The North Atlantic Bloom
The Big Picture ...
Every March, one
of the oceans grandest biological events begins just north of the
Sargasso Sea and Bermuda. As days in the northern Hemisphere begin to
lengthen, phytoplankton respond by initiating heightened photosynthetic
activity, leading to the explosive growth of phytoplankton populations called a
"bloom". Utilizing nutrient concentrations that have increased
over the winter, this explosion of phytoplankton growth sweeps from the
Sargasso northward like a green wave, until the entire northern Atlantic Ocean
is covered with a blanket of teeming, microscopic oceanic plant life. The
wave rolls northward, past Iceland, into the far reaches of the North Sea,
toward Spitzbergen and the fjords of Norway.
Sounds simple, doesnt it?
Truthfully, the spring phytoplankton bloom is one of the most widespread
changes in the oceanic biosphere, and it occurs every spring in the North
Atlantic. That much is clear and obvious. But the picturesque (and
exaggerated) language of the opening paragraph smooths over the actual facts of
the North Atlantic Bloom it proceeds not as a simple wave, but in bursts
and outbreaks that wax and wane from day to day, depending on the clouds, the
currents, the winds, and the waves. The North Atlantic Bloom is an ideal
illustration of the importance of resolution in remote sensing, and of the
importance of scale in the consideration of phenomena in the marine
environment. From a distance, or when averaged over longer periods of time and
larger areas of the sea, oceanic processes can appear fairly uniform and easy
to understand. But when the scale is reduced, the picture becomes a bit more
muddled, and complete understanding is harder to obtain.
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The progression of the North Atlantic
Bloom during the months of spring and early summer in the Northern Hemisphere
is easily seen in reduced-size images of monthly SeaWiFS ocean color data. The
resolution of the SeaWiFS Level 3 monthly data is about 9 kilometers. (Each
picture is linked to a full size image of chlorophyll concentration.) Take a
look at one of these full-resolution images, even if it requires a few seconds
for the image to load. Notice that even in the monthly images, large patches of
the ocean are more productive than nearby regions. This
"patchiness" is obvious, even though SeaWiFS daily observations have
been averaged over time to produce the monthly images. Patchiness is even
more pronounced in SeaWiFS 8-day composite images, though there are larger
areas where no data was obtained due to the presence of persistent clouds.
Oceanographers employ the word "patchiness" to describe this
characteristic pattern of ocean biological productivity. Another term for it
is "mesoscale phenomena", which describes processes that generate
features between 10-1000 km in size. "Mesoscale" actually means
"intermediate size". SeaWiFS provides the opportunity to watch the
variability of mesoscale phenomena in the oceans for two reasons: the spatial
extent of its observations, and the frequency of observations over a particular
area. |
In an article published in 1981 in the
magazine Oceanus , Dr. Wayne Esaias discussed the difficulty of
observing patchiness in the oceans. Esaias used a simple diagram (shown in
modified form here) to illustrate the temporal and spatial range of various
phenomena in the oceans, and the capability of various observational platforms
(research vessels, instrumented buoys, aircraft, satellites, etc.) to observe
them.
This diagram illustrates the approximate spatial and temporal ranges for a
variety of oceanic phenomena, along with estimated observational
capabilities of various research platforms. Three basic trophic levels
(phytoplankton, zooplankton, and fish) are shown. Thus, aircraft
observations are ideal for large-scale, high resolution regional (synoptic)
characterizations, encompassing a single tidal cycle and population
variability at a scale of 10-1000 meters. In contrast, satellites can
best observe variability over 10-1000 kilometers, with a maximum temporal
resolution of about 1 day. Ship observations can be at very high spatial
resolution but are more limited in spatial range than aircraft or satellites.
... and Some Closeups
To get an idea of what "mesoscale" means, a fairly good analogy can be made with
meteorological processes, i.e., the weather. For example, a cold front is a
meteorological feature that is larger than the mesoscale. Yet the
thunderstorms which form as a cold front moves across a continent are mesoscale
phenomenon. Current weather forecasting models do a very good job of
forecasting the movement of cold fronts and other large- (macro) scale features
in the atmosphere. Where the models have trouble is at the mesoscale;
accurately describing the location and intensity of the thunderstorm-sized
features embedded in the cold front is still beyond the ability of current
models.
Much like the ability of meteorologists to recognize the factors
contributing to the formation of a weather front, oceanographers can describe
the factors which foster the formation of the biological "front" that
is the large-scale manifestation of the North Atlantic Bloom. The most
important factors are increased daily solar irradiance, increasing sea surface
temperature, and higher nutrient concentrations in surface waters.
Two high-resolution SeaWiFS images depicting
chlorophyll-a concentrations provide an even better illustration of the
variable nature of this biological event. The first image (full size) is from May 4, 1999, south of
Newfoundland (the roughly triangular island near the top left.) For
comparison, an image made by combining the data from bands 1 (412 nm), 5 (555
nm) and 6 (670 nm) is also shown. This pseudo-true color image gives a good
representation of how the human eye would view the scene from the altitude of
the satellite. This comparison illustrates the sensitivity of the SeaWiFS
radiometer, as the high concentrations of chlorophyll are virtually invisible
to our eyes.
The second image of
chlorophyll concentrations is of an area in the open Atlantic ocean, south of
Greenland. This image (full
size) was obtained on May 21, 1999, two weeks later than the image near
Newfoundland. Note that the prominent area of high productivity near the top
left of the image wraps around a roughly circular circulation feature called an
eddy.
The data for both of these images was obtained by the receiving station at
the Bedford Institute of Oceanography in Dartmouth, Nova Scotia. These
images provide a remarkable close-up of the North Atlantic Bloom in full bloom
and show how difficult it is to capture the daily progression of this
event.
One of the main reasons for a mission like SeaWiFS, which provides daily
observations of the ocean, is to observe progressively changing phenomena like
the North Atlantic Bloom. The goal of oceanographers is to model these
phenomena at increasingly higher and higher resolution, which will provide
better quantification of the amount of carbon that is produced each spring in
the North Atlantic. The only way to determine if the models match with
reality, and if they are sensitive to the same variables that drive the
phenomenon in nature, is to compare their predictions with the actual event.
SeaWiFS provides data that can be used for such a comparison.
Reference
Esaias, Wayne E., 1981: Remote Sensing in Biological
Oceanography. Oceanus, Vol. 24, Number 3, 32-38.
Link
Data from the Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom
Experiment (NABE): http://www1.whoi.edu/mzweb/nabe.htm
A note on the images: Data from SeaWiFS is broadcast to any station
within receiving range. Because this data is obtained directly, occasionally
the signal is lost, resulting in missing scan lines, which appear in the images
shown here. Some of the factors affecting reception are atmospheric
conditions and the distance from the satellite to the receiving station.
ACKNOWLEDGMENT
We gratefully acknowledge the assistance of Dr. Hugh
Ducklow of the JGOFS program and the Virginia Institute of Marine Science and
Dr. Wayne Esaias of NASA Goddard Space Flight Center in the
preparation of this Science Focus! Web page.
SeaWiFS image produced by James Acker, Raytheon ITSS.
Page design by Robert Simmon, Research and Professional Services. Accompanying text by James Acker, Raytheon ITSS.
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