This month’s image comes courtesy of glider RU10, which was deployed by Rutgers University undergraduates Shannon Harrison and Dave Kaminksy.  The glider was deployed out of Tuckerton, NJ on the Endurance Line from March 23, 2012 to April 10, 2012 and was equipped with a fluorometer that measures the amount of chlorophyll a in the water. Chlorophyll a is a pigment found in all phytoplankton and oceanographers use it to measure the concentration of phytoplankton in the ocean.

The concentration of chlorophyll a is proportional to the number of healthy phytoplankton cells in the water column. Phytoplankton are small (0.2-100 microns) plant cells that capture sunlight (photons) using chlorophyll a and other accessory pigments. The photons are used to break bonds in water (H₂0) and converts carbon dioxide (CO₂) into organic molecules and produce oxygen (O₂). This process is known as photosynthesis. These small ocean plants account for half of the oxygen on Earth. A chlorophyll molecule can only absorb a finite number of photons. If a cell receives more photons then the molecules can use, the absorbed photons can be re-emitted as red light (“fluoresced”).  This is how the fluorometer on the glider works. It shines a blue light on the phytoplankton cells (more than the cells can use), which is absorbed by chlorophyll a and measures the amount of red light that is fluoresced back.

The top image displays the entire chlorophyll a cross-section gathered by RU10 during this deployment. The bottom image is zoomed into the time period between March 27 and March 31.  Keep in mind that all times on the x-axes are in GMT (Greenwhich Mean Time), which is 4 hours ahead of Eastern Daylight Time (EDT). The first thing to notice is there are regions of high and low concentrations of chlorophyll a. This shows the variability in phytoplankton concentrations in the Mid-Atlantic Bight.

However there is some interesting features in the fluorescence data. Notice the areas enclosed by the red boxes that occur daily beginning around 12:00 GMT (08:00 EDT). At first glance, it would appear that these areas are associated with lower chlorophyll a and, thus, less phytoplankton. However this is not the case.  These time periods correspond to passage of the sun overhead leading to variability in light intensity as it varies over the day (the sun is brightest at local noon when the sun is overhead).  To avoid be “sun burned” the cells tune their photosynthesis to amount of light intensity present, decreasing the efficiency that cells can “use” the absorbed light to avoid burning out their photosynthetic machinery when light intensities are high. The amount that the chlorophyll fluorescence decreases is proportional to the changes in light intensity.  This is a natural process. The amount that cells “tune” their machinery is a function of their health and oceanographers can use these processes to get a snap shot of the cell’s physiology. Therefore chlorophyll fluorescence measured during the day provides information on the cells physiology and the measurements at night provide the cleanest data about the concentration of chlorophyll a (and thus phytoplankton).