1. Projected changes to the tropical Pacific Ocean

2. Based on Chapters 3 & 4

3. Outline
Projected temperatures and currents: surface and vertical structure
Implications for oceanic nutrients
Acidification
Food web for tuna
Influence of temperature on tuna

4. Sea surface temperature
Change in SST °C ( )
2000: 27.4°C
2035: 28.1°C (+0.7°C)
2100: 29.9°C (+2.5°C)
Ocean is heat sink!
Validation process;
: 27.4°C
: 28.1°C (+0.7°C); model spread +/-0.3°C
: 29.9°C (+2.5°C) ; model spread +/-0.6°C

5. Vertical structure and stratification5
Vertical structure and stratification }
Warm, mixed-layer }
Thermocline depth }
Cold, deep ocean

6. Vertical structure and stratification6
Vertical structure and stratification } } }

7. Vertical structure and stratification7
Vertical structure and stratification
Most warming occurs at surface
Leads to increase in stratification

8. Projected changes in vertical currents
Less downwelling
Less downwelling
Less UPWELLING
Less downwelling

9. Implications for nutrients
Warm, mixed-layer
Thermocline acts as a barrier
Limits the supply of nutrients
Cold, deep ocean
Low nutrient
Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface
High nutrient

10. Implications for nutrients
Warm, mixed-layer
Cold, deep ocean
Low nutrient
Increased stratification makes it harder to bring nutrients upwards
Link with Valerie’s talk. The main nutrients associated with biological productivity are nitrates, phosphates and silicates. The maintenance of this productivity can be fragile, however, because nutrients are not distributed evenly they are depleted near the surface, where they are needed, but abundant in the deeper ocean. This variation occurs because the phytoplankton use up the available nutrients in the photic zone, where there is sufficient light for photosynthesis and, although a small part of the nutrients pass down the food web, most of them eventually sink as organic matter into the deep ocean. There, bacteria remineralize the organic matter, releasing nutrients. As a result, concentrations of nutrients are much greater at a depth of 100 m than they are at the surface
High nutrient

11. Why is this important?
Nutrients support the food web for tuna

12. Image: Marc Taquet, FADIO, IRD/IFREMER

13. Projected changes in major currents
Pacific Current system
Very stable; the currents are projected to remain similar in the future
However their strength is projected to change, especially near the equator.
For instance, the equatorial undercurrent, in blue … Iron

14. Eddies
Amount of eddies related to strength of currents

15. Nutrient supply by eddies
Eddies temporarily lift the nutrient-rich waters
Eddy activity is related to current strengths

16. Eddies and nutrients

17. Convergence zone

18. Similarities with ENSO
Skipjack tuna catch
Source: Lehodey et al. (1997)

19. Ocean acidification

20. Projected acidification
Saturation >4 healthy conditions Saturation falls below 3.3 causing problems for some corals Saturation decreases to 2.4

21. Sea level rise Projection 2035 2050 2100 IPCC 8 cm 18-38 cm 23-51 cm
Semi-empirical
20-30 cm cm cm

22. Summary Increases in sea surface temperature Stronger stratification
Nutrient supply reduced due to increase in stratification, with effects on tuna food web
Slowdown of equatorial currents and upwelling
Aragonite drops below critical levels
Sea level rise, > 1 m cannot be ruled out