Kelps (order Laminariales) are some of the most prolific primary producers on Earth, and create ecosystems that provide myriad benefits to coastal communities such as nursery habitat for commercially important fishes and shoreline protection. On the Pacific coast of North America, the dynamics of Macrocystis pyrifera have been well-studied for decades, while those of Nereocystis luetkeana, the dominant canopy-forming kelp from Santa Cruz, California to Alaska, are far less understood (Springer et al., 2007). This study aims to (1) document interannual variability, and (2) elucidate the local (herbivory, disturbance) and regional (climate cycles) drivers of this variability in N. luetkeana populations in southern Oregon. Supervised classification and multiple endmember spectral mixing analysis were used to quantify canopy area and density of N. luetkeana from Landsat imagery over the past 30 years.
Results/Conclusions
Maximum annual canopy area of six kelp beds was quantified for about half the years from 1985-2011. These 6 beds ranged across about 70km and represented around 75% of the N. luetkeana in Oregon. The results of these canopy area measurements confirmed anecdotal reports of enormous interannual variability in N. luetkeana, with total canopy area ranging two orders of magnitude from a minimum of 72,000m2 in 1990 to a maximum 2,655,900m2 in 1999. These dramatic changes in canopy area could take place over very short time periods as well. For example, the maximum canopy area in 1999 was 2,655,900m2, while in 2001 it was 99,900m2, a ~26-fold decrease in two years time. This interannual variability is far greater than that experienced by M. pyrifera which varied by < 1 order of magnitude in Southern California and the Channel islands from 1989- 2016 (CDFW, 2016).
This variability is not easily explained by some of the environmental factors that would be expected to drive kelp population size. While the year with the greatest canopy area, 1999, was a La Nina year, there was no clear link between ENSO phase and kelp canopy area. Furthermore, canopy area did not correlate well with red and purple urchin densities. Future studies will assess the role of upwelling variation as a driver of kelp abundance.