Total organic nutrients in Drake Passage

Approximately 400 paired measurements of total organic nitrogen (TON) and t otal organic phosphorus (TOP) were made on a hydrographic transect across D rake Passage by a uv photooxidation technique. Latitudinal variations in TO N and TOP concentrations were observed, with the continental margins having surface-water concentrations approximately 50% greater than those in the c entre of Drake Passage. This may be due to enhanced primary production on t he continental shelves leading to increased production of organic nutrients . The vertical distributions of TON and TOP were characterised by surface m axima, declining to approximately constant levels of about 2.5 mu M TON and levels of TOP below detection by about 800 m. TON and TOP can be related b y the equation TON = (15.7 +/- 1.7)TOP + (2.48 +/- 0.17), r(2) = 0.44, n = 397. This is interpreted as reflecting two weakly connected pools of organi c material, the first a substantial refractory pool containing about 2.5 mu M TON and undetectable levels of TOP, which dominates in deep water, and t he second a surface labile pool containing TON and TOP in quasi-Redfield st oichiometry. This relationship between TON and TOP is not significantly dif ferent from a similar regression of data from the HOTS site in the central Pacific, implying that the composition of organic material, and hence the p rocesses controlling its formation, are not substantially different in thes e two biogeochemically very different environments. Our postulation of two different pools of organic material with different chemical compositions an d residence times leads us to suggest that the cycles of TON and TOP are mo re strongly coupled than is often thought. (C) 2000 Elsevier Science Ltd. A ll rights reserved.