Question 1

What is eutrophication and why do rivers turn green?

Accepted Answer

**Eutrophication** is the over-enrichment of water with nutrients — principally **nitrogen and phosphorus** — which triggers explosive algal growth and can ultimately strip the water of oxygen.

**The sequence:**

**Nutrients enter the water** from agricultural fertiliser run-off, livestock manure, treated and untreated sewage, and detergents.

**Algae and cyanobacteria bloom**, because nutrients were the limiting factor on their growth. The water turns green, sometimes with visible surface scum.

**Light is blocked.** Dense surface algae shade out submerged plants, which die.

**The bloom dies and decomposes.** Bacteria break down the enormous quantity of dead organic matter, and **decomposition consumes oxygen**.

**Oxygen crashes.** This is the damaging stage. Fish and invertebrates suffocate, producing die-offs. Severe cases create **hypoxic dead zones** where little can survive — the Gulf of Mexico and the Baltic Sea have large seasonal ones.

**Positive feedback** follows, as oxygen-poor sediments release stored phosphorus, fuelling further blooms.

**Why it is worse in warm, still conditions.** Higher temperatures accelerate growth, and warm water holds **less dissolved oxygen** — so heat and nutrients compound. This is why blooms appear in summer and why climate change is expected to worsen them.

**Cyanobacteria (blue-green algae)** are a particular concern because many produce **toxins** harmful to people, livestock and dogs. Dogs have died after swimming in affected water, and warning signs on lakes exist for this reason.

**Where the nutrients come from.** Agriculture is generally the largest source in rural catchments — **diffuse pollution** from fertiliser and manure spread on land, which is hard to regulate because it has no single outflow. Sewage is the dominant source in urban areas, and **storm overflows** discharging untreated sewage during heavy rain have become a major public issue in the UK.

**What helps:** buffer strips along watercourses, precision fertiliser application, cover crops, wetland restoration and improved phosphorus removal at treatment works.

Question 2

What is a carbon sink, and how much CO₂ do the oceans absorb?

Accepted Answer

A carbon sink is any reservoir absorbing more carbon than it releases. The two dominant natural sinks are the ocean and land vegetation and soils, and together they absorb roughly half of human CO₂

Question 3

What is ocean acidification?

Accepted Answer

The fall in ocean pH caused by seawater absorbing carbon dioxide from the atmosphere — sometimes called "the other carbon problem", because it is a direct chemical consequence of emissions that has

Question 4

Where does tap water come from, and how is it treated?

Accepted Answer

From rivers, reservoirs or underground aquifers, through a treatment sequence that has changed remarkably little in principle for a century — and which is among the largest public health

Question 5

What is blue carbon?

Accepted Answer

Carbon captured and stored by coastal and marine ecosystems — principally saltmarsh, mangroves and seagrass — which sequester carbon at rates far exceeding most terrestrial habitats, per unit area.

Question 6

What is overfishing, and do sustainable seafood labels mean anything?

Accepted Answer

Overfishing is taking fish faster than a population can replace itself — and the reason it persists despite being understood for a century is that it is a textbook common pool resource problem, where

Question 7

What is permafrost, and why does thawing matter?

Accepted Answer

Ground that has remained frozen for at least two consecutive years — covering roughly a quarter of the northern hemisphere's land — and its thaw matters because of what has been locked inside it for

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