Wetland condition assessment and mitigation - guiding stewardship of wetlands

Background/Question/Methods

We propose a framework for wetland mitigation that places each wetland in the context of surrounding watershed, the complexity of the type of wetland relative to other types of wetlands, and a local site assessment. We also propose a calculated mitigation debit/credit ratio.  A watershed profile prioritizes wetlands by assessing the proportional abundance and condition status based on national standard classifications such as NWI or NatureServe’s Ecological Systems and remotely sensed Ecological Integrity or Landscape Condition Model Assessments. For individual wetlands, the wetland type can be prioritized by a “complexity rating” and the current ecological integrity assessment (EIA) status. Each wetland can then be rated by three criteria: the ecological integrity score, the “complexity rating,” and the watershed priority weight. This combined rating can be used as the potential “debit value” on an acre by acre basis if impacted or destroyed, or a “credit value” when mitigation improves wetland integrity. We can compare any two wetlands (impact and mitigation site) regardless of type and calculate a mitigation ratio that reflects the wetland’s relative ecological priority. We propose this method as a framework to prioritize wetlands and the resulting calculated ratio to steer impact away from high priority wetlands and direct mitigation that will most benefit the watershed. This method is based on the ecology and functions of the wetlands involved rather than set standard USACOE ratios. We can also apply the EIA method to assess and monitor performance standards of the mitigation site.

Results/Conclusions

Our “complexity rating” considers the complexity of soil and vegetation based on wetland type (NWI or Ecological System) and incorporates the degree of difficulty (including length of time) for successful mitigation. It is based on a maximum achievable ecological integrity score and a limit to the increment or amount of increase in the integrity, assuming that less complex wetlands (e.g. freshwater herbaceous) are more easily mitigated then very complex wetlands (e.g. woody peatlands). These guidelines follow the USACOE and EPA 2008 Final Rule to use a watershed approach to “maintain and improve quantity and quality of wetlands and other aquatic resources in watersheds through strategic selection of compensatory mitigation project sites.” We demonstrate a watershed profile and mitigation framework for Southeast Alaska, from a pilot study for the Juneau watershed.