Ecosystems have complex internal connections. Who woulda thought it?

BBC NEWS | Science/Nature | Cod fall may speed ‘toxic tide’

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.

The scientific team – which also involved researchers from Germany and Latvia – assessed three decades of data on the Baltic Sea food web.

Basically, zooplankton (tiny marine animals) eat phytoplankton, and sprat (small fish) eat zooplankton. Finally, cod eat the sprat.

“Right now, 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,” explained Dr Casini.

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

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