Micro-organisms as hydrological indicators in seas and oceans—IV

The quantitative distribution of heterotrophs in the water column of the Atlantic Ocean between Greenland and the Tropic of Capricorn (along 30°W), in the Norwegian, and in the Greenland Seas was studied. In the tropics the abundance of micro-organisms which assimilate slightly decomposed, non-humus organic matter is great, while in the subarctic and arctic areas it is low.

Equatorial-tropical water, rich in heterotrophs, was identified in the subatropic and subantarctic zones of the Atlantic Ocean, in the Norwegian and in the Greenland Seas at various depths. It occurred not only down to 1000 m but also much deeper at 2000–2500–3000 m.

Judging by the finding of equatorial-tropical water layers or ‘islands’ in the Atlantic Ocean, in the western Norwegian Sea and in the Greenland Sea at the same levels, it may be supposed that the circulation of these waters at certain depths is stable over extensive areas.

Most of the Atlantic Water (along 30 °W) from Denmark Strait to the Tropic of Cancer, is of arctic origin, i.e., water with few heterotrophs. These waters penetrate into the tropics and cross the equator. However, in the equatorial-tropical zone they do not form as thick a layer as Defant (1957) estimates.

Microbiological data indicate that waters in the equatorial-tropical zone of the Atlantic are significantly enriched by slightly decomposed, non-humus organic matter. In this respect they are similar to the Equatorial Water Masses of the Indian and Pacific Oceans. They, therefore, cannot be considered as merely transitional between the Central Water Masses of the northern and southern Atlantic.