Assessment of the condition of Southern California depressional wetlands

Application of Macroinvertebrate, Diatom and Overall Condition Indices for Assessing Southern California Depressional Wetlands

Depressional wetlands are the most abundant, yet most threatened wetland type in California. Despite their relatively ubiquitous nature, they are poorly characterized, and unlike streams, they are not subject to any systematic monitoring and assessment. Consequently, decisions regarding protection, restoration, and management are usually made without the benefit of any regional context of condition, knowledge of predominant stressors, or rigorous documentation of reference conditions.

Ambient monitoring of depressional wetlands can be a critical tool to inform management decisions. However, before such a program can be implemented, biological assessment tools must be evaluated, validated, and modified (if necessary). Once these steps are completed, preliminary assessments can begin to provide answers to critical questions regarding wetland condition and key stressors.

The goals of this study were

1) to adapt three readily available bioassessment tools for assessing depressional wetland condition in southern California. This included a statewide rapid assessment method that had been calibrated and validated for depressional wetlands (CRAM) but had no prior use in a regional assessment program, a macroinvertebrate index developed for depressional wetlands from a different geography (northern California), and a benthic diatom index which had been developed for a different habitat type (wadeable streams).

2) To evaluate the regional condition of depressional wetlands in southern California using the adapted tools.

3) To evaluate the relationship between condition and stress by sampling both local stressors (intensity of direct wetland use, water chemistry and sediment toxicity) and landscape stressors (adjacent land use, flow diversions, road density). Once achieved, these goals should establish the foundation for developing a robust and ongoing depressional wetland ambient monitoring program.

This study included perennial or seasonal depressional wetlands as defined by Brinson (1993) located within the boundaries of the Los Angeles, Santa Ana, and San Diego Regional Water Quality Control Boards. Wetlands were not considered for this study if they were concrete-lined, marine-influenced, treatment ponds, livestock wastewater ponds, riverine (i.e. dominated by riverine hydrology), dry (i.e. not seasonal), or on-military bases (due to access issues). Vernal pools were also excluded; while these are considered a subclass of depressional wetlands, they represent a distinct wetland habitat that is typically evaluated with specific methods. Furthermore, given their rarity and ecological sensitivity they are often assessed through focused studies.