Trends in floating kelp along the Strait of Juan de Fuca and the outer coast (WA, USA): patterns and links to environmental drivers

The Salish Sea hosts one of the most diverse kelp communities in the world. This critical biogenic habitat is known to respond to both environmental drivers and human stressors, but regional trends are not well documented. The most conspicuous species, bull kelp (Nereocystis luetkeana) and giant kelp (Macrocystis pyrifera), form floating canopies that can be easily monitored with airborne remote sensing. This study explores links between floating kelp canopy area since 1989 and a series of environmental drivers using the Washington State Department of Natural Resources’ (DNR) longterm monitoring dataset along the Strait of Juan de Fuca and the outer coast. Many findings support widespread understanding of kelp ecology, while other results are surprising. Both species showed high interannual variability in canopy area, while the annual species (bull kelp) showed substantially greater variability. During the monitoring period, canopy area increased more than three-fold from 727 ha in 1989 to 2,574 ha in 2000, and then decreased. The 2014 estimate is similar to the lowest values in 1989 and 1997, which could represent either a loss or a return to previous levels. Climate has been hypothesized to be an important driver of kelp abundance, and we found significant correlations between canopy extent and two regional climate indices (North Pacific Gyre Oscillation and Pacific Decadal Oscillation). However, other patterns argue against a simple coupling between floating kelp abundance and climate indices. Greater declines in some areas can be attributed to local stressors, such as extreme losses adjacent to the Elwha River mouth following the release of sediment associated with dam removal. Other factors that were outside the scope of this study likely played a role as well, including changes in grazer populations due to sea otter predation and human harvest of sea urchins