Question 21N.2.HL.TZ0.d
| Date | November 2021 | Marks available | [Maximum mark: 2] | Reference code | 21N.2.HL.TZ0.d |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Suggest | Question number | d | 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.]
The graph shows the mean biomass of autotrophs and heterotrophs in the eight mesocosms. The horizontal lines indicate the initial biomasses.
[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.]
Chlorophyll concentration was used as an estimate of the photosynthetic capacity of the community. The rate of photosynthesis and mass of chlorophyll per unit volume were measured in a mesocosm at three different temperatures.
[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.]
Suggest reasons for the decreases in biomass of autotrophs as temperature rises, despite the increases in photosynthesis.
[2]
biomass of autotrophs decreases as temperature rises because of:
a. more herbivory/grazing/feeding by (zooplankton/heterotrophs);
b. higher populations/numbers/biomass of zooplankton/heterotrophs;
c. more mortality/more decomposition/decay of autotrophs/phytoplankton;
d. respiration (rate higher than photosynthesis rate in autotrophs/phytoplankton);
Enzymes do not denature at the temperatures used in this experiment. Also, the autotrophs are phytoplankton living in seawater so transpiration cannot be the cause of biomass reductions at higher temperatures. The data in previous graphs showed higher biomasses of heterotrophs at higher temperatures and thus greater rates of herbivory were the obvious explanation for reduced autotroph biomass.
