| Date | May 2015 | Marks available | 2 | Reference code | 15M.3.HL.TZ1.10 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 10 | Adapted from | N/A |
Question
During the 1980s, a tiny invasive crustacean Bythotrephes cederstroemii entered the eastern Great Lakes from Europe (probably via freshwater or mud in the ballast water of merchant ships) and eventually colonized Lake Michigan. Bythotrephes reproduces very quickly and eats common zooplankton, disrupting the food web by directly competing with small juvenile resident fish. Bythotrephes avoids predation by larger fish through the timing of its activities which have been investigated in offshore waters of Lake Michigan at various depths during the day and night.
State the depth range showing the most Bythotrephes during the night.
Describe the distribution of Bythotrephes during the day.
Deduce the responses of Bythotrephes to temperature and light.
Explain the change in distribution of Bythotrephes between day and night in terms of its position in the lake food chain.
Markscheme
0–10 meters
a. Bythotrephes found at all depths down to 20–30 m/none below 30 m;
b. greatest number/density (of organisms) at 10–20 m;
c. least number/density (of organisms) at 0–10 m;
a. avoids/driven away by light (to colder water);
b. in absence of light attracted to warmer water;
c. can tolerate a wide range of temperature (accept numbers in range of 4/5–20/25°C);
a. zooplankton found in (warmer) surface water where small plants / algae/phytoplankton are found due to light;
b. as predator, Bythotrephes moves up to the surface to feed on zooplankton at night;
c. as prey, Bythotrephes moves to lower/darker depths during the day to avoid being (easily) seen by predators/fish;
Examiners report
Almost all candidates were able to get the 1 mark for reading the graph correctly.
This question also involved reading the graph correctly and most were able to get 1 mark. Many received a second mark as well.
The association between light, temperature and depth was problematic for many candidates. Many were able to get 1 mark usually for light avoidance.
There was poor reference made to the role of Bythotrephes as predator or prey. Many confused “position in food chain” for position in the lake in terms of depth.
