The transport and budget of sterols in the western North Atlantic Ocean1,2

Abstract

A general correlation exists between total free sterols, particulate organic carbon, particulate organic nitrogen, and chlorophyll a in the upper 300 m of the water column in the western North Atlantic Ocean. High values are found in shelf waters and in the subsurface chlorophyll a maximum in the Sargasso Sea, lower values at a Gulf Stream meander or ring fringe station.A diatom sterol, 24‐methylcholesta‐5,22‐dienol, appears to be produced only within the euphotic zone. Below this, consumption or removal processes dominate and the concentration of this compound decreases with depth. On the other hand, a zooplankton sterol, cholesterol, occurs throughout the water column, exhibiting maxima at the base of the mixed layer and at depths where particles tend to accumulate because of favorable chemical or density gradients.Flux calculations show that at most 0.05–0.3% of the sterols produced by phytoplankton in Sargasso Sea surface waters is deposited to the ocean floor. The sterol residence time (the average lifetime of a sterol molecule before it is metabolized) in the euphotic zone was calculated to be about 1 month, whereas the deep water residence time is 20–150 years.