Reference: Goldenberg, S.U., Nagelkerken, I., Ferreira, C.M., Ullah, H., Connell, S.D. 2017. Boosted food web productivity through ocean acidification collapses under warming. Global Change Biology 23: 4177-4184 https://doi.org/10.1111/gcb.13699
Feature Image: The ocean absorbs one quarter of the carbon released in to the atmosphere, leading to ocean acidification. Photo credit: Pixabay.
How are our oceans changing?
Since the industrial revolution, which began in 1760s, the pH of the ocean has decreased by approximately 30% (or 0.1 pH unit) as a result of the ocean absorbing about one quarter of the carbon dioxide released by humans into the atmosphere in a process called ocean acidification. While an increase in carbon dioxide in seawater may benefit photosynthetic algae that require carbon dioxide to grow, it makes it harder for organisms with shells to thrive. This means that organisms such as corals, mussels, and oysters are going to have a harder time as the ocean continues to acidify.
Climate change is not only causing the chemistry of the oceans to change, but also increasing the average temperature of the oceans. Increased temperatures are also a threat to ocean ecosystems, acting as a stress on organisms and speeding up their metabolism. This means that as temperatures rise, species will have need more food in order to balance their increased metabolic needs. In order for scientists and coastal managers to understand how climate change is impacting the ocean, it is necessary to look at changes in the food web as a result of both ocean acidification and increased temperatures.
How will ocean acidification and warming impact food webs?
Silvan Goldenberg and his colleagues from the University of Adelaide in Southern Australia recently published a study that examined how a food web consisting of microscopic photosynthetic algae (i.e. phytoplankton), herbivorous copepods that eat the algae, and predatory fish that eat copepods were affected by ocean acidification and warming. Copepods are very small shrimp-like animals that live throughout the ocean and eat even smaller algae and animals. The researchers set up 475 gallon tanks that mimicked the natural coastal environment. In each tank, the researchers added algae, copepods, and fish.
To test for the effects of ocean acidification on the food web, the researchers had tanks the mimicked the present conditions in the ocean (control) and tanks that had ocean water with a lower pH to mimic future predictions (ocean acidification). To test for the effects of ocean warming the researchers set up tanks that had a higher temperature predicted under future warming (warming). Lastly, the researchers set up tanks that had both decreased pH and elevated temperature to see how ocean acidification and warming may interact (warming + ocean acidification). The researchers then measured changes in the amount (i.e. biomass) of algae, copepods, and fish at the end of 3.5 months in each treatment.
Algal biomass increased under ocean acidification, warming, and warming + ocean acidification. Lower ocean pH corresponds to an increase of carbon dioxide in seawater, which can increase photosynthesis. Increased temperatures can also speed up the rate of photosynthesis and increase algal growth. Interestingly, despite the increase in algae under all climate treatments, an increase in herbivorous copepods was only observed under ocean acidification. Under warming and warming + ocean acidification the biomass of copepods was similar to the control, which represented current ocean conditions. An identical pattern was also observed in the predatory fish. Through another experiment, the authors were able to determine that warming acted as a stress on the fish, increasing their food demand under warming and warming + acidification. Ultimately, this resulted in an imbalanced food web that had more algae, but similar copepod and fish biomass under warming and warming + ocean acidification compared to the control. For a visual schematic of the results, check out Figure 2 in the study.
Goldenberg and his colleagues showed that under future ocean warming and acidification, food webs may become imbalanced, which may have consequences for fisheries. While future research is needed to determine whether this result is similar in other ocean ecosystems, it does highlight the fact that climate change is a complex mix of stressors that interact in unanticipated ways to impact our oceans.