Nutrient upwelling and threats to coral reefs

Ecosystems (Continuity and change) · Climate change · note 3 of 5

Spec D4.3.5, D4.3.7Download all Climate change notes (PDF)Download all notes (PDF)6 pages

Nutrient upwelling and threats to coral reefsSpec D4.3.5, D4.3.7

In short

Changes in ocean currents alter the timing and extent of nutrient upwelling: warmer surface water can prevent nutrients rising, decreasing ocean primary production and energy flow through food chains. Coral reefs are threatened with ecosystem collapse because increased carbon dioxide causes ocean acidification and suppresses calcification, and higher water temperatures cause coral bleaching.

Nutrient upwelling

In upwelling zones, currents bring cold, nutrient-rich water from the deep ocean to the sunlit surface, where phytoplankton use the nitrate and phosphate for growth. These zones support some of the most productive fisheries.

Warmer surface water is less dense and forms a stable layer that does not mix with the cold water below. This can prevent nutrient upwelling to the surface. Ocean primary production decreases, so less energy flows through marine food chains to fish, seabirds and marine mammals. Changes in currents also alter the timing of upwelling, so it may not match the breeding seasons of consumers.

Two ocean cross-sections beside the land: normally wind moves surface water offshore and cold, nutrient-rich water rises, supporting many phytoplankton and fish; with warmer surface water, a warm, stratified layer means upwelling is prevented and there are fewer phytoplankton and fish. — Warm, stratified surface water can prevent nutrient upwelling, lowering ocean primary production.

Threats to coral reefs

Ocean acidification: increased CO₂ dissolves in seawater, forming carbonic acid. The pH falls and fewer carbonate ions are available, which suppresses calcification, so corals build calcium carbonate skeletons more slowly.
Coral bleaching: increases in water temperature cause corals to expel their symbiotic algae (zooxanthellae). The corals turn white and lose their main source of food; if high temperatures last, they die.

Corals build the reef structure that provides habitat and food for very many species. Loss of corals causes the collapse of reef ecosystems, so coral reefs are an example of potential ecosystem collapse.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

What are positive feedback loops in climate change?

Positive feedback loops are cycles in which warming causes changes that cause more warming. Examples are loss of reflective snow and ice, so more solar radiation is absorbed; thawing permafrost releasing CO₂ and methane; CO₂ released from the deep ocean; and more droughts and forest fires releasing carbon.

How does climate change affect coral reefs?

Climate change threatens coral reefs in two ways. Increased carbon dioxide causes ocean acidification, which suppresses calcification so corals build skeletons more slowly. Higher water temperatures cause coral bleaching, when corals expel their symbiotic algae and may die. Loss of corals causes collapse of the whole reef ecosystem.

Why are species moving to higher altitudes and towards the poles?

Species are moving because warming makes the warm edge of their range too hot, while cooler places further upslope or poleward become suitable. Tropical montane birds in New Guinea have shifted upslope, and some North American trees show contraction at their warm edge and northward spread.