Download Plankton: The Water Drifters

Plankton: The Water Drifters
A Brief, but In-Depth Summary
The term plankton is used by scientists to describe a group of organisms found in both freshwater
and marine habitats. The word plankton comes from the Greek word planktos which means
“drifter” or “wanderer.”
Some plankton spend their entire lives drifting with the currents. Other plankton are simply the
larvae (larv-ee or lar-vay) of various aquatic animals and will no longer be plankton once they
become large enough to settle to the bottom or strong enough to swim against the current. These
larval plankton are called meroplankton. One way to remember this is that these organisms are
plankton merely part of their lives. Plankton that never grow large enough or strong enough to
swim against the current are called holoplankton. Holoplankton are plankton their whole lives.
Most plankton are microscopic (or at least pretty small ~ like
krill). However, jellies (jellyfish) are also considered to be
planktonic since they are not strong enough to fight the
currents. Organisms that are too big or too fast to be
considered plankton are called nekton. Nekton include fish,
whales, seals and aquatic birds.
Krill – 1 to 6 inches
“Plants” and “Animals”
Another common way to divide plankton is by how they get their energy. Phytoplankton are
organisms that photosynthesize just like plants do. They create their own food from absorbed
nutrients, carbon dioxide and sunlight. Phytoplankton are sometimes referred to as “plant
plankton” even though they are not truly plants – they are “plant-like.” Some examples of
phytoplankton include diatoms and dinoflagellates.
Zooplankton are organisms that must eat to survive. Some zooplankton are truly
animals such as larval crabs or fish, or copepods (The “Plankton” character in the
Sponge Bob Squarepants show is a copepod.) Zooplankton that are not animals
still need to “eat” (rather than photosynthesize) to get their energy. These nonanimal zooplankton include bacteria, viruses and protists, a hodgepodge group of
living things that have a true nucleus, (unlike bacteria, but don’t qualify as plants,
fungi or animals! Wow, isn’t science fun!?
Copepod
In the Zone
In the ocean, all are the phytoplankton and most of the zooplankton live in the
photic zone. The photic zone is the upper 300 feet of the ocean. In freshwater
habitats, you will find plankton just about anywhere in the water where light can
penetrate.
Phytoplankton
Structurally Speaking
Many plankton are denser than the seawater that they live in, so in order to keep from sinking
completely into the abyss, their bodies are structured to keep them near the surface. In other
words, they are buoyant. However, plankton don’t want to float on the surface of the water, they
want to drift within the water column. This would make them neutrally buoyant, meaning that their
body mass is equal to the mass they displace in the water.
Plankton have evolved many ways to keep themselves from sinking quickly, since sinking deeper
would take them farther from their energy source. Remember, phytoplankton need to stay where
the light is so they can photosynthesize. Herbivorous zooplankton want to be where the
phytoplankton are so they can eat them. Carnivorous plankton may never come near the surface
since they only eat other zooplankton.
Size (small is good!)
When compared to larger organisms, plankton have much more surface area on their bodies in
relation to their volume. The more surface area an organism has in relation to its volume, the
greater the friction will be between its body and the water – this slows the sinking process. Some
plankton have a large cavity (space) in the center of their bodies that adds to the surface area on
their bodies. However not all planktonic species are small. Jellies are also
considered plankton and the largest recorded was about 120 feet long!
Projections (spines and stuff!)
Having a body covered with projections, such as spines, also helps distribute
their weight over a larger surface area. Some plankton even store oil or fat in
their bodies to help them be more buoyant.
Phytoplankton
Swimming
Although plankton cannot swim against the current (that’s what makes them plankton), they can
swim here and there as they are carried along in the current. Many zooplankton will even perform
nightly vertical migrations whose distance could be compared to a human climbing Mt. Everest!
Some plankton have tail-like structures, called flagella, to propel them along – even including some
phytoplankton.
Life without Plankton = No Life at All
Not only is plankton vital to the survival of all marine life as a source of energy, but phytoplankton
(along with kelp and other algae (al-gee) is responsible for the production of about 70% of the
oxygen we breathe. Rainforests are responsible for only about one-third of the Earth’s oxygen.
According to marine scientist Sylvia Earle, Today, Dr. Earle notes that just one type —
Prochlorococcus, so small that millions can fit in a drop of water – has achieved fame as perhaps the
most abundant photosynthetic organism on the planet. It releases countless tons of oxygen into the
atmosphere. The tin organism is estimated to provide the oxygen in “one in every five breaths we
take…at it is just one of thousands of types of marine algae and photosynthetic microbes –
everything from invisible cells to plant-like growths to kelp forests.
http://www.nytimes.com/2009/01/13/science/earth/13ocea.html?pagewanted=all&_r=0