Deep Breathing: Climate Stressors in the Coastal and Open Sea

Lisa Levin, Ph.D.
Scripps Institution of Oceanography

Lecture Summary

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Most organisms need a certain minimum amount of oxygen to survive, and this critical threshold of oxygen varies considerably among species. In the ocean, where the concentration of free (usable) oxygen in water, known as dissolved oxygen, is much lower than it is in the atmosphere, it is phytoplankton and various algae and submerged vegetation that create a steady supply of dissolved oxygen in the well-lit zone of the ocean during photosynthesis. At night and in deeper waters below the well-lit, or photic, zone, some organisms, even photosynthetic ones, function like animals by consuming more oxygen than they produce. This depletes the dissolved oxygen supply.

Further depletion of dissolved oxygen at depth comes from the breakdown of organic material by bacteria. These processes can result in potentially vast regions of hypoxia, a condition of low oxygen concentration in the water, that vary in size, duration, and oxygen concentration.

In her lecture for Metcalf Institute on June 5, 2012, at the University of Rhode Island Graduate School of Oceanography, Dr. Lisa Levin of Scripps Institute of Oceanography and the University of California at San Diego, discussed hypoxia and other climate stressors in coastal and ocean ecosystems. She explained that there are natural processes that cause hypoxia, such as nutrient influx from water that upwells from lower depths, seasonal separation of water layers, and poorly oxygenated waters that come from other natural sources, and there are man-made influences like eutrophication and global warming.

In the marine environment, these oxygen minimum zones (OMZs) naturally occur at depths between 100 and 500 meters and can extend deeper than a thousand meters. OMZs are particularly prevalent in the eastern boundaries of the Pacific and Southern Atlantic Oceans and in the Indian Ocean. Because of the natural production of carbon dioxide, OMZs are not only characterized by low levels of dissolved oxygen but also by acidic waters–with high concentrations of carbon dioxide.

Contrary to the expectations of many scientists, there are a surprising number of organisms that live in and around OMZs, including bacteria, worms, nematodes, and some molluscs such as squid. These organisms have greater tolerances to hypoxia than crustaceans and fishes. While some organisms relocate or decrease food consumption in response to hypoxic conditions, others have evolved adaptations that enable them to obtain sufficient oxygen to tolerate hypoxia. Interestingly, some research has shown that hypoxia influences the structure of a marine ecosystem, where a single species lives out its different life stages at different depths.

Levin pointed out that recent research shows that OMZs are expanding by extending to shallower waters and to deeper waters. Simultaneously, an increase in atmospheric carbon dioxide is acting to acidify surface waters. These two phenomena are expected to compress habitats as hypoxia-sensitive species are constrained to narrowed depths.

Hypoxia can shape communities from the bottom-up, Levin concluded, from responses of plankton to their effect on fish and other large predators. She advised researchers to pay closer attention to regions that are not yet hypoxic. Finally, Levin argued that the interactions between hypoxia and global warming, ocean acidification, and ocean currents needs to be better understood. In the end, she warned, we should all prepare for potentially large changes in the oceans caused by expanding regions of hypoxia.

Lisa Levin is director of the Center for Marine Biodiversity and Conservation and distinguished professor at the Scripps Institution of Oceanography in La Jolla, California. Before moving to Scripps in 1992 she was associate professor in the Department of Marine Earth and Atmospheric Sciences at North Carolina State University in Raleigh. Levin is a marine ecologist who studies benthic ecosystems in the deep sea and shallow water. Together with her students she has worked with a broad range of taxa, from microbes and microalgae to invertebrates and fishes. Her recent research has emphasized three major themes: (1) the structure, function and vulnerability of continental margin ecosystems, particularly those subject to oxygen and sulfide stress; (2) wetland biotic interactions as they mediate marsh function, invasion and restoration; and (3) larval ecology of coastal marine populations with emphasis on connectivity and response to ocean acidification and deoxygenation. Her research has been conducted over the past three decades on the margins of the Pacific, Indian and Atlantic Oceans using ships, submersibles and remotely operated vehicles to sample and conduct experiments. She has participated in over 30 oceanographic expeditions around the world and served as Chief Scientist on 12 of these. She is the author or co-author of more than 160 scientific publications.

Lecture summary prepared by Al Nyack, Ph.D.