Marine heatwaves are a pressing hot topic in coastal ecology as these anomalously warm ocean events have become both more severe and more common over the last few decades. Heatwaves have already been linked to die-offs in organisms as small as plankton to as large as marine mammals and shorebirds. In many places, the effects of heatwaves are obvious; kelps disappear, seagrasses die, and corals bleach. In others, the damage is a little less obvious. Along the northeastern Pacific, giant kelp, an iconic large species of brown macroalgae, forms lush and highly diverse kelp forest habitats that are home to algae, invertebrates, fish, and marine mammals. During the unprecedented multi-year marine heatwave that affected the eastern Pacific from 2014-2016, colloquially called the “warm blob”, giant kelp experienced die-offs across its northeastern Pacific range. Much to the surprise of researchers at the Santa Barbara Coastal Long Term Ecological Research (SBC LTER) project, local Santa Barbara Channel populations were particularly resilient during this heatwave. It is still not clear why local kelp survived, but below the surface canopy, were all species within kelp forests as resilient?
To understand kelp forest change, one must first don a SCUBA tank and dive down to the seafloor to observe which species are covering the rocky bottom or swimming through the kelp fronds. Taking stock of which species are present, and which are notably absent, helps us to understand how our kelp forests are changing and what this ecosystem may look like in a warmer ocean. To make such predictions, we first need to establish a baseline – who is usually present in the kelp forest each year and how do these communities change on an annual or seasonal time scale? The SBC LTER was established in 2000 to understand the dynamics of kelp forest habitats over both space and time. Scientific divers at the SBC survey 9 kelp forest sites along the mainland coast of the Santa Barbara Channel every July, while also collecting continuous temperature and oceanographic measurements from permanent moorings. Some of our divers have been exploring these kelp forests for decades; Dr. Dan Reed, a co-author of this study, has been diving the SBC LTER reefs for most of his life and was the founding principal investigator of the SBC LTER. He established our permanent transects 26 years ago.
In the spring of 2020 when my field work for another project was put on hold during the Covid-19 lockdown, I started to dig into our long-term kelp forest datasets. Initially, I was exploring changes in the abundance of a nonnative species of bryozoan, Watersipora sp., and noticed that this species was much more common after 2016, the year following the 2014/2015 marine heatwave. Did other invertebrates show a similar response to the heatwave? What about during the heatwave? I began exploring the response of all sessile invertebrates, which are animals that spend their lives permanently attached to the rocky bottom. We noticed a dramatic decline of invertebrates in 2015 which was driven by warm water, but also by a reduction in their main source of food, phytoplankton, during heatwave years. Which species were present also changed significantly from before to after the heatwave. Of course, there are more than just animals covering the bottom in kelp forests; there are also many species of macroalgae that grow below the surface canopy formed by giant kelp. These “understory” species are in constant competition with animals for space while simultaneously competing for light that filters through the surface canopy formed by giant kelp.
We then wondered whether species composition shifted in understory in the same way it had for the invertebrates. Did any species increase during the heatwave years? What about after the heatwave? The Santa Barbara Channel experienced another significant heatwave in 2018, just two years after the blob subsided. At first, it seemed like the understory were quite resilient from 2014 to 2018; we didn’t see the same die-offs in understory that were very apparent in the invertebrates. Maybe some species that do well during warm years were replacing species that don’t do very well. We dug into the annual monitoring data to explore the responses of species within this important group.
A few patterns were apparent: first, one of the dominant understory kelp species, Pterygophora californica, did not fare well during the 2014/2015 heatwave. In addition to dramatic reductions in biomass, divers, including technician extraordinaire Clint Nelson and co-author and current lead PI of the SBC LTER, Dr. Bob Miller, observed dying Pterygophora individuals with decaying fronds. The tips of the stipes were turning black and disintegrating. We still do not know what caused this phenomenon. Was it disease? Was it just normal decay? Regardless, the kelp forest looked dramatically different at sites where Pterygophora had previously carpeted the bottom.
Secondly, there was a noticeable increase in geniculate coralline algae, which are a group of red algae that build tough calcium carbonate laden tissues (think pink tufts of crunchy algae). One species, Corallina officinalis, increased in abundance across our kelp forest sites and is now far more abundant than before the heatwave years. Similar increases were observed in other corallines. This got me thinking about the consequences of this shift for herbivores in kelp forests. Once when diving at our warmest site, Carpinteria reef, which is now covered in Corallina, I collected a few red urchins to harvest their gonads as a mid-day snack (uni, for the sushi connoisseurs). When I opened the animal, its gonads were insignificant, but I was struck by how full the animal was of coralline algae. Clearly, they are eating it, but is it as nutritious for them as kelps? Are they eating it because that is all that is available to them? How will shifts away from understory kelps during heatwaves affect the quality or quantity of food for these consumers? These are questions that should be explored.
Even in the eight years I have been diving in the Santa Barbara Channel, I have seen changes in this habitat first-hand. Of course, kelp forests are characteristically dynamic, but our baseline seems to be shifting towards species that are more warm tolerant. With increasing frequency of heatwaves, including another heatwave that is currently affecting California, it is critical that we continue to monitor kelp forests to understand how these ecosystems will respond in the face of climate change and ocean warming.