US EPA

934459 

Journal Article 

Rayleigh distillation equations applied to isotopic evolution of organic nitrogen across a continental shelf 

Van Hale, R; Frew, RD 

2010 

1 

Marine and Freshwater Research
ISSN: 1323-1650
EISSN: 1448-6059 

61 

3 

369 

English 

10.1071/MF09041 

WOS:000276030500012 

http://www.publish.csiro.au/?paper=MF09041
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The isotope enrichment of nitrate due to uptake by phytoplankton has previously been measured in a variety of ways, both in the field over time or space, and in vitro. This study provided an unusual opportunity to measure the isotopic fractionation of nitrate over an extended spatial scale. Results are presented from the Otago Continental shelf, New Zealand. delta(15)N-PON and [NO(3)(-)] was correlated using Rayleigh distillation equations to obtain isotope enrichment factors for the uptake of nitrate. The decrease in nitrate concentration during advection landward across the shelf was consistent with removal by phytoplankton, with an average enrichment factor (epsilon) of 8.2 per mil, as calculated using the Rayleigh accumulated product equation. The spatial evolution of delta(15)N-PON and [NO(3)(-)] across the Otago continental shelf led to the conclusion that cross-shelf advection is the predominant process bringing nutrients to the shelf. No Rayleigh distillation-type process is observable from monthly observations. However, in subantarctic surface water (SASW), a seasonal variation in d15N-PON was seen that was in the opposite phase to that expected for isotopic enrichment of the nutrient pool. In SASW, delta(15)N-PON peaked at 4 parts per thousand in winter months and fell to -2 parts per thousand in the spring and summer. 

field measurements; nitrogen isotopes; particulate organic nitrogen; subantarctic zone (SA); subtropical water (STW)