DESCRIPTION OF PHYTOPLANKTON AND NUTRIENT IN SPRING IN THE WESTERN NORTH-ATLANTIC OCEAN

Abundance of diatoms in the northern Sargasso Sea in spring has been observed in a 4-year period between 1981 and 1985 and then again in 1987. A theory is presented that if stratification blocks get-through of deep nutrients to the surface, as in the southern Sargasso, then in-situ cycling via non-diatoms and ammonia dominates. The theory is that if in-situ cycling does not dominate, as northward, then stratification does not block get-through of deep nutrients to the surface, which, in combination with the marked growth capacity of diatoms, produce an abundance of cells. This abundance consumes nitrate, phosphate and silicate at the surface and ultimately this nutrient-depleted water arrives in the southern Sargasso via the gyre circulation. Thus the nutrient-depleted water of the western Atlantic gyre is considered to be primarily achieved only by diatoms, in spring and in the northern Sargasso. A transect during April 13-16, 1985, north from the Virgin Islands along 63°-64°W shows a great change in the diatom species plus Emiliania huxleyi from negligible concentrations in the southern half of the section to large concentrations in the northern half. This unevenness of distribution can be rephrased by indicating that these species are narrow-niched with regard to nutrient availability. The other species, coccolithophores and dinoflagellates, are evenly distributed at low concentrations south-north (though some diminish slightly) and so they are broad-niched with respect to nutrient. High species diversity depends on broadness of nutrient niche and equilibrium conditions of hydrographically stable water (i), implying that when non-equilibrium conditions occur (ii), when niche breadth is less (iii) or both (iv), then diversity is less marked, (i) is confirmed by broad-niched species southward, (ii) is confirmed by broad-niched species northward, (iii) is confirmed by narrow-niched species southward and (iv) by narrow-niched species northward. © 1990 Oxford University Press.