IB ESS SLTopic 4 — Aquatic Food Production SystemsPaper 1 & 2Core idea~11 min read
Warming Seas and Ocean Acidification
Two separate consequences come from the same cause. Extra carbon dioxide in the air warms the planet, and some of that carbon dioxide dissolves into the sea and changes its chemistry. Keep them apart in your head and this topic becomes straightforward; blur them together and you will lose marks describing bleaching.
📚 What you need to know
Climate change is significant change in global temperatures and weather patterns over time, driven mainly by burning fossil fuels, deforestation and industry.
Ocean acidification is the ongoing decrease in the pH of the oceans, caused by absorption of excess CO2 from the atmosphere.
Dissolved CO2 forms carbonic acid, which lowers the pH.
Acidification impacts: weaker calcium carbonate shells in oysters, clams and sea urchins.
Coral bleaching is caused by warming and acidification together: stressed coral expels the algae in its tissues, turns white, and dies if the stress continues.
The Great Barrier Reef is the case study to know, including the back-to-back bleaching events of 2016 and 2017.
Two problems, one cause
Climate change refers to significant changes in global temperatures and weather patterns over time. It is mostly driven by human activity — burning fossil fuels, deforestation and industrial processes — and it leads to global warming, a rise in Earth’s average surface temperature.
Ocean acidification is a separate consequence of the same emissions. The ocean absorbs a large share of the excess CO2 we release. When CO2 dissolves in seawater it forms carbonic acid, which releases hydrogen ions and lowers the pH.
“Acidification” does not mean the ocean has become an acid — seawater is still slightly alkaline. It means the pH is falling, and for organisms that build calcium carbonate shells, that shift is enough to matter.
What warming does
Temperature rise. Warmer water alters habitat ranges. Many fish populations are moving towards cooler water, which disrupts the fishing industries that depended on them being where they were.
Melting ice caps. Polar ice is essential for many species: polar bears and seals need it for hunting, avoiding predators and raising young. Walruses forced to rest on land instead of ice crowd together, and mortality rises.
Sea level rise. Melting ice and thermal expansion threaten coastal ecosystems. Rising seas are encroaching on the coastal mangrove forests of Bangladesh, which are both habitat and protection against coastal erosion.
Hurricane damage. More intense and frequent hurricanes physically break coral reefs. Hurricane Irma in 2017 caused widespread coral destruction around the Florida Keys and the Virgin Islands.
What acidification does
Shellfish vulnerability. Acidic conditions weaken the calcium carbonate shells of oysters, clams and sea urchins, making them more vulnerable to predation, disease and environmental stress. That leads to population decline and disruption of marine food webs. Oyster populations in the Pacific Northwest of the USA are declining partly for this reason, and oyster farms there have struggled with reduced harvests as shells degrade.
Coral weakening. Coral skeletons are also calcium carbonate, so acidification makes reef structures more prone to erosion and breakage — on top of the bleaching caused by heat.
Coral bleaching
Coral gets most of its energy from algae living inside its tissues. Warmer temperatures and acidification stress the coral, and it expels those algae. Without them the coral turns white — bleached — and if the stressful conditions persist it dies.
A bleached reef can still recover if conditions return to normal quickly enough. What it cannot survive is repeated events with no gap between them — which is exactly what happened on the Great Barrier Reef in 2016 and 2017.
Case study: the Great Barrier Reef
Off the coast of Queensland, Australia, the Great Barrier Reef is the world’s largest coral reef system and home to enormous marine diversity. It also supports tourism and fishing industries that are central to the local economy.
Pressure
What it does
Coral bleaching events
Repeated events have caused significant damage; back-to-back bleaching in 2016 and 2017 affected around two thirds of the reef
Rising sea temperatures
Even very small temperature increases are enough to trigger a bleaching event
Ocean acidification
Weakens the coral structure itself, making it more susceptible to erosion and breakage
Knock-on effects
Biodiversity loss. Reefs support huge numbers of species; bleaching removes their habitat and food sources, so biodiversity falls.
Fishing industry. Reduced fish populations affect the livelihoods of local fishers and the wider economy.
Tourism. The reef draws millions of visitors a year. Bleached, lifeless coral reduces its appeal, and snorkelling and diving operators lose business, cutting tourism revenue.
Coastal protection. Reefs act as natural barriers against storm surges and erosion. Damaged reefs buffer waves less effectively, so communities along the Queensland coast face higher flood and property risk.
Food web disruption. Reefs are integral to marine food webs; bleaching removes key species and alters predator-prey relationships throughout the system.
The ESS point. A damaged reef is not only an ecological loss. It removes an ecosystem service — coastal protection — and two industries at the same time. That triple link between environment, economy and society is what an evaluation question is really asking for.
Bleaching is a stress response, not a disease and not pollution. If a question asks you to explain it, the sequence is: warmer and more acidic water, coral expels its symbiotic algae, coral loses its colour and its main energy source, coral dies if the stress persists.
Worked examples
EXAM Q1
Explain how increased atmospheric carbon dioxide causes ocean acidification. [3]
Step 1: absorption
The ocean absorbs excess CO2 from the atmosphere at its surface.
Step 2: reaction
Dissolved CO2 reacts with seawater to form carbonic acid.
Step 3: result
This releases hydrogen ions, so the pH of the ocean falls.
More CO2 in the air means lower pH in the seasay “lower pH”, not “the ocean becomes acidic” — it is still alkaline
EXAM Q2
Explain why ocean acidification is a particular problem for shellfish. [3]
Point 1: what their shells are made of
Oysters, clams and sea urchins build shells from calcium carbonate.
Point 2: the chemistry
Lower pH makes carbonate less available and weakens existing shells.
Point 3: the consequence
Weaker shells mean more predation, disease and stress, so populations decline and food webs are disrupted.
The organisms most affected are the ones that build with carbonatethe Pacific Northwest oyster farms are a ready-made named example
EXAM Q3
Discuss the impacts of coral bleaching on the Great Barrier Reef. [6]
Ecological
Loss of habitat and food sources for reef species, so biodiversity falls, and food webs are disrupted as key species disappear.
Economic
Fish populations decline, hitting fishers’ livelihoods, and tourism revenue drops as bleached reefs lose their appeal.
Social and protective
Weakened reefs buffer waves less well, so Queensland coastal communities face greater flooding and property damage.
Scale and evidence
The 2016 and 2017 back-to-back events affected around two thirds of the reef, leaving little recovery time between them.
Ecological, economic and social costs, all from the same causea 6-mark “discuss” needs three or more distinct strands plus evidence
💡 Exam tip
Keep warming and acidification as separate mechanisms with a shared cause. Questions often ask for one specifically.
Use the sequence CO2 → carbonic acid → lower pH whenever acidification comes up.
Describe bleaching as the expulsion of algae, not as the coral dying.
Learn the Great Barrier Reef with two facts: 2016 and 2017, and roughly two thirds of the reef affected.
Cover ecological, economic and social impacts for any high-mark question.
Link back to ecosystem services — coastal protection is one that reefs supply for free.
⚠ Common mix-up
Saying the ocean is becoming acidic. Its pH is falling but it remains slightly alkaline.
Thinking bleaching means the coral is dead. It is stressed and can recover if conditions improve quickly.
Blaming bleaching on pollution. The trigger is temperature and acidity, not dumped waste.
Treating acidification as a warming effect. It happens because CO2 dissolves, and would occur even without a temperature rise.
Forgetting sea level rise is partly thermal expansion, not only melting ice.
Listing only ecological impacts. ESS wants the economic and social consequences too.
Up next: Water Pollution — what we put into aquatic systems, how it is measured, and what it does to the food webs from the start of this sub-topic.
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