Burek, K. E., Brien, J. M., & Scheibling, R. E. (2018). Wasted effort: recruitment and persistence of kelp on algal turf. Marine Ecology Progress Series, 600, 3-19.
Washed up on the beach, kelp looks a lot like green lasagna. The thick, smooth strips of Saccharina latissima wave delicately at the sides, the bottom of the alga funneling into a stem and holdfast. Aptly named, the holdfast is what holds the kelp to the bottom of the sea; it branches out like grabby fingers, desperately reaching for something to hold on to. When the holdfast fails, losing its grip, the entire kelp rises, floating to the top of the water and drifting away. In recent years, kelp has begun to decline in the northwestern Atlantic, and turf algae have moved in to take its place. Researchers at Dalhousie University in Nova Scotia set out to determine if kelp can make a comeback from seas filled with turf algae. What they found is that the future of the species points to an uphill battle, one that is waged at the holdfasts of the kelp.
Kelp has had a few rough decades that have led to the decline that threatens not only kelp but the other species that use the algae for habitat. Firstly, kelp are cold-water algae, and increasing ocean temperatures can limit the growth and reproduction of kelp. Additionally, an invasive species came to Canada in the early 1990s, a bryozoan called Mebranipora membranacea. When this bryozoan grows on kelp, it looks like crusty, thin, fishnet stockings growing over the leaves of kelp, and ultimately it destroys the blades that do manage to grow.
At the same time, when kelp are weakened, conditions are optimal for turf algae. Turf algae do better with warmer water; they can grow and reproduce faster. When humans pollute waters by releasing more nutrients, turf algae grow even faster, while kelp at best only maintain their growth. If water is stirred up and sediment floats before settling onto the algae, other algae can be choked out, but turf-algae is much hardier. When the kelp dies, the turf algae moves in. After that, it can be hard for the kelp to come back.
In order to investigate if kelp have a chance of returning, the researchers layed out an ambitious plan to look at kelp in five sites along the Atlantic Coast of Nova Scotia. For each site, they calculated how much wave activity there was as well as how much of the sea floor at each site was covered in turf algae or rock. They also collected samples of kelp that were attached to both rocks and those that attached themselves to turf algae and examined their holdfasts for size and complexity. Finally, they performed a series of 2 experiments. In the first, they placed kelp that was attached to either rock or turf algae in the same channel near the mouth of the cove and recorded if the kelp survived or not. If the kelp died, the noted if it was because the holdfast failed, if the stem of the kelp broke, or if the blades of the kelp degraded. In the second experiment, the researchers examined kelp survival over a 40-week period at one of the sites to determine how waves and weather impacted kelp survival.
In general, kelp struggles to survive when it attaches to turf algae. S. latissima that attached to turf algae was generally smaller than kelp that attached to rocks. In fact, the kelp attached to the turf algae was almost entirely in the smallest size group at each of the five sites, while all of the largest kelps, which were over 100cm long, were attached to rock. Additionally, kelp attached to turf algae had holdfasts with a longer length to width ratio and that were more complex structurally compared to those that grew on rocks. Kelps on turf algae also had a larger proportion of their mass dedicated to the holdfast. This increase in mass, however, didn’t seem to protect kelp as much as it should; the researchers found that kelp attached to turf algae died mostly from the holdfasts detaching from the turf algae. Kelp that had been attached to turf algae was over three times more likely to die than kelp attached to rocks. Regardless, all kelp had a hard time making it through the winter. After a period in October with 3-4m waves, almost all of the large kelp was lost, regardless of if it had been attached to rock or turf algae. By April, there was very little kelp at all.
The findings are bad news for kelp. Researchers found that turf algae covered 97.6% of the sea floor in the three sites they looked at in St. Margaret’s Bay. That means that kelp in Nova Scotia may have very few places to grow besides rocks covered in turf-algae. When kelp does manage to grow on turf algae, extra energy gets invested in making a longer, more complex holdfasts, and yet the kelp is more likely to dislodge and die. This kelp never grows large enough to reproduce, and the cycle of kelp decline does not improve. It’s a discouraging conclusion, but it does paint an interesting picture. This is maybe the first evidence that kelp will try to grow on top of turf algae and return to its prior habitat. What it shows is that if possible, it may be easier to try to maintain a healthy kelp ecosystem than to try to return once it has degraded. For the moment at least, turf algae is here to stay.
I am a PhD student studying Biological Oceanography at the University of Rhode Island Graduate School of Oceanography. My interests are in food webs, ecology, and the interaction of humans and the ocean, whether that is in the form of fishing, pollution, climate change, or simply how we view the ocean. I am currently researching the decline of cancer crabs and lobsters in the Narragansett Bay.