Southern Ocean GasEx Blog

Dispatches from the Southern Ocean Gas Exchange Experiment

Peaks and Valleys at Sea

Posted by sogasex on March 11, 2008

By Bruce Hargreaves, Lehigh University

Peaks and valleys are familiar features of land. But these terms can also describe properties of ocean waters. Different colors (wavelengths) of sunlight that cause us to marvel at the sight of a rainbow can also tell us how the ocean may influence climate change.

The energy of the sun fuels the metabolism of phytoplankton (and through food web connections, most living organisms in the sea). On this cruise a collection of old and new methods for measuring absorption of light by photoplankton is combined with simultaneous measurements of metabolism, carbon dioxide concentrations, and satellite data.

The biological oceanographers on this cruise are seeking to learn more about how phytoplankton metabolism can soak up carbon dioxide from the atmosphere, and how satellites might some day be able to measure this process across the entire surface of the world oceans. Bob Vaillancourt and Veronica Lance, with colleague John Marra (all from LDEO, Columbia University), are using several methods to measure photosynthesis and light absorption by phytoplankton. I am making several types of optical measurements, including a new method to measure the absorption of light by freshly collected phytoplankton to compare with measurements of frozen or chemical extracted samples.

hargraves-graph.jpg

An absorption spectrum of the pigments of the phytoplankton community from our study site shows characteristic peaks and valleys. The broad peak in the middle of this chart absorbs blue light to provide most of the energy for photosynthesis. The smaller peak on the right side absorbs red light. The valley between these two peaks transmits green light (the green color we expect to see in living plants is the result of this valley). The prominent peak at the left side of the chart represents a natural sunscreen chemical produced by phytoplankton that live near the surface where UV-B radiation can cause damage. Phytoplankton from a depth of 30 meters had less absorption by this chemical. At our study site where this sample was collected the phytoplankton were equally abundant at 10 and 30 meters but were scarce at a depth of 80 meters (the sun is too weak at that depth to support life).

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