Characterizing biological integrity using benthic macroinvertebrate communities as indicators within the San Gabriel River watershed

The health of stream ecosystems is summarized by the river continuum concept which describes the predictable flow of energy, in the form of organic material, from watershed headwaters to the larger order streams and rivers of the lower watershed, and the corresponding succession of microbial, algal and benthic macroinvertebrate (BMIs) communities along this continuum. Disruption of this stream continuum through anthropogenic inputs and/or physical habit disturbances can be detected by biological community indicators. The SGRRMP has collected BMI, chemical, physical habitat and toxicity testing data from 2005 to 2011 using a multiple lines of evidence approach to determine stream health. This presentation focuses on the changes in the biological community dynamics of the San Gabriel River Watershed using BMIs as indicators of pollution. The Southern California Index of Biological Integrity (IBI) is based seven biological metrics and was developed using reference conditions found throughout the southern California region. The IBI and its component metrics are used to quantify the biological condition of streams in the San Gabriel watershed and to assess the magnitude of potential stressors on these communities. Streams located in the higher elevations near the headwaters of the San Gabriel River have BMI communities that are similar to reference conditions and thus have a higher IBI scores than those in the more highly urbanized lower watershed. The composition of the BMI community follows a predictable stressor gradient that is most clearly defined by physical habitat disruption associated urban development and actively managed flood control which has severely disrupted the hydrology of the watershed. These changes have reduced streambed habitat complexity and the integrity of the riparian zones which are both important to the integrity of BMI communities. Identifying these sources of stress using BMIs is helping watershed managers to focus and prioritize their restoration efforts.