Describe the effect of temperature on the total biomass.

Compare and contrast the effects of temperature on the biomass of autotrophs and heterotrophs with added nutrients.

Explain the effect of temperature on the rate of photosynthesis in this mesocosm.

Suggest reasons for the decreases in biomass of autotrophs as temperature rises, despite the increases in photosynthesis.

Describe the effects of temperature and nitrate concentration on biomass.

Suggest two abiotic factors, other than temperature and nutrient supply, that may affect the production of biomass of the grasslands.

The first study used mesocosms and the second study was carried out in natural grassland. Discuss the use of mesocosms as opposed to a study in a natural environment.

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);

a. autotroph biomass decreases and heterotroph biomass increases with higher temperatures;
b. decrease in autotrophs is greater/larger/more than increase in heterotrophs
OR
little difference in biomass (between auto and heterotrophs) at highest temperature/27 °C;
c. autotrophs show smaller and smaller gains in biomass from initial as temperature rises/WTTE;
d. heterotrophs no gain in biomass at 21 °C then larger and larger gains as temperature rises;

rate of photosynthesis increases as temperature rises because:
a. temperature is the limiting factor for photosynthesis;
b. higher temperatures increase enzyme activity;
c. faster molecular motion/more molecular kinetic energy/more frequent enzyme-substrate collisions;
d. Calvin cycle/light independent reactions (of photosynthesis) speed up;

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);

a. increased temperature raises biomass;
b. increased nitrate raises biomass more than increased temperature;
c. increased nitrate and temperature raises biomass by same amount as nitrate alone;

a. water availability/rainfall/humidity;
b. light/sunlight (intensity) / daylength;
c. salinity of soil / high/low soil pH;
d. chemical pollution/herbicides/allelopathy/parasitic weeds;

Mark the first two answers only.
Do not accept carbon dioxide or weather conditions.

advantages of mesocosms/converse problems with studies in natural environments
a. easier to manipulate/control variables/conditions / less susceptible to outside influences
OR
easier to replicate
OR
take up less space;

disadvantages of mesocosms/converse opportunities with studies in natural environments
b. some trophic levels missing/incomplete food chains in mesocosms
OR
large animals cannot be included / ethical concerns about enclosing animals in mesocosms
OR
some variables lacking in mesocosms / doesn’t show what happens in natural ecosystems;

Allow only one mark for an advantage and one mark for a disadvantage as this is a discuss question.

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.

Most candidates stated that autotroph biomass decreased and heterotroph biomass increased as temperature increased, but this contrast was only given one mark. Far fewer candidates gave a second worthwhile comparison or contrast. Very few candidates realised that at all temperatures the autotrophs had gained biomass from the initial level, but the gains were less as temperature rose. Similarly, few candidates stated that the heterotrophs had not gained mass at 21°C but gained increasing amounts of biomass as the temperature rose.

About half candidates merely stated the relationship shown in the graph, rather than actually explaining it. An explanation based on enzyme activity were expected. Performance in this question and in (d) correlated well with the overall performance of each candidate on the paper, probably because biological understanding was required. Other parts of question 1 were less well correlated, as is typical for data analysis questions.

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.

This was another question where many candidates’ answers lacked discernment. The increase in biomass with nitrate was clearly greater than that with temperature alone, but the increase with nitrate and temperature combined was not significantly different from nitrate alone. A useful way of thinking about answering questions such as this is ‘If I read out my answer to someone over the phone, would they correctly sketch the relative size of the bars without seeing the actual bar chart?’ With many candidates the answer to this would have been no.

About half of answers given were accepted. Carbon dioxide concentration was unlikely to vary enough to affect grassland biomass production. Vague answers such as ‘pH’ were not accepted but soil pH was.

Because this was a ‘discuss’ question, one mark was awarded for arguments in favour or mesocosms and one mark for counter arguments. The best answers weighed up the relative advantage of these two approaches, rather than just singing the praises of one of them.