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D4.3.5—Ocean current changes

Ocean warming can alter circulating currents and reduce nutrient upwelling, lowering primary productivity and food supply near the surface waters.

Syllabus
First assessment 2025
Objective
D4.3.5
Level
SL

Surface Warming Can Suppress Nutrient Upwelling

Warmer surface water strengthens density stratification and can change the timing and extent of ocean upwelling.

Upwelling brings cold nutrient-rich deep water into the sunlit surface. Stronger stratification resists vertical mixing, so fewer nutrients reach phytoplankton.

Surface warming → stronger stratification → reduced/delayed upwelling → lower surface nutrients → lower phytoplankton primary production → less energy entering marine food chains.

If a seasonal upwelling pulse weakens, phytoplankton and then zooplankton production can fall, reducing food available to migrating consumers.

Currents also respond to wind and salinity; one local season is insufficient to attribute a long-term circulation shift.

Ocean current changes

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice.

What earns marks

Build the answer around this relationship: Warmer surface water can strengthen stratification and reduce vertical mixing.

Representative question

Question 1

[Maximum number: 1]

What is a consequence of ocean water having a very high temperature?

A

Decreased bleaching of coral reefs

B

Increased production of oxygen

C

Increase in energy flow through food chains

D

Reduced nutrient upwelling to the surface

Retrieve the SL Climate Chain

Core D4.3 is secure when every climate impact is explained as a chain: human greenhouse-gas sources or feedbacks change climate conditions, which alter habitats, oceans, carbon stores, or species distributions. Carbon sequestration is the mitigation chain that stores atmospheric CO2.

  • human gases and positive feedbacks amplify warming
  • boreal forests, ice habitats, upwelling and reefs shift through specific mechanisms
  • species may move poleward, upslope, contract, or lose ice/reef habitat
  • afforestation, agroforestry, regeneration and peatland rewetting store CO2

Climate Effects on Ecosystems

Core climate-change transfer answers should not list endangered examples. They should identify the climate driver, explain the physical or chemical mechanism, then state the biological consequence. Use this for greenhouse gases, feedbacks, boreal forests, ice-dependent species, upwelling, range shifts, reefs, and carbon sequestration.

  • Link human activities to increased greenhouse gases and enhanced warming.
  • Explain ecosystem impacts using mechanisms such as positive feedback, carbon sink/source shifts, habitat ice loss, reduced upwelling, range shifts, bleaching or acidification.
  • Explain carbon sequestration by naming the storage pathway in biomass, forests, soils or peatlands.

Concept essentials

  • Warmer surface water can strengthen stratification and reduce vertical mixing.
  • Reduced upwelling limits nutrients available to surface phytoplankton.
  • Changes in ocean currents can alter productivity and marine food-web support.