Fig. 1.

Sectional data obtained during a cruise from Woods Hole, MA to St. George’s, Bermuda in April of 2012. Gray shading indicates bathymetry. The Gulf Stream, separating productive northern waters from the oligotrophic subtropics, was crossed at ∼37.4°N. (A) Soluble reactive phosphorus (SRP) decreased sharply across the shelf-break and then fell to mostly <10 nmol⋅L⁻¹ in the Sargasso Sea. (B) Sulfolipid (SQDG) to phospholipid (PG) molar ratios and (C) total alkaline phosphatase activity increased concomitantly from north to south. (D) The ratio of polyphosphate (polyP) to total particulate phosphorus (TPP) also increased; note that this ratio is a relative measure expressed as nanoequivalents of a synthetic polyP standard. An identical latitudinal trend with fivefold increase in polyP:TPP from the offshore TWNA to the Sargasso Sea was found using NaOH instead of enzymatic polyP extraction, although with lower absolute polyP values. Data from the enzymatic polyP method are presented for consistency to Fig. 3 and because the method is more sensitive, which allowed greater coverage of the transect (SI Materials and Methods).