Humans rely on marine ecosystems for economic and nutritional sustenance—including about 16% of animal protein consumed by humans—making it especially important for natural scientists, economists, conservationists and long-term policy planners to understand how climate change is likely to affect oceanic food webs. Yet the general effects of warming on food web productivity are completely unknown. The productivity of consumers (such as zooplankton), in food webs is determined in large part by their metabolic rates and the availability and productivity of their limiting metabolic resources. A general theory relating food web dynamics to temperature suggests that fundamental differences between consumers and primary producers (such as phytoplankton) may lead to predictable shifts in their relative abundance and productivity with warming. A team of UNC scientists led by my former PhD student Mary O’Connor experimentally tested the effects of warming on food web structure and productivity under two resource supply scenarios. Our results show that warming alone can strengthen the role of consumers in the food web, increasing consumer biomass relative to producer biomass, and reducing the total biomass of the food web despite increases in primary productivity. In contrast, when resources were less available, food web production was constrained at all temperatures. These results demonstrate that small changes in water temperature could drive dramatic shifts in marine food web structure and productivity, and potentially provide a general, species-independent mechanism of ecological response to climate change.
By Erik Stokstad, 26 August 2009
Teasing apart the complex ways in which global warming will affect ocean life has been tough. But new research suggests that a simple ecological theory may explain at least one piece of the puzzle: the effect on marine food webs. And the news may not be all bad.
New experiments confirm that phytoplankton, which form a bottom rung of oceanic food chains, will become less productive in warmer, nutrient-rich water. However, the results also show that zooplankton should boom in these warmer areas, which could benefit certain fisheries.
The food-web theory hinges on the assumption that temperature affects the metabolism of organisms that rely on other creatures for food, like zooplankton, while not having much of an impact on photosynthetic organisms like phytoplankton. That suggests that in warmer waters, zooplankton should generally grow faster and start reproducing sooner than they do in cooler waters. As zooplankton become more abundant and eat more phytoplankton, the population of phytoplankton should shrink.
Mary O’Connor, now a postdoc at the National Center for Ecological Analysis and Synthesis in Santa Barbara, California, and her colleagues set up an experiment to test the theory. They put zooplankton and phytoplankton into 4-liter tubs and let them sit for 8 days. Some were kept at the ambient temperature; others were heated by 2°, 4°, or 6°C. Recognizing that nutrient levels vary in the ocean, they added extra nitrogen and phosphorous to half the tubs in each group.
As temperatures rose, the productivity of the communities without extra nutrients hardly changed. Nor did the food web. This suggests to O’Connor and her colleagues that nutrient-poor food webs may be relatively resilient to global warming. The tubs that got additional nutrients were another story: The zooplankton in warmer water became more abundant while the numbers of phytoplankton fell. In fact, the ratio of zooplankton to phytoplankton rose 10-fold, the team reports in a paper posted online on 25 August in PloS Biology. “It matched our predictions really well,” O’Connor says. She adds that even though overall biological productivity declined as temperature rose, the increase in zooplankton could benefit fish that eat them in nutrient-rich waters.
There is a great synopsis of the article in PLoS which starts out with a pretty politically charged perspective:
While politicians like United States Representative Michele Bachmann (RMinn.) rail against efforts to curb human contributions to global warming—she thinks carbon dioxide, a ‘‘natural byproduct of nature,’’ could not possibly be harmful—scientists are documenting the damage. Numerous studies describe how climate change is threatening the persistence of a broad range of plant and animal species across diverse taxa, geographic regions, and trophic levels, from the polar bear at the top of the food chain to the shrimp-like krill at the bottom. As they catalog the ecological casualties.
Mary also published the study-related haiku in the New Yorker, which originally appeared at the website Dissertation Haiku.
It’s warm; feel your tummies growl?
Graze down hot seaweed.
See the related story in Scientific American here
O’Connor MI, Piehler MF, Leech DM, Anton A, Bruno JF (2009) Warming and Resource Availability Shift Food Web Structure and Metabolism. PLoS Biol 7(8): e1000178. doi:10.1371/journal.pbio.1000178
Download the PLoS Synopsis: http://www.plos.org/press/plbi-07-08-OConnorSynopsis.pdf