Quarterdeck 4.1
Recent Graduate
Micro-phytoplankton in the Equatorial Pacific
by Duan Liu
Carbon dioxide (CO2) is the most prevalent type of man-made pollution. Ocean
phytoplankton absorb CO2 during normal life processes and sink to the ocean
floor when they die, where they are gradually buried under accumulating
sediment. This is one of the most important ways by which oceans remove
CO2 from the atmosphere. To assess the effect of this we need to know how
much phytoplankton lives in the ocean.
My thesis project was designed to measure the amount of phytoplankton in
the equatorial Pacific Ocean, and assess their variability and characteristics.
Sizes of marine phytoplankton usually range from one micron to one millimeter.
The group I studied consists of species larger than 20 microns, called micro-phytoplankton.
My samples were taken from thirteen locations along 140°W from 12°N
to 12°S. When the research vessel arrived at a designated sample location,
researchers dropped a water sampler into the ocean. The sampler has multiple
containers which collect water at different depths. My samples were collected
at eight depths from three to 200 meters below the surface, preserved, and
shipped to our lab at Texas A&M University.
In the lab, we left the sample still for about one day to let the micro-phytoplankton
settle to the bottom of the container. Then I used a microscope to count
and measure the phytoplankton present. From the number and sizes we can
estimate the biomass of the phytoplankton, often expressed as milligrams
of carbon per liter of seawater.
I found three major groups of phytoplankton: dinoflagellates, diatoms, and
coccolithophorids. Dinoflagellates have two flagella, and make up about
55% of the micro-phytoplankton cells in our samples. Both diatoms and cocco-lithophorids
cells each contributed nearly 20%.
Most phytoplankton are found near the surface because they need light to
grow. In the equatorial Pacific deeper than 100 meters the light is too
low for phytoplankton. In surface water, however, there is usually a shortage
of plant nutrients. There is normally a layer of maximum phytoplankton biomass,
called the near-surface maximum, where phyto-plankton find optimal amounts
of both light and nutrients. In this study the near-surface-maximum layer
was between three and sixty meters deep. In the near-surface maximum at
each station we found 1400-4500 cells per liter. Converted to biomass this
equals 0.4-1.4 micrograms of carbon per liter. At 200-meter depths, the
micro-phytoplankton cell numbers were as low as 92 cells per liter.
Micro-phytoplankton biomass in equatorial areas is lower than in coastal
areas and higher than in each hemisphere's central gyre. The cell numbers
we found during an El Niño event are about one order of magnitude
lower than those during non-El Niño conditions in the same area.
We concluded that this was the result of El Niño. The low phytoplankton
biomass was accompanied by a high number of species. At each station we
found 81 to 137 species, although two to seven dominant species contributed
more than 50% of the cell numbers.
We know little about ocean phytoplankton compared to our knowledge of their
terrestrial counterparts, plants. This study provided a better understanding
of the phyto-plankton in the equatorial Pacific area, especially during
an El Niño event.
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Oceanography, Texas A&M University
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URL=http://oceanography.tamu.edu/Quarterdeck/QD4.1/Grads/liu.html
Updated May 27, 1996