A second stock solution of nitrate and nitrite was commercially available (1000mg L−1;Merck, CertiPur®).To ensure accuracy of the calibration and dilutions both solutions were
used for crosschecking during each run (except experiments). Low nutrient
seawater (LNSW) was provided by OSIL (www.seawatersolutions.
com) and artificial seawater (ASW) was prepared according to the
procedure given in Doe (1994). DOC spiking was done by dilution
of a stock solution of potassium hydrogen phthalate (1000 mg L−1,
Merck, CertiPur®). All dilutions were carried out with ultrapure water
(N18 MΩ cm−1, Sartorius).