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IB DP ESS — Topic 4: Water | SL & HL | Paper 1 & Paper 2A/2B
🎯 Learning Intentions By the end of this note, you will be able to:
- Define marine and freshwater food systems, including key terms such as trophic level, food web, biomass, and productivity in aquatic contexts.
- Explain how energy flows through marine and freshwater food webs and why efficiency decreases at each trophic level.
- Describe the structure and function of marine and freshwater ecosystems as food-producing systems, including the roles of producers, consumers, and decomposers.
- Analyse the impacts of human activities — including overfishing, aquaculture, pollution, and climate change — on marine and freshwater food systems using ESS analysis parameters: scale, sustainability, trade-offs, and multiple perspectives.
- Evaluate the sustainability of different approaches to managing marine and freshwater food systems, including fisheries management strategies and aquaculture practices.
- Discuss contrasting worldviews (ecocentric, technocentric, anthropocentric) in relation to the exploitation and conservation of aquatic food systems.
- Compare and contrast marine and freshwater food systems in terms of their productivity, biodiversity, and vulnerability to human pressures.
Real-World Context & Relevance
In 1992, the Canadian government made one of the most consequential fisheries decisions in history: it imposed a moratorium on Atlantic cod fishing off the coast of Newfoundland. The Grand Banks, once described by explorer John Cabot as so full of fish that they could be caught in baskets, had been fished commercially for over 500 years. By the early 1990s, the cod population had collapsed to less than 1% of its historic biomass. An estimated 40,000 people lost their livelihoods overnight. Three decades later, the cod population has still not recovered to commercially viable levels, demonstrating that the collapse of a marine food system can be effectively irreversible on human timescales.
This collapse was not an isolated event. According to the Food and Agriculture Organization of the United Nations (FAO), approximately 35.4% of the world's marine fish stocks are currently fished at biologically unsustainable levels — a figure that has risen steadily since the 1970s (FAO, "The State of World Fisheries and Aquaculture 2022"). Meanwhile, freshwater systems face their own crisis: the WWF Living Planet Report 2022 recorded an average 83% decline in freshwater vertebrate populations since 1970 — the steepest decline of any biome on Earth. Understanding how aquatic food systems function, and why they are so vulnerable, is one of the most urgent challenges in environmental science today.
🌍 Why This Matters Aquatic food systems — marine and freshwater — provide approximately 17% of the global population's animal protein intake and support the livelihoods of over 600 million people. Their collapse would trigger cascading ecological, economic, and food security crises, disproportionately affecting low-income coastal and riparian communities in the Global South. Understanding the ecology of these systems is therefore inseparable from understanding human sustainability.
Core Concepts & Definitions
To understand why the Grand Banks cod collapse happened — and why it was so difficult to reverse — we need to understand how energy moves through aquatic ecosystems. All food systems, whether on land or in water, are built on the same fundamental principle: energy enters through photosynthesis, flows through a series of feeding relationships, and is lost at each transfer. In aquatic systems, this process has distinctive features that make these ecosystems both extraordinarily productive and extraordinarily fragile.
📖 Key Definition — Aquatic Food System An aquatic food system is the network of organisms, ecological processes, and human activities involved in the production, harvest, processing, and consumption of food derived from marine or freshwater environments. It encompasses the full ecological food web — from primary producers to apex predators — as well as the human systems of fishing, aquaculture, and trade that extract food from these environments.
The foundation of any aquatic food system is primary production — the conversion of solar energy into organic matter by photosynthetic organisms. In marine systems,