Eutrophication and Biochemical Oxygen Demand

Eutrophication is the process by which a water body becomes progressively enriched with plant

nutrients -- mainly nitrogen and phosphorus compounds -- and, as a direct consequence, becomes

progressively depleted of the dissolved oxygen that aquatic animals need to survive. It proceeds

through a fairly predictable sequence of stages:

1. Sewage, detergents and fertiliser runoff carry large amounts of nitrate and phosphate compounds into a lake or pond.
2. These nutrients act like fertiliser for the water's algae and other aquatic plants, triggering explosive growth -- an algal bloom -- that can cover much of the water's surface.
3. The dense mat of algae blocks sunlight from reaching submerged plants below, which die back for lack of light.
4. As the bloom itself, and the shaded-out plants, eventually die, their remains are broken down by aerobic (oxygen-using) decomposer bacteria, whose population explodes to match the sudden glut of dead organic matter.
5. This burst of bacterial decomposition consumes dissolved oxygen from the water far faster than it can be replenished, and the water body becomes progressively hypoxic (oxygen-depleted).
6. Fish and other oxygen-dependent aquatic animals, unable to survive the falling oxygen levels, die off in large numbers, and the water body's overall biodiversity collapses.

Biochemical Oxygen Demand (BOD) is the standard laboratory measure used to track exactly this

oxygen-depleting potential of a water sample. It is defined as the amount of dissolved oxygen (in

milligrams per litre, or parts per million) that aerobic micro-organisms consume while breaking

down the biodegradable organic matter present in a water sample over a fixed period, conventionally

measured over five days at $20^{\circ}\text{C}$.

BOD is used as a water-quality indicator precisely because it directly measures how much organic

pollution load a sample carries: a high BOD value means the sample contains a large amount of

biodegradable organic matter, so decomposing it will draw down a correspondingly large amount of …