| Date | May 2015 | Marks available | 2 | Reference code | 15M.3.HL.TZ2.10 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Distinguish | Question number | 10 | Adapted from | N/A |
Question
The filamentous green alga (Cladophora) forms mats along the shore of certain fresh water lakes. When the mats become stranded on beaches, they produce a bad odour from the action of decomposers. A study was undertaken on the abundance and persistence of fecal indicator bacterium Escherichia coli in the mats and in the water at three beach sites on one lake. The number of colonies (colony-forming unit or CFU) that grew from bacteria in a 100 gram sample of mat or water collected on day 0 at the three sites was measured on four days over an eight day period to test the survival of E. coli.
Identify the site with the lowest average CFU of E. coli in the water samples.
Distinguish between the trends in the survival of E. coli on mat samples and in water samples over time.
Scientists formerly related the population of Cladophora to changes in phosphorous levels in the lake. However, phosphorous quantities have decreased but Cladophora has recently increased along the shore. Suggest two reasons, other than phosphorous, for the change in population growth of Cladophora in the lake.
Discuss the possible ecological relationships between E. coli and Cladophora.
Markscheme
(site) 1
a. (CFU of) E. coli on mats remains higher/almost 102 more than in the water samples;
b. over time in mats the values do not change much while in water they decrease/disappear;
a. excess nitrogen from fertilizers as run-off from agricultural lands;
b. excess organic matter from sewage overflow;
c. change in temperature/global warming;
d. change in pH;
Do not accept a general statement of minerals or fertilizers in the water.
a. Cladophora provide a habitat for E. coli so more E. coli/CFUs (in mats);
b. Cladophora provide more food for E. coli so more E. coli/CFUs (in mats);
c. Cladophora in mats are dead and decomposed by E. coli /Cladophora in water are alive so not decomposed by E. coli;
Examiners report
Most candidates were able to identify "site 1" and the trends in data, although some got lost in details for the latter.
The other parts of the question caused problems for most, showing that many candidates had not understood the relationship described in the stem, that the mats were formed of decomposing Cladophora and E. coli.
Suggestions were generally very vague and most candidates did not distinguish between conditions that would apply to a lake, such as changes in temperature and pH, and those in laboratory conditions, such as changing light intensity of CO2 concentrations.
There were many wild guesses about the relationship between the two species, namely parasitism or mutualism without any justification for either.
