Long-term changes in bull kelp in South and Central Puget Sound

Understanding the historical extent of biogenic habitats can provide insight into the nature of human impacts and also inform restoration and conservation actions. This talk summarizes two linked studies that used historical records to explore long-term changes in bull kelp (Nereocystis luetkeana) extent in South Puget Sound (SPS) and Central Puget Sound (CPS), areas of concerns for losses. We synthesized historical charts, surveys and ecological studies in order to understand patterns over time in bull kelp distribution, from the late 1800s to recent years. In SPS, we identified a basin-wide baseline from the 1870s, early in the period of European settlement. There, the linear extent of bull kelp decreased 67% between the 1870s and 2017, with large losses in all areas except the Tacoma Narrows, where healthy beds persist. In CPS, dramatic losses along Bainbridge Island and other shorelines contrast strongly with persistence along Southern Whidbey Island and other locations. These trends are distinct from the exposed coast and the Strait of Juan de Fuca, where canopy area has generally been stable over a similar time period. Multiple natural and human factors that are known to impact kelp could have driven the observed losses in SPS and CPS, but limited data exist at the spatial and temporal scale of this study. Elevated water temperatures and low nutrient concentrations could have played a role in some areas, where the observed values approached thresholds associated with physiological damage to kelp. In recent years, bull kelp observations predominately occurred along shorelines with high current velocities, especially along the Tacoma Narrows and Admiralty Inlet. Areas of intense tidal currents and mixing may provide spatial refugia from stressors