Most of the talk about kelp and marine heatwaves is about adult forests: the canopies that thin out, the urchins that move in, and the reefs that turn bare. A new study from UC Santa Barbara looks at a much earlier stage, the first few weeks of a kelp’s life. That stage matters more and more, because both restoration projects and kelp farms depend on young plants raised in hatcheries and then set out at sea. If those juveniles meet a heatwave just after leaving the nursery, a whole season’s work can be lost.
The team, led by Lauren Smith with corresponding author Halley Froehlich, published the work in Frontiers in Climate this autumn. They grew juvenile giant kelp (Macrocystis pyrifera) on “mini-spools”, small versions of the seeded lines used on farms. The spores came from Ocean Rainforest, the company behind the large Ventura County farm plan that California approved last month. In a first lab experiment, they held juveniles for a week at temperatures from 5°C to 30°C. The pattern was clear: above 20°C, the young kelp did worse on every measure the team tracked. Growth was fastest at around 10°C, and cover peaked at around 15°C.
The second experiment tested whether the nursery could prepare the kelp for trouble. During a week-long hatchery stage normally held at about 10°C, some spools were given a brief warm spell (a day at 12°C, then a day at 15°C), some were given full-strength rather than half-strength nutrient medium, and some got both. All the spools then went through the same simulated heatwave, five days at 18°C, followed by a recovery period. The spools that had both a warm start and extra nutrients did best. Their average growth rate was 0.478% per day, against 0.266% for warmth alone and 0.303% for nutrients alone. Both the fully primed and the nutrient-primed spools ended up with more than twice the dry weight of the unprimed controls. The authors say the results support a “nutrient rescue” idea: well-fed kelp copes better with heat. They also say California’s current advice to wait for water of about 15°C before outplanting is reasonable, although cooler water closer to 10°C may suit young plants even better.
The authors are frank about the limits. These were short lab trials with small samples: four spools per temperature, and twelve per priming treatment. The priming experiment had to run in two batches in different months, and a bacterial outbreak in the hatchery came between them. At 25°C and 30°C, cyanobacteria overgrew the spools, so those results are shaky. All the kelp came from one source population, faced one simulated heatwave, and was grown on miniature spools rather than real farm lines. Nobody has yet tested whether the benefit carries over into the open ocean, or what is happening inside the kelp to produce it. Those are the next questions. Still, the idea is appealing: some of the resilience restoration needs might be built in cheaply, in the nursery, before a single plant goes in the water.