Why are one of the world's richest fishing grounds situated around the coast of Iceland?

Answer

The rich fishing grounds around Iceland depend on highly fertile phytoplankton communities. Phytoplankton are tiny water plants. In the summer, they live in the upper photic zone of the sea where they are carried by currents. Like plants on land, phytoplankton have the ability to use sunlight to make organic matter out of non-organic. This process is called photosynthesis and provides the main source of organic material for other organisms. Phytoplankton is food for zooplankton, which in turn is food for young fish and for pelagic fish. So the phytoplankton is the basis for all organic life in the sea, and thus for the fishing stocks.

In order to flourish and grow, phytoplankton need nutrients such as nitrate, phosphate and silica. Deep sea is normally richer in nutrients than surface sea, and the most fertile seas in the world are found where the nutrient-rich deep sea water has easy access to the surface layers. This can be found in the upstream areas off the west coast of South America and Africa and in high latitudes, where winds and ocean currents promote the mixing of deep sea with surface layers, thus renewing nutrients near the surface.

Phytoplankton seen under a microscope

To the south and west of Iceland the nutrient-rich deep sea streams up by the continental shelf and enriches the surface waters. At the sea current divide, southeast and northwest of the island the warm sea from the south and the cold sea from the north meet and draw up nutrients from the deep, throughout the whole summer. So the sea is also very fertile in those regions. Unstable weather conditions around Iceland also contribute to the blending of deep sea water with surface water which replenishes the nutrient content in the upper layers. This means that conditions for phytoplankton, which is the base for animal life in the sea, are very good off the shores of Iceland.

Icelandic seas are among the most fertile in the world. It has been estimated that phytoplankton within Icelandic fishing limits fix about 120 million tons of carbon in organic material every year, which approaches the amount in the Barent Sea, another area with fertile fishing grounds. But there is another side to the coin. Because nutrients are used so inefficiently between levels in the complex marine food chain, only a fraction of phytoplankton productivity is returned in productivity of fishing stocks.

Because the country is located at the confluence of warm and cold ocean currents, the environmental conditions here are very changeable, both in time and space. One can say that the condition of the sea off the coast of Iceland is rather unstable, particularly off the north coast where the influx of warm Atlantic water varies greatly from year to year. But it is exactly this variation that contributes to the level of fertility that is evident in the Icelandic seas. Both wind and sea currents help bring new supplies of nutrients up to the surface layers where they are used by phytoplankton. But this same variation causes annual fluctuations in stocks to be relatively large.

Translated by Paul Richardson.

Picture: University of Liverpool Earth and Ocean Sciences