Improved ultraviolet photo-oxidation system yields estimates for deep-sea dissolved organic nitrogen and phosphorus

Abstract

Photolysis of dissolved organic matter using high-intensity, ultraviolet (UV) light has been utilized since the 1960s as a method for the oxidation and subsequent quantification of dissolved organic nitrogen and phosphorus (DON and DOP) in both freshwater and marine water. However, conventional UV systems yielded variable and sometimes unreliable results; consequently, the method fell out of favor throughout much of the oceanographic community. Researchers turned to other oxidation methods such as persulfate oxidation or high-temperature combustion, even though they have difficulty when DON and DOP are <10% of the total dissolved N and P (for example, in the deep sea and in surface waters at high latitudes). Here, we revive the UV oxidation method using modernized light-generating equipment and high-precision colorimetric analysis of the oxidation products, resulting in the most well-constrained full ocean depth profiles of DON and DOP that are available to date. At Station ALOHA, in the North Pacific Subtropical Gyre, in the depth range of 900–4800 m, we find that DON is 2.2 ± 0.2 μmol L −1 (n = 49), DOP is 0.049 ± 0.004 μmol L −1 (n = 19), and the DOC : DON : DOP molar stochiometric relationship is 759 : 45 : 1. Preliminary estimates for the global ocean inventories of refractory DON and DOP are placed at 43.6 Pg N and 2.14 Pg P.