April 25, 2017

California's Kelp Forests Undergoing Massive Changes

Sunflower star, Photo by David Horwich

By Dr. Jan Freiwald, Director of Reef Check California

As Reef Check California begins its 2017 training and survey season, we are keen to find out how the massive changes in the kelp forest ecosystem witnessed over the last couple of years will continue to affect California's nearshore ecosystem. Since 2013, we have documented the west coast-wide decimation of many sea star species due to sea star wasting disease. Sunflower stars (Pycnopodia helianthoides) and giant spined stars (Pisaster giganteus) declined massively in 2013 and 2014 statewide. Sunflower stars, which can grow up to one meter in diameter and are voracious invertebrate predators on California's rocky reefs, have been all but absent from RCCA sites since 2014 (Figure 1). At sites between Point Arena, Mendocino County in the north and San Luis Obispo in the south, where this species has been abundant in the past, the population experienced a massive decline within one year from the disease's onset.

In northern California, the species had experienced a slow decline in previous years and was completely decimated by 2014. In the first year after the decline, anecdotal evidence of many very small individual sea stars provided hope for a recovery of the population. Unfortunately, we have not seen any recovery in 2016. In fact, we saw only two sunflower stars during our statewide surveys – one in the Big Creek State Marine Reserve along the Big Sur coast, and another at our Caspar North site in Mendocino County.

In northern California, sunflower stars are the main predators of sea urchins and other invertebrates, and their decline has had wide-spread consequences for the kelp forest ecosystem. Concurrently with the onset of the wasting disease, the canopy-forming bull kelp (Nereocystis luetkeana) disappeared. It is likely that the unusually warm oceanographic conditions referred to as the “Warm Blob” played a role by either reducing growth or leading to recruitment failures. Regardless of the initial reason, the consequent changes to the community are likely a result of this concurrence. The loss of canopy kelp and sea stars was followed by a sharp increase in purple urchin abundance (Figure 2).


Figure 1. Density of sunflower stars observed in all Marine Life Protection Act study regions monitored in California during annual RCCA surveys.

  Figure 2.Changes in kelp forest community at RCCA sites in Sonoma County. While purple sea urchins have increased over 100-fold, canopy-forming kelps have all but disappeared at these site.
Kelp forest decimated by purple urchins, Sonoma County. Photo by Jan Freiwald

Starting in 2014 at RCCA's Ocean Cove site, purple urchins (Strongylocentrotus purpuratus) began increasing in abundance; densities that had been well below 10 individuals per transect in previous years increased to several hundred per transect. The following year, purple urchins were present in similar densities at two of the other long-term monitoring sites. Concurrently, densities of understory kelps such as Pterygophora declined. Over the last three years the kelp forests along the Sonoma Coast have been transformed into urchin barrens — areas devoid of kelp, resembling a clear-cut forest, where little other than rocks and urchins remains.

This transition from kelp forests to urchin barrens has been observed in other places, but not yet at the same scale seen along the northern California coast. This ecosystem change, from lush forests to barren rock, will have consequences for other species on the rocky reefs, as they depend on the food and shelter provided by the kelp. We have started to see similar changes at sites in central California, where urchin barrens have developed in some areas, as well. Surveys in 2017 will show if these barrens are spreading and what the effects on other species are. Continued, regular long-term monitoring will allow us to understand these changes, and our data can be used to mitigate these dramatic changes through targeted management measures.