With marine heatwaves facing our oceans as a climate stressor, UC Santa Barbara’s Hofmann Lab draws attention to just how drastically marine organisms may change in response.  As a relatively recent phenomenon, marine heatwaves, which are periods of anomalous ocean warming, are profoundly reshaping marine biodiversity along California’s coast. From local changes in biodiversity to significant physical changes in the ocean’s physical state, the climate change-driven stressor of marine heatwaves is pushing marine life to their limits. 

The Hofmann Lab studies global change biology in the local ocean and is headed by principal investigator and interim director of the Marine Science Institute professor Gretchen Hofmann. In her 20 years of residence at UCSB, Hofmann’s hierarchical research aim focused on the ecological and economic impact a changing ocean has on organism-environment interactions. 

Her lab’s goal is to uncover what’s happening below the surface and use these findings as a basis to inform decision makers and marine ecosystem managers on mitigating the impacts of the ocean warming crisis.

The Nexus spoke with Hofmann about the significant changes occurring below the ocean’s surface. With marine heatwaves (MHWs), alongside other climate stressors, predicted to increase in frequency and intensity in the coming decades, the beaches and oceans might feel sterile one day from the climate-driven loss  of biodiversity. 

A recent study defines a MHW as a prolonged period of anomalously high seawater temperatures persisting for more than five consecutive days. Anomalously high seawater temperatures are those categorized as 90th percentile relative to local long-term climatology. The phenomenon of MHWs is relatively recent to the scientific community. Researchers only began to record their presence as early as 2013 according to Hofmann; however, their impact on marine ecosystems in the past decade has been profoundly detrimental to biodiversity.​​The vitality of the ocean and land are intrinsically linked. According to Hofmann, ocean productivity is responsible for delivering nutrients to support life on beaches. 

“Over time if we lose this, the whole [beach] will feel … more sterile,” Hofmann said regarding ocean productivity. 

With a future of empty beaches looming, the Hofmann Lab’s research is critical for informing mitigation efforts. In the last several years Hofmann and her team of graduate and postdoctoral students have been comprehensively studying the ecological and economic impacts of MHWs in California’s marine ecosystem. The Hofmann Lab’s research alongside others has shown that MHWs are pushing cold-blooded organisms to their thermal limits, causing species range shifts, impacting reproductive success and exacerbating disease outbreaks. 

Professor Gretchen Hofmann holds up an urchin. Courtesy of UCSB.

During MHWs, some organisms are affected disproportionately. Sessile organisms, which are those fixed in one place, cannot escape MHWs. Whereas mobile organisms, such as fish and some marine invertebrates, can migrate poleward to cooler waters. Under the thermal stress of MHWs, entire populations of mobile species sometimes leave, while sessile species populations drastically decline locally. Combined, these MHW effects are restructuring ecosystem dynamics often in significantly negative ways, yet the long term ecological impacts of MHWs are still largely unknown. To encapsulate this issue, Hofmann says that “climate change is shaking up the biodiversity puzzle of our coasts.”

A MHW, most notably known as “the Blob,” struck the northeastern Pacific Ocean in 2014, with anomalously high seawater temperatures for 711 days. In that time, coastal researchers throughout the Pacific documented the mass mortalities of crucial mammal and bird species due to the Blob. In the local Santa Barbara region, research from the Santa Barbara Coastal Long Term Ecological Research (SBC LTER) shows that MHW-induced declines and changes in species composition have persisted for nearly six years post-blob. 

Even now, when speaking about the  long-term impacts of the Blob, Hofmann said that   there can be a shift in biodiversity. 

Since the Blob, Hofmann says the Santa Barbara region has seen MHWs in the years 2018, 2020 and again this past summer. 

To better understand MHWs as an emerging phenomena, the lab collaborates with SBC LTER to detect changes in both ocean surface and floor temperature. The data indicate MHWs affect all levels of the water column, spanning from the surface to the benthic floor, which motivates many of the lab’s current research efforts on marine invertebrates. 

Erin de Leon Sanchez is one of the graduate students working in the Hofmann lab on this project. Courtesy of Gretchen Hofmann.

Some of the lab’s most recent research explores the impact of MHWs on the economically and ecologically important red urchin and Kellet’s whelk sea snail species. The lab also focuses on how MHWs affect the fertilization and different life stages of invertebrates such as the purple urchin in addition to their plasticity. In the context of their research, plasticity refers to the capacity of a species ability to respond to environmental stress. 

A former doctoral student in Hofmann’s lab, Xochitl Clare, found that under MHW conditions, the larvae of the Kellet’s whelk exhibited significant developmental abnormalities. Clare’s research implies that MHWs have the potential to cause population decline of Kellet’s whelk locally, a detriment to the aquaculture industry. Another study, conducted in the Hofmann Lab by former doctoral student Juliet Wong, indicates there is a significant link between the alteration of gene expression and increased temperature in the development of red urchin.  As an economically significant species, red urchins are primarily harvested for their roe. The alteration of red urchin development from MHWs could potentially have a negative impact on the aquaculture industry of uni.

Another species studied by the Hofmann Lab is the purple urchin, an integral species in maintaining the equilibrium in California’s kelp forests. Research from the Hofmann Lab suggests that under MHW conditions, the purple urchins have a smaller, more variable body size in development compared to normal temperatures. 

This phenotypic change was found to be inherited by purple urchin offspring suggesting that thermal stress has the potential to shift species morphology on a short timescale.  In the face of MHWs, the lab found purple urchins have lower reproductive success, however, their plasticity to thermal stress exhibits the species’ potential resilience to MHWs. 

With the element of uncertainty that  MHWs are bringing to oceans ecologically, research like Hofmann’s Lab is critical to understanding the changes below the surface. 

“[The lab’s current research is] telling us a story that the ocean is changing and that we should start to care immediately,” she said. 

But for Hofmann, finding a solution is not exactly the right term. Instead, according to Hofmann, scientists and policymakers need to adjust and mitigate in response to the current ocean warming crisis. Hofmann meets this challenge with optimism.

 “We’ve got a lot of fight in us. There are a lot of ideas that can be leveraged… Mother Nature has a lot of resilience,” she said.

 For the rest of her research career, Hofmann aims to focus on how thermal stress affects the vast California kelp forests and their organisms, which are just a few hundred yards away from her lab. 

A version of this article appeared on p. 11 of the Nov. 16, 2023 print edition of the Daily Nexus.