Third Annual Report: 2006 Pre-Construction Eelgrass Monitoring and Propagation for King County Outfall Mitigation

摘要

King County proposes to build a new sewer outfall discharging to Puget Sound near Point Wells, Washington. Construction is scheduled for 2008. The Point Wells site was selected to minimize effects on the nearshore marine environment, but unavoidable impacts to eelgrass (Zostera marina) beds are anticipated during construction. To mitigate for these impacts and prepare for post-construction restoration, King County began implementation of a multi-year eelgrass monitoring and restoration program in 2004, with the primary goal of returning intertidal and shallow subtidal habitat and eelgrass to pre-construction conditions. Major program elements are: (a) pre-construction monitoring, i.e., documenting initial eelgrass conditions and degree of fluctuation over 5 years prior to construction, (b) eelgrass transplanting, including harvesting, offsite propagating and stockpiling of local plantstock, and post-construction planting, and (c) post-construction monitoring. The program is detailed in the Eelgrass Restoration and Biological Resources Implementation Workplan (King County 2006). This report describes calendar year 2006 pre-construction activities conducted by Pacific Northwest National Laboratory (PNNL) in support of King County. Activities included continued propagation of eelgrass shoots and monitoring of the experimental harvest plots in the marine outfall corridor area to evaluate recovery rates relative to harvest rates. Approximately 1500 additional shoots were harvested from the marine outfall corridor in August 2006 to supplement the plants in the propagation tank at the PNNL Marine Sciences Laboratory in Sequim, Washington, bringing the total number of shoots to 4732. Eelgrass densities were monitored in the five experimental harvest plots established in the marine outfall corridor. Changes in eelgrass density were evaluated in year-to-year comparisons with initial harvest rates. Net eelgrass density decreased from 2004 post-harvest to 2006 in all plots, despite density increases observed in 2005 in some plots and at some harvest rates. Eelgrass densities within individual subplots were highly variable from year to year, and the change in density in any interannual period did not correlate to the initial 2004 harvest rate. Continued monitoring should help project managers determine an optimum harvest rate that supports rapid recovery of donor eelgrass beds.