Heading photo by photographer, filmmaker, and ocean enthusiast Nathan Minatta
Photo credit: Yale E360
Look around you.
Trees tower above your head. Sparse rays of sunlight filter through the canopy, casting dancing beams on a textured landscape of rocky ledges and dense underbrush. The rasp of your breath is loud, almost intrusive, in the quiet world. Around you, animals creep and lurk in silence, hiding among the leaves of trees glowing gold. Movement in the corner of your eye brings your attention to your right, where a silver tail disappears rapidly into the dense forest. You look down, and realize you’ve floated a little too high: as you exhale and descend, the forest welcomes you back into its embrace.
No, you’re not dreaming. You’re in an underwater forest, a world supported by sky-reaching columns made of a humble type of brown algae: Macrocystis pyrifera. Wow. Even the name inspires awe. But for simplicity’s sake, let’s call it by its common name: kelp.
Photo credit: Nathan Minatta
Photo credit: Nathan Minatta
Kelp forests provide the foundation for most of the life you’ll find in coastal, temperate seas: basically, anything colder than coral reefs but warmer than the Arctic. In these forests, kelp provide the 3D structure for organisms to hide in, to live on, and to eat: supporting an ecosystem in constant motion. Fish, octopuses, and sharks fly through the forests, while urchins, starfish, snails and lobsters patrol the seafloor, perpetually munching away. Every one of these organisms depend on kelp for something: without it, their populations dwindle and collapse.
Unfortunately, in several regions of southern California, the native kelp is being out-competed by an invasive species of brown algae called Sargassum horneri. This species of Sargassum is native to Japan and Korea, but when it was brought here (likely on the hull of some boat), its populations exploded and started to take over available space in the kelp forests. The result: the kelp is slowly dying and being replaced. This is not good for business.
A Sargassum dominated site. Photo credit: Jason Baer
A Macrocystis, or kelp dominated site. Photo credit: Jason Baer
You see, Sargassum doesn’t provide the same services that kelp does. It’s not tall, like kelp, or handsome, like kelp, or tasty, like kelp. You may think I’m kidding, but those are the real problems the ecosystem is facing. Sargassum is a short, bushy algae that doesn’t grow taller than a few feet off the substrate: providing a poor habitat for larger fish and sharks. It’s slimy, dense and easy to get tangled in, making it difficult for animals on the seafloor to move around and for kelp juveniles to obtain light to grow.
In a healthy kelp forest, the seafloor is kept relatively clear of algae that compete with kelp by grazers, such as urchins. Photo credit: Laís Lima
To make matters worse, Sargassum produces toxins called polyphenols which the natural community in southern California is not accustomed to. Even the urchins, which live on a diet of almost exclusively brown algae and possess an appetite rivalling Godzilla, don’t want to eat it. So, with its growth unchecked, Sargassum continues to take over, and where there were once great forests of kelp, there’s now dense thickets of Sargassum. Or ‘slime meadows,’ as I like to call them.
We’re not sure yet how the Sargassum is gaining such a competitive advantage over the kelp, but we think it might have something to do with microbes. Algae are covered in bacteria, just like us, and the bacteria on this “skin” are constantly interacting with other bacteria in the water and on the organisms around them. Often times these interfaces between bacterial communities create turf wars. We call this microbial warfare, and it’s happening more often, and more significantly than you think. In fact, it’s probably literally happening right under your nose (no, there’s nothing you can do about it).
Think of it like an elementary school playground. Remember when Stacy practically owned that nice swing, the one that flew the highest, had the perfect butt-to-thigh ratio, and didn’t sound like nails on a chalkboard as you pumped your legs vigorously for the heavens? Yeah, we all remember that swing. It was a nice damn swing. And Stacy guarded it tooth and nail because of it. She wouldn’t hesitate to throw a death glare your way, maybe even a couple rocks if you got too close.
Kelp and Sargassum growing in close proximity to each other compete for space, nutrients, and light. Photo credit: Jason Baer
Stacy knew the value of good territory, and bacteria do too. They produce antibiotics to ward off other bacteria (like Stacy’s rocks), act as pathogens to attack the other host, and secrete compounds onto the substrate that make it harder for baby kelps to settle and grow. And it’s not just in kelp forests: the microbial communities associated with all the organisms around us are constantly facing off, and the results of these interactions change the environment around them.
The invasive Sargassum horneri brings a new, unique microbial community to the table here in California, and the lack of appropriate defenses on the kelp’s part may be one of the reasons why it is losing the battle. To investigate this, the Dinsdale lab and I took a trip to Catalina Island, a beautiful and oddly-placed paradise off the coast of bustling Los Angeles, to explore more pristine kelp ecosystems that are currently losing the war to Sargassum.
There, we spent a week alternating between being wrapped in neoprene and lab coats: scuba diving allowed us to collect data and samples of kelp and Sargassum, and back in the lab we ran experiments on the health of each species, and looked at the microbial communities associated with each species alone compared to when they were interacting. When we have analyzed this data, we hope to be able to better identify the specific factors that are giving Sargassum the upper hand in this constant battle for the watery, metaphorical swing.
Dr. Elizabeth Dinsdale collecting kelp and Sargassum samples for later analysis. Photo credit: Jason Baer
PhD student Laís Lima collecting data about the frequency of kelp and Sargassum in the ecosystem using a transect line. Photo credit: Jason Baer
A curious schooling shark, commonly known as soupfin sharks, circled us as we worked. Photo credit: Jason Baer
Undergraduate Hayden Bursch swimming through a site in which kelp is losing the battle with Sargassum. Photo credit: Jason Baer
Unfortunately, Sargassum isn’t the only threat to kelp: warming southern California waters also present a grave danger to the survival of kelp, and the ecosystem it supports. Rising ocean temperatures stress and eventually kill native kelp species, who are accustomed to colder waters, and free up space for invaders like Sargassum to begin taking over the ecosystem. Understanding how these environments shift, and how bacteria might be mediating this, is more important now than ever, as climate change causes huge shifts in ecosystem community structure across the globe (see this article for a similar situation happening on coral reefs).
Photo credit: Jason Baer
In any event, the diving in Catalina was beautiful, and is always the best part of field science. Kelp still reigns sovereign in many sites, sharks still prowl in the outfield, and despite the shifts in dominance from kelp to Sargassum, the ecosystem is still very much alive. Field work involves long days, short nights and a lot of delirium, but meaningful data, a mojito here and there, and the hope of answered questions keeps us coming back for more. That, and the humbling reverence of diving in a kelp forest, where, if only for a few short minutes, you can become a player in a story entirely different from your own. Addicting, really.
-Jason