Plankton
Equipment needed for each student group:
compound microscope
dissecting microscope
reference manual for phytoplankton genera
one petri dish with a zooplankton subsample
Equipment needed for general class use:
video of various plankton specimens
video camera mounted on compound microscope
one or more monitors (depending on class size) for demonstration
materials and samples to prepare phytoplankton wet slides
assorted specimens with large labels placed in phylogenetic order
DISCUSSION:
Phytoplankton as a Trophic Level
It is important to realize that all life on this planet is connected. The emissions from
automobiles in California effect polar habitats a thousand miles away. The droughts in the plains
affect the price of beef, since without rain there would be nothing for cows to graze. Perhaps the
most simple and direct example of connectivity is one we all probably learned years ago: the food
chain. A food chain, like the simple one depicted in figure 1, is a simple, unidirectional diagram
that shows one organism feeding upon another. However, in reality, it is rare for an animal to
only feed on one type of food its whole life.
FIGURE 1
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Today the linear food chain depiction has been replaced by the more realistic representation
of food webs (Fig. 2). Webs illustrate the vast complexity of the trophic interactions in nature by
allowing for multiple pathways from the lower tropic levels to the upper trophic levels. We will
examine food webs again in a later lab. For now we will focus on the lowest level of any trophic
level: autotrophs. Autotrophs are capable of creating their own food by utilizing an outside
energy source such as sunlight. The most common autotrophs are plants and they are necessary
for converting the energy from the sun into a source of energy that all other organisms can utilize.
This is accomplished by all plants through photosynthesis.
FIGURE 2
When we consider the plants of the ocean, most of us who have had limited contact with the
oceans think of the medium to large-sized plants that we have seen in the rocky intertidal zone or
on pier pilings, often generalized as ‘seaweed’. Some of us may have seen kelp beds along the
West Coast of the United States or the New England States. If we were to get on a ship and head
out to sea, these larger plants would no longer be seen. They cannot grow below the photic zone
since they need light to survive. In the North Atlantic or the Gulf of Mexico we might see the
occasional patch of sargassum, a brownish floating plant, but otherwise plants would not be
visible. How can this be? We know that plant life has to be present to support the animal forms
of life that we see in the water.
The reason we see no plant life despite the many larger animals, is that most are microscopic.
The plants in the open ocean are similar to plants on or near land in some ways. They all need
nutrients and light to grow. However, without soil, plants in the ocean must have a low surface
area to volume ratio to absorb nutrients from the surrounding water. These drifting ocean plants
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are also affected by ‘sinking’ which is a function of size and density difference between a
particular species and the seawater in which it lives. This means that the same species will sink
more slowly in colder water near the poles than in warmer water near the equator of the same
salinity because the density difference will be less (remember the density lab).
If we were to take a water sample and let it settle for a few hours, we would find the plants of
the ocean by drawing off a few drops from the bottom of the column. We could put the drops on
a microscope slide and place a cover slip on top. When viewed through a compound microscope
(capable of magnifications up to 100), we would see many different small organisms, some round
in shape and others in chains with long spines holding the ‘units’ together. These are the plants
of the sea, the phytoplankton, or plant plankton. Plankton means ‘drifter’. The phytoplankton
are drifting plants of the ocean.
The procedure described in the previous paragraph is how we will be looking at some of
these phytoplankton through the microscope in class. We will look at how they are grouped and
classified based on differences in their sizes, shapes and inorganic components. Keep in mind
that all phytoplankton are plants and they all photosynthesize, removing carbon dioxide from the
water that comes from respiration and producing oxygen and organic compounds that are
necessary for all organisms higher on the trophic pyramid.
DISCUSSION:
Diatoms
Diatoms can be put into two broad categories depending mainly on shape: centric diatoms
and pennate diatoms. The basic body plan of centric diatoms is cylindrical while that of pennate
diatoms is more elongated or elliptical. Both groups are non-motile, but the pennates can have a
gliding, back and forth, motion due to their shape, much like a leaf falling from a tree. Many
centric diatoms form chains of cells that are held together by long spines or short threads. These
chains will often break up when they are collected in a towed net. The diatom cell is surrounded
by a complex inorganic structure made of silica. This silicate structure has importance to
paleontologists because it persists in marine sediments and can be identified to species depending
on its many smaller features. They can be used to look at the fossil record in terms of extinction
and evolution events as well as ocean temperature of past times (using oxygen isotopic analyses).
These silicate structures are called the frustule. These silica frustules are very ornamented with
pores. In centric diatoms, they are basically like the two halves of a petri dish, one being slightly
smaller than the other so that they fit together. Around the circumference where they fit together
is an intercallery band which holds them together (Fig. 1).
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FIGURE 1
Diatoms divide when the cell splits in half and each half forms a new upper and lower valve
inside the older valve. As a result, diatoms get slightly smaller every time they divide. Growth
rates are affected mainly by light availability and nutrient concentrations, but are also affected by
temperature. Cell division takes place every 1-2 days under normal conditions.
DISCUSSION:
Dinoflagellates
Dinoflagellates are usually armored by cellulose plates that fit together like pieces of a
jigsaw puzzle. They have the ability to be slightly motile because they have two flagella, or
hairlike structures, which can wave in the water and cause the cell to move. A typical horned
dinoflagellate is seen below in Figure 2. One flagellum is in a median groove or cingulum, and
the other one is in a transverse groove that runs more or less perpendicular to the median sulcus.
Not all dinoflagellates have the cellulose plates. In the less highly evolved specie the flagella
originate from one end of the cell. Some dinoflagellates produce bioluminescence or ‘living
light’ and are responsible for red tides. Red tides are unusual increases in the dinoflagellates
concentrations in seawater and often lead to shellfish poisoning and fish kills. Sometimes the
cells get so concentrated that they actually clog the gills of the fish and suffocate them. When
these red tides die out, the oxygen in the water can be seriously depleted due to bacterial
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decomposition. This can also cause mortality in water column and benthic species. Reproduction
is usually by binary fission
FIGURE 2
DISCUSSION:
Coccolithophores
Coccolithophores are typically much smaller than most diatoms and dinoflagellates. In this
group the cell is covered by small, calcareous plates called coccoliths. These coccoliths can
either adhere closely to the cell or be on long stems. They usually have ornate sculpture, which
aids in their identification in sediment samples. As in the case of the diatoms, coccolithophore
remains in the sediments can be used to study past ocean temperatures and the extinction and
evolution of species. A typical coccolithophore is shown below in Figure 3.
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FIGURE 3
EXERCISE 2:
Viewing Phytoplankton slides
For this exercise, use the compound microscope on your lab bench. Your T.A. will instruct
you in its use for those of you who may be unfamiliar with its operation. The main features are:
the stage where the slide will go, the eyepieces you will look through, the objective lenses that
allow you to change the magnification and the coarse and fine focus adjustments. On the stage
you will find a spring-loaded clip which holds the slide. Just below the stage are a pair of knobs
that allow you to move the slide in the ‘x and y’ directions. You will also see small rulers that
run in each direction on the edges of the stage. These are used so that when someone finds
something of interest that they want to come back to later, or show someone else, they can note
the pair of numbers which will serve as coordinates in navigating the slide much like a Cartesian
coordinate system
You will each have a reference guide to help you identify the phytoplankton you see. The
guide will tell you what the name of the genus is and what larger group it belongs to. There may
be small animal plankton on the slide as well because the collecting technique used (a towed net
with small meshes) does not discriminate between plant and animal, just on size.
The primary slide you will view will be one recently collected from either a coastal area or
from the Gulf of Mexico. The sample used and its composition will vary from semester to
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semester. You may be using premade slides or making your own from a sample. To make a
slide, get a drop or two of the sample and place it on a microscope slide. Get a cover slip, a
smaller, very thin piece of glass and place one end of the cover slip on the microscope slide near
the edge of the drop. Carefully lower the cover slip from the opposite edge so that it spreads out
the drop as it comes to rest on the microscope slide. Put the slide in the stage clip of your
microscope and use the knobs to position the cover slip area under the 10X microscope objective.
Using the coarse focus adjustment, lower the objective towards the slide to its lowest position.
BE CAREFUL NOT TO RUN THE OBJECTIVE INTO THE SLIDE. Looking through the
eyepiece, start to back the objective upwards until you see the slide in focus. It may help to use
the stage knobs to move the slide while you are looking through the eyepieces to see the motion
of the cells as they come into focus. Use the fine focus knob to achieve better focus. Remember
that the cells are three-dimensional, even though they are small, and the entire cell may not be all
in focus at once.
Your T.A. will use the microscope camera and monitors to display a slide from the same
sample, or another. They will show you some of the more common genera.
1. Sketch five different types of cells that you see and label them with a genus name. These
sketches are not based on artistic ability, but on your ability to see the details of the organism. Be
sure to draw only phytoplankton, and not the animal plankton, which will appear slightly larger
and more complex.
a.
b.
c.
d.
e.
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DISCUSSION:
Zooplankton as a Diverse Group
Plankton are organisms that drift with the currents. Zooplankton are the animal
plankton found in freshwater as well as marine waters. Size alone is not the best criteria
to assign organisms to this category as seen by the large jellyfish in the aquarium on one
of the side benches (it is a zooplankter).
Holoplankton is a term used for those zooplankton that spend their entire life cycle
in the plankton. Meroplankton are those zooplankton that spend only a portion of their
life cycle in the plankton - usually the early developmental stages of organisms that are
benthic (bottom dwelling) as adults.
We will be covering a number of different zooplankton groups during this lab. Some
will be shown by video and others will be demonstrated by specimens that will be set out
and labeled around the lab. These groups include:
A. Protozoa - single celled animals
1. Foraminifera, or "forams", are small, but common members of the zooplankton.
They have spherical ‘tests’ made of CaCO3 in which the cell lives (see Fig. 4).
These tests form part of marine sediments (remember the lab on sedimentation)
and they can be used for age-dating by stable isotopes and also record the
extinction and origin of various species.
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FIGURE 4
B. Coelenterates
1. Periphylla is a genus of jellyfish that lives deep in the water column in tropical
and subtropical seas, but fairly shallow (0-200m) in colder waters such as the
Southern Ocean. There is a large specimen on display in an aquarium on the
side bench.
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2. Siphonophores - These are colonial coelenterates that live from the very surface
to several hundred meters. Physalia , also know as the Portuguese Man-O-War,
is a siphonophore that many of you may be familiar with. The often wash up on
the beach and have a large, bluish, gas-filled float that keeps the colony
suspended from the surface. The colony has long, trailing tentacles that have
numerous stinging cells, called nematocysts that kill actively struggling prey
such as copepods of small fishes. They can also cause painful stings to humans,
so beware. Subsurface siphonophores have the same general morphology and
are shown in Fig. 6 below.
FIGURE 6
http://www.divediscover.whoi.edu/images/hottopics-deepsea.jpg
1. Ctenophores are soft-bodied organisms that are commonly called "sea walnuts"
or "comb jellies". They are more common in coastal waters, where they can be
abundant, but are also found from the near surface to several 100m in the open
ocean. They have eight rows of small ctenes, or combs, and they are radially
symmetrical (Fig. 7).
FIGURE 7
http://www.oceanlab.abdn.ac.uk/news/news_pics/ctenophore.jpg
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D. Molluscs
1. Pteropods are small to medium sized molluscs with external shells (Fig. 8) made
of CaCO3 (aragonite - more soluble than calcite). They are sometimes called
"sea butterflies". Under the microscope you will see several of these in your
subsamples. Other specimens are in labeled jars on the side bench.
FIGURE 8
https://www.msu.edu/course/isb/202/snapshot.afs/tsao/images/Pteropod2.jpg
E. Crustacea
2. Copepods are small crustaceans that come in three main subgroups, but all of the
same general shape (Fig. 9). Some species are herbivores, some are carnivores
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and some are omnivores. Many species undergo diel vertical migration. They are
important food items in the diets of many squids, fishes and other copepods.
FIGURE 9
http://blog.oregonlive.com/environment_impact/2008/09/large_copepod09.JPG
2. Euphausiids are also called "krill", or "whale food". The species that lives in the
Southern Ocean can form large swarms of over one million metric tons. These and
smaller swarms are fed on by filter feeding whales and fished commercially by ships of
several nations. This species reaches up to 60mm in length, but most species are much
smaller (Fig. 10). A jar of antarctic krill may be on the side bench.
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FIGURE 10
http://www.google.com/imgres?imgurl=http://www.zuckerspeicher.de/ecoscope/krill/krill666.jpg
&imgrefurl=http://www.zuckerspeicher.de/ecoscope/krill/&h=459&w=591&sz=22&tb
nid=M1jiIVpUXbepM:&tbnh=105&tbnw=135&prev=/images%3Fq%3Deuphausia%2Bsuperba&u
sg=__YMaMfXHf6cs1nx5tj2sAIPN64_s=&ei=SlCsSsVa0eCUB9iq5bsG&sa=X&oi=i
mage_result&resnum=7&ct=image
3. Amphipods are also crustaceans that sometimes swarm in considerable numbers.
A jar of these will be on the side bench.
F.
Chaetognaths are also called "arrow worms". They are carnivores that live in all
oceans from the surface waters to the bottom. They can be small; several mm’s to
several cms (Fig. 11). On the right is a schematic chaetognath to show its overall
morphology. On the left is a Scanning Electron Microscope (SEM) photograph to show
the hooks associated with several sets of teeth that border the mouth.
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FIGURE 11
(Right) From Introductory Oceanography, Eighth Edition, by Harold V. Thurman. Copyright
1997 by Prentice Hall, Inc., Upper Saddle River, N.J. Photo courtesy of Howard J. Spero.
(Left) http://www.cmarz.org/CMarZ_RHBrown_April06/images_press/Eukrohnia_sp1b.jpg
G. Urochordates
1. Pyrosomes are colonial tunicates that are 1 to 10 or more centimeters. They are
shaped like a cucumber and even have bumpy surfaces like some kinds of
cucumber. They have an opening at one end through which water is pumped to
propel the colony through the water. They are bioluminescent; that is, they
produce ‘living light’ which is blue-green in color. Specimens might be in a jar
on the side bench.
2. Salps (Fig. 12) are almost transparent cask shaped organisms that have openings
at both ends. Water comes in one end, passes through a filter, and goes out the
other end. This propels the salp through the water. Sometimes they occur in
long chains more than a meter in length. Each individual in the chain is
independent, so it a towed net or a large swimming organism breaks the chain,
all the fragments survive.
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FIGURE 12
http://www.whoi.edu/cms/images/oceanus/salp_550_59911.jpg
There will be a short video of living zooplankton with narration. It comes from several
different sources. Many zooplankton are very delicate and are easily damaged when
caught by a net. The video will show a gentle technique for collecting delicate forms
from a Remotely Operated Vehicle (ROV).
DISCUSSION:
Collection, Preservation and Analysis of Zooplankton
Your T.A. will show you a zooplankton net and a flowmeter, which is placed in the
mouth of the net to measure the distance the net travels during a tow. With this open net,
only a single sample is collected from the time the net enters the water until it is pulled
out. There are more sophisticated nets, which allow us to open and close individual nets,
so we could sample different layers in the ocean without any contamination from the
water above it or below it.
When the net returns to the surface, it is rinsed with a hose to wash organisms in the
net itself down in to the ‘cod’ end or the container at the end of the net. The cod end is
then removed and rinsed into a sample jar. The jar is nearly filled with seawater and
formaldehyde, a preservative, is added to make a 10% solution. Sodium borate is added
to buffer the sample and keep the pH around 8. This is done to prevent the dissolution of
any calcareous shells in organisms like foraminifera and pteropods. Although the pH of
seawater is already slightly basic, fluids in the zooplankton themselves and the
formaldehyde are acidic and will, over time, turn the pH acidic unless it is buffered.
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In the lab we use a Folsom Plankton Splitter to make smaller, but equal subsamples.
Your T.A. may point one out to you in the lab. Samples are split because there are
usually many ten’s of thousands of individuals in a typical zooplankton samples that
comes from, say 300m3, and it would take an inordinate amount of time, and therefore
money, to process the entire sample. We often split the sample down until we have
approximately 1,000 individuals. This still takes an experienced technician about 1.5-2
hours to identify and count all the individuals. To determine the amount of water filtered,
you multiply the mouth area of the net by the distance traveled. The distance traveled is
calculated by multiplying the total flow counts for the tow on the flowmeter by the
flowmeter calibration factor. We determine this by going over to the swimming pool and
towing the flowmeter 10 lengths in the 50m pool. This gives us a flowmeter calibration
factor for each flowmeter in m/count.
EXERCISE 1:
Counting Your Own Zooplankton Subsample
Each student will get a small petri dish with a known fraction of the original sample.
The lid should remain on the sample. The samples are in water. Your T.A. will show
you a lab counter to illustrate how we count samples. Using a subsample from a larger
sample you will count the number of individuals of several different groups. Your T.A.
will tell you which groups you will be counting and you will have handouts as reference
guides. In addition, your T.A. will show you representatives of each group on the
microscope monitors and there are color pictures of each group on each bench. Since
each subsample should have the same fraction of the total sample, you will enter your
counts into a computer. Your T.A. will give you a copy of the class results, which will
have means and variances for each group.
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