Main         Home  |  CO's Welcome  |  Sea Cadet Corps  |  Our Unit  |  Junior Section  |  Things we get up to
Menu :     Ship's Company  |  Parents  |  Adult recruitment  |  Find Us  |  FAQ  |  British Navies  |  Site map

Adapted from a BBC online article and other sources: 7 May 2008

Declining fish stocks could be partly responsible for algal blooms in the oceans. Scientists found that the fall in cod stocks in the Baltic Sea in recent decades has been associated with increased numbers of the tiny marine plants that produce the blooms.

	Blue-green algal bloom in the Baltic Sea (Photo: R Lumiaro)

Algal blooms - sometimes known as toxic tides when they produce toxins - can be poisonous to people, fish and other wildlife, and seem to be on the increase worldwide.

The main cause of the blooms has been thought to be increasing levels of nutrients in the sea, with a second factor being sea temperatures driven higher by climate change. Nutrients such as nitrogen and phosphorus wash into the seas from agricultural land, and are also produced by some types of industry - a particular problem in largely enclosed waters such as the Baltic. These nutrients stimulate the growth of types of phytoplankton - varieties of algae - that can form blooms.

As well as the toxins they produce, the process takes oxygen out of the water. Basically, zooplankton (tiny marine animals) eat phytoplankton, and sprat (small fish) eat zooplankton. Finally, cod eat the sprat.

It seems that in the last 30 years, cod have been the top predators in the Baltic, after populations of seals and other marine mammals declined because of hunting.

The data gathered by the researchers showed a simple correlation. As the cod population declined sharply from the early 1980s, the sprat population rose; zooplankton declined, and phytoplankton increased.

	Red tides are sometimes a serious problem in northern Australia. (Red tide is a name sometimes given to coastal algal blooms that are reddish in colour.)
	Atlantic Cod: declining stocks are thought to cause algal blooms (International Cod Genomic Consortium)

The blue-green algae Nodularia spumigena and Aphanizomenon spumigena are the most common harmful phytoplankton species, forming spectacular blooms nearly every summer.

What to do if you come across an algal bloom

• Never drink water containing high concentrations of algae.
  Boiling the water doesn’t help – it doesn’t get rid of the toxins.
• Avoid bathing.
• Don’t let children swim or play near the shore.
• Keep pets and livestock away from affected shores and waters.

The blue-green alga Nodularia spumigena, common in the Baltic