6. Plankton Blooms
Changes through the year
Global average
The image to the right is the average ocean chlorophyll concentration measured by the ocean colour sensor SeaWiFS on SeaStar from 1997 to 2004. There is a close relationship between physical and chemical ocean processes - like temperature differences and nutrient upwelling from the deep ocean that happens near coastlines - and ocean plant life. See the tutorial on Ocean Currents for more details.
As the image shows, chlorophyll (and therefore phytoplankton) concentrations are higher in coastal areas than they are in the open ocean. They also are higher in the northern oceans than the southern.
An image like this smooths out variations through the year and over the seven years of data. We can use daily, weekly and monthly images to look at variations on different time-scales. The other images on the page show the monthly chlorophyll concentrations for June, September, December 2007 and March 2008. There is also an image showing all the monthly images from June 2007 to June 2008
From season to season
Some of the changes through the year are linked to the way the ocean circulation varies, others to the amount of light available. These seasonal cycles result in characteristic seasonal variations in different parts of the oceans. The satellite images on this page show these variations clearly in some regions, particularly the N.Atlantic and near the Equator.
Spring bloom
During the winter, waters in the high latitude oceans are cold at the surface. The temperature difference between surface waters and deep waters is small, and the water can mix vertically. Rising deep water brings nutrients to the surface. So when the amount of light available increases with the spring, the surface waters have both light and nutrients, and phytoplankton thrive. As the available nutrients are used up and also as the phytoplankton are eaten by zooplankton, the annual bloom fades away until the right conditions return the next year.
Equatorial upwelling
Nutrients can also be brought to the surface waters by upwelling, which often occurs along coasts and is also important near the equator. The images show a second global pattern in chlorophyll near the Equator, particularly in the Pacific Ocean. A turquoise streak along the Equator shows that phytoplankton were growing. Surface waters are pushed away from the Equator by the prevailing winds and cooler water with a higher nutrient content rises to replace it.
East-blowing winds along the Equator push water North in the Northern Hemisphere, and South in the Southern Hemisphere. See the Ocean Currents module for more details.
See the links page for animations on plankton bloom dynamics.
Ocean colour data from the MODIS sensor on the NASA Aqua satellite.
Source: NASA
Ocean colour data from the MODIS sensor on the NASA Aqua satellite.
Source: NASA
Source: NASA
Ocean colour data from the MODIS sensor on the NASA Aqua satellite.
Source: NASA
What else do the images tell us?
The black areas near the north and south poles tell us about light levels in these regions during the year.
The satellite passes over at pretty much the same time each day. The equator crossing time is 1.30 pm local time, so a rough estimate of the crossing time near the poles will be about 30 minutes either side of this.
Does the fact that an image is black mean that there is no light at all in those regions? Actually it can mean that there is not enough light to make useful measurements of ocean colour at the time that the satellite passed over.
The patchy black areas over the oceans are cloud. This means that the clouds are present throughout a month. But does it mean that the clouds are there all day long each day? This is quite likely is some areas, but we need other data to tell us if this is the case.
Can phytoplankton grow under cloud? Just as on land plants grow on cloudy days, we expect phytoplankton to carry on growing under cloud.
Ocean colour data from the MODIS sensor on the NASA Aqua satellite.
Source: NASA
Ocean colour data from the MODIS sensor on the NASA Aqua satellite.
Source: NASA
