Stressor impacts on the health and persistence of the bull kelp Nereocystis luetkeana in south Puget Sound

Bull kelp (Nereocystis luetkeana), is a critical, habitat-forming, floating seaweed found along Salish Sea’s shorelines. East of the Strait of Juan de Fuca, Nereocystis is the sole canopy forming seaweed. Unfortunately, its abundance in Puget Sound is declining for unknown reasons. Most existing information about abiotic and biotic stressors influencing kelp in our region come from populations of the giant kelp Macrocystis pyrifera in open coast upwelling systems. Little research exists on Nereocystis within Puget Sound, an inland sea with distinct environmental conditions and stressors. In addition, stressors often interact synergistically in marine environments, compounding negative impacts more than would be predicted from single stressors alone. This study examines Nereocystis bed condition and candidate stressors in a region of extensive Nereocystis losses, South Puget Sound. Monitoring parameters for candidate stressors include substrate, temperature, suspended sediment concentrations, nutrient availability, light levels and grazer abundances. Three sites were selected along a gradient in environmental characteristics and apparent bed condition from Budd Inlet to the Tacoma Narrows. Preliminary data shows highest temperatures and lowest salinities at the most interior site. Additionally, WA-DNR documented losses to Nereocystis extent and health at this site from 2013-2016, suggesting a relationship between abiotic conditions and Nereocystis health. Environmental monitoring data will be compared to health indices of individual Nereocystis morphometrics and bed-wide characteristics compiled during spring and summer 2018. Comparative analysis will focus on variation of plant and bed health between sites and identification of key stressor variables that best explain differences. Information obtained from this analysis will help target key variables for future monitoring and aid in site selection for restoration projects