What are dead zones and why do they stink of farts?

There are many terms thrown around in the climate debate of what many may be incomprehensible gibberish to you. This is an explanation of the term “dead zones”, a term commonly used in the media when marine, brackish (mix between salty and fresh water e.g. the Baltic Sea) and lake systems are discussed.

“Dead zones” are “dead” areas in marine- or limnetic- (lakes) systems. Well, they are not completely dead, it is just that the “normal” flora and fauna can no longer survive in the oxygen-free conditions that have developed.

So why does this happen? Often it is a combination of too high nutrient concentrations, high temperatures together with the right (or wrong) circulation patterns. High temperature and nutrients may result in a high production of organic material in the so-called “euphotic zone”.

The euphotic zone is the zone the sun’s rays can penetrate and hence where photosynthesis can be carried out by the phytoplankton (the plankton that can carry out photosynthesis and that are so called “producers”). We may call them the “plants” of the ocean, though not entirely correct. The deep sea is dark, so there is only a small belt of water at the top of the water column where this photosynthetic production can occur.

The phytoplankton settle (sink) out of the water column as they die or lose buoyancy (lose their ability to float around in the euphotic zone) and eventually reach the bottom of the lake or ocean where they continue to be broken down by bacteria and other benthic organisms.


The benthic organism Glycera alba (image credit: Hans Hillewaert)

Benthos is another name for the bottom of a lake or the ocean. Some of the benthic organisms look like something out of a horror film. The most amazing, creepy things crawl around down there, and they are important: very important!

The organisms “eat” the material and respire. The respiration process is the same that occurs in us humans when we eat:


Respiration: we eat, we breathe, and we create energy and carbon dioxide. We are quite amazing when you think of it!

Oxygen is needed in this process. However, when too much material settles down on benthos, the oxygen runs out! No energy can be made, and the benthic organisms die.

Albeit, the world is full of opportunists, and there are indeed weird bacteria that can live under these conditions. Instead of using oxygen in this process, they use sulfur. To use more technical terms: they use sulfur instead of oxygen as terminal electron donor during anaerobic photosynthesis. Unfortunately, the side product of this reaction is hydrogen sulphide, which smells of farts. In a way, we can see the dead zones as giant egg-smelling fart-factories. They are not good.


The major dead zone areas around the world labeled in red. You can clearly see the impact of humans when the hydrographic conditions are unfavourable (image credit: Robert Simmon & Jesse Allen (NASA Earth Observatory))

You can do a lot to prevent these dead zones from spreading: buy eco-labeled washing powder and other detergents and if you have a boat: do not dump your sewage straight into a lake.

Science Safely

Dr. Anna of The Imaginarium

likes beer

is a marine biologist


  • Marine benthic hypoxia: A review of its ecological effects and the behavioural responses of benthic macrofauna. By: Diaz, RJ; Rosenberg, R. Edited by: Ansell, AD; Gibson, RN; Barnes, M. OCEANOGRAPHY AND MARINE BIOLOGY – AN ANNUAL REVIEW, VOL 33  Book Series: Oceanography and Marine Biology   Volume: 33   Pages: 245-303   Published: 1995
  • Life after death in Lake Erie: Nutrient controls drive fish species richness, rehabilitation. By: Ludsin, SA; Kershner, MW; Blocksom, KA; et al. ECOLOGICAL APPLICATIONS  Volume: 11   Issue: 3   Pages: 731-746   Published: JUN 2001
  • Anaerobic Oxidation of Sulphur Compounds as Electron Donors for Bacterial Photosynthesis. H. G. Truper and U. Fischer. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. Vol. 298, No. 1093, Sulphur Bacteria (Sep. 13, 1982), pp. 529-542
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