Download Water density and marine organisms

Title: Water density and marine organisms
Summary:
Students expand on concepts they have previously learned about seawater density, ocean stratification,
and density-driven circulation by connecting these physical concepts to the biology in the ocean. It is
designed to help students gain a more comprehensive understanding of how physical processes in the
ocean strongly influence the distribution of marine organisms.
Grade level: 7-12
Time:
Building estuary models: 1-2 hours (using a hot glue gun reduces the time it takes to build the models)
Prep time: 20 – 30 min.
In class lesson: 2 x 50min periods
Key Concepts:
1. Water masses of different densities can interact to form stable layers and drive circulation in the
ocean.
2. Water density strongly influences the vertical and horizontal distribution of marine organisms.
Lesson format: Hands on activity, PowerPoint presentation, worksheets, and class discussions.
Materials
For teacher:
PowerPoint slides – “Density Currents and Biology”
Table top estuary models – see instructions on how to build
1 box Kosher or Pickling salt (dissolve more quickly than other types of salts)
Food coloring, two colors
2 large plastic pitchers
Eggs *test out to make sure they sink
3 large jars, beakers, or clear plastic soda bottles with the top cut off
For students – in groups of 4-5
1 Estuary model per group, with the “dam” secured in place with Vaseline
2 pitchers of water (per group)
Worksheets for each student: “Water Density and Plankton Distribution” and “Sink or Float?”
Background:
Prior to this lesson students should have been introduced to how temperature and salinity affect water
density, and how the interaction of two bodies of water with different densities can drive water
circulation (eg. current formation). The central concepts of stratification and density-driven water
movement have already been identified and discussed
Session 1
Preparation: 15-20 min
1. Set out two large plastic pitchers filled with fresh water, blue and yellow food coloring, and
measure out 34g/L of Kosher salt for each group of students.
2. Set up the Sink or Float demo in the front of the classroom. Fill one large pitcher with fresh
water and mix in a couple of drops of yellow food coloring. Fill another large pitcher with fresh
water, thoroughly mix in 34g/L of Kosher salt, and mix in a couple of drops of green food
coloring.
Fill a large 1000mL+ beaker with the fresh, yellow water and label “Fresh”.
Fill another beaker with the salty, green water and label “Salty”.
In the third beaker, fill ½ of the beaker with salty, green water. Float a sponge on
top of the salty water and carefully and SLOWLY pour the fresh water over the
sponge, filling the beaker up the rest of the way. Carefully remove the sponge and
you should have two defined layers. Label this beaker “Layered”.
Teacher Directions: Review and Invitation
1. Students are first asked to recall how ocean currents are formed (review of density-driven water
movement). In groups of 4/5, students are asked to think about and discuss where on earth we
would likely see examples of both temperature and salinity driven current formation.
2. Each group will share examples they have come up with. These examples are written up on the
board so that a few can be focused on in more depth.
i. Poles (ice formation)
ii. Where rivers run into the ocean (estuaries)
iii. When tropical waters come into contact with colder (temperate/polar waters)
iv. Mediterranean sea
Note: If one of the above is not mentioned by students, probe them with questions.
e.g. Can you think of bodies of water that are much saltier then the ocean? - Black Sea,
Mediterranean sea. Do any of these bodies of water come in contact with the ocean?
Where?
3. Show the PowerPoint images of some of the major areas in our world’s ocean where density
driven circulation occurs: Polar ice formation, Ocean Conveyor (NADW formation),
Mediterranean output, and the Amazon River outflow (a very large estuary), and briefly describe
what is occurring each area. Focus student attention on the Amazon estuary and show how
fresh water from rivers can be observed far out into the surface water of ocean (as indicated by
the murky sediment laden waters) depending on the amount of outflow from the river. Students
should be asked to think about why this is observed while moving into the exploration stage.
Student Exploration: Estuary Models
1. Students are instructed to mix in yellow food coloring to one of their pitchers. In the other
pitcher have the students thoroughly mix in the salt (until dissolved) and blue food coloring.
2. Have the students carefully pour the pitcher of yellow water into one section of the tub,
“estuary model” and pour the pitcher of blue water in the other section filling both sides up to
approximately one inch from the top. Tell the students that the yellow represents fresh river
water and the blue represents salty ocean water
3. Have the students predict what will happen when they release the “dam” and the two bodies of
water meet. The students can be asked to consider the following:
•
•
•
•
Which water is denser, why?
What will the denser water want to do?
What will the less dense water do in response?
What will be the final outcome of the salt and fresh water?
4. After predictions are made, students should then be directed to make close observations when
they release the dam and the two bodies of water to interact. Each group should carefully note
down, step-by-step, what they observed. The following steps should be observed:
I.
II.
III.
IV.
Salty water pushed into the fresh water
Salty water sank to the bottom displacing the freshwater.
The fresh water is pushed up and starts to flow on top of the salty water
The waters flow in opposite directions forming layers (stratification) on
both the river and the ocean side of the model.
Note – if this system was not closed, but rather an open system like a real estuary the
countercurrent flow would be continuous (the two bodies of water would not stop flowing).
1.
2.
3.
Classroom Demonstration: Sink or Float?
This classroom demonstration is designed to get students thinking about how an organism’s
vertical position in the water column can be affected by water density and layering (as observed
in the estuary models). The students will observe and make predictions on their worksheet.
1. Have the students predict what will happen to the marine organism (egg) when placed
in the three different environments: fresh, salty, and layered.
2. In front of the class, add the egg to each of the 3 beakers (or you can have a student
volunteer). A large mixing spoon works great to scoop out the egg. Make sure to add
the egg to the layered environment last!
3. Have the students complete the worksheet.
* Students should keep both their group observations and worksheets for the next session of
the lesson.
Session 2
Preparation: 2 min - setup the PowerPoint “Density Currents and Biology”
Teacher Directions: Review and Concept Introduction:
1. Start the lesson by asking the students to recap and describe what they observed in each
of the demonstrations from the previous session. Ask the students to share their
observations and ideas about what they think is occurring, providing evidence supporting
their explanations. The instructor should write down student observations and ideas on
the board under the headings “Estuary Models” and “Sink or Float?”
2. After students have a chance to share their observations and ideas, a clear explanation
should be provided to the students as to what occurred in each of the demonstrations and
why. The key concepts (as stated above) should also be introduced at this time.
3. Lead a discussion about ways in which marine organisms may be adapted to dealing with
encountering density differences in their environment: swimming, buoyancy control, and
osmoregulation.
Application/Assessment using the “Plankton Dispersal Model” simulations
Students will been shown three movie clips generated from a plankton dispersal model in an estuary
system – San Francisco Bay. The graphic shown in each simulation represents the flow in SF Bay, the red
and yellow layers represent surface freshwater flowing out to the mouth of the estuary, while the
darker blue colors represent ocean bottom water flowing into the estuary. The light blue layer
represents flow that is not moving in or out, but being maintained in the estuary. Each movie clip
simulates plankton, represented by the white dots, having different swimming behaviors: only
swimming up, only swimming down, and swimming both up and down. Each video clip clearly shows
that plankton swimming behaviors interact with hydrography (flow) to determine overall plankton
distribution. Plankton only swimming up are washed out of the estuary and into the ocean. Plankton
only swimming down are washed far up into the estuary and away from the ocean. Plankton swimming
both up and down are mostly maintained in their original location within the estuary. The histogram at
the top shows final plankton distribution after the 10 day simulation.
1. Show the students the three plankton distribution model simulation movie clips (embedded into
the PowerPoint), asking the students to pay close attention to how the plankton are moving and
where they finally end up at the end of the 10 day simulations.
2. After showing the model simulations lead the students through a Think, Group, Share. Students
are first asked to think about what they observed in each simulation and why, then students talk
about their observations and ideas within their groups. Each group should write down: i) how
the plankton moved, ii) final distribution, iii) ideas about of why the plankton were distributed
differently between each of the three simulations.
3. The students can then be lead in a class discussion where each group contributes the
observations and ideas that they wrote down. The question map can be used by the instructor
to guide the class through the discussion about the model simulations.
4. Hand out the “Water Density and Plankton Distribution” worksheet to each student and have
them independently complete the work sheet. Each student should be required to turn in a
work sheet for teacher assessment of student understanding.
Question Map for Plankton Dispersal Model Discussion:
1. Why would the bottom water be flowing into the estuary and the top water being flowing out
of the estuary? – This is an application stage question asking students to think about how
waters with very different densities interact when they converge (review question).
2. Do you think that water masses of different densities flowing into each other would have an
effect on the organisms in that area? – This is a broad question used to generate interest about
how density-driven circulation might affect marine organisms.
3. Did you make any interesting observations about what happened to the plankton in the two
simulations? – A broad question to find out what the students observed.
4. Were there any differences between the simulations? What did you notice that was different?
– Another broad question aimed at finding if the students were able to identify the initial and
final distributions of the plankton in each simulation. Did anyone notice anything different?
5. Did the plankton start out in the same spot in each simulation? – A more focused question in
case students did not mention that in all simulations the plankton had the same initial position.
6. Were the plankton transported in the same way in each simulation? – A focused question to
get students to identify that the plankton had different ending distributions in each simulation
based solely on their swimming behaviors.
7. How should the plankton swim if they want to be maintained in the estuary? – Problem
solving question. Anyone think something different?
8. Why might plankton be up at the surface despite the risk of being washed out to sea? –
challenge question to promote deeper thinking.
Lesson References :
•
Instructions for building the table top estuary models for this lesson can be found at:
http://fermi.jhualp.edu/student/currents/waterfall_apparatus.htm
•
The computer model simulations of plankton distribution in an estuary system were generated
using a tidally-averaged estuary model of San Francisco Bay developed by Parker Mac Cready
(flow dynamics) and Danny Grünbaum (biological component) at the School of Oceanography University of Washington.
Water Density and Plankton Distribution:
Up Stream
Mouth of the Estuary
Above is a computer graphic representing current flow in San Francisco Bay (an estuary) use this graph
to answer the questions below.
1. a) Label the fresh water current. Which way is the current flowing and why is it flowing in that
direction?
b)
Label the salt water current. Which way in the current flowing and why is it flowing in that
direction?
2. a) Where would the plankton living in this estuary end up if they choose to always swim up
toward the surface?
b) Where would they end up if they always choose to swim down?
3. If you were a plankter living in this estuary and wanted to survive in the estuary how would you
decide to swim? Explain your reasoning. (remember you are not strong enough to swim against
the current, only up or down)
Sink or Float
Imagine that the egg is a marine organism that is not swimming. In this activity we are going to explore
how an organism’s vertical position in the ocean can be determined by its environment. Three different
environments are provided, a fresh water environment, a salt water environment and a stratified
(layered) environment. Gently place the egg in each of the environments and observe what happens to
your marine organism.
1. Make predictions about where your marine organism will end up when added to each of the
environments. Explain why you think that.
Fresh –
Salty –
Stratified (layered) –
2. Write down or draw what you observed.
Fresh –
Salty –
Stratified (layered) –
3. Provide an explanation for why your marine organism ended up where it did in each
environment.
Fresh –
Salty –
Stratified (layered) –