Name:________________________________
Date:________________
Per:______
WS 3.1 – Analyzing Marine Food Webs
Antarctica is the most inaccessible and least-studied of all the
Earth’s continents. To get a better idea of how the organism in
Antarctica interact, scientists analyze naturally occurring stable
isotopes of nitrogen. All organisms are born with a certain amount
of these isotopes in their bodies, about 3.8 parts per thousand. As
these organisms eat they retain not only the nitrogen isotopes (δN)
they were born with but also the isotopes of nitrogen in the
organism that are being consumed. Therefore, as you analyze
organisms going up a food web the concentration of nitrogen
isotopes (δN) in organism’s increases by a predictable amount and
can be used to help determine what trophic level an organism is.
So, dividing the δN composition of different organisms by 3.8
will give an indication of the relative trophic position of each
organism in a food web.
Organism Name
Diatoms
Copepods
Krill
Amphipods
Petrel
Adelie Penguin
Crab Eater Seal
Baleen Whale
Small Fish
Large Fish
Emperor Penguin
Skua
Weddle Seal
Leopard Seal
Killer Whale
Amount
of δN in
organism
(ppt)
δN / 3.8
3.8
7.6
7.6
11.4
11.5
11.4
11.4
11.4
12.6
13.68
14.09
14.4
18
22.32
25.56
1.0
2.0
2.0
3.0
3.0
3.0
3.0
3.0
3.3
3.6
3.7
3.8
4.7
5.9
6.7
1. What are the apex predator(s) on the food
chain above?
2. Does a high δN value necessarily mean an
organism is a apex predator?
3. What makes these organism apex predators?
4. According to the δN data, how many 3rd level
consumers are there? (remember just because
there is a 3 in front does not necessarily make
them a 3rd level consumer)
5. Provide a rational for why small fish may have a
slightly higher δN value than other organisms in
their trophic level. What does this indicate
about the trophic levels of the organisms they
consume?
6. What organism(s) would be considered
zooplankton?
7. Which of the 2 penguins obtains more energy
through the food they eat?
8. What would happen to crabeater seal
populations if the Krill population increased?
9. What would happen to the large fish population
if the Weddle seal population increased?
10. What would happen to amphipod populations if
the copepod population decreased?
11. What would happen to emperor penguin
populations if the leopard seal population
decreased?
12. How would a increase in copepods effect Krill
populations?
13. What is the only group of organisms that would
cause a universal decline in all populations?
14. Can any organism in the food web vary without
affecting all other organisms?
15. Why are many marine ecologists
recommending against fishing smaller fish lower
on the food chain?