The frequency of marine heatwaves (MHWs) is increasing due to climate change. Although seaweeds are resilient to environmental changes, an increasing body of evidence shows that rising sea surface temperatures have deleterious effects on temperate kelp species. However, information on the vulnerability of juvenile kelp to these stressors and their population stability is limited. This study summarizes findings on the ability of juvenile sporophytes of Macrocystis pyrifera to survive and recover from simulated MHW conditions (22°C, 5 d) in combination with nitrate limitation (<1 µM) by evaluating photosynthetic capacity, nitrate uptake, tissue composition, bio‐optical properties, and oxidative stress of single‐blade juvenile sporophytes (<20 cm). Temperature, nitrate availability, and their interaction had significant effects on the physiological status of juvenile sporophytes after the exposure and recovery periods. Overall, as expected, the photosynthetic capacity of juvenile sporophytes decreased with increased temperature and lower nitrate availability. Short‐term exposure to simulated MHWs resulted in oxidative damage and reduced growth. The termination of the experimental warming allowed partial recovery to control values, indicating high physiological resilience. However, the interaction of both high temperature and nitrate scarcity induced irreversible damage to their photosynthetic capacity, with an increase in compensation irradiance, highlighting potential limitations in the carbon balance of juvenile sporophytes.

Due to climate change, the incidence of marine heat waves (MHWs) has increased, yet their effects on seaweeds are still not well understood. Adult sporophytes of Macrocystis pyrifera, the species forming the iconic giant kelp forests, can be negatively affected by thermal stress and associated environmental factors (e.g., nutrient depletion, light deprivation); however, little is known about the tolerance/vulnerability of juvenile sporophytes. Simultaneously to MHWs, juveniles can be subjected to light limitation for extended periods of time (days–weeks) due to factors causing turbidity, or even because of shading by understory canopy‐forming seaweeds. This study evaluated the effects of a simulated MHW (24°C, 7 d) in combination (or not) with light deprivation, on the photosynthetic capacities, nutrient uptake, and tissue composition, as well as oxidative stress descriptors of M. pyrifera juvenile sporophytes (single blade stage, up to 20 cm length). Maximum quantum yield (Fv/Fm) decreased in juveniles under light at 24°C, likely reflecting some damage on the photosynthetic apparatus or dynamic photoinhibition; however, no other sign of physiological alteration was found in this treatment (i.e., pigments, nutrient reserves and uptake, oxidative stress). Photosynthetic capacities were maintained or even enhanced in plants under light deprivation, likely supported by photoacclimation (pigments increment); by contrast, nitrate uptake and internal storage of carbohydrates were strongly reduced, regardless of temperature. This study indicated that light limitation can be more detrimental to juvenile survival, and therefore recruitment success of M. pyrifera forests, than episodic thermal stress from MHWs.

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Eisenia arborea is the kelp species that is distributed furthest south in the northern hemisphere. Although much is known about how the giant kelp, Macrocystis pyrifera, responds to nutrient-poor conditions, very little is known about Eisenia, even though these two kelps share much of their habitat. The response of E. arborea to environmental nitrogen shortage was explored by determining short-term nitrogen uptake in the laboratory, duration of internal nitrogen reserves in nutrient-poor tanks, and recovery of reserves during weekly pulse fertilization with nitrate. Nitrate uptake was linear for at least 3 h at all concentrations and did not exhaust nitrate in the media. In both experiments in nutrient-poor outdoor tank cultures, regardless of initial tissue nitrogen concentrations, tank nitrate fell to undetectable levels after 4 weeks, and internal tissue nitrogen reserves fell to 1.0% of dry weight within 5 weeks. Low internal reserves were replenished in fertilized treatments, which had significantly greater dry weight tissue nitrogen (up to 1.3%) than thalli in non-fertilized tanks, in which algae tissue nitrogen fell to and remained constant at 1%. We conclude that the nitrogen physiology of E. arborea is adapted to survive a maximum of 5 weeks in the field under nutrient-poor conditions, similar to the giant kelp M. pyrifera.

The spread of non-indigenous and invasive seaweeds has increased worldwide, and their potential effects on native seaweeds have raised concern. Undaria pinnatifida is considered among the most prolific non-indigenous species. This species has expanded rapidly in the Northeast Pacific, overlapping with native communities such as the iconic giant kelp forests (Macrocystis pyrifera). Canopy shading by giant kelp has been argued to be a limiting factor for the presence of U. pinnatifida in the understory, thus its invasiveness capacity. However, its physiological plasticity under light limitation remains unclear. In this work, we compared the physiology and growth of juvenile U. pinnatifida and M. pyrifera sporophytes transplanted to the understory of a giant kelp forest, to juveniles growing outside of the forest. Extreme low light availability compared to that outside (~0.2 and ~4.4 mol photon ⋅ m −2 ⋅ d −1 , respectively) likely caused a "metabolic energy crisis" in U. pinnatifida, thus restricting its photoacclimation plasticity and nitrogen acquisition, ultimately reducing its growth. Despite M. pyrifera juveniles showing photoacclimatory responses (e.g., increases in photosynthetic efficiency and lower compensation irradiance, E c ), their physiological/vegetative status deteriorated similarly to U. pinnatifida, which explains the low recruitment inside the forest. Generally, our results revealed the ecophysiological basis behind the limited growth and survival of juvenile U. pinnatifida sporophytes in the understory.

Eisenia arborea (sensu Ecklonia arborea) is the kelp with the greatest latitudinal distribution on the Northeastern Pacific Coast. It is harvested in Mexico in small amounts for abalone farm fodder and occasionally exported to Asia for human consumption. Because the high-energy environment where it naturally grows limits its sustainable harvest, we explored the domestication and cultivation of this kelp on the west coast of Baja California, Mexico. The life history of E. arborea was completed in the laboratory following traditional methods for kelp cultivation. Gametophytes became reproductive approximately 50 days after spore release and sporophytes were visible to the naked eye after 80 days. When sporophytes reached 2–3 cm, they were transplanted to two sites on long-line systems: in an open bay (Todos Santos Bay) and in a shallow coastal lagoon (San Quintín Bay). At both sites, full differentiation occurred 6 to 8 months after deployment, with fertile blades occurring within a year. Once individuals reached maximum size, blades were harvested by pruning. Monthly, regrowth of pruned blades and new blade addition were measured. Pruned individuals reached the same weight as controls in three months. On the basis of these results, a seasonal seeding program was implemented at a third site where the presence of natural Eisenia is rare due to the lack of hard substrate but was hypothesized to provide appropriate conditions for kelp cultivation (Santa María Bay). Cultures were deployed at three different depths and growth was correlated with season, depth, and temperature. Only cultures seeded in winter developed successfully at this site. This study shows that the culture of E. arborea in a variety of ocean conditions is possible; however, site selection and seasonality are important considerations. Because E. arborea is perennial, it can be harvested periodically during the year thereby increasing yield, and because it is the warmest tolerant kelp species, it is an ideal target for ocean farming and commercial cultivation, considering climate change.