| Date | May 2012 | Marks available | 2 | Reference code | 12M.3.HL.TZ1.10 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Suggest | Question number | 10 | Adapted from | N/A |
Question
Mercury is a toxic substance and its biomagnification in aquatic food chains is a global concern. A study tested the effects of inorganic phosphorus levels on both algal growth and mercury accumulation by Daphnia mendotae feeding on the algae which absorb mercury from the water. Daphnia may subsequently be eaten by fish.
Experiments were performed in six different tanks with increasing phosphorus concentrations. The same amount of mercury was added to each tank. The quantity of algae, determined by measuring the amount of chlorophyll a, and the accumulation of mercury by Daphnia was measured in each tank. Chlorophyll a and mercury levels were measured twice at each of the six different phosphorus concentrations.
Deduce the tank in which the quantity of algae was highest.
Deduce the tank in which the level of mercury accumulation in Daphnia was lowest.
Outline the relationship between phosphorus concentration in the water and the accumulation of mercury by Daphnia.
Using the data, suggest reasons for the relationship between phosphorus concentration in the tanks and mercury concentration in Daphnia.
Markscheme
(tank) 6
(tank) 4
increasing phosphorus decreases mercury accumulation by Daphnia;
increasing concentration of phosphorus above tank 4 has little effect;
highest levels of mercury accumulation recorded at low phosphorus concentrations;
increased phosphorus concentrations produce algal blooms/increased algae;
larger populations of algae result in smaller concentrations of mercury in the (individual) algae;
less mercury taken in/accumulated by Daphnia from their food;
Examiners report
Almost all candidates correctly indicated that tank 6 had the highest quantity of algae.
many incorrectly stated in (ii) that tank 6 had the lowest mercury accumulation in Daphnia. Tank 6 has the least difference between the two measurements of mercury accumulation but the lowest level was in tank 4.
While many candidates were able to get one mark for stating that increasing phosphorous decreases mercury accumulation in Daphnia, few were able to get a second mark in this outline question.
The better candidates were able to get two marks here by suggesting increased phosphorous concentrations led to algal blooms which absorbed the mercury and the result of this on mercury levels in Daphnia.
