Nitrogen-phosphorus model worksheet answers

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A schematic of the modelled nitrogen and phosphorus cycles. The unlabelled arrows leading into the phytoplankton (O and NF) indicate the biological uptake of nitrate and phosphate.
The article provides answers to the questions posed in the nitrogen-phosphorus model worksheet.

Worksheet answers

  1. Yes.
  2. Yes.
  3. P behaviour is near-identical in the two models (slight differences, but not important).
  4. They do agree. Only the N2-fixation flux is different. Yes, the changes act to return the N:P ratio to its steady-state value (negative feedbacks).
  5. Without N2-fixation the imbalance is 488 + 16 – 30 – 38 = 436 Tg N y-1. N2-fixation takes up the slack (increases to that value, in steady-state). Nitrogen-fixation increases because denitrification removes reactive nitrogen (represented by nitrate in the model) from the system, and so conditions are made more favourable for those phytoplankton able to get nitrogen from other sources, i.e. N2 (which is always plentiful).
  6. Again nitrogen-fixation increases to take up the slack and bring extra nitrogen into the system.
  7. Increasing RP increases TPP proportionately. Increasing RN has no effect.
  8. Increased RP → more PO4 in the system → increased nitrogen fixation (because extra PO4 not initially accompanied by extra NO3) → more NO3 brought into the system → more of both PO4 and NO3 → increased TPP. Conversely, increased RN → more NO3 in the system → decreased nitrogen fixation (because extra NO3 not initially accompanied by extra PO4) → less NO3 brought into the system → no net increase in either PO4 or NO3 → no effect on TPP.
  9. It is because the population of nitrogen-fixers is limited and controlled by phosphate: in the model, nitrogen-fixer growth rate (μ = μmax * PLIM) is dependent only on phosphate. Although nitrogen-fixers typically make up only a small fraction of the total phytoplankton community, they also act as a conduit for bring new nitrate into the system and in this way their activities also influence the nutrient levels for the other phytoplankton.

Further information


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