Most people probably think of kelp as scraggly beach plants they see dried out on the sand. It turns out, though, that not only is kelp a vital part of the marine ecosystem, but it’s got some fascinating characteristics.
Kelp are large brown seaweeds that typically have flat structures similar to leaves, called blades, anchored to a stem-like structure called the stipe. They typically grow in large underwater forests in cold, shallow waters that are rich with nutrients. And they can grow quite fast. In a healthy ocean system, sea urchins eat back the kelp, keeping the seaweed from overtaking the ocean food system. The urchins, in turn, are eaten by sea otters, keeping their numbers in check and allowing kelp to continue moderate growth.
Unfortunately, our ocean system may not be as healthy as it once was. The reasons are complex and not yet well understood. One contributing factor is likely climate change, which causes rising water temperatures that adversely impact kelp, and extensive storms that can rip out entire underwater forests. Pollution and overfishing are other factors that cause significant marine damage.
Scientists are currently studying another factor: the sea star wasting disease of 2013.
Thought to be caused by a densovirus, the disease resulted in the mass mortality of around 40 different species of sea stars. “The year after these sea stars got hit hard, they also had a major el niño event which created really warm seawater,” said Josh Smith, a graduate student at the University of California, Santa Cruz’s Ecology and Evolutionary Biology Department. “As a result of that warm water, kelp productivity declined.” Remember, kelps grow best in those nutrient-rich cold waters.
“What that means,” Smith says, “is that in the absence of these major sea star predators and the absence of food, sea urchins came out of their crevices storming around looking for all the remaining kelp to eat.” It turns out there are no sea otters on the northern coast of California, so the sea urchins had an all-you-can-eat buffet with no one to charge them for their meal. “And, so, for literally hundreds of kilometers of coastline, we've seen complete kelp deforestation and ended on this central good.”
Down in the Monterey Bay, where Smith conducts his research, sea urchin dietary patterns have created patchy landscapes of what’s known as urchin barrens, where the urchins have consumed all of the kelp, among areas of healthy kelp forest. “On a single dive,” says Smith, “you could literally swim through a sea urchin barren, swim through a patch of kelp forest, and then through another urchin barren. Here, just like on the north coast, we had the major sea star disease and so there are no sea star predators here either.”
The one difference between Monterey Bay and the Northern California coast, though, is that Monterey does have sea otters. “Predation by sea otters is sort of contributing to the resistance, helping to maintain kelp forests.”
Though Smith is looking at other factors contributing to the problem, such as the decimation of the sunflower star, which preys on sea urchins, and the effect of worsening storms, he remains interested in how the sea otters fit into the story.
“As I said, we were hypothesizing that sea otters are the reason there's remaining forest patches. But one of the things that really puzzled us was, well, how did these urchin barrens even happen? How did they occur in sea otter country?” The Monterey Peninsula has the highest density of sea otters in the state. “But yet we still saw these urchin barren outbreaks.” Smith and his group started tagging individual sea otters in order to record their foraging behavior. What they found was that sea otters seem to be eating more urchins than ever, even though the barrens keep expanding.
It turns out that looking at the health of the sea urchin is another critical piece of the puzzle. When kelp is plentiful, sea otters consume that food and put their energy into reproduction. But when food stores are lean, urchins can exist in a low energy state, living but not procreating. Meaning otters might crack one of these urchins open, see very little meat inside, and discard it. “Instead they are preferentially going to these patches of kelp forests where the urchins are really healthy,” says Smith, “and they're choosing those urchins over the ones that are inside of barrens.” Practically that means that, while the otters aren’t doing much to help kelp forest persistence in the barrens, they are curbing urchin population within the patches of kelp forest.
Meredith McPherson, also a UCSC graduate student studying coastal science and policy, is studying kelp through aerial satellite platforms, looking at kelp canopy coverage in Northern California over time. “I’m trying to get at things like biomass and density of the plants,” she says, “so that we can have a better estimation of the amount of carbon in the system and what the impacts of recent losses have been to the system.” The kelp canopy, she explains, is essentially the part of the kelp that are floating just below the surface of the ocean. Bull kelp is a particular focus for her, she says, “because it’s a species that is present for the most part in Northern California. They have these bladders that allow them to grow towards the surface, so they they're really tall. They're able to form this canopy that just kind of floats at the surface and it’s a really bright target compared to the water.” That means the canopy coverage can easily be detected using satellites or other remote sensing platforms.
McPherson is currently developing a time series of kelp over the last 33 years from a satellite called Landsat. “That satellite is great because it has a really long time record. It's been flying for decades,” she says. What her research has shown is that between 1986 and 2012, there have been oscillations around a mean canopy coverage level. “But then after 2012 we see this huge drop in the kelp canopy coverage and that persists until 2018.” Like Smith, McPherson looks at the sea star wasting disease and lack of natural sea urchin predators as probable causes for the kelp die-off. “It's probably due to a combination of a lot of different factors, including high sea surface temperature conditions which lead to low nutrient conditions as well as physiological and ecological stressors.”
Author and photographer Josie Iselin also studies the science of seaweed, though for her, answers come from close examination of individual specimens. “About ten years ago or so,” she says, “I started experimenting with seaweed. I was out in the cool air and I held a piece of seaweed up to the sky. It was like, oh my gosh, this incredible color, this incredible form. And I knew I had to get it back to the studio.” She put her samples directly on the bed of her scanner and looks at her images with the eye of an artist and an investigator.
“The imagery came first and it prompted all of these questions,” says Iselin. A beach walker, she has made a habit of noticing patterns when seaweed shows up on the sand and what it looks like. “During those years of drought there were no winter storms. They just didn't come. And it's the winter storms that blow the colder weather in. “Last year,” she says, “we got cold again and the seaweeds rebounded. The intertidal seaweeds you eat that you find all over here are really happy, really robust.” Sadly, the same isn’t true for bull kelp.
“Bull kelp has been vastly reduced,” she says. “The coverage of the kelp forest is just a tiny fraction of what it has been in past years.” She, too, credits the lack of sea otters in Northern California and the explosion of sea urchins for the decline in local kelp. The sunflower star and the sea star wasting disease, in her mind, are other contributing factors. While some strategies for reducing urchins, such as having urchin collecting and smashing parties, suctioning urchins out from the ocean floor, and even relocating sea otters have been floated, not all of these approaches are practical. “A big problem with translocating otters,” Iselin explains, is that they have a social instinct to go right back to where are from.” That’s not a viable solution.
It’s going to take a lot of different studies looking at various different angles to fully understand what’s going on with our oceans and whether we can set the balance right again. The first step is to understand how the lowly seaweed fits in with the life cycle and health of our ocean system.