were collected from 30 random and 6 targeted sites within the San Gabriel watershed. A subset of algae samples were also analyzed for species composition. Collateral data collection included water chemistry  and toxicity, BMI, and physical habitat.


Significant findings were:

• Diversity of algal taxa in southern California appeared to be sufficient to support development of an IBI.

• Southern California algal taxa exhibited trends in sensitivity and tolerance that were similar to those identified in other regions.

• Diatom and soft algae taxonomic data told consistent “stories” about physical habitat & water quality. For both assemblages, there was decreasing overall diversity, and a relative increase in nutrient-tolerant taxa, with increased nutrient concentrations.

• Statistical analysis of algae community composition revealed three site groupings, corresponding to anthropogenic disturbance.


This pilot was conducted in collaboration with the University of Colorado at Boulder and California State University at San Marcos.

For more information on San Gabriel River Watershed Pilot Study: Bioassessment Using Algae, contact Betty Fetscher at (714) 755-3237.
This page was last updated on: 7/1/2014