A new study of Red sea urchins, a species of commercial value, looked at how different populations respond to changes in their environments. The results show that red sea urchin populations in northern and southern California are adapted to their local conditions but differ in their vulnerability to environmental changes expected to occur in the future due to global climate change and ocean acidification.
The new findings, published Jan. 20 in Science advances, suggest that red sea urchin populations in southern California may be more vulnerable to climate change than those in northern California. Although Southern California sea urchins have already adapted to warmer conditions, researchers believe the warming of their environment may be more than they can handle.
“Red sea urchins from southern California were more sensitive to environmental changes than those from northern California, and we think this is likely because they are already closer to some kind of thermal limit,” said lead researcher Kristi Crocker, associate professor of ecology. and evolutionary biology at the University of California Santa Cruz.
First author Emily Dunham led the study as a graduate student at UCLA and is now a postdoctoral researcher at UCSB. “Red sea urchins are an important species for fisheries along our coast, so understanding how they are likely to be affected by climate change is very important,” she said.
The study looked at the effects of three major environmental variables in sea urchin coastal habitats: water temperature, dissolved oxygen, and pH (a measure of ocean acidity). Climate change caused by increased carbon dioxide in the atmosphere is warming the oceans and reducing oxygen levels in the water, while increased seawater uptake of carbon dioxide is causing ocean acidification.
According to Kroeker, most studies looking at the ability of species to adapt to climate change have focused on one aspect of environmental change, such as warming or ocean acidification. “But all of these species that we’re concerned about are embedded in environments with multiple variables that will be affected by climate change,” she said.
Using a network of sensors spread along the coast, the researchers first characterized current conditions in kelp forests in northern and southern California. There are large differences between the coastal waters of northern and southern California because of the strong seasonal upwelling in the north, which brings cold deep waters to the surface with low levels of dissolved oxygen and a low pH (closer to the acidic end of the scale). Coastal sewage is much weaker in Southern California.
As a result, sea urchin populations in Northern California are already exposed to conditions that are more acidic, less oxygenated, and colder than the waters of Southern California. However, in the future, both regions will see warmer, more acidic, and less oxygenated waters compared to current conditions.
To study the sensitivity of red sea urchins to these changing conditions, the researchers grew juvenile sea urchins from the two regions in outdoor tanks at UCLA’s Long Marine Laboratory, where they could control the conditions in each tank.
The experiments exposed sea urchins from both groups to the average conditions in each of the two regions for temperature, dissolved oxygen, and pH. The results clearly show that populations of red sea urchins adapt to their home environments and have increased mortality when reared under different conditions. Southern California sea urchins performed poorly under Northern California conditions, and vice versa.
The researchers also exposed Red Sea urchins to projected future conditions for their home regions based on regional climate projections for the year 2100. These future conditions do not generally overlap with the set of conditions currently measured along the coast.
Although mortality was increased in populations from both regions under projected future conditions, northern California sea urchins experienced lower mortality and had better body condition than southern California sea urchins.
“Although Northern Californians will be in more acidic conditions with less oxygen in the future, Southern Californians will be the most affected,” Crocker said.
This was a surprise, she said, because Northern Californians are acclimating to seasonal changes in which temperature, dissolved oxygen, and pH drop together, whereas with climate change, this tightly bound variance, or “heterostructure,” will break. The dissolved oxygen and pH will drop further, but the temperature will rise.
“They break down the ecological heterostructure they’re adapted to, so our expectation was that this would make them more vulnerable. But that’s not what we found,” Crocker said.
The results indicate that water temperature is an important environmental variable for red sea urchins. With warmer temperatures to begin with, Southern California coastal waters may not have to get much warmer to reach temperatures unsuitable for red sea urchins.
“We should not assume that the vulnerability of species to climate change is the same across their entire range,” Crocker said. “Each population group adapts to local conditions, and not all populations will respond identically to global climate change.”
In addition to Dunham and Crocker, co-authors include Iris Flores, Alexis Huber, and Evan O’Brien at UC Santa Cruz. Kate Villette and Jan Freywald of the Reef Check Foundation; and Yoichiro Takeshita at the Monterey Bay Aquarium Research Institute. This work was supported by the UCLA Institute for the Study of Environmental and Evolutionary Climate Impacts (ISEECI) and the California Ocean Conservation Council.