Phytoplankton
Like the grass of the fields
and the leaves on the trees, most life on earth ultimately depends on plants
for nourishment. In the ocean, microscopic animals called zooplankton
graze on the pastures of plankton. These little animals are ultimately eaten
by larger zooplankton, by fish and as amazing as it may sound, by the largest
creature of all - the blue whale.
There is never a shortage of
water in the ocean and generally, there is ample light for phytoplankton to
grow in the surface waters. Carbon dioxide is plentiful in the ocean and
whenever the supply in the surface waters goes down because the phytoplankton
have used it during photsynthesis. it is replenished from the atmosphere
above. In the ocean, the major limiting factor regulating phytoplankton growth
is the availability of nutrients. Nutrients (nitrates, phosphates, silicates,
etc.) are found in great quantities in the deeper, colder depths of the ocean.
Whenever those waters are brought near the surface, the oceans are essentially
fertilized (just like you do to your lawns to green them up) and the plankton
bloom.
Following on what was just
said, wherever mixing takes place in the ocean, or where currents bring the
colder, nutrient-rich waters up from the depths, the surface waters will be
enriched with added nutrients which may stimulate phytoplankton growth. In
looking at the large-scale distributions of phytoplankton in the ocean, we can
see how closely they are related to areas where nutrients are being supplied
to the surface waters.
Because the animals all the
way up the food chain ultimately depend on the supply of phytoplankton at the
base, if the plankton disappear, the chain is broken and eventually the
animals will suffer. Life in the ocean is a delicate balance between the
physical conditions that set the stage for life to exist, and the ability of
life to act upon that stage.
Although phytoplankton
account for approximately 50% of the photosynthesis on this planet, over 99.9%
of all the carbon dioxide that has been incorporated into living things over
geologic time is buried in marine sediments, indicating that phytoplankton
play a very important part in regulating the amount of carbon in the
atmosphere.
Furthermore, not all phytoplankton are green. They come in a variety of shapes and
colors and while most of them are harmless, some can bloom in such large numbers
and produce toxins that can be quite harmful to marine life and in some cases,
to humans as well. When phytoplankton growth is stimulated by an overabundance
of nutrients from sources such as sewage discharge or runoff of agricultural
fertilizers used on land, the consequences can be quite serious. Dense blooms of
phytoplankton can essentially block sunlight from reaching the bottom in shallow
areas of bays or estuaries and can cause the massive decline in the Submerged
Aquatic Vegetation (SAV) that has been taking place in places like Chesapeake
Bay. These grasses are vital nursery grounds for many species of fish and
invertebrates and their loss can have dire ecological results. In addition, when
these blooms die and the plankton sink to the bottom, bacterial decomposition of
all this organic matter essentially strips the water of oxygen. Fish, shellfish
and most other living things require oxygen to survive and decaying
phytoplankton blooms have been the cause of many massive fish kills over the
years. Of the phytoplankton that can be directly harmful on their own, the most
commonly known form of these, dinoflagellates, are the source of red
tides.
courtesy of daac.gsfc.nasa.gov