Plankton Exercise

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Plants and animals that drift through the ocean are called phytoplankton and zooplankton, respectively. The plankton are distinct from animals that swim through the ocean, like fish, which are called nekton. Just as plants on land form the foundation of the terrestrial food chain, phytoplankton concentrate energy from the sun in a form that supports much of the marine food chain.
One difference between land plants and phytoplankton is that the phytoplankton are much more efficient at converting sunlight into the carbohydrates (sugars, for example) that fuel herbivores. While most land plants convert only 1-2% of the sunlight that falls upon their leaves into carbohydrates, phytoplankton can convert up to 55%.
Zooplankton that eat phytoplankton convert only about 10% of the energy stored in the phytoplankton tissue into tissues in their own body. This is a typical metabolic efficiency for herbivores (plant eaters) and carnivores (meat eaters). Omnivores like yourself turn only 10% of your meals into part of your body; the remaining 90% is either not absorbed by your gastrointestinal tract (wasted) or metabolized into muscle power and heat. The low metabolic efficiency of herbivores and carnivores means that an average ecosystem must have lots of plants supporting a small number of herbivores, which in turn feed only a few carnivores.
Plankton have a lot of potential limitations in life. The availability of light and nutrients determines the health of phytoplankton. The presence of predators is a strong constraint on whether both phytoplankton and zooplankton live or die. The specific physical forms and behaviors of plankton are adaptations that have proven advantageous for survival in the marine environment. Evolution has created a spectacular diversity of planktonic shapes, structures, and behaviors.
In this lab you will use microscopes to examine the diverse adaptations of plankton. The microscope is necessary because plankton are very small. Generally, phytoplankton have diameters of 1-20 micrometers (0.001-0.02 mm), not much bigger than a bacterium! Zooplankton are much larger, typically 1-100 mm across. If you look closely at the preserved samples, you should be able to see some zooplankton without magnification.

Plankton Lab

1) Understand energy flow through a food chain.

2) Guess and calculate the number of plankton in a ml of typical seawater.

3) Use microscopes to study plankton adaptations.

On April 10, 1999, plankton were collected using a net towed by the Research Vessel Point Sur at three locations in Monterey Bay. As the boat pulled the net at a fixed speed through the water, plankton were strained out of the water that flowed into and through the net, accumulating in a collection cup. After a measured amount of time, the net was hoisted onto the deck, and the cup contents were emptied into a sample jar. Shortly after the cruise, the samples were preserved (using formalin). Some of the preserved sample was set aside for analysis of species composition with a microscope, and the remaining fluid was passed through a coffee filter. Finally, the coffee filters and their plankton contents were dried.
A basic question about plankton that can be answered using the information gathered that day is "How many plankton live in a cubic centimeter of typical sea water?" Another way of phrasing this question is "What is the concentration of plankton in the ocean?" Concentration is the number of things in a certain volume. In this part of the lab, we will first calculate the volume of water that passed through the plankton net at each station. Then we will divide an estimate of the number of plankton (based on the measurement of the mass of plankton in the coffee filters) by the volume of water filtered to find the concentration of plankton in the waters of Monterey Bay.
1. First, test your intuition. How many plankton do you think are present in 1 cubic centimeter (the volume of a sugar cube) of sea water? If you've ever swum in the ocean, you've probably swallowed this amount of water! Write your guess in the space below.

2. Here is the information you will need to calculate the volume of water that was strained by the plankton net at the three stations in Monterey Bay.


Speed of ship

Duration of tow

Radius of net opening

1: Inner Bay

50 cm/s

300 s

30 cm

2: Outer Bay

75 cm/s

210 s

30 cm

3: Kelp Forest

30 cm/s

330 s

30 cm

Working within your small group, discuss how to calculate the volume of water that flowed through the net at each station. Draw a picture of the net moving through the water to help you understand the calculation, labeling the radius of the net opening, as well as the speed of the ship (and net). Write the equation you use (and show your work) below. Enter the volumes filtered in the 2nd column of the following table.


Volume filtered by net


Number of plankton filtered


Number of plankton per cubic cm

1: Inner Bay


2: Outer Bay


3: Kelp Forest


3. Finally, estimate the plankton concentration (number of plankton in each cubic centimeter) by dividing the 2nd column values (in the table above) by the 1st column values. Write your results in the 4th column of the table above.

4. Is your guess (from question 1) more or less than the calculated plankton concentrations?

5. Rank the stations in order of plankton abundance, from highest to lowest. Give two reasons of why the plankton concentrations may have differed between the three stations in Monterey Bay.

6. Like land plants, phytoplankton use sunlight to convert nutrients (nitrates and phosphates) into carbohydrates. During this process, called photosynthesis, they consume CO2 and produce O2. Little sunlight penetrates deeper than about 100 meters into the oceans, so if plankton sink beneath this depth (the bottom of the photic zone), then they will die and decompose as they slowly sink into the deep ocean. Considering these processes, explain the major features in the vertical profiles of oxygen and nutrients in the oceans, pictured below.

7. Plankton have many adaptations that enable them to remain near the surface of the ocean, where the phytoplankton can grow in the sunlight. Using the microscopes, examine a sample of water from the Monterey Bay plankton nets and identify at least four different types of organisms. For each organism, (a) draw a simple sketch, (b) specify its common name , and (c) describe any adaptations that you think help to prevent it from sinking beneath the photic zone.

Organism 1:
Sketch: Name:

Descriptions of adaptations for remaining suspended:

Organism 2:
Sketch: Name:

Descriptions of adaptations for remaining suspended:

Organism 3:
Sketch: Name:

Descriptions of adaptations for remaining suspended:

Organism 4:
Sketch: Name:

Descriptions of adaptations for remaining suspended:
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