IB Environmental systems and societies 4.3D Impact on Freshwater and Marine Ecosystems Notes

Climate Change and Ocean Acidification Are Reshaping Aquatic Ecosystems

Climate Change

1. Long-term shifts in temperature, rainfall, and wind patterns across the Earth's climate system.
2. Driven mainly by burning fossil fuels, deforestation, and industrial emissions, which strengthen the enhanced greenhouse effect.
3. Causes warming, melting ice, sea-level rise, and disrupted ocean currents.

Ocean Acidification

1. Excess CO₂ dissolves into seawater and forms carbonic acid (H₂CO₃).
2. The acid releases hydrogen ions (H⁺) and lowers the supply of carbonate ions (CO₃²⁻) that organisms need to build shells and skeletons.
3. Surface ocean pH has fallen from about 8.2 to 8.1 since pre-industrial times, making the ocean around 30% more acidic.

Coral Bleaching

1. Corals live in symbiosis with photosynthetic algae called zooxanthellae.
2. Under heat or acidity stress, corals expel the algae and lose their colour, a process called bleaching.
3. Bleached corals grow and reproduce poorly, and die if the stress lasts.

Warming, Hypoxia, and Weaker Shells

1. Warming and habitat shifts
   1. Warmer seas push species poleward or into deeper, cooler water.
   2. Marine species have narrow thermal ranges, so even a 1 to 2°C rise can force migration or cause die-offs.
2. Lower oxygen
   1. Warm water holds less dissolved oxygen, worsening hypoxic (low-oxygen) conditions.
3. Weaker shells and skeletons
   1. Molluscs, sea urchins, and plankton build shells from calcium carbonate (CaCO₃).
   2. As pH falls, CaCO₃ is harder to form and can dissolve, which destabilises whole food webs.

Designing an Experiment: Acidification and Shell Growth

1. Aim: investigate how acidification affects shell growth in molluscs.
2. Independent variable: water pH, controlled by bubbling CO₂.
3. Dependent variable: shell mass or thickness of oysters or snails.
4. Procedure:
   1. Keep temperature, salinity, and light identical across groups.
   2. Expose matched groups to different pH levels, such as 8.1, 7.8, and 7.5.
   3. Measure shell mass after several weeks.
5. Expected result: lower pH gives slower growth and thinner shells.