57

key concepts

57.1

Ecosystem Science Considers How Energy and Nutrients Flow through Biotic and Abiotic Environments

57.2

Energy and Nutrients in Ecosystems Are First Captured by Primary Producers

57.3

Food Webs Transfer Energy and Nutrients from Primary Producers to Consumers

57.4

Nutrient Cycling in Ecosystems Involves Chemical and Biological Transformations

57.5

Ecosystems Provide Important Services and Values to Humans

Food Webs in an Acidic and Warming Ocean

There is no dispute that the burning of fossil fuels over the last two centuries has increased Earth’s atmospheric CO₂ concentrations. Estimates show that oceans are absorbing about 48 percent of all atmospheric CO₂. Marine primary producers use some of the human-caused CO₂ in photosynthesis, but the remainder reacts chemically with seawater, lowering its pH and causing oceans to become more acidic. Ocean acidification can have negative effects on calcifying organisms such as corals, mollusks, and crustaceans, which rely on calcium carbonate for the accretion and maintenance of their external shells. But the negative effects of increasing CO₂ and acidification are not universal. For example, primary producers such as phytoplankton, algae, and sea grasses are known to increase their production under elevated CO₂. Increasing photosynthesis increases O₂ production, which can potentially counteract growing acidity in surrounding seawater. Research is under way on the effects of ocean acidification on single species, but studies on how ocean acidity affects marine ecosystems and food webs are also needed. Moreover, along with the acidification of oceans, water temperatures are rising, potentially creating multiple stressors for organisms.

Estuaries are places where the dual effects of ocean acidification and temperature on primary producers and their food webs may be particularly relevant. Among the most productive ecosystems on Earth, estuaries support a diverse array of marine organisms, including commercially important fish species. Because estuaries are at the interface between land and sea, they have been highly affected by humans through intense overfishing, habitat destruction, and pollution from rivers. Estuaries also experience warm water temperatures, a condition that could intensify with global warming. But estuaries may also be places where the effects of ocean acidification could be mitigated if primary producers such as macroalgae and sea grasses are able to sequester CO₂ and provide more food and habitat for higher trophic levels.

Research on estuaries was conducted by a group of researchers on the western coast of Sweden. The group focused on a food web that includes algae primary producers and invertebrate consumers and exposed this food web to different conditions of CO₂ and temperature. What they found suggests that ocean acidification and temperature affect food webs in complex ways that may be hard to predict.

How will food webs respond to the multiple effects of ocean acidification and warming in marine ecosystems?