Module 4: Impacts on Local Species
Now that you have a better understanding of the processes that impact ocean acidification in Alaska and a sense of how OA is showing up in regions around the state, let’s take a closer look at how OA impacts species that call Alaska’s waters home.
Marine Food Webs
Food webs describe who eats who in an ecological community. Food webs are made of interconnected food chains. Phytoplankton, plants, bacteria, and algae form the base of the marine food web. These organisms are called primary producers because they make their own energy directly from sunlight and chemicals in the ocean, through photosynthesis and chemosynthesis. As we saw in Module 2, phytoplankton play a major role in the marine carbon cycle and seasonal blooms have a big impact on ocean acidification in Alaska. Phytoplankton’s role at the base of the marine food web is another way these incredible organisms contribute to the health of our planet.
Primary producers are eaten by primary consumers like zooplankton, shellfish, krill, small fish, and crustaceans. They are called consumers because they must eat other organisms for energy and nutrients. Primary consumers are in turn eaten by fish, corals, sea stars, small seabirds, and baleen whales. Top ocean predators include large seals, large seabirds, toothed whales, walruses, and polar bears. Humans consume aquatic life from every section of the marine food web.
Different levels of the food web are called trophic levels. Trophic means “related to eating”. In these feeding relationships, the energy stored in prey flows to predators, up through trophic levels. This is known as a trophic flow. Because each organism may have multiple food choices, a food web is created. Take a look at some of the different feeding relationships of an Arctic food web:
Pteropods
One species of zooplankton that is especially important to marine food webs around Alaska is the Pteropod. These microscopic sea snails swim through ocean water using two wing-like structures that protrude from their foot – in fact, pteropod means “wing-foot”. Sometimes they are called sea butterflies because of how they appear to flap their “wings” as they swim through the water column. Pteropods are eaten by a variety of organisms ranging in size from tiny krill to seabirds to whales. These fat-rich plankton are an important source of food for North Pacific juvenile salmon (pink, sockeye, and chum), herring, and other fish. At higher trophic levels, fish like tuna, salmon, and walleye pollock consume other creatures that rely on pteropods as a food source.
Watch this short video from the Monterey Bay Aquarium Research Institute to learn more about the types of pteropods and their importance to ocean health.
Shelled pteropods are very sensitive to changes in ocean chemistry. Like other shellfish, pteropods extract calcium ions and carbonate ions from seawater and combine them into solid crystals of calcium carbonate that are laid down to make shells. Watch this interesting video to learn how this process works:
It is important to remember that there are two types of calcium carbonate crystals that shell-builders can form: calcite and aragonite (uh-RAG-o-nite). Unlike some other shell-builders, pteropods don’t use both – pteropods build their thin shells out of aragonite. In Module 1, you learned about aragonite saturation, which tells us how much of this carbonate mineral is available in seawater. Ocean acidification lowers the concentration of this building block in the ocean. As aragonite saturation goes down, it gets harder for shelled organisms to build their shells and exoskeletons. We refer to waters with low aragonite saturation as corrosive.
In more corrosive water, pteropods can have trouble growing their shells. In extreme cases when aragonite situation is very low, their shells can dissolve. Pteropods also have physiological and neurological sensitivity to more acidic water. When you combine ocean acidification with warming ocean temperatures, the sensitive pteropod becomes even more vulnerable.
There is current evidence that corrosive conditions are already harming pteropods in the Gulf of Alaska and in the Bering Sea. Researchers have documented extensive shell dissolution in juvenile pteropods exposed to corrosive seawater for prolonged periods of time.
From the Gulf of Alaska and the Bering Sea to the Beaufort Sea, scientists are finding pteropods with dissolved shells. Dr. Nina Bednarsek, a biogeochemist with the Southern California Coastal Water Research Project, recently presented some of these findings at the Alaska Marine Science Symposium.
Dr. Bednarsek’s latest field observations show dissolving pteropod shells from a broad hotspot of corrosive water that encompasses their spawning grounds in the western Gulf of Alaska. In the Beaufort Sea, the picture is even more troubling. There, where ocean acidification is exacerbated by glacial meltwater, up to 70% of the pteropods have corroded shells.
The region of pteropod-friendly water near the surface is shallower in cold water (which you learned can absorb more carbon dioxide) and in places like the northern Pacific Ocean, where circulation and upwelling brings naturally more acidic water closer to the surface. The rapid increase in anthropogenic (human-caused) carbon dioxide is expanding pteropods’ exposure to harmful conditions.
Image depicts the difference in thickness between two specimens of pteropods, one collected in more acidified coastal waters, the other collected offshore for this study. Credit: Lisette Mekkes, Naturalis Biodiversity Center.
Modeling work by University of Alaska Fairbanks oceanographer Dr. Claudine Hauri shows how these corrosive hotspots in high-latitude oceans have been growing in recent decades. “In 1980, in the Gulf of Alaska, there were already some regions exposed, but when you look at a more recent year, these areas [of corrosive water] are much broader, and much more widespread throughout the year,” Dr. Hauri explains.
High-latitude pteropods evolved in an environment where acidity shifts with the seasons. “Species exposed to naturally high variability might be more adaptable,” Dr. Hauri points out. “But as acidification increases, the extremes will be more than they have ever seen before.”
Monitoring Other Species
Because shelled pteropods are so sensitive to ocean acidification and other climate stressors, they are widely regarded as bioindicators of ocean health.
As an important part of the food web in Alaskan waters, researchers are interested in monitoring pteropods because a decline in pteropod quality and quantity could negatively impact local fisheries.
As the ocean continues to become more acidic, researchers are expanding the range of species they are investigating for OA impacts. Here in Alaska, seventeen species have been studied. Visit the Alaska OA Network website to explore what is known so far about species in Alaska based on lab experiments. To see big-picture results in one view, click to expand the poster on the left.
If you're looking for a more technical exploration, this annotated bibliography provides a detailed look into species response through peer reviewed literature.
Most of the Alaska species that have been studied so far show negative effects in the lab setting with respect to calcification, growth, reproduction or survival, as seen in the poster. However some species are showing signs of resiliency in the lab. We still have a lot to learn about OA impact on ecosystems and the marine food web.
Ocean acidification is not a silver bullet that explains all of the current changes we are seeing in species populations. Instead, ocean acidification is one of many components of ocean health. Mitigating other stressors may help species and ecosystems better deal with the impacts of OA. We are still investigating which physiological mechanisms help species survive and thrive in changing conditions.
Looking at whole ecosystems, we have learned there is a lot of natural variability throughout regions of Alaska. We know that, in the future, there will be periods of the year in which conditions will drop below the threshold of what is favorable for shell-builders. Creatures that are mobile may relocate in search of cooler temperatures or less acidic waters. Researchers are trying to figure out when departure from natural variability in ocean basins may be more important for species health and survival than specific pH or aragonite saturation states.
In the next two modules, we will take a closer look at recent research on the impacts of OA on species of crab and salmon in Alaska. In the wake of the recent collapse of Bering Sea crab populations and subsequent closures of both Bristol Bay red king crab and snow crab fisheries, a comprehensive understanding of current research findings will be essential to effectively manage fisheries in the future.
Extra Credit: Want to learn more about the chemistry of OA? Explore this interactive module on how ocean acidification impacts shell-building on a molecular level.