Dying organisms and nutrients sink because they are heavier than water. You might think that all the useful material would be lost, but it doesn’t necessarily stay in the deep zone.

In polar and temperate oceans, as the surface water gets cold it sinks and forces the nutrient-rich water up from the depths. This process is called overturn. Storms also mix up the layers and this constant churning process is why the waters around the poles are such rich feeding grounds. The nutrients support huge populations of plankton in the summer months. This is also why the temperature at the poles is so crucial because if the surface water warms up, this massive food factory is effectively shut down.

Upwelling
Overturn doesn’t happen in tropical and subtropical oceans, but a similar effect can be seen where currents pull surface water after them and cause an upwelling. Once again, nutrient-rich water from the deep replaces the top layers. One example of this happens off the coast of Namibia in southern Africa where the extra plankton attracts vast numbers of sardine, that in turn feed dolphins, whales, fur seals, sea birds and sharks. The key current here is the Benguela current.

Regions of upwelling are among the richest biological areas of the world and explain why the coastal waters off California and Peru are prime fishing areas. The Peruvian upwelling is caused by the Peruvian current, a branch of the Humboldt current and the increased productivity is important for vast numbers of anchoveta and sardines. Fifty per cent of Pacific fisheries production comes from the Peruvian upwelling zone.

Another example of upwelling is generated by monsoon winds and happens off the coast of Somalia in October. The colder water that is pushed up here can be some 18 degrees (centigrade) colder than the surface. This particular upwelling happens in October and is associated with large populations of sardinella.

Upwelling is caused by the effects of wind and the Coriolis effect. During El Niño years the process of upwelling can be disrupted, causing knock-on effects for many species.