Question 21N.2.HL.TZ0.a
| Date | November 2021 | Marks available | [Maximum mark: 2] | Reference code | 21N.2.HL.TZ0.a |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Describe | Question number | a | Adapted from | N/A |
A community living in the water of an estuary was used to investigate how climate change may affect ecological systems. The food web in this community included phytoplankton (producers), zooplankton (consumers) and saprotrophic bacteria. Small plastic mesocosms were set up with water from the estuary containing only these three groups of organisms. The mesocosms were subjected to four different temperatures and two nutrient levels (control and nutrients added) to replicate local variations of the conditions in the estuary during springtime warming.
The graph shows the biomass of the community for each of the eight mesocosms at the end of the experimental period. Biomass was measured in terms of the amount of carbon present. The horizontal line indicates the initial biomass.
[Source: adapted from O’Connor, M.I., Piehler, M.F., Leech, D.M., Anton, A. and Bruno, J.F., 2009.
PLOS Biology, [e-journal] 7(9). https://doi.org/10.1371/journal.pbio.1000178.]
Describe the effect of temperature on the total biomass.
[2]
a. negative correlation/decrease (in biomass) as temperature rises in added-nutrients (mesocosms);
b. little/no (significant) change in biomass as temperature increases in control (mesocosms);
The word ‘total’ confused some candidates who tried to write about both the control and added-nutrients without distinguishing between them. Of those candidates who realised that ‘total’ must mean the total biomass within a mesocosm, almost all got the decrease in ‘with nutrients’ mesocosms as temperature increased, but many were not discerning enough with the control mesocosms. The differences were less than the error bars, so were clearly insignificant and should have been ignored Question 2.
