How Climate Change Reshapes Kelp Forests and Urchin Grazing
Beneath the ocean's surface lies a world remarkably similar to terrestrial forests, with towering giant kelp (Macrocystis pyrifera) forming dense underwater groves that shelter countless marine species.
Among the most productive ecosystems on Earth, rivaling tropical rainforests in biodiversity.
Voracious grazers whose behavior is transformed by changing ocean conditions.
Warming waters and increasing acidity threaten the delicate balance of marine ecosystems.
These submerged ecosystems face an invisible threat—our changing climate is simultaneously warming ocean waters and increasing their acidity through carbon absorption.
This double-edged sword doesn't just affect the kelp itself but also transforms the behavior of one of its most voracious consumers: the purple sea urchin (Strongylocentrotus purpuratus). The delicate balance between these foundational species is shifting in ways scientists are just beginning to understand, with profound implications for the future of our coastal ecosystems.
You might assume that increasing ocean acidity would universally harm marine vegetation, but giant kelp reveals a more complex story. Adult kelp possesses a remarkable ability to withstand elevated carbon dioxide levels.
Research shows that when exposed to future acidification conditions (pH reduced by 0.3-0.4 units), kelp maintains its photosynthetic rates and growth nearly unchanged 1 8 .
While kelp may tolerate acidification, rising temperatures present a more formidable challenge. When exposed to elevated temperatures alone, kelp experiences significant reductions in both growth and photosynthetic carbon uptake 2 .
The effect is particularly pronounced during kelp's vulnerable early life stages. Studies on kelp spores reveal that combined high temperature and high CO₂ conditions significantly decrease germination rates and increase mortality 4 .
Perhaps most counterintuitive is how these stressors interact. When kelp faces warmer and more acidic waters simultaneously, something remarkable happens: the combined effect differs dramatically from either stressor alone.
One study found that while elevated temperature alone reduced kelp growth and photosynthesis, the combination of high temperature and high CO₂ actually enhanced both growth and photosynthetic carbon uptake compared to ambient conditions 2 . This unexpected synergy demonstrates the complexity of climate change impacts—simple narratives rarely capture the full story.
| Stress Condition | Effect on Growth | Effect on Photosynthesis | Impact on Early Life Stages |
|---|---|---|---|
| Elevated CO₂ alone | Minimal change 1 | Minimal change 1 | Reduced germination 4 |
| Elevated temperature alone | Significant reduction 2 | Significant reduction 2 | Increased mortality 4 |
| Combined high CO₂ & temperature | Increased 2 | Increased 2 | Severe developmental restrictions 4 |
| Marine heatwaves | Canopy loss | Not measured | Not measured |
While kelp shows mixed responses to climate stressors, purple urchins face more consistent challenges. Unlike kelp, urchins—especially during their early developmental stages—experience significant physiological impacts from ocean acidification.
Urchin larvae reared under high CO₂ conditions show developmental delays, reduced growth rates, and elevated metabolic demands 5 .
The core issue for urchins in acidified waters appears to be energetic. Maintaining internal pH balances and other physiological functions under high CO₂ conditions requires additional energy at the expense of growth 5 9 .
One study documented how urchin larvae in high CO₂ environments had similar feeding rates but developed more slowly, suggesting they were redirecting energy from growth to physiological maintenance 9 .
Climate change affects not just urchin physiology but also their behavior—with potentially dramatic consequences for kelp ecosystems. Research reveals that urchin feeding behavior responds to changing conditions, but these responses are complex and context-dependent.
In a fascinating twist, urchins fed kelp that had been grown under future climate conditions showed higher growth rates than those fed kelp grown under current conditions, partially ameliorating the negative direct effects of climate stressors on the urchins themselves 2 .
The predator-prey dynamic also shifts under climate influence. Under current conditions, the presence of spiny lobsters—natural urchin predators—significantly reduces urchin feeding activity. However, this protective effect diminishes under future ocean conditions, as urchins become less responsive to predator cues 3 . This behavioral change could lead to increased grazing pressure on kelp forests as urchins' natural behavioral controls break down.
| Stress Condition | Effect on Larvae | Effect on Adults | Behavioral Changes |
|---|---|---|---|
| Elevated CO₂ alone | Developmental delay, reduced growth 5 | Reduced gonad development 6 | Altered predator response 3 |
| Elevated temperature alone | Not studied | Reduced growth 6 | Increased foraging 6 |
| Combined high CO₂ & temperature | Not studied | Reduced survival 6 | Reduced predator avoidance 3 |
| Low food availability | Not applicable | Amplified negative effects of stressors 6 | Increased active grazing 6 |
To understand how climate change will truly affect kelp forest ecosystems, we need to examine both kelp and urchins together under simulated future conditions.
A groundbreaking study conducted by Matthew Brown and colleagues investigated the independent and combined effects of elevated temperature and pCO₂ on both giant kelp physiology and purple urchin grazing behavior 2 .
The researchers designed a comprehensive experiment with a factorial design that allowed them to tease apart the effects of individual stressors and their interactions:
Giant kelp sporophytes were collected and acclimated to laboratory conditions before being randomly assigned to one of four treatment combinations.
Purple urchins were similarly distributed across the same four climate treatments, with some fed kelp from different treatments.
Researchers measured kelp growth rates, photosynthetic performance, and urchin growth, gonad development, and feeding rates.
The findings revealed complex interactions that would have been missed in single-stressor studies:
Elevated pCO₂ alone had no effect on growth or photosynthesis, while elevated temperature alone reduced both. However, the combination of high temperature and high CO₂ resulted in significantly increased growth and photosynthetic carbon uptake compared to ambient conditions—an unexpected reversal 2 .
Urchins held under future conditions generally exhibited reduced growth and smaller gonads. However, urchins fed kelp grown under future conditions showed higher growth rates, suggesting that changes in kelp nutritional quality might partially compensate for direct negative effects 2 .
| Organism | Treatment | Growth Response | Other Key Observations |
|---|---|---|---|
| Giant Kelp | Elevated pCO₂ alone | No significant change | Photosynthetic carbon uptake unchanged |
| Elevated temperature alone | Significant reduction | Photosynthetic carbon uptake reduced | |
| Combined high temperature & high pCO₂ | Significant increase | Enhanced photosynthetic carbon uptake | |
| Purple Urchin | Future conditions (combined) | Reduced growth | Smaller gonads |
| Fed future-condition kelp | Higher growth rates | Partial amelioration of direct effects |
Understanding how these complex ecosystems respond to climate change requires sophisticated tools and methods. Researchers in this field employ an array of approaches to simulate future conditions and measure biological responses.
| Research Tool | Primary Function | Application in Experiments |
|---|---|---|
| pCO₂ dosing systems | Precise control of carbon dioxide levels in seawater | Creating acidification conditions (e.g., 1200 µatm pCO₂ for future scenarios) 1 |
| Temperature-controlled water baths | Maintaining stable elevated temperatures | Simulating ocean warming conditions 2 |
| Carbonic anhydrase activity assays | Measuring enzyme activity in kelp | Assessing carbon concentration mechanism efficiency 1 8 |
| Photorespirometers | Quantifying photosynthetic rates | Evaluating kelp photosynthetic performance under different conditions 1 |
| Time-lapse photography | Documenting animal behavior over time | Tracking urchin emergence and feeding activity 3 |
| Gonad somatic index | Measuring reproductive investment | Assessing urchin fitness and energy allocation 6 |
The intricate dance between giant kelp and purple urchins under climate change reveals a fundamental truth about our changing planet: ecological outcomes are rarely simple.
While giant kelp possesses surprising resilience to ocean acidification—and may even benefit from increased CO₂ in some contexts—its vulnerability to warming temperatures remains concerning. The contrasting responses to single versus combined stressors highlight the importance of studying climate change impacts in all their complexity.
For purple urchins, the future appears more consistently challenging, with their physiology and development compromised by both warming and acidification. Yet their adapted feeding behavior and potential benefits from nutritional changes in stressed kelp create additional wrinkles in predicting future grazing impacts.
What emerges is a picture of an ecosystem in transition, where relationships are being rewritten by changing environmental conditions. The fate of kelp forests will depend not just on how individual species respond to climate change, but on how these responses alter their interactions—who eats whom, who competes with whom, and who depends on whom.
As research continues, particularly on the vulnerable early life stages of both kelp and urchins, we gain a clearer window into the future of these precious marine ecosystems. One thing remains certain: the decisions we make today about carbon emissions will reverberate through these underwater forests for generations to come, reshaping the hidden landscapes that support so much marine life along our coasts.